Stability in marine fish communities

Abstract

Fisheries-independent surveys sample major portions of marine systems and can be considered proxies for many important ecosystem components. A striking feature of many independent trawl-survey datasets is the relative stability of total biomass over several decades, despite overfishing of several commercial species and marked changes in the relative abundance of different species. This relative stability suggests a pattern of species replacements related to compensatory dynamics in the total production of fish biomass. In contrast, sustained declines in overall biomass are evident in other ecosystems. We used trawl-survey data to analyze the mechanisms that regulate stability across 19 marine ecosystems around the world. A linearized, multispecies Gompertz model within a state space framework was fitted to the survey data to quantify interactions among diet-based functional groups and external drivers. The total biomass in 7 of the 19 systems lacked uni-or multidirectional trends and was stationary. The stable ecosystems were associated with lower levels of aggregate community fishing pressure enabling differential rates of growth, immigration and emigration among the different populations to be the important factors regulating community dynamics. Higher levels of fishing pressure tended to synchronize the community response to the external driver resulting in large changes in total biomass. Evidence for interactions among functional groups was relatively weak; however, density dependence at the functional group level suggested within-group compensation as an important stabilizing mechanism.