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What Influences Fisheries Production? Comparing the Effects of Environmental, Fishing, and
Food Web Forcing Across Large Marine Ecosystems

With growing interest in taking an ecosystem approach to fisheries management, it is increasingly important to understand the complex forces regulating ecosystem dynamics. In particular, how do climate forcing and food web structure interact to support fisheries production, and what processes amplify, dampen, or obstruct the production that ecosystems provide? AFSC researcher William Stockhausen and several colleagues organized a symposium at the American Fisheries Society 141st Annual Meeting on this topic. In the symposium, results of an international workshop were presented that focused on applying multiple surplus production models to widely diverse ecosystems of the world's oceans to understand how multiple drivers of productivity in fishery ecosystems simultaneously interact to determine overall production levels. These drivers reflect a triad of factors influencing fisheries production including fisheries, the environment, and trophodynamics (food web interactions).

The presentations in the session described a common methodological framework (i.e., surplus production models) that was applied across multiple levels of aggregation (e.g., single species, taxonomic, trophodynamic, functional and whole ecosystem) for 14 large marine ecosystems in the northern hemisphere and examined model outputs from multiple production modeling packages. They also estimated management-relevant metrics and ecosystem attributes and compared them across populations and ecosystems and described the utility of applying surplus production models in single-species, multi-species, and aggregate species group frameworks. The results particularly elucidate those links between the biogeochemistry, ecology, and harvesting in these ecosystems that are globally consistent. They also highlight some challenges of fitting such production modeling approaches to similar species or functional guilds in contrasting arrangements (different species within ecosystems and similar species between ecosystems) to better delineate what controls ecosystem fisheries productivity. Implications of these results for future work relevant to operational oceanography, population and community modeling, and ecosystem-based fisheries management were also discussed.

William Stockhausen coauthored and presented at the meeting the paper "Taking the Final Step: Can a Full Multispecies Production Model Tell Us Anything Single-Species Models with Covariates Can't?" The abstract follows: We define extended single-species production models (ESSPMs) as single species production models that incorporate time series of principal prey and/or predator species as biological covariates. One advantage to ESSPMs over single species production models without biological covariates is that these models can be used to test the existence and direction of (one-way) species interactions. However, estimates of actual interaction strengths are confounded with scaling of the covariates (i.e., catchability). In addition, biological reference points (BRPs; e.g., maximum sustainable yield, MSY) that can be calculated in ESSPMs may be of limited value compared to those estimates that more directly incorporate species interactions because of a lack of feedback between prey and predator species in ESSPMs. We thus developed a full multispecies production model (MSPM) to estimate biological interaction strengths and examine tradeoffs in multispecies MSY. As a preliminary demonstration, we fit this model to functionally analogous cod and herring species for the eastern Bering Sea. We compared results from the MSPM with those from ESSPMs that examined the same species grouping. Estimated species interactions from the two approaches were consistent regarding significance and direction; in contrast, the two approaches produced different MSY estimates. Future work will be to apply the MSPM to functionally analogous cod and herring species across multiple large marine ecosystems in the northern hemisphere. At this point, the answer to the question posed in the title is a qualified "yes", but the tradeoffs between modeling approaches to estimating BRPs for cases where biological interactions are known to be important merit further examination before use in a fisheries management context.

By William Stockhausen

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