Measuring biodiversity in marine ecosystems

Abstract

The use of catch data to determine indicators of biodiversity such as 'mean trophic level' does not adequately measure ecosystem changes induced by fishing. Improved ways to assess those changes are required. See Letter p.431 It is often claimed that industrial fisheries are 'fishing down marine food webs' by depleting top predators (such as tuna) before targeting their prey species (plankton feeders such as oysters and sardines). But new global data reveal little evidence for this pattern of sequential depletion, working downwards through the trophic levels of the marine ecosystem. Rather, comparison of model predictions of the widely adopted marine indicator, mean trophic level (MTL) derived from reported catches, with actual ecosystem MTL suggests that fishing has intensified throughout all levels of marine food webs. The trend can be masked by the use of data based on catches, and if we are to accurately monitor future fisheries collapses — and recoveries — we may need to shift focus from catch-based indicators to tracking true abundance trends using scientific surveys and models.