International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 87 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".
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