International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 85 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
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