Evaluating catch mean trophic level as an indicator of ecosystem change

The mean trophic level (MTL) of catch has been proposed to track changes in marine ecosystems resulting from fishing. Despite the ongoing debate surrounding its validity, catch MTL is a key indicator for measuring progress toward global biodiversity goals. Evaluations of catch MTL have found no linear correlation between trends in the indicator and the ecosystem state. I use simulation models and a method common in epidemiology to evaluate catch MTL as a strategic indicator for ecosystem changes even though it is not linearly related. The performance of catch MTL was ‘fair’ when applied globally, but varied considerably across individual simulated ecosystems. Catch MTL performed most reliably when the composition of the catch reflected the ecosystem and fishing pressure was constant over time. The inconsistent performance of catch MTL suggests it is not a reliable indicator of ecological change, but it still provides useful information about fisheries catch over time.