Indexing starvation mortality to assess its role in the population regulation of Northern cod

Abstract

Starvation is a ubiquitous process in nature as all animals depend on finite resources to survive. Limited food resources can lead to starvation-induced mortality and, depending on the scale of the limitation, population-level consequences may emerge. However, attempts to model starvation-induced mortality are rare as it is difficult to isolate the effects of specific sources of natural mortality. Using commonly collected data on body condition, we develop a statistical approach to index starvation-induced mortality by estimating the proportion of individuals experiencing severe emaciation in a fish population. This approach accounts for seasonal patterns in body condition, allowing data collected at any time of year to be included. Using Canada's Northern cod (Gadus morhua) stock as a case study, we found a positive association between rates of natural mortality estimated by an integrated population model and the index of mortality derived from proportions of cod in poor condition. This index is also associated with the availability of two key prey species, capelin (Mallotus villosus) and Northern shrimp (Pandalus borealis). Taken together, our results indicate that starvation-induced mortality represents an important component of the natural mortality experienced by Northern cod, supporting the idea that insufficient food contributed to the collapse of the stock in the early 1990s and, subsequently, limited its recovery. These implications highlight the practical relevance of explicitly considering prey availability (e.g., through body condition or prey abundance indices) to improve stock assessment modeling and aid the development of actionable ecosystem-based fisheries management strategies.