Extirpation of Atlantic Cod from a Northwest Atlantic ecosystem in the absence of predator control: inference from an ecosystem model of intermediate complexity

TL;DR

This study uses an ecosystem model to show that grey seal predation is a major cause of Atlantic cod decline in the Gulf of St. Lawrence, and that reducing seal numbers is crucial for cod recovery.

Contribution

It provides the first ecosystem-based analysis demonstrating seal predation as the primary factor in cod decline and the necessity of seal management for cod recovery.

Findings

Seal predation accounts for most cod mortality.

Cod likely to be extirpated without rapid seal reduction.

Seal reduction does not significantly impair herring biomass.

Abstract

Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence (sGSL) declined to low abundance in the early 1990s and have since failed to recover due to high natural mortality, which has been linked to grey seal (Halichoerus grypus) predation. Increased grey seal harvests have been suggested to improve cod survival, however, predicting the response of cod to seal abundance changes in the sGSL is complicated by a hypothesized triangular food web involving seals, cod, and small pelagic fishes, wherein the pelagic fishes are prey for cod and grey seals, but may also prey on young cod. Grey seals may therefore have an indirect positive effect on prerecruit cod survival via predation on pelagic fish. Using a multispecies model of intermediate complexity fitted to various scientific and fisheries data, we found that seal predation accounted for the majority of recent cod mortality and that cod will likely be extirpated without a strong and rapid reduction in grey seal abundance. We did not find evidence that reducing grey seal abundance will result in large increases to herring biomass that could impair cod recovery.