Predator-prey interactions altered by fishing-induced decreases in predator body size

Background/Question/Methods

In the last few decades, the cascading consequences of overharvesting top marine predators have become increasingly clear. However, even when it does not cause dramatic reductions in predator populations, fishing can disrupt the food web in a way rarely considered—by shrinking predator body size distributions. Because marine predators almost universally change which species and sizes they eat as they grow, many species do not begin eating certain prey until they reach a size threshold. As a result, reductions in size may cause the same loss of the functional role of the predator population as complete removal.

We developed a general size-structured predator-prey model to examine the predator characteristics that make predator-prey interactions vulnerable to disruption by fishing-induced changes in size distributions. We applied this model to California kelp forest ecosystems in which predation by sheephead, Semicossyphus pulcher, on sea urchins, Strongylocentrotus spp., depends on size. Predation rates were compared at sites where sheephead size varied and densities were comparable. Simulations were used to examine the reduction in urchin mortality relative to unfished populations due to the combined effect of reductions in sheephead size and density, and compare the magnitude of the effect to that of density alone.

Results/Conclusions

Fishing caused the greatest reductions in prey consumption for prey eaten late in the life history. Predators with the lowest overlap in diet between juvenile and adult classes experienced the biggest fishing-induced shifts in the prey consumed by the predator population. Specifying ontogenetic shifts in diet composition is critical to accurately predicting changes in predation due to harvesting the predator.

Within California kelp forests, urchin mortality rates are lower, particularly so for the largest urchins, when the largest sheephead are absent from field populations. In simulations, sustainable fishing on sheephead led to a reduction in the ability of these predators to control urchin densities, and the combined effect of decreases in predator size and abundance may increase the likelihood that kelp forests will be converted to urchin barrens. Such a conversion results in considerable losses in biodiversity, underlining the importance of considering the effects of fishing on changes in predator size as well as numbers.