Age structure or functional response? Reconciling the energetics of surplus production between single-species models and ECOSIM

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Abstract
Whole-ecosystem foodweb models, specifically the dynamic model ECOSIM, contain specific hypotheses for surplus production that differ from traditional single-species management models. Specifically, ECOSIM begins with an assumption that all species are tightly connected and energetic surplus does not arise through fishing, whereas single-species fishing theory implies that fishing leads to surplus by removing larger, older, less-productive fish from populations. Although ECOPATH production ratios and single-species estimated production levels are both derived from the dynamics of von Bertalanffy consumption and growth equations, the dynamics of ECOSIM differ from the implied bioenergetics of fishing as applied to age-structured populations. Specifically, while the ECOSIM "Arena" functional response and the von Bertalanffy equations both lead to the appearance of densitydependence in predator consumption per unit biomass, the difference in starting assumptions between the models leads ECOSIM to "fix produc...

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