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

Marisa Vedor,António Dos Santos,Pedro Afonso,Gonzalo Mucientes,Jorge Fontes,Rui Rosa,Frederic Vandeperre,David W Sims,Ana Couto,Ivo Da Costa,Nicolas E Humphries,Nuno Queiroz

doi:10.7554/elife.62508

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.

Highlights

Water temperature and Dissolved oxygen (DO) can be important in determining shark dive depths, we found a less marked effect of temperature at depth with blue sharks diving into cold waters during deep dives both in the oxygen minimum zone (OMZ) and adjacent waters, and the vertical change in temperature (DTC) below the mixed layer depth (MLD) being similar in magnitude both inside and outside the OMZ
The eastern tropical Atlantic (ETA) OMZ is characterised by higher sea surface temperatures (SSTs) than adjacent waters (e.g. Figure 2A) and, we found that maximum daily dive (MDD) depth increased with increasing SST up to 24 ̊C, with SST >24 ̊C being observed along blue shark tracks across the core OMZ area that coincided with the shallowest MDD depths
Our findings suggest that blue sharks above the ETA OMZ will undergo habitat compression as they respond by avoiding lower DO concentrations and associating with high SST and Net primary production (NPP) in the uppermost 100 m layer, and show preference for suitable DO gradients along the margins of OMZ areas – potentially due to the physiological limits of sharks (Payne et al, 2015; Coffey et al, 2017; Sims, 2019), to habitat compression of prey species (Gilly et al, 2013; Childress and Seibel, 1998), or even hypoxia-related visual impairment (McCormick and Levin, 2017)

Summary

Introduction

Dissolved oxygen (DO) content of the global ocean is declining (ocean deoxygenation) due to sea temperature warming, increased stratification, and changing circulation, and the interactions of these processes with hypoxia-inducing biological activity (Keeling et al, 2010; Gilly et al, 2013; Watson, 2016; Schmidtko et al, 2017; Breitburg et al, 2018; Levin, 2018; Laffoley and Baxter, 2019; Stramma and Schmidtko, 2019). For large tropical pelagic fishes such as tunas and billfishes, water layers with DO concentrations 3.0–3.5 ml O2 lÀ1 have been recognised as a generalised lower habitat boundary associated with waters overlying OMZs that limits depth distributions (Lowe et al, 2000; Ekau et al, 2010; Stramma et al, 2012) This predicts shifts in predatory fish distributions even with mild hypoxia occurring in waters above OMZs (Vaquer-Sunyer and Duarte, 2008; Ekau et al, 2010; Sims, 2019). The increased susceptibility of large pelagic fishes to fisheries due to shoaling OMZs has been hypothesised (Gilly et al, 2013; Prince and Goodyear, 2006; Prince et al, 2010; Stramma et al, 2012) but has not been quantified directly

Methods

Results

Discussion

Conclusion