Evaluating the potential impacts of grey seal predation and fishery bycatch/discards on cod productivity on the Western Scotian Shelf and in the Bay of Fundy

TL;DR

This study assesses how grey seal predation and fishery bycatch/discards impact Atlantic cod mortality on the Western Scotian Shelf and in the Bay of Fundy, using a statistical model to evaluate their relative effects.

Contribution

It introduces a statistical catch-at-age model that incorporates grey seal predation and bycatch/discards to better understand cod mortality drivers.

Findings

Most cod mortality remains unexplained unless cod makes up over 45% of seal diet.

Reported bycatch estimates suggest discards have a minor impact, but data uncertainty is high.

Predation by grey seals could be a significant factor in cod mortality if diet proportions are high.

Abstract

The recovery of many groundfish stocks throughout the Northwest Atlantic has been impeded by elevated natural (i.e., non-fishing) mortality (M) among older/larger individuals. The causes of elevated mortality are not well known, though predation by rapidly growing grey seal herds and unreported fishing are thought to be possible drivers of mortality for Atlantic Cod (Gadus morhua) on the Western Scotian Shelf and in the Bay of Fundy (known as "4X5Y cod") and in nearby ecosystems. We developed a statistical catch-at-age model for 4X5Y cod that accounted for both grey seal predation and estimated bycatch/discards to evaluate the degree to which either of these factors may influence cod mortality. The model was fit over a range of predation and discarding scenarios to account for uncertainties and a lack of data for these processes. We found that most cod M remained unaccounted for unless cod comprised a large proportion (>0.45) of the grey seal diet by weight. If the reported bycatch estimates are taken as accurate, then the magnitude of cod discards from non-directed fisheries was minor, though these estimates are highly uncertain.