An Ecosim model for exploring ecosystem management options for the Gulf of Mexico: implications of including multistanza life history models for policy predictions

Abstract

An Ecopath-Ecosim ecosystem model under development for coastal areas of the Gulf of Mexico simulates responses of 63 biomass pools to changes in fisheries and primary productivity. 10 key species are represented by detailed, multi-stanza population dynamics models (31 of the biomass pools) that attempt to explicitly account for possible changes in recruitment rates due to changes in bycatch rates and trophic interactions. Over a 1950-2004 historical reference period, the model shows good simulated agreement with time series patterns estimated from stock assessment and relative abundance index data for many of the species, and in particular offers an explanation for apparent nonstationarity in natural mortality rates of menhaden (declining apparent M over time). It makes one highly counterintuitive policy prediction about impacts of management efforts aimed at reducing bycatch in the shrimp trawl fishery, namely that bycatch reduction may cause negative impacts on productivity of several valued species (menhaden Brevoortia patronus, red drum Sciaenops ocellatus, red snapper Lutjanus campechanus) by allowing recovery of some benthic predators such as catfishes that have been impacted by trawling but are also potentially important predators on juveniles of the valued species. Recognition of this policy implication would have been impossible without explicit, multistanza representation of juvenile life histories and trophic interactions, since the predicted changes in predation regimes represent only very small overall biomass fluxes.