Abstract
In recent years, much attention has been focused on the impact of climate change, particularly via ocean acidification (OA), on marine organisms. Studying the impact of OA on long-living organisms, such as sharks, is especially challenging. When the ocean waters absorb anthropogenic carbon dioxide (CO2), slow-growing shark species with long generation times may be subjected to stress, leading to a decrease in functionality. Our goal was to examine the behavioral and physiological responses of sharks to OA and the possible impacts on their fitness and resilience. We conducted a systematic review in line with PRISMA-Analyses, of previously reported scientific experiments. We found that most studies used CO2 partial pressures (pCO2) that reflect representative concentration pathways for the year 2100 (e.g., pH ~7.8, pCO2 ~1000 μatm). Since there is a considerable knowledge gap on the effect of OA on sharks, we utilized existing data on bony fish to synthesize the available knowledge. Given the similarities between the behaviors and physiology of these two superclasses’ to changes in CO2 and pH levels, there is merit in including the available information on bony fish as well. Several studies indicated a decrease in shark fitness in relation to increased OA and CO2 levels. However, the decrease was species-specific and influenced by the intensity of the change in atmospheric CO2 concentration and other anthropogenic and environmental factors (e.g., fishing, temperature). Most studies involved only limited exposure to future environmental conditions and were conducted on benthic shark species studied in the laboratory rather than on apex predator species. While knowledge gaps exist, and more research is required, we conclude that anthropogenic factors are likely contributing to shark species’ vulnerability worldwide. However, the impact of OA on the long-term stability of shark populations is not unequivocal.
Highlights
Despite the shark’s long evolutionary history and being cosmopolitan in nature [1,2], they face substantial new threats, namely climate change and its resulting environmental impacts [1]
The added papers included bibliographic reviews on the behavior and physiology of elasmobranch and bony fishes [35,36], papers with more information on other stress factors reflected in shark behavior [1], papers from the personal files of the authors (e.g., [8]), relevant papers found in selected articles during the third step of the systematic review, and more recent papers published after the search (e.g., [4])
One of the main challenges of this paper was to review and explore changes to sharks’ behavior and physiology due to ocean acidification (OA) based on available scientific data, as there is a lack of knowledge on sharks and OA, we examined possible effects on other fishes as well
Summary
Despite the shark’s long evolutionary history and being cosmopolitan in nature [1,2], they face substantial new threats, namely climate change and its resulting environmental impacts [1]. New global reassessment reveals that 167 out of 536 (31.2%) sharks are threatened by extinction, and up to 75.7% of the coastal sharks [6].Shark biodiversity, abundance, and distribution have already been altered, with dramatic declines through direct mortality (overexploitation and bycatch, habitat loss, and more), and via the sub-lethal effects of environmental stressors [2,4,8,9]. These not fully understood sub-lethal stressors may have fitness consequences [10], leading to a continuing decline in vitality and adaptability to the new environments. With the rapid increase in the rate of global change, phenotypic plasticity is likely to be especially important in responding to the climate changes as a result of sharks’ long-life span [12]
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