Dynamic Models of Size Spectra: Insights into Productivity and Exploitation of Fish Communities

This talk outlines a model for the dynamics of size spectra in which fish increase in body size by eating other organisms, including other fish.  The model starts with bookkeeping of biomass at the level of individual predation events, and builds up to biomass flow at the macroscopic level of fish populations and communities.  This helps understanding of ecosystem processes, such as the balancing of mass at steady state.  Productivity typically decreases with increasing body size in these models.   When fishing mortality is brought more in line with productivity, the models show that biomass yields can be substantially increased.  At the same time, coexistence of exploited species is enhanced, disruption to size structure reduced, and resilience to perturbations increased.  We suggest that the relatively benign effects of balanced harvesting could stem from fishing mortality tending to replace predation mortality, rather than adding to it.