Quantifying Critical Points in Ecological Indicator Responses to Fishing and the Environment

Ecosystem-based fisheries management (EBFM) is a more holistic management strategy that concurrently addresses human, ecological, and environmental factors influencing living marine resources and evaluates these considerations collectively on a system level. Ecological indicators seek to develop decision criteria for EBFM as keyed to quantifiable attributes of ecosystem status. For EBFM, indicator reference points associated with management action need to be quantified, analogous to single species decision criteria (e.g., B_MSY). Ecological indicator thresholds would in principle capture responses to both fishing and environmental pressures. Theoretical and quantitative methods have been developed to assign decision criteria to ecological indicators’ response to human-use pressures; yet few efforts have established decision criteria in response to the combined influence of human-use and environmental pressures. Here, we seek to identify ecological thresholds at which a small change in fishing and environmental pressure results in an abrupt change in ecosystem status. We applied multiple analytical techniques including bivariate generalized additive threshold models and gradient forest models to determine more broadly (i.e., with global and national representation) if ecological indicators have common inflection points in response to fishing and environmental pressures. Our findings highlight levels of pressure where the magnitude of indicator response might differ from our expectations.