A Tale of Two Systems: Testing the Robustness of Mean Trophic Level of the Catch As an Ecosystem Indicator

Just over fifteen years ago in their seminal work, Pauly and his colleagues coined the phrase “fishing down the food web”.  That phrase describes how fisheries systematically target fish species sequentially down the food web as the higher trophic levels are depleted.  This behavior, measured by the mean trophic level of the catch, can have dramatic impacts on an ecosystem and be a potentially useful indicator for ecosystem-based management.  We tested the relative strength of this indicator using two different systems: Georges Bank and the Gulf of Alaska.  Each system has experienced different exploitation histories with Georges Bank being exploited in the more classical sense of fishing down the food web while the Gulf of Alaska was exploited sequentially from the bottom up.  Using mass balance food web models we tested the robustness of the mean trophic level of the catch indicator by engineering scenarios that could be catastrophic to the ecosystem and then evaluated the response of the indicator.  A robust indicator would identify, and ideally warn, of the undesirable ecosystem state.  In addition to the mean trophic level of the catch, alternative trophic indicators were identified as potential indicators relative to ecosystem-level management objectives.