Are We Catching What They Eat? - Assessing Mean Trophic Level of Fisheries Catch and Predator Consumption Globally

The mean trophic level of fisheries catch (mTL_Y) is commonly used as an index to evaluate the state of fisheries in ecosystems. Here, it was used simply to estimate the mean trophic level of catches for 1) all fisheries, 2) all finfish fisheries, and 3) all forage fisheries. In a similar manner, the mean trophic level of predator consumption (mTL_q) index was used to estimate the trophic level of prey items that marine predators were consuming in a given ecosystem. These indices were used in order to screen for potential ecosystems or latitude groups where fisheries and the major predator categories (i.e. seabirds, marine mammals, and large predatory fish) may be targeting prey at similar trophic levels. We utilized 43 marine ecosystem models (Ecopath) from an existing database and from other sources, which represented ecosystems within the last 40 years and included all the major predators groups. Few differences were found between the mean trophic levels targeted by fisheries and predators, and no differences were found between forage fisheries and predators on latitudinal scales. Our results show where potential conflicts for forage species may arise between these ecological stakeholders, providing information that can inform an ecosystem-based approach to management.