Research Focus
Cascading effects of overfishing marine systems

https://doi.org/10.1016/j.tree.2005.08.018Get rights and content

Profound indirect ecosystem effects of overfishing have been shown for coastal systems such as coral reefs and kelp forests. A new study from the ecosystem off the Canadian east coast now reveals that the elimination of large predatory fish can also cause marked cascading effects on the pelagic food web. Overall, the view emerges that, in a range of marine ecosystems, the effects of fisheries extend well beyond the collapse of fish exploited stocks.

Introduction

Although the role of fishing in the collapse of exploited stocks is beyond doubt, it has been less easy to determine whether there are indirect effects on other ecosystem components. Fish are the main predators in most marine systems and one would expect that removing them might have an impact on lower trophic levels. However, assessing the relative impact of predators has long been a difficult problem in ecology.

Section snippets

When do predators make a difference?

The classic dilemma is nicely illustrated by the account of the Italian scientist Lorenzo Camerano published in 1880 [1] explaining how naturalists in those days were divided in two categories. According to Camerano, the first category reasoned: ‘Birds feed to a great extent on insects; so if we increase the numbers of birds, the number of insects will decrease’. This is what we now call top-down regulation. The second category had a ‘bottom-up’ perspective: ‘the number of birds is high

Cascading effects of a Canadian cod collapse

The findings of Frank et al. are based on the analysis of a time series that shows a remarkable coincidence of changes in the Atlantic shelf ecosystem off the coast of Nova Scotia, Canada. During the late 1980s and early 1990s, numbers of cod and other large-bodied predators in the benthic fish community declined sharply. This appeared to result in the near elimination of the ecological role of this group in the ecosystem (i.e. as top predators). Indeed, the biomass of benthic invertebrates,

Future questions

Unraveling the interplay of bottom-up and top-down forces will remain a major challenge in marine research over the coming years. Intensive fishing and ongoing climatic change imply that we are heavily modifying both forces, and good management should be based on an understanding of how this affects the ecosystem. The issue is a difficult one as there is much at stake and it is not easy to get the balance right. For instance, some have argued that fishermen should be ‘let off the hook’ because

Conclusions

Overall, the observations on the Scotian shelf and the Caribbean reefs are in line with the emerging view that marine communities are characterized by strong non-linearities 13, 14. Such an ecosystem view [15] suggests that there is a need to look differently at management of marine ecosystems. It implies that sharp irreversible change can sometimes result from gradually increasing fishery pressure, and that the critical threshold for such change will vary with climatic conditions. Although the

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