Continued decline of a collapsed population of Atlantic cod (Gadus morhua) due to predation-driven Allee effects

Abstract

Most stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed in the early 1990s, with little sign of recovery since then. In the southern Gulf of St. Lawrence (sGSL), the failed recovery is due to severe increases in the natural mortality of adult Atlantic cod. We examined the role of predation by grey seals (Halichoerus grypus) in this failed recovery by directly incorporating grey seal predation in the population model for Atlantic cod via a functional response. Estimated predation mortality of adult Atlantic cod increased sharply during the cod collapse and has continued to increase, comprising the majority of mortality since the late 1990s. While predation by grey seals appeared to play a minor role in the collapse of Atlantic cod, we found it to be the main factor preventing recovery. Our results are consistent with the hypothesis that failed recovery is due to predation-driven Allee effects, a demographic effect due to the decline in cod abundance and an emergent effect resulting from increasing grey seal abundance. Under current conditions, extirpation of sGSL Atlantic cod appears likely unless there is a large decline in the abundance of grey seals.

Risk of extinction of a unique skate population due to predation by a recovering marine mammal

“…These results contrast with the Type II responses obtained for grey seals preying on Atlantic cod in the sGSL (Neuenhoff et al. ) and west of Scotland (Cook et al. , Cook and Trijoulet ).…”

“…This phenomenon is widespread among large‐bodied species in the demersal fish community of the sGSL and has been attributed to increased predation by the recovering grey seal population in this area (Benoît and Swain , Swain and Benoît , Neuenhoff et al. ).…”

“…Foraging by these seals is most intense on the eastern Scotian Shelf and in the sGSL (Breed et al 2006, Harvey et al 2008. Dramatic increases in the natural mortality of adults of large-bodied demersal fishes are widespread in these ecosystems and are thought to reflect increased predation by these seals (Benoît et al 2011a, Sinclair et al 2015, Swain and Benoît 2015, Neuenhoff et al 2019.…”

Implications of fisheries‐induced evolution for population recovery: Refocusing the science and refining its communication

“…These modelling approaches have been heavily criticized (e.g. Perälä & Kuparinen, ; Shelton & Healey, ) amidst empirical evidence that a reduction of F is not always a sufficient condition for recovery (Hutchings & Kuparinen, ) and that the population dynamics of some stocks at low size are consistent with the presence of Allee effects (Keith & Hutchings, ; Neuenhoff et al, ; Perälä & Kuparinen, ).…”

“…But even if the relative contribution of FIE to phenotypic change is small, it may still have influence, as demonstrated by research that partitioned the sources of phenotypic change in the growth of horns in male bighorn sheep ( Ovis canadensis , Bovidae), a species hypothesized to be subject to hunting‐induced evolution (Douhard et al, ). An additional consideration is that there may be instances where FIE synergistically contributes to, or magnifies the effects of, other factors known to negatively affect recovery, such as high and prolonged F , excessively small population size (Kuparinen et al, ), low r max (Neubauer et al, ) or short‐term (<3 generations) increases in M (generated, e.g., by smaller size at maturity [Hutchings & Kuparinen, ] or altered predator‐prey interactions [Audzijonyte et al, ; Neuenhoff et al, ]).…”