The influence of ocean warming on the natural mortality of marine fishes

Climate change is anticipated to have multiple consequences for aquatic ectotherms. Warming temperatures, for example, are predicted to reduce body size in many fish species. Smaller sizes may be caused by physiological constraints associated with respiration, life-history responses to faster growth and concomitant earlier age at maturity, and interactive effects of fishing and climate change on evolution. Here, using a phylogenetically broad dataset of 100 marine species, we explore how natural mortality might respond to a 10% reduction in asymptotic length (L_∞). We find that this decrease in size (predicted to be associated with a 1 °C ocean temperature increase) is likely to exacerbate natural mortality (M) for most marine fishes, albeit not all, on Canada’s Scotian Shelf. Across all bony fishes (Actinopterygii), the median proportional increase in natural mortality is 10.5%; limited data suggest that chondrichthyans are less affected. Smaller-bodied fish species experienced greater absolute increases in M than larger-bodied species. Among commercially exploited species for which sufficient data are available, M is predicted to increase by 0.14 for Atlantic mackerel (Scomber scombrus), 0.08 for Silver hake (Merluccius bilinearis), 0.04 for Atlantic herring (Clupea harengus), and 0.02 for Atlantic cod (Gadus morhua) and Pollock (Pollachias virens). The present study offers a simple means of exploring the mortality consequences of reduced body size hypothesized to result from globally warming water temperatures, thus contributing a potential tool for climate-change vulnerability assessments of marine fishes.