Compensatory Dynamics in Marine Fish Communities

Specifying effective ecosystem-based management strategies requires an understanding of the status, trends and interactions among key ecosystem components.  Fishery- independent surveys provide the longest and most comprehensive data sets on important components of marine ecosystems.  A striking feature of many independent trawl-survey data sets is the relative stability of biomass over several decades, despite overfishing of several commercial species and marked changes in the relative abundance of different species groups.  This relative stability suggests a pattern of species replacements related to compensatory dynamics in the total production of fish biomass or within functional groups.  In contrast, sustained declines in overall biomass are evident in other systems.  Our goal was to determine if there were consistent interactions among diet based functional groups that could provide insight into the general ecological organization of marine systems.  We quantified the interactions among functional groups from 20 trawl surveys located around the world with state space models and included proxies for the environment and aggregate fishing pressure.  Fishing pressure was a major driver regulating the biomass of demersal piscivores, but was not an important factor for benthivores and planktivores.  Benthivores exhibited a higher degree of internal regulation, suggesting compensation.  As a functional group, they also generally experience lower overall removals.  The importance of environmental factors varied by location, exhibiting little impact in the western Atlantic, but significant forcing in the eastern Atlantic.  Environmental factors also had greater impact in areas with lower fishing pressure.  The examination of compensation within and among functional groups can help identify emergent properties of fish communities at higher levels of organization.