Climate-driven deoxygenation elevates fishing vulnerability for the ocean's widest ranging shark

Abstract

Climate-driven expansions of ocean hypoxic zones are predicted to concentrate pelagic fish in oxygenated surface layers, but how expanding hypoxia and fisheries will interact to affect threatened pelagic sharks remains unknown. Here, analysis of satellite-tracked blue sharks and environmental modelling in the eastern tropical Atlantic oxygen minimum zone (OMZ) shows shark maximum dive depths decreased due to combined effects of decreasing dissolved oxygen (DO) at depth, high sea surface temperatures, and increased surface-layer net primary production. Multiple factors associated with climate-driven deoxygenation contributed to blue shark vertical habitat compression, potentially increasing their vulnerability to surface fisheries. Greater intensity of longline fishing effort occurred above the OMZ compared to adjacent waters. Higher shark catches were associated with strong DO gradients, suggesting potential aggregation along suitable DO gradients contributed to habitat compression and higher fishing-induced mortality. Fisheries controls to counteract deoxygenation effects on shark catches will be needed as oceans continue warming.

Elasmobranch Responses to Experimental Warming, Acidification, and Oxygen Loss—A Meta-Analysis

“…Global warming is further driving changes in precipitation patterns, ocean circulation and stratification, and sea-level rise, entailing the potential disruption of key habitats for these animals (Chin et al, 2010;Bindoff et al, 2019). At the same time, OD, which is mostly driven by OW, is expected encompasses the expansion of oxygen minimum zones and the concurrent compression of the available habitat for pelagic sharks, altering ecosystem dynamics while increasing the exposure of these animals to fishing pressure (Vedor et al, 2021). Moreover, these effects are further likely to cascade into considerable ecological consequences, such as changes in productivity, shifts in food availability, deterioration of foundation species, and the potential increased prevalence of harmful algae blooms (Bindoff et al, 2019;Leggat et al, 2019;Smale et al, 2019)-all of them likely to affect elasmobranchs to an extent.…”

“…In fact, climate change is likely to affect this group both through direct impacts over their biological responses (Chin et al, 2010;Rosa et al, 2017;Bouyoucos et al, 2019;Wheeler et al, 2020) and indirect effects due to the disruption of ecological dependences, including changes in habitat quality, ocean productivity, and prey availability (Chin et al, 2010;Schlaff et al, 2014;Bindoff et al, 2019). Moreover, the expected changes may increase their exposure to other anthropogenic threats, namely fishing pressure (Vedor et al, 2021), or compromise the efficacy of conservation measures (Davies et al, 2017). Additionally, their k-selected life strategy, with long generation times and low fecundity, reduces their margin for adaptation to rapid environmental change (Wheeler et al, 2020).…”

“…As such, the identification of knowledge gaps, along with the targeted focus toward the most pressing concerns, is particularly critical for this group. On the other hand, while experimental research provides a central stepping-stone to understand the impacts of climate change over this taxon, it is crucial that we further integrate these insights with the information collected through different approaches (e.g., Chin et al, 2010;O'Brien et al, 2013;Lighten et al, 2016;Birkmanis et al, 2020;Vedor et al, 2021).…”

Ten new insights in climate science 2021: a horizon scan

“…For example, due to warming and an excessive nutrient input, the oceanic oxygen content has demonstrably declined since around 1960, leading to an expansion of deep sea oxygen minimum zones and more frequent hypoxia in coastal systems (Oschlies, 2021). Deoxygenation accelerates the emission of greenhouse gases such as nitrous oxide and methane, reduces habitat quantity and quality for many species, elevates vulnerability to fishing, for example, for the ocean's widest ranging sharks (Vedor et al, 2021) and threatens ocean ecosystem services at large. Warming waters shift species distributions and reduce biomass across trophic levels, and heat waves threaten, for example, the survival of coral reefs and possibly temperate kelp forests.…”

Shark Conservation – Analysis and Synthesis

“…The species dominates the bycatch of longline fsheries (Oliver et al 2015) and is considered to be at high risk due to its distribution, which overlaps heavily fshed regions (Queiroz et al 2019). Further, as oxygen minimum zones (OMZs) expand due to global warming, blue sharks may be shifting their distribution patterns into surface waters to avoid deeper, oxygen depleted waters (Vedor et al 2021). Therefore they are at even higher risk of being caught by surface longliners, who operate mostly above those OMZ depths (Vedor et al 2021).…”