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

In the early 1990s, Atlantic cod, a major component of the Newfoundland-Lab rador ecosystem, suffered a stock collapse, and other groundfish stocks suc h as American plaice and yellowtail flounder seriously declined. This paper explores whether the relative effects of predation and fishing alone can a ccount for these ecosystem changes. The Newfoundland-Labrador ecosystem was first modelled with a mass balance model for a time period in the mid-1980 s when the groundfish biomass was relatively stable. This provided the star ting 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 pre dation 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 collapse d, seal populations and invertebrates such as shrimp and snow crab increase d in abundance. The model predicted these increases, while a simulated incr ease in harp seals further repressed the recovery rate of cod. It was concl uded that these results are consistent with the hypothesis that the collaps e of cod was caused by excess fishing and that cod recovery is retarded by harp seals.