Fishing on ecosystems: the interplay of fishing and predation in Newfoundland-Labrador

In the early 1990s, Atlantic cod, a major component of the Newfoundland-Labrador ecosystem, suffered a stock collapse, and other groundfish stocks such as American plaice and yellowtail flounder seriously declined. This paper explores whether the relative effects of predation and fishing alone can account for these ecosystem changes. The Newfoundland-Labrador ecosystem was first modelled with a mass balance model for a time period in the mid-1980s when the groundfish biomass was relatively stable. This provided the starting point for simulations using a trophodynamic simulation model, Ecosim. A series of simulations were run, under different assumptions about energy control, to address the larger question "can the effects of fishing and predation account for the changes observed in the ecosystem?" The collapse and nonrecovery of cod was replicated, assuming top-down energy control. Other control assumptions were less successful. While groundfish stocks collapsed, seal populations and invertebrates such as shrimp and snow crab increased in abundance. The model predicted these increases, while a simulated increase in harp seals further repressed the recovery rate of cod. It was concluded that these results are consistent with the hypothesis that the collapse of cod was caused by excess fishing and that cod recovery is retarded by harp seals.