The consequences of fishing for size-dependent predator-prey interactions
The consequences of fishing predators for food webs have been documented in many marine and aquatic systems. Most of the ecosystem changes due to fishing have been attributed to declines in predator abundance. However, fishing also reduces the body size of predators. Because many predators change the species and sizes they eat as they grow, reductions in body size may render the predator population functionally extinct for a given prey even without changes in predator abundance. Predictions based on reductions in predator biomass alone may dramatically underestimate the effects of fishing on predation where diet changes with size are strong.
We examined the effects of fishing for a range of predator life histories, diet changes with size, fisheries selectivity patterns and fishing effort. We quantified the reduction in predation due to fishing and evaluated how the tradeoff between predation and catch varied with ontogenetic shifts in diet. We simulated the consequences of these changes in predator consumption for prey under various scenarios.
Fishing at maximum sustainable levels reduced predation to 40% of the unfished level for prey eaten uniformly by all predator size classes, and this was consistent across predator life histories and fisheries size selectivity patterns. For a given reduction in predation, fisheries that were more selective for large fish achieved a higher catch rate, but had a greater tradeoff in catch to achieve higher levels of predation. Predation on prey eaten late in life history declined more steeply with catch, leading to significant under-estimates of the effect of fishing when ignoring changes in diet with size. The discrepancy was magnified for more size-selective fisheries, and for predator life histories with higher mortality rates relative to growth.
The consequences for prey dynamics were greatest when the focal predator represented a large fraction of the total predation mortality, but could be mediated by fishing the prey. Where shifts in diet composition occurred between size classes of the same species rather than different species, the effects of fishing on prey were reduced, except in the case of fast-growing prey in which a large fraction of the population could achieve sizes at which they could be consumed only by large predators.