Incorporating Food Web and Habitat Interactions in a Forage Fish Stock Assessment

Some researchers have suggested that because of the multiple sources of mortality that forage fish face, fisheries managers could use other biological reference points – e.g. total mortality (Z) – as a benchmark for making management decisions.  Doing so would require accounting for other sources of natural mortality. In this paper, we outline the potential for using ecosystem and habitat models to estimate natural mortality for forage fish stocks.  We use the Chesapeake Bay Fisheries Ecosystem Model (CBFEM) to generate various sets of a time series for predation mortality on Atlantic Menhaden.  In alternative runs of the CBFEM, we use forcing and mediation functions to influence physical factors-species interactions and generate additional time series of natural mortality.  Then using a simple stock assessment model and the time series generated with the CBFEM, we compare the fit of the various formulations (i.e., constant natural mortality, variable natural mortality based on trophic effects alone, and variable mortality based physical factors and trophic interactions) to determine the utility of incorporating physical and trophic interactions into stock assessment models. These model scenarios and stock assessment approaches are presented to gain insight into the possible responses of menhaden to the fishery management and water quality management actions.