A spatiotemporal model of condition to derive a novel starvation mortality index that improves a state-space stock assessment model for Atlantic cod on the Southern Grand Bank of Newfoundland

Atlantic cod on the Southern Grand Bank (SGB) collapsed in the early 1990’s and a fishing moratorium has been in place since 1994. I investigate how fish condition influences the natural mortality rate (M) and the prospects for rebuilding the SGB cod stock. I developed a body condition model and derived a starvation mortality rate (Mₖ) index. I modelled weight as a function of length, with deviations (i.e., change in condition) for survey strata, year, month, length, and their interactions. The Mₖ index was higher in the spring than in the fall, higher for cod between 55–80 cm in length and for cod larger than 120 cm, and higher during 1991–1993 when the stock experienced a substantial decline. The Mₖ index was incorporated into an age-based state-space stock assessment model (SSAM) as a component of M. This led to a significant decrease (62%) in the size of the cohort process errors (i.e., their standard deviation). This reduction suggests that Mₖ significantly improved the model of stock productivity. Therefore, I conclude that Mₖ explains a substantial portion of the variation in M, and that starvation mortality is an important component of the productivity of SGB cod.