Marine fisheries as ecological experiments

There are many examples of ecological theory informing fishery management. Yet fisheries also provide tremendous opportunities to test ecological theory through large-scale, repeated, and well-documented perturbations of natural systems. Although treating fisheries as experiments presents several challenges, few comparable tests exist at the ecosystem scale. Experimental manipulations of fish populations in lakes have been widely used to develop and test ecological theory. Controlled manipulation of fish populations in open marine systems is rarely possible, but fisheries data provide a valuable substitute for such manipulations. To highlight the value of marine fisheries data, we review leading ecological theories that have been empirically tested using such data. For example, density dependence has been examined through meta-analysis of spawning stock and recruitment data to show that compensation (higher population growth) occurs commonly when populations are reduced to low levels, while depensation (the Allee effect) is rare. As populations decline, spatial changes typically involve populations contracting into high-density core habitats while abandoning less productive habitats. Fishing down predators may result in trophic cascades, possibly shifting entire ecosystems into alternate stable states, although alternate states can be maintained by both ecological processes and continued fishing pressure. Conversely, depleting low trophic level groups may affect central-place foragers, although these bottom–up effects rarely appear to impact fish—perhaps because many fish populations have been reduced to the point that they are no longer prey limited. Fisheries provide empirical tests for diversity–stability relations: catch data suggest that more diverse systems recover faster and provide more stable returns than less diverse systems. Fisheries have also provided examples of the tragedy of the commons, as well as counter-examples where common property resources have been managed successfully. We also address two barriers to use of fisheries data to answer ecological questions: differences in terminology for similar concepts and misuse of records of fishery landings (catch data) as a proxy for biomass trends.