Trends in population starvation mortality based on a spatiotemporal model of condition: Part 1: A case study of Atlantic cod on the Southern Grand Bank

Abstract

Fish condition is often defined as a deviation in the relationship between fish weight and length, indicating if the fish is leaner or fatter than the average. The proportion of a stock in critically poor condition may indicate a component of the total natural mortality rate M, which has been called the starvation mortality rate (i.e., M_K ≤ M). The weight-length relationship may vary spatially and temporally (both between and within years). Hence, M_K may also vary the same way. We developed a spatiotemporal condition model to derive a spatiotemporal and length-specific index of M_K. We aggregated M_K across space and months to produce an annual and length-specific M_K index for the entire stock, as a potential input to assessment models. We applied the model to survey data for cod on the Southern Grand Bank of Newfoundland. Our results indicated that M_K was: 1) higher in the spring than the fall, 2) higher for cod between 55 and 80 cm and cod ≥ 120 cm, and 3) higher during 1991–1993 when the stock declined substantially, but was also high in 2016. We discuss potential drivers of starvation mortality as well as how this information can be included in a stock assessment model to improve fisheries management advice.