Using Fish Diets As Ecosystem Indicators: Are Fish Feeding Down the Food Web on Georges Bank?

The science behind “fishing down the food web” (Pauly et al. 1998) suggests marine food web structure has shifted over the past half-century.  The assumption is that mean trophic level (MTL) from fisheries catch echoes ecosystem MTL and biodiversity, and findings imply global declines in ecosystem health.  Questioning the suitability of this indicator, we compared time series of MTL of fish diets from 21 species (1973-2012) and trawl survey catches (1963-2012) on Georges Bank, northwest Atlantic.  Objectives were to identify temporal trends in diet and survey catch MTL, identify common trends among these time series, and determine whether fish are feeding down the food web.  The hypothesis being that fish diets should reflect similar trends in ecosystem MTL as shown by survey catches.  Contrary to Pauly et al. and fisheries catch, modeled diet and survey catch MTL on the Bank has not shifted dramatically over the past 4+ decades supported by AIC.  One common pattern among diet and survey catch MTL time series was identified, suggesting these series are time-invariant.  Diets from opportunistic generalists shed a novel light on monitoring ecosystem change.  Accordingly, the collective diet stability and feeding strategies of these predators on Georges Bank may strengthen ecosystem resilience.