Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes

Abstract

Habitat compression and associated potential habitat loss was validated using electronic tagging data from 47 blue marlin. This phenomenon increases vulnerability to surface fishing gear for billfishes and tunas 8,9 , and may be associated with a 10-50% worldwide decline of pelagic predator diversity 10 . Further expansion of the Atlantic OMZ along with overfishing may threaten the sustainability of these valuable pelagic fisheries and marine ecosystems.Dissolved oxygen is critical for sustaining most marine animal life. When dissolved oxygen is minimized, widespread mortality 11,12 or avoidance 13 of affected areas can result. OMZs in the eastern tropical seas represent the largest contiguous areas of naturally occurring hypoxia 9 in the world's oceans. In the present climate change cycle, characterized by anthropogenic CO 2 emissions 2 and global warming, these areas are expanding and shoaling 3,12,14 . Possible consequences of OMZ expansion to the marine ecosystem 14 include loss of vertical habitat for high-oxygen-demand tropical pelagic billfishes and tunas and the associated increased risk of overfishing of these species by surface fishing gear 8,9 .Large-scale expansion of OMZs over the past 50 years 3 poses a challenge for predicting impacts to pelagic fish stocks and their ecosystem. Although oceanographic modelling and ocean observations for retrospective analyses are useful for examining past trends, understanding future OMZ expansions and the concurrent impacts on billfish and tuna populations is essential for preventing overfishing. We analysed recent hypoxia data associated with OMZ expansion in the eastern tropical Atlantic (ETA) to examine possible habitat loss of the near-surface layer. We also present vertical habitat use data of Atlantic blue marlin (Makaira nigricans) monitored with electronic tags (Fig. 1). Changes in habitat use were validated by maximum daily depths

Major impacts of climate change on deep-sea benthic ecosystems

“…Some effects will be direct; for example, we expect alterations in the distributions and health of open-ocean and deep-sea fish populations and commercially exploited stocks. This impact will result from warming-induced changes in metabolism (Deutsch et al, 2015) and body size (Cheung et al, 2013) linked to latitudinal or depth shifts in species distributions, in addition to vertical habitat compression from OMZ expansions (Prince and Goodyear, 2006;Stramma et al, 2010Stramma et al, , 2012. Less clear are the impacts of acidification stress and multiple stressors on deep-sea fish populations and fisheries production (Rosa and Seibel, 2008).…”

“…Though not resolved at the grid resolution shown in Figure 1, tropical and subtropical bathyal waters between 200 and 700 m, including those bathyal margins of all major eastern boundaries, have already lost considerable amounts of O 2 over the last half-century, and many OMZs have expanded in volume (Stramma et al, 2008(Stramma et al, , 2010(Stramma et al, , 2012Gilly et al, 2013;Deutsch et al, 2014). In the abyssal realm, seafloor habitats under areas of deep-water formation (e.g., those in the North Atlantic and Southern Oceans) could experience a maximum decline in O 2 concentration of 0.03 mL L -1 by 2100 (i.e., a 0.5% drop from current levels; Tables 2, 3; Figures 2, 3).…”

“…Warming of the oceans will enhance thermal stratification and density gradients, which will reduce vertical mixing. Combined with a reduction in O 2 solubility in warmer water, increased thermal stratification is predicted to create widespread ocean deoxygenation (Keeling et al, 2010;Long et al, 2016), with the greatest effect in intermediate waters (100-1000 m; Stramma et al, 2012;Bopp et al, 2013). Already, distinct deep-water masses in the Southern Ocean (Helm et al, 2011), eastern North Atlantic (e.g., Sub-polar Mode Water, the Intermediate Water and the Mediterranean Outflow Water; Stendardo et al, 2015), and in the West Pacific (Levin, 2003;Helly and Levin, 2004).…”

“…Many observational studies are showing that presentday climate change is already impacting deep-sea environments, as evidenced by increased deep-sea temperature (Purkey and Johnson, 2010), deoxygenation (Stramma et al, 2008(Stramma et al, , 2010(Stramma et al, , 2012Keeling et al, 2010;Helm et al, 2011), lowered pH of intermediate deep-waters (Byrne et al, 2010), and altered POC flux to the seafloor (Ruhl and Smith, 2004;Smith et al, 2013). Despite emerging evidence that climate-driven changes in deep-sea environmental conditions may perturb the functioning of oceanfloor ecosystems (Danovaro et al, 2001;Smith et al, 2007Dunlop et al, 2016;, our understanding of the extent to which projected physical and chemical changes will lead to deleterious ecological consequences is still very poor (Philippart et al, 2011).…”

Short-term movements and habitat preferences of sailfish, Istiophorus platypterus (Istiophoridae), along the southeast coast of Brazil

“…However, other oceanographic features have also been shown to affect the depth behavior of pelagic fishes. For example, Prince et al (2010) and Stramma et al (2012) demonstrated that the vertical distribution of sailfish and blue marlin was directly related to the amount of dissolved oxygen, which may act as a physical barrier to limit vertical movement (in the eastern Atlantic). However, considering the much higher concentrations of dissolved oxygen in the western Atlantic (Castro & Miranda, 1998;Braga & Niencheski, 2006), such influence is unlikely in the present case.…”