The Consequences of Fishing-Induced Changes in Size on Predator-Prey Interactions: Implications for Multi-Species Management

Even without selecting for large fish, the added mortality of fishing means fewer fish make it to larger sizes, leading to distributions shifted towards smaller individuals.  The consequences of these shifts for single-species management have long been recognized. Because body size limits the sizes of prey a predator can eat, truncated size distributions may cause equally dramatic changes in predator-prey interactions. As a result, multispecies models based on biomass alone, without considering changes in size distributions, may over- or under-estimate predation rates. These potential trophic effects have largely been ignored when managing these fisheries, but will be critical to fulfilling the mandate for multispecies management.  I developed a size-structured predator-prey model to evaluate how fisheries-induced changes in predator size distributions affect prey population dynamics.  I parameterized this model within California kelp forests using observations of size-specific predation of California sheephead (Semicossyphus pulcher) on urchins (Stronglyocentrotus spp.).  When predation success varied with predator size, and predators were size-selective, shifts in size distributions toward smaller predator sizes decreased overall prey mortality rates, with disproportionate reductions for larger prey, and increased prey densities.  These patterns suggest changes in predator size structure will lead to previously unexpected changes in prey size structure and abundance.