Evaluating the conservation risks of aggregate harvest management in a spatially-structured herring fishery

Despite broad recognition of the potential for significant spatial complexity in morphological, behavioral, and life-history traits within many fish populations, fisheries are commonly managed across large spatial scales that aggregate interacting sub-populations into single management units. Such mis-match between the ecological and management scales may lead to a loss of spatial diversity and a restricted ability of populations to adapt and persist in the presence of changing environmental conditions. Developing harvest strategies for spatially complex fish populations therefore remains a major challenge. In this study, we evaluate whether managing spatially complex fish stocks as large-scale aggregates leads to greater conservation risks. We develop a closed-loop simulation model that represents a range of dispersal scenarios and includes imperfect management knowledge about the abundances and dynamics of interacting Pacific herring (Clupea pallasi) sub-populations, as well as weak management control of how exploitation is allocated among sub-populations. The latter is driven by the spatial dynamics of the fishing fleet as it seeks to optimize profitability. Simulated management outcomes did not always lead to increased risks under all scenarios of dispersal, fishery spatial dynamics, and management errors. Instead, these processes interacted to either mediate or intensify the impact of inappropriate management assumptions and stock assessment errors. Management strategies aimed directly at limiting exploitation risk consistently protected spatially complex populations in the presence of incorrect management assumptions about stock structure, high fishing power, and persistent stock assessment errors. Given the pervasiveness of these errors in fisheries, we recommend further evaluation of spatio-temporal refugia for tactical management of spatially complex fish populations.