1 Introduction

The genus Scomberomorus, commonly known as Spanish mackerel or seerfish, comprises medium to large-sized predatory fish in the family Scombridae. These species are widely distributed in coastal and continental shelf waters of tropical and temperate regions around the world. The genus includes approximately 18 recognized species, such as the well-known Japanese Spanish mackerel (S. niphonius), narrow-barred Spanish mackerel (S. commerson), Indo-Pacific king mackerel (S. guttatus), and Atlantic Spanish mackerel (S. maculatus). They are found across the Asia-Pacific, Indian Ocean, and Atlantic coasts. Renowned for their rapid swimming and aggressive predation, Scomberomorus species are key targets in many coastal fisheries and hold significant importance in global fisheries (Zeng et al., 2022).

The genus Spanish mackerel has both paleontological research value and practical fishery management significance. On the one hand, its extensive geographical distribution and highly migratory characteristics make it a typical representative species for studying the origin and diffusion of marine animals; through molecular phylogenetic, its evolutionary path in the geological historical period can be traced to understand the impact of marine geographical changes on species differentiation (Jeena et al., 2022; Abdussamad et al., 2023). On the other hand, Spanish mackerel are also important catches for humans, and fishermen in many coastal countries rely highly on such fish resources to make a living. In recent years, with the increase in fishing intensity and changes in the climate environment, the population of Spanish mackerel in some sea areas has decreased significantly. Therefore, studying the origin and evolution, biological characteristics and response laws of Spanish mackerel fish is not only of scientific significance, but also helps to formulate effective fishery management and protection strategies.

This study aims to review the "original and diffusion" of the genus Spanish mackerel, and comprehensively summarize the latest scientific research progress: mainly introduce the classification status, phylogenetic relationship and origin hypothesis of the genus Spanish mackerel; discuss the impact of paleogeography and climate change on the origin and distribution of genus Spanish mackerel; summarize the ecological physiological mechanism of Spanish mackerel adaptation to the environment; through the above content, we strive to reveal the survival picture of Spanish mackerel under the interweaving of the dual factors of evolution and human activities, and provide a scientific basis for fishery management and ecological protection of related species.

2 Phylogenetic and Origin of Scomberomorus spp.

2.1 Classification status and species diversity of the genus Spanish mackerel

The Spanish mackerel belongs to the suborder of the Mackerel family Tuna, which is a specialized branch in the Mackerel family. The genus was first named in 1801 by French naturalist Lacepède based on specimens collected by Martinique. The Mackerel family is traditionally divided into several families according to its molecular and morphological characteristics. The Spanish mackerel belongs to the Scomberomorini family, which is parallel to the Mackerel (Scomber, such as Mackerel) and Tuna family (Thunnini). Currently, it is recognized that the genus Spanish mackerel contains about 18 effective species, commonly known as "mackerel" or "foxmackerel", and is distributed in warm waters around the world. These species are morphologically slender and streamlined, with sharp teeth and well-developed tail shank bulges, making a living by swiming at high speeds and preying on small fish. The main distinctive characteristics between different species include body lateral markings, scales and vertebrae number. For example, the body of the Japanese Spanish mackerel (S. niphonius) has blue-gray wavy markings, mainly produced in the northwest Pacific; the body of the narrow-band Spanish mackerel (S. commercial) has black stripes, which are widely distributed in the Indo-Western Pacific and invaded the Mediterranean Sea; while the Atlantic Spanish mackerel (S. maculatus) is scattered on the side, distributed along the Western Atlantic coast (Widayanti et al., 2024). Molecular phylogenetic analysis supports the effectiveness of species composition of Spanish mackerel species, but also suggests the possibility of hidden species. This suggests that species diversity in the genus Spanish mackerel may be underestimated, and that some widely distributed species have genetic differentiation within them.

2.2 Application of molecular phylogenetic analysis in origin research

The development of modern molecular systems provides a powerful tool for exploring the origin and evolution of Spanish mackerel. Through mitochondrial DNA and nuclear gene sequence analysis, researchers were able to reconstruct the phylogenetic relationship within the genus Spanish mackerel and with other Spanish mackerel fishes, and calculate the differentiation time based on the molecular clock. Early phylogenetic research is mostly based on single gene sequences such as mitochondrial COI barcode or 16S. In recent years, it has gradually expanded to the combined analysis of multiple mitochondrial and nuclear genes. The results support the monolinearity of each known species of the genus Spanish mackerel, with clear lineage differentiation between Atlantic species (such as the Caribbean Mackerel S. regalis group) and Indo-Pacific species. Molecular clock analysis shows that the genus Spanish mackerel is likely to differentiate from other branches of the Spanish mackerel family from the late Paleogene to the early Neogene (about 30~25 million years ago), similar to the differentiation of the genus Tuna and Mackerel (Jeena et al., 2022). The genus has since experienced faster species radiation, forming the major current lineages during the Miocene to Pliocene. Genetic distance analysis of mitochondrial DNA sequences also reveals that some regional populations may have hidden taxons. For example, the narrowband mackerel samples collected in Australian waters are divided into two genetic branches, corresponding to the Australian endemic species S. queenslandicus and the wider narrowband mackerel, but these two species with similar morphology are difficult to distinguish molecularly. This finding prompts morphological review and re-evaluation of species boundaries (Zeng et al., 2022).

2.3 Origin hypothesis: from tropical shallow sea to global spread

Based on phylogenetic and paleogeographic evidence, scholars suggest that the genus Spanish mackerel originated from the ancient tropical shallow sea environment and spread to the world with the formation of marine channels and climatic events. Spanish mackerel is now particularly diverse in western Indo-Pacific, such as the Southeast Asia-Western Pacific Ocean region, which has gathered multiple species and is believed to be the center of origin of the genus. During the Cenozoic period, the closure of the Tetis Sea and the settlement of the Sunda ancient continent created vast tropical shallow seas such as the Indo-Taiwan Warm Pond, providing a suitable environment for the early evolution of Spanish mackerels. Subsequently, with the sea route between Africa and South American continents, some Indo-Pacific ancestor Spanish mackerels entered the Western Atlantic through the South Atlantic, achieving trans-ocean diffusion. For example, the Atlantic Ocean's king Spanish mackerel (S. cavalla) and the Spanish mackerel (S. maculatus) may have originated from the ancestral populations entering the Atlantic through the Cape of Good Hope in the late Miocene, and are closely related to the narrow-band Spanish mackerel in the Indo-Pacific (Herrera et al., 2015). The molecular phylogenetic tree shows that Atlantic species are phylogenetically embedded between Indo-Pacific species, suggesting that they originate from one or more westward spread events in the Indo-Pacific lineage. Meanwhile, some Spanish mackerel species may spread westward through the ancient Mediterranean-Tettis Sea channel, or cross the Eastern Pacific before and after the closing of the Panama Isthmus. Species existing in the Eastern Pacific (west coast of the Americas), such as S. sierra, are believed to have common origins with the Atlantic species, and their ancestors may have detoured through the southern end of the Central and American isthmus during the Pliocene. In addition, when sea levels in the Pleistocene ice age fell, the distribution of some nearshore species was limited by the shelf exposure, and the rise in the late ice age caused its re-expansion.

3 The Impact of Paleogeography and Climate Change on the Origin of Spanish mackerel

3.1 Shaping of distribution during the quaternary ice age and sea level changes

The repeated ice-interglacial cycles during the Quaternary period had a profound impact on the geographical distribution of marine organisms. For species like Spanish mackerels that mainly live in shallow coastal coastal seas, a significant drop in sea levels during the glacial period has significantly changed their habitat area and connectivity. During the last glacier period (about 20 000 years ago), sea level fell by about 120 meters from today, with a large area of continental shelf exposed, and many of the now-separated sea areas were once connected (Schuerch, 2017). This environmental change may shrink the Spanish mackerel's habitat range and lead to population isolation. For example, the Sunda Shelf of Southeast Asia was connected to large land during the ice age, and the area of shallow waters such as the South China Sea and Java Sea reduced, which led to the differentiation of some nearshore populations. However, the rapid sea level rise after ice (the so-called “Flandrian transgression”) reconnects the previously isolated shallow sea areas, providing opportunities for Spanish mackerel population spread and genetic exchange (Figure 1) (González‐Wevar et al., 2023). The genetic diversity analysis of mitochondrial DNA supports this: For example, the proportion of samples sharing haplotypes in various places such as the Brazilian coastal Spanish mackerel (Brazilian Spanish mackerel S. brasiliensis) has a high proportion of samples and extremely weak genetic structure. It is inferred that it experienced population expansion about 10 000 years ago. This coincides with the time when seawater rises and re-expands habitat during the last period of ice elimination. Changes in the paleomarine environment not only affect horizontal distribution, but also vertical distribution. During the ice age, sea water temperature drops, and warm-water Spanish mackerels may migrate south to avoid cold. After the interglacial period, they will go north to occupy high-latitude waters again. This climate-driven distribution recombination has shaped the genetic pattern and adaptability of populations in various regional areas of modern Spanish mackerels (Oosting et al., 2022). Therefore, the fluctuations in the Quaternary climate and the rise and fall of sea level during the ice age are important background factors for understanding the history of the origin and diffusion of Spanish mackerel.

3.2 Changes in connectivity between current systems and habitats

Marine circulation plays a media role in biological diffusion and gene exchange, and its historical changes have also affected the diffusion path and population connection of Spanish mackerel. During the Miocene-Pliocene, plate tectonic events such as the closure of the Tethis Sea and the changes in the Indo-Pacific Strait reshape the global current pattern. For example, with Australia moving northward and New Guinea joining with Asia, the equatorial circulation was blocked, forming a pattern of separation between the modern Indian Ocean and the Pacific Ocean. The ancestor populations of the genus Spanish mackerel may be driven by changes in these currents and expand into new sea areas under the influence of the current system. Many Spanish mackerels today lay eggs nearshore and rely on ocean current diffusion to expand their habitats for the floating larval stage (Pan et al., 2020). According to research, the early life history of Spanish mackerels is drifting, and eggs and juveniles can spread hundreds of kilometers away with the surface flow. During the Pleistocene climate change, the intensity and direction of ocean currents also changed. For example, changes in prevailing winds lead to intensity adjustments such as black tides and tides, which in turn affects the distribution of fish in the northwest Pacific. Long-term fluctuations in climate modes such as Pacific Interdecadal oscillation (PDO) will cause north-south swings and changes in the upstream intensity of the current path, thereby periodically changing the connectivity of Spanish mackerel habitats (Manral et al., 2023). For example, in the cold phase of PDO, the black tide tributaries in the East China Sea in China are weak, which may reduce the mixing frequency of Spanish mackerel populations in the north and south seas; while the warm phase is the opposite. This change in marine processes is more severe in the geological age: during the last ice-interglacial transition, the current system underwent reorganization, and the South Asian monsoon and equatorial countercurrent were all changed. Therefore, as a "diffusion highway", its temporal and spatial changes directly affect the opportunity and scale of Spanish mackerel's spread across regions.

3.3 Fossils and ancient DNA evidence for early distribution patterns

Compared with other marine fishes (such as sharks or bonefish fossils), the fossil record of Spanish mackerels is not rich because they mostly live in remote waters and their bones are not easy to preserve. However, limited fossil evidence and ancient DNA analysis still provide some clues. The suspected Spanish mackerel fossils reported mainly include vertebrae or teeth found in the Neocentric strata, but the precise ownership is still questionable. Because the bones of Spanish mackerel lack obvious species characteristics, it is difficult to analyze its evolutionary history by traditional paleontology. Relatively speaking, the development of ancient DNA technology has provided new ideas for the study of paleogenetics of fish. If environmental DNA fragments of Spanish mackerels can be extracted from deep-sea sediments or polar ice cores, or DNA sequences can be detected from fish bones at medieval human sites, it is expected to compare differences in modern populations (Zeng et al., 2022). Paleoclimatic marine sediment indicators (such as organic matter markers) show that after the last ice age, productivity in tropical upwelling areas may have promoted the prosperity of middle and upper-class fish such as Spanish mackerel. In addition, some ethnic archaeological records indirectly reflect the changes in the distribution of Spanish mackerels in historical periods. For example, the emergence of narrow-band Spanish mackerels has been recorded in the eastern Mediterranean since the end of the 19th century, which is believed to be the result of migration to the north via the Suez Canal. This type of prehistoric/historical data combined with modern genetic diversity patterns can verify the reliability of molecular clock inference.

4 Ecological Adaptation Mechanism of Spanish mackerel

4.1 Evolution of temperature and salinity tolerance

As a large-scale migratory fish spread throughout the temperate to tropical areas, Spanish mackerel has formed a widespread adaptation to environmental factors in its long-term evolution. The first is tolerate and adapt to water temperature. Most species of the genus Spanish mackerel prefer warm waters, and the temperature range is generally between 18 ℃and 30 ℃, but the tolerance of different species to low temperatures has evolved. The northernmost Japanese Spanish mackerel can tolerate seawater temperatures as low as about 10 °C in winter, so it can overwinter in the Yellow Sea and Bohai Seas (Harada et al., 2021). On the contrary, some strictly tropical species such as Borneo mackerel (S. multiradiatus) retreated to the waters near the equator when the water temperature dropped slightly. This difference stems from the differences in population selection and metabolic regulation capabilities of cold adaptive genes. Studies have found that Japanese Spanish mackerel can maintain body function by increasing basal metabolic rate and heat production at low temperatures, while tropical species lack this mechanism. In terms of salinity, Spanish mackerels usually live in normal salinity seawater offshore (about 30‰~35‰), but can tolerate a certain range of salinity fluctuations. For example, in the estuary area, Spanish mackerels in the juvenile stage can enter mixed waters with slightly lower salinity for feeding. Its gills and kidneys have good osmotic regulation functions and can cope with the impact of salinity changes on fluid balance. These physiological adaptation features allow Spanish mackerels to utilize extensive nearshore niches (Eaton et al., 2024), distributed from the tropical coral sea to the temperate continental shelf. In recent years, environmental DNA and transcriptomic studies have further revealed the molecular basis of Spanish mackerel's adaptation to warm salt. Some cold-resistant populations show high expression of anti-cold-related genes (such as anti-freeze protein genes), while high-temperature-resistant populations have heat shock proteins used to deal with heat stress. These evolutionary adaptations make Spanish mackerel a "versatile" in dealing with changing marine environments, and its distribution adjustment ability is also relatively strong in the context of global climate change.

4.2 Efficient predation and migration ability

Spanish mackerel is a typical middle and upper-class swimming predator, and has evolved a series of morphological and physiological characteristics that are conducive to predation and migration. First, its streamlined body, high proportions of white muscles and developed tail shanks are conducive to rapid swimming, with burst speeds exceeding 10 meters per second, which allows it to quickly chase prey and migrate long distances. The tail fin of the Spanish mackerel is crescent-shaped, and the fork tail helps reduce drag and generates strong thrust. Coupled with the tiny round scales on the surface and the skin recessed structure, it can reduce turbulence, making it an ideal design for high-speed swimming. Secondly, Spanish mackerel has sharp teeth and strong jaw muscles, and is good at preying on middle and upper fish schools, such as sardines, anchovies, mullets, etc. They often hunt in groups, forcing the prey fish to the water and rushing hard to bite, and feeding efficiently. In an uneven distribution of food resources in the marine environment, Spanish mackerels rely on rapid movement to search for bait and capture it, and their digestive tracts are short to quickly process high-protein foods. This “high predation-high metabolism” strategy requires strong circulatory and respiratory support (Planas et al., 2017). The gill filaments of Spanish mackerels have a large area and high oxygen capacity in the blood, which allows them to meet their oxygen needs when they move at high speed. Again, Spanish mackerels showed obvious seasonal migration behavior. Take Japanese Spanish mackerel as an example. Every spring, as the sea water heats up, it migrates from south to north to the Yellow and Bohai Sea to lay eggs, and goes south to overwinter in autumn and winter (Fauvelot and Borsa, 2011). This migration model is conducive to the use of highly productive sea areas in each season. The migratory ability of Spanish mackerels is partly derived from their perception of environmental cues such as geomagnetic fields, ocean currents, and temperature gradients, as well as the guidance of group behavior. Genetic studies have shown that high migration leads to gene mixing among different egg-laying populations, reducing geographical differentiation.

4.3 Reproductive strategies and ecological adaptation of reproductive cycles

Spanish mackerel has r-countermeasure reproductive characteristics, namely high egg laying rate and long-distance egg laying migration to increase the survival chances of offspring. Most Spanish mackerels mature early and have strong fertility. For example, Japanese mackerel females mature sexually at about 2 years old and can lay eggs at about 40 cm in length. The spawning season usually chooses summers with suitable water temperatures and abundant baits, and is carried out in warm waters along the coast or near islands. Spanish mackerel is a multiple-spawn type, and ovulates multiple times in batches throughout the breeding season. Each mature female can lay hundreds of thousands to millions of eggs in one season (Weng et al., 2020). These eggs are floating eggs with a diameter of about 1 mm, and contain oily balls that can spread to a wide area with the current, thereby increasing the distribution range of juvenile fish. High egg laying number and diffusion at the slab stage are a kind of adaptation of Spanish mackerel to unstable marine environments - even if locally unfavorable conditions are present, drifting larvae may still reach better sea areas, thereby ensuring population continuation. However, this strategy also leads to possible confounding of offspring between spawning grounds, reducing the isolation of population structure. For example, a study on the microchemistry of larvae otoliths of East China Sea and Yellow Sea Spanish mackerel found that juvenile fish hatched in different spawning grounds could share the same fishery and were jointly supplemented into multiple local populations rather than independently supplemented. This shows that the breeding strategies of Spanish mackerel tend to be "source-aggregation" dynamics, that is, multiple spawning grounds serve as sources to transport young fish to a wide range of habitats. In addition to high egg laying, Spanish mackerel also exhibits synchronization of environmental cycles in the reproductive cycle. For example, when the water temperature of Japanese Spanish mackerel rises above 20 ℃ from April to June each year, the peak is in early summer, which is consistent with the peak of bait small fish during this period, which is conducive to improving the survival rate of juvenile fish. After laying eggs, adult fish will return to the fattening grounds (such as the northern waters) in time to recover their strength. Reproductive input and feeding growth alternately to maintain a healthy body condition cycle. In addition, some low-latitude species may lay eggs multiple times throughout the year, but there is still a tendency to reproduce concentratedly during periods of good conditions such as rainy season and estuaries.

5 Global Distribution Pattern and Migration Route

5.1 Native populations of the Western Pacific and Indian Oceans

The Western Pacific-Indian Ocean region is considered to be the origin and differentiation center of the genus Spanish mackerel, and is also the area with the most diverse species and the most prosperous population today. In the Western Pacific, China's East China Sea, South China Sea, Japan's offshore and Southeast Asian Archipelago waters are inhabited by several Spanish mackerel species, especially those represented by Japanese mackerel (S. niphonius) and narrow-band Spanish mackerel (S. commercialson) (Vineesh et al., 2018). Japanese mackerel is mainly distributed in the coasts of China, Japan and South Korea in the northwest Pacific, and is an important fishery resource in this sea area in spring and summer. Its typical migration route is to overwinter in the East China Sea and the northern part of the South China Sea in winter, and to migrate northward to the Yellow Sea and Bohai Sea as the water temperature rises, and then return south to overwinter in late summer and early autumn. This migration period leads to obvious abundance fluctuations in populations in different sea areas at different times, and fishermen from coastal countries often chase migrating Spanish mackerel populations according to seasons (Pan et al., 2020). There are also a wide variety of Spanish mackerels in the Indian Ocean region. For example, the famous narrow-band Spanish mackerels are widely distributed in the Red Sea, Arabian Sea, Gulf of Bengal and East Africa coasts, and are the center of local fishery species. The annual monsoon conversion drives its migration along the East Coast of Africa and the Indian subcontinent, following the bait fish in the upward high-yield areas (Sougueh et al., 2023). In the tropical waters of Southeast Asia, spotted Spanish mackerels, Queensland mackerels (northeast coast of Australia), Indian mackerels (population of S. guttatus in the Indian Ocean), etc. are living. Most of these fish are active along the coast and near island reefs, and are of considerable scale. For example, the Bay of Bengal and the Andaman Sea have huge spotted Spanish mackerel fisheries that support local livelihoods.

5.2 The transmission channel into the Atlantic Ocean and the mediterranean

There are relatively few Spanish mackerels in the Atlantic Ocean and mostly belong to the "New World" genealogy, which is believed to be the result of independent evolution in the Atlantic Ocean after migrating through sea routes. The main species include: Spanish mackerels (S. maculatus) and king Spanish mackerels (S. cavalla) distributed in the northern part of the Western Atlantic Ocean, cero Spanish mackerels (S. regalis) distributed in the Caribbean Sea and the Gulf of Mexico, and three-tooth Spanish mackerels (S. tritor) distributed along the eastern Atlantic Ocean and West Africa. These species have a close relationship and together form the "Atlantic mackerel species group" (i.e., the S. regalis population), which has a slightly larger genetic distance from the Indo-Pacific population. There may be two ways to appear at the Atlantic mackerel: one is to enter the Atlantic coast from the Indian Ocean through the southern tip of Africa (Cape of Good Hope). The current model shows that from the end of the Miocene to the early Pliocene, the warm currents in the western Indian Ocean could transport organisms along the coast of West Africa through the then warmer South Atlantic. The second is to enter the Western Atlantic Ocean from the Eastern Pacific Ocean through the Central American waterway (before the closing of the Isthmus of Panama). However, the Panama Isthmus was completely closed about 3 million years ago, and the main differentiation of the genus Spanish mackerel may be slightly later, so more evidence supports the first pathway. After entering the Atlantic Ocean, Spanish mackerel successfully settled on the nearshores of North and South America and formed a trend of expansion toward temperate zones (Rohde and Hayward, 2000). The Mediterranean Spanish mackerels are likely to originate from the "invaders" of the Red Sea. Since the opening of the Suez Canal in 1869, more than 300 Red Sea species have migrated to the eastern Mediterranean Sea, and narrow-band Spanish mackerel is one of the representative large migratory fish. Narrow-band Spanish mackerel was recorded along the Palestine coast as early as the 1930s, and by the end of the 20th century the species had established populations in the Eastern Mediterranean. In recent years, reports have also pointed out that narrow-band Spanish mackerels have been found in the western Mediterranean Sea, such as Tunisia (Figure 2) (Weng et al., 2021). This indicates that it is continuing to spread into the Mediterranean, which may have an impact on local ecology and fisheries.

5.3 Secondary diffusion paths caused by human activities

In addition to natural geographical spread, human activities have also promoted the secondary diffusion and introduction of the genus Spanish mackerel in modern times. This influence mainly occurs in two ways: one is the opening of canal waterways, and the other is the transplantation and introduction of species. The aforementioned Suez Canal case is the first. The artificial canal has opened up barriers between the Red Sea and the Mediterranean, allowing Red Sea fish such as narrow-band Spanish mackerels to enter. This "Lessepsian migration" is an unintentional artificial spread, but has a significant impact. For example, the Panama Canal (navigated in 1914) connects the Pacific Ocean and the Atlantic Ocean. Although it is mediated by freshwater lakes, it is generally believed that marine species are difficult to cross directly, but occasionally reports indicate that "unusual" fish appear in the waters near the canal. If the climate warms in the future, the new possibility of Spanish mackerel spreading through the canal cannot be ruled out. The second pathway is direct proliferation, release or transplantation. Compared with salmon, Spanish mackerels have not been widely transplanted and cultivated, but there are proliferation and release practices in some countries. These release juvenile fish may spread to other adjacent waters, thereby changing the local population structure (Malik and Muzaffar, 2024). In the United States, there are also coordination of proliferation projects for Atlantic Spanish mackerel and interstate release management. Furthermore, with the development of global marine trade, ship ballast water has become a new medium for the trans-ocean transmission of marine organisms. The larvae or egg may be sucked into the chamber compartment and released when discharged from other ports. If a new environment is suitable, they have the opportunity to build populations. Although the probability of large predatory fish diffusing through ballast water is low, it cannot be completely ruled out. In addition, the "invasion" of populations originally distributed adjacent to each other in the context of climate change can also be regarded as part of secondary diffusion. For example, in recent years, reports of narrow-band Spanish mackerels in the Indian Ocean entering the southwestern Atlantic Ocean along the southern end of Africa have gradually increased, which may be related to the increase in water temperature.

6 Population Structure and Genetic Diversity

6.1 Regional population division and gene flow pattern

The wide-area distribution and highly migratory life history of Spanish mackerel make its population structure characterized by "large-scale weak differentiation", but there are also substructures in the local area. Genetic studies usually analyze the degree of gene communication and differentiation levels between Spanish mackerels in different sea areas through mitochondrial DNA sequences, multi-site microsatellites and single nucleotide polymorphisms (SNP) markers. The results generally show that Spanish mackerels exhibit significant gene flow: populations hundreds or thousands of kilometers apart tend to share the same haplotype or alleles, and the F_ST isovariate index is extremely low, making it difficult to distinguish independent population units (Broderick et al., 2011). For example, the genetic distance between Brazilian horse Spanish mackerels collected at eight sites along the Brazilian coast was small, and AMOVA analysis failed to detect significant intergroup differences, supporting it as a single pan-MiG population. For example, the microsatellite analysis of Japanese Spanish mackerels along the coast of China also found that the samples in the Yellow Sea, East China Sea and South China Sea did not cluster clearly, and the genotype frequency differences in different places were not obvious. These are all attributed to the strong swimming ability and long-distance migration of Spanish mackerels, allowing individuals in different regions to cross geographical barriers and undergo reproductive mixing. This situation is particularly evident in open waters and is called "genetic homogeneity". However, not all cases are completely structured. In some oceanic islands, semi-enclosed seas or environments with obvious ecological differences, Spanish mackerels may form relatively independent subgroups. For example, due to shelf restrictions and monsoon circulation characteristics, the Arabian Sea and the Bay of Bengal groups have experienced certain genetic differentiation. Even within the same species, weak structures may be caused if multiple spawning grounds exist and adults have certain re-entry habits. For example, narrowband Spanish mackerels are separated from the spawning grounds on the east and west coasts of Australia, and the two groups have weak allelic frequency differences.

6.2 Application of microsatellite and SNP markers in genetic structure analysis

High-resolution molecular markers are a powerful tool for analyzing the structure of fine populations. For high-gene flow species such as Spanish mackerel, structural differences were often difficult to detect in the past single mitochondrial markers. In recent years, researchers have used more microsatellite and SNP markers of multiple loci to increase detection statistical effectiveness. Microsatellites are simple repeat sequences with variable lengths, and their high polymorphisms make them well suited for population-level studies. By developing species-specific microsatellite sites and genotyping different populations, allelic frequency distributions can be obtained for calculating genetic distances and differentiation indicators. As a new generation of markers, SNP is more dense and distributed throughout the genome, which helps capture population adaptability differences. Joy et al. (2020) used ddRAD sequencing to obtain thousands of SNP sites in narrowband Spanish mackerels, which were used to analyze population structures such as the South China Sea and the Java Sea. They found that overall these groups were mixed, but two genetic clusters could still be identified through F_ST weak values and STRUCTURE clusters, corresponding to the South China Sea-Java Sea and the Bali Sea-Sulawesi Sea, which may be related to the marine circulation pattern. In addition, SNP data can also be used to detect local adaptation signals, such as screening differential sites associated with environmental variables such as water temperature and salinity, to help identify the genetic response of populations to the environment. For a wide range of species like Spanish mackerel, this method can reveal adaptive genetic differentiation in different regional populations, and even if neutral markers show no structure, differences in functional genes may be found.

6.3 The association between gene diversity and ecological adaptability

Genetic diversity is the basis for population adaptation to environmental changes and long-term survival. The species of Spanish mackerel are generally highly genetically diverse, which is related to their large populations and extensive genetic exchange. Sequence analysis of mitochondrial control zones showed that most Spanish mackerel populations had high haplotype diversity (>0.8) and medium nucleotide diversity, suggesting that they had experienced population growth in history and maintained a large effective population size. For example, the analysis of mitochondrial COII genes of Spanish mackerel fish at different locations in Indonesian waters has 8 haplotypes, with haplotype diversity reaching 0.89 and nucleotide diversity reaching 0.072, which is a relatively high level (Widayanti et al., 2024). High genetic diversity means that there are abundant genetic variations in the population, which can be selected for natural selection and screening, which is conducive to adapting to environmental changes. Therefore, genetic diversity is often positively correlated with ecological adaptability. On the one hand, diverse genotypes improve populations’ ability to respond to different environmental conditions. For example, traits such as temperature tolerance and disease resistance are often regulated by multiple genes. Population with high genetic variation is more likely to contain genotypes that are tolerant to extreme conditions, thus having a higher survival rate when the environment changes dramatically. On the other hand, genetic diversity also reflects the effective scale and mobility of the population in history. Large populations with high connectivity are usually more robust and less likely to become extinct due to local disasters. Large-scale gene exchange of Spanish mackerels has buffered the overall impact of local resource fluctuations to a certain extent. However, there are also studies that local genetic differentiation may promote the evolution of special traits when populations adapt to specific niches. For example, two narrow-band Spanish mackerels along the coast of Australia have evolved different spawning sequences despite overall genetic exchange to adapt to local temperature and seasonal changes.

7 Global Fisheries Development Discovery Status and Management Challenges

7.1 The economic status of Spanish mackerel in fisheries in various countries

7.1.1 Comparison of major catch countries and yields

Spanish mackerel is an important part of the marine fishing industry in many countries due to its excellent meat quality and high market value. In Asia, China, Japan, Indonesia and other countries have long been among the forefront of the world in terms of Spanish mackerel fish catches. The Japanese Spanish mackerel fishery along the coast of China has a long history. The Spanish mackerel production was once very considerable in the 1980s and 1990s, accounting for about 4% of the national marine fish fishing volume. According to statistics, the production of Chinese mackerel (mainly Japanese mackerel) in 2019 was about 35 000 tons, and about 36 000 tons in 2020. Production has fluctuated in recent years but remains high, ranking among the top in China's marine fishing (about fifth). The Spanish mackerel production in Southeast Asian countries such as Indonesia is also outstanding. Indonesian waters are rich in narrow-band Spanish mackerels and spot Spanish mackerels, which are one of the main target fishes of local offshore fishermen (Tarigan et al., 2019). On the Indian Ocean coast, fishery statistics from countries such as India, Iran, and Oman show that Spanish mackerel accounts for a large proportion of fish production along its coast. African coastal countries such as Nigeria and Ghana also have a certain scale of Spanish mackerel fishing. In the Western Hemisphere, the United States has a dedicated Spanish mackerel and king Spanish mackerel fishing industry in the southeastern Western Atlantic (Florida area), adopting a coexistence of commercial longline fishing and recreational fishing, with an average annual output of thousands of tons. Mexico, Brazil, Venezuela and other countries also catch Spanish mackerel for domestic consumption.

7.1.2 The role of Spanish mackerel in coastal fisheries and small-scale fisheries

Since most Spanish mackerels live on the nearshore and continental shelf, they have always been an important source of income for small coastal fishermen. Many fishermen in developing countries use small-scale sailboats, windsurfings, canoes, etc. to set up gill nets and fishing lines in the near sea to catch Spanish mackerels. This type of small-scale fishery is usually carried out seasonally. For example, when Japanese Spanish mackerels migrate to a certain place, local fishing villages collectively dispatched to round up, forming a tradition of "chasing Spanish mackerels" (Krismatama et al., 2020). In Southeast Asia, the Philippines, Malaysia and other places, Spanish mackerel (locally known as "mackerel") is also an important target species for fishermen along the coast to make at sunrise. The meat can be sold freshly or made into dried fish and fish sauce to meet local protein needs and generate income. Studies have shown that Spanish mackerels often account for a large proportion of the total catch value, which has a significant impact on livelihoods in fishing areas. Relatively speaking, industrialized large and medium-sized fisheries have a slightly lower dependence on Spanish mackerels, because Spanish mackerels swim quickly and are not easily caught in large quantities by trawls. They are more selectively fished through fixed nets, fences and longline fishing. For example, Taiwan mainly uses a combination of fixed flow nets and trolling for narrowband Spanish mackerels, and Spanish mackerel accounts for about 20% of the total fishermen's income. It can be seen that Spanish mackerel is not only a "granary fish" for small-scale coastal fisheries, but also an important supplement to medium-sized commercial fisheries, and plays an economic pillar role in fisheries of different scales (Chaves et al., 2021).

7.1.3 International trade and Spanish mackerel product export market

The meat of fresh Spanish mackerel is tender, but it is not easy to transport long distances; therefore, it often appears in the form of frozen or processed products in international trade. For example, Southeast Asian countries freeze the excess Spanish mackerel to the East Asian market, and Japan imports some Indonesian narrowband Spanish mackerels every year to meet consumer demand. Spanish mackerel can also be processed into fish fillets, fish balls, fish pine and other products, and circulate in regional trade. China's domestic Spanish mackerel dumplings, Spanish mackerel balls and other foods are well-known. Processing plants need a large amount of frozen Spanish mackerel raw materials, and some of them are imported from Vietnam, India and other places to supplement. In addition, in the aquatic markets in Europe and the United States, frozen Spanish mackerels from Latin America or West Africa are occasionally sold to Europe to serve immigrant communities or specialty catering. Overall, the scale of international trade in Spanish mackerel is relatively limited, and most of the catch is used for intra-regional consumption. However, with the development of global cold chain logistics, the cross-regional circulation of Spanish mackerel products has a growing trend. For example, India has expanded its exports of Spanish mackerel to the Middle East in recent years, and Oman has also frozen exports of its rich Spanish mackerel to East Asia (Amani and Yawar, 2025). This means that Spanish mackerel resource management is no longer just a national affairs, but is gradually linked to international markets, and needs to consider the issues of fair use and sustainable trade under the framework of regional cooperation.

7.2 Overfishing and habitat degradation

Although Spanish mackerels have strong reproductive and resilience capabilities, some populations have shown signs of decline under high-intensity fishing and environmental pressures. Overfishing is one of the main problems facing Spanish mackerel resources at present. In many coastal areas, Spanish mackerels are subject to long-term intensive fishing due to their high value, often lacking effective fishing quotas and body length restrictions, resulting in resource decline. Taking the Japanese Spanish mackerel in China's Yellow and Bohai Sea as an example, its population dropped sharply at the end of the 20th century. Historical data shows that the annual output of Bohai Spanish mackerel gradually shrank after reaching its peak in the 1970s, and by the early 2010s it was less than one-tenth of its peak. Overfishing leads to insufficient parent fish and a reduced reproductive success rate, causing resources to fall into a vicious cycle of "recession-harder recovery". The narrow-band Spanish mackerel fishery in the Taiwan Strait also experienced a significant production decline at the beginning of this century, falling from 6 600 tons in 2002 to about 500 tons in 2018, and the fishery department evaluated that two of the three local populations were nearly collapsed (Ahmed Al-shehhi et al., 2021). These data reveal that overfishing has brought some Spanish mackerel populations close to or reach dangerous levels. If measures are not taken in time, continuing to fish will seriously overdraw its regeneration capacity.

At the same time, degradation of habitat environment has also exacerbated the pressure on Spanish mackerel resources. Spanish mackerel egg laying and cabbage rely on healthy nearshore ecosystems such as estuaries, coral reefs and upstream areas. However, coastal pollution, mangrove disappearance, coral bleaching and other phenomena caused by human activities are damaging these key habitats. For example, the Pearl River Estuary and the Yangtze River Estuary were once important fattening sites for young Spanish mackerels in Japan, but now the eutrophication of water quality and industrial pollution have reduced the diversity of fish in these waters and reduced bait fish, which is not conducive to the growth of young Spanish mackerels. For example, due to climate change and offshore engineering, some upwelling fishery have changed their nutrient salt delivery model and reduced primary productivity, which will also reduce the supply of Spanish mackerel. In addition, overfishing is often accompanied by the problem of fishing gear destroying habitats, such as the damage to the seabed environment by bottom trawls indirectly affecting the food chain structure of middle and upper fish (Weng et al., 2021). Although Spanish mackerels do not directly contact the seabed, the destruction of low-value fish habitats in the food chain may also reduce their prey.

7.3 Dilemma of fishery quotas and cross-border management mechanisms

Given that Spanish mackerel is a migratory fish shared by multiple countries, achieving sustainable use requires coordinated management of coastal countries and regions. At the national level, many developing countries lack clear fishing quotas or limit management for Spanish mackerels. Small-scale fisheries are scattered and law enforcement is difficult, and even if the official quota is set, it is difficult to effectively monitor and implement. In addition, Spanish mackerel resource status assessments are often insufficient: catch reports may be inaccurate and lack independent scientific investigations. This makes it difficult for management departments to formulate scientific quota standards. Mainland China mainly protects offshore fish, including Spanish mackerel, through annual fishing moratorium measures, but there is a lack of special management regulations for Spanish mackerel in addition to fishing moratorium, and illegal fish fishing and other phenomena still exist.

At the international level, the genus Spanish mackerel is not currently included in the jurisdiction of any regional fisheries management organization (RFMO). ICCAT in the Atlantic Ocean is responsible for the management of tuna and some Spanish mackerels (such as the genus Tuna), but does not cover Spanish mackerels. There is also no regional management mechanism specifically for Spanish mackerels in the Indian Ocean and the Western Pacific. This means that transnational populations lack a unified management agreement, and all countries act independently, and it is easy to have "tragedy of the commons". Another problem is that management standards are inconsistent. For example, different countries have different regulations on minimum catchable sizes, fishing gear mesh sizes, etc., making it difficult to provide continuous protection of the population (AlMusallamami et al., 2025). Even if RFMO tries to take Spanish mackerel into control in the future, it will face difficulties in implementation coordination, because Spanish mackerel mainly operates in coastal waters and does not fall into the scope of high seas fisheries, and it is difficult to force countries to comply with unified quotas. Another manifestation of the transnational management dilemma is the inadequate monitoring and law enforcement. The fishing industry of Spanish mackerel fish are mostly small nearshore ships, and lacks VMS (ship monitoring system), etc., making it difficult to eliminate stealing and cross-border fishing. There is often a lack of information sharing channels among countries, and fishing catches and efforts cannot be exchanged in time, making resource assessments difficult to accurately carry out.

8 Ecological Risks in the Context of Climate Change

8.1 The impact of ocean warming on migration behavior

Global warming is causing significant increases in ocean temperatures, with profound impacts on fish such as Spanish mackerels that rely on water temperature to trigger migration and breeding. The study predicts that as sea water warms, the geographical distribution of Spanish mackerels may undergo range transfer and timing changes. First, the trend of the distribution range extending to high latitudes. Temperature is one of the main environmental driving forces for Spanish mackerel migration. Every spring, when the water temperature along the coast rises to the appropriate range, Spanish mackerel begins to breed northward. As global warming, spring and summer temperatures in high-latitude waters will provide new habitat for Spanish mackerels. Some species that were originally limited to the tropical region may gradually spread to subtropical and even temperate waters. For example, Japanese Spanish mackerel in the North Pacific has observed signs of a spawning ground moving north. Simulation studies show that under the moderate emission scenario (RCP4.5), the appropriate habitat area of Japanese Spanish mackerel will be reduced by about 33% in the summer of 2050, while its distribution center will move northward, and the degree of northward migration will be more significant by 2100 under the high emission scenario (RCP8.5) (Figure 3) (Go et al., 2025). This means that the current temperate waters may become a new major gathering area for Spanish mackerels in the future, while some traditional tropical fishery may attract fewer fish schools due to the high water temperature. Second, warming will change the migration clock of Spanish mackerels. Increased water temperatures usually advance and extend the breeding season. In some areas, if winter becomes warmer, Spanish mackerels may no longer migrate southward to overwinter, but can overwinter at mid-latitude, thereby shortening the migration distance. This has been observed in Spanish mackerels in North America: As the nearshore winter temperatures become higher, its overwintering distribution has advanced. In addition, ocean warming may also lead to changes in Spanish mackerel migration paths, as temperature gradients affect ocean currents and food chain pattern.

8.2 The threat of acidification and hypoxic events to survival

The other two major marine environmental problems brought about by climate change are seawater acidification and mid-level hypoxia expansion, which have potential impacts on the early development and habitat depth of marine fish such as Spanish mackerel. Seawater acidification refers to the decrease in ocean pH caused by an increase in atmospheric CO₂. Experimental studies have shown that acidification may damage the sensory system and balance function of fish and affect the development of juvenile fish. For Spanish mackerels who live on high-speed swimming and precise predation, if they interfere with the auditory and smell of their young fish, they may reduce their ability to forage and avoid enemies, which in turn affects survival rate (Wexler et al., 2023). In particular, young Spanish mackerels often live on the surface and are sensitive to pH changes. The model predicts that the pH of some surface sea areas will drop by more than 0.3 at the end of this century, which may increase the deformity rate of juvenile fish and reduce the survival. Although there is currently a lack of acidification exposure test data for Spanish mackerels, we can refer to studies similar to middle and upper-level fish to speculate that there is a risk. If acidification leads to a decrease in population replenishment, it will directly impact the fishery's subsequent replenishment.

The problem of ocean hypoxia is also worth paying attention to. Global warming has caused ocean stratification to strengthen and ventilation to weaken, and the middle-level hypoxic zones have expanded and surged in some areas. Spanish mackerel is a highly metabolic aerobic fish and is more sensitive to the decrease in dissolved oxygen. When the oxygen content of the middle water body decreases, the Spanish mackerel is forced to compress and move in shallower surface waters to obtain sufficient oxygen. This will change its daytime vertical distribution and predation behavior and may increase its chances of encountering surface longline fishing gear, which in turn increases the risk of being arrested. Hypoxia caused by eutrophication along the coast is also a similar problem. For example, hypoxia in the bottom layer of the estuary in summer may reduce the breeding of bait fish, thereby indirectly affecting the Spanish mackerel fish that feeds here.

8.3 Trends of northward habitat migration and population redistribution

Based on the influence of the aforementioned factors such as warming, acidification, and hypoxia, it can be foreseen that the habitat pattern of Spanish mackerels will undergo systematic northward migration and redistribution. This trend has already appeared in some areas and is called "tropicization" or "species change". For example, in the Yellow Sea and the East China Sea, the abundance of middle and upper-class fish with high economic value (such as Spanish mackerel) has decreased in the past few decades, while low-value fish (such as hairtail and Spanish mackerel) once flourished. However, in recent years, with overfishing and environmental changes, the entire community has further succeeded to smaller mixed fish. Among them, the share of Spanish mackerels fluctuates greatly, showing signs of the transfer of resources to the sea and high latitudes. In North America, Atlantic Spanish mackerel was recessed due to overcapacity and a cold winter resource decline in the 1970s. It was restored through management and recently expanded northward due to rising water temperatures, constantly breaking the record of its northernmost distribution (Yang et al., 2022). This reminds us that redistribution of Spanish mackerel populations will become a new topic in fishery management in the 21st century. Northward migration will bring about new ecological interactions: Spanish mackerels enter the originally colder ecosystem and may compete with local predators (such as blue fish, salmon, etc.) or prey on local baitfish, causing changes in the structure of the food web. Meanwhile, fishermen in the south who originally relied on Spanish mackerels may face a decline in catches and have to operate more ocean-distance or catch other fish instead. For cross-border shared populations, this redistribution may cause fishing rights disputes: for example, assuming that narrow-band Spanish mackerels appear more in the waters of high-latitude countries in the future, low-latitude countries may claim historical fishing rights, and thus need to renegotiate fishing distributions. We need to strengthen long-term monitoring and model prediction to predict the trend of changes in abundance of Spanish mackerels in various sea areas in advance. Using niche models to predict suitable areas under different climate scenarios can provide a scientific basis for fishery adjustment. At the same time, fishery management in various countries should be more flexible and be able to dynamically adjust quota areas according to changes in resource centers. International cooperation must also be strengthened to avoid disorderly competition in "fishing by fishing".

9 Future Outlook: Sustainable Utilization and Protection Path

9.1 Potential distribution prediction based on niche model

Faced with the future of climate change and changing environment, scientific prediction of the potential distribution and changing trends of Spanish mackerel is very important for formulating forward-looking management strategies. In recent years, niche models (such as the maximum entropy model MaxEnt, Biomod integration model, etc.) have been widely used to predict the geographical distribution of species in climate change scenarios. For marine fish such as Spanish mackerel, historical fishing locations and biological survey data can be correlated with environmental variables (temperature, salinity, primary productivity, etc.), a species distribution model can be constructed, and then environmental variables in future climate scenarios can be input to simulate the changes in their suitable habitats. Yang et al. (2022)'s study is an example. They used the biomod2 model to predict the eligible areas of Japanese Spanish mackerel in different seasons and future years. The results show that under the moderate emission scenario RCP4.5, the suitable habitat area in summer will be reduced by about one-third in 2050. By 2100, under the high emission RCP8.5, the suitable area in summer may shrink by nearly 90%, and the distribution center will obviously move northward. This type of model provides quantifiable predictions and is of great reference value for resource management. If the model consistently indicates that a certain sea area will no longer be suitable for Spanish mackerel breeding and laying eggs in the future, the local area should adjust the fishery structure as soon as possible to reduce its dependence on Spanish mackerel. At the same time, the new suitable areas pointed out by the model may become key fishing grounds in the future and require pre-planning and management. In addition, in addition to climate factors, the niche model can also be included in human impact variables (such as fishing intensity) for comprehensive simulation to evaluate the changes in Spanish mackerel resources under various scenario combinations. Model accuracy should be further improved in the future, such as using higher resolution marine environmental forecast data, taking into account interspecies relationship impacts (such as predation, competition) to improve prediction reliability.

9.2 Cross-regional collaborative management strategy suggestions

The migratory characteristics of Spanish mackerels determine that management measures in a single country are often difficult to work, and cross-regional and cross-border collaborative management mechanisms are urgently needed to achieve sustainable use of resources. In response to this need, the following are some suggestions: Establish a Spanish mackerel management working group or alliance at the regional level. For example, in East Asia, the Joint Management Committee for the Yellow-Bohai Spanish mackerel Resources is formed by China, Japan, South Korea and other countries, which regularly share scientific research data, coordinate fishing moratoriums and fishing quotas, and jointly crack down on illegal fishing. Secondly, promote the standardization and linkage of management measures. With the support of scientific data, countries should agree on a common minimum catchable size and unify the mesh size requirements of the mesh to avoid the management vacuum caused by strict looseness of one country. At the same time, the fishing moratorium system and fishing ban zones of various countries should be as closely linked as possible. Again, establish a network of cross-border marine protected areas. Important spawning grounds and juvenile habitats such as estuaries and coral reefs should be listed as protection priorities. Strengthen joint scientific research and monitoring. Scientific research institutions in various countries can carry out collaborative research on Spanish mackerels, such as jointly conducting resource assessment cruises, marking release experiments, and genetic population structure analysis. Through data sharing and joint model evaluation, more comprehensive and accurate resource dynamic information can be obtained. New technologies such as satellite telemetry and electronic tags can also be used to track the cross-border migration routes of Spanish mackerels to provide a basis for managing the demarcation. Finally, collaborative management also needs to consider the issue of fishermen's livelihood transformation. Countries should cooperate to provide technical and financial support to help fishermen affected by reducing Spanish mackerel fishing turn to alternative industries or increase value chain revenue.

9.3 Genetic resource protection and population restoration construction

In order to achieve the long-term sustainable utilization of Spanish mackerel resources, in addition to fishing management, it is also necessary to pay attention to the protection of its genetic diversity and population recovery. In terms of genetic resource protection, a germplasm resource library of Spanish mackerel fish should be established to preserve live or frozen sperm eggs, tissue samples, etc. for important populations in different sea areas for future breeding and research purposes. Live Spanish mackerels can be established to protect populations based on aquatic research institutions or marine aquariums. At the same time, high genetic diversity populations in the wild should be protected. In terms of population recovery, artificial proliferation and release can be used as an auxiliary measure. Although it is difficult to breed Spanish mackerel artificially, China and other countries have made breakthroughs in the breeding of Spanish mackerels in Japan, and have deployed a certain number of young fish to the Bohai Sea and the Yellow Sea every year. And strengthen the marking of release fish to track and evaluate survival and recovery rates. Proliferation and release must be coordinated with fishery management. If fishing is not restricted after release, the juvenile fish will be caught soon, in vain. Public education and fishermen's participation are also important parts of conservation and restoration, and strive for fishermen and consumers' support by promoting the value of Spanish mackerels in the ecosystem, current resource status and the importance of management initiatives. Establish a benefit-sharing mechanism, such as the government or NGO compensates fishermen for losses during fishing moratoriums and fishing bans, so that protection and recovery become joint action. Only when genetic diversity is maintained and populations steadily recover can Spanish mackerel resources truly achieve sustainable utilization.

Acknowledgements

We thank the anonymous reviewer for their valuable comments that will help improve the quality of this manuscript. At the same time, thank you to colleagues and technicians who provided support and assistance during this research process.

Conflict of Interest Disclosure

The authors confirm that the study was conducted without any commercial or financial relationships and could be interpreted as a potential conflict of interest.

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