Background and Interpretation of the ‘Marine Trophic Index’ as a Measure of Biodiversity

D. Pauly, R. Watson, (2005). Philosophical Transactions of the Royal Society B: Biological Sciences 360, 415-423.

Since the demonstration, in 1998, of the phenomenon now widely known as ?fishing down marine food webs?, and the publication of a critical rejoinder by FAO staff, a number of studies have been conducted in different parts of the world, based on more detailed data than the global FAO fisheries statistics originally used, which established the validity and ubiquity of this phenomenon. In this contribution, we briefly review how, rather than being an artifact of biased data, this phenomenon was in fact largely masked by such data, and is, indeed, more widespread than was initially anticipated. This is here made visible by comparing two global maps of trophic level (TL) changes from the early 1950s to the present. The first presents the 50 year difference of the grand mean TL values originally used to demonstrate the fishing down effect, while the second is based on means above a cutoff TL (here set at 3.25), thus eliminating the highly variable and abundant small pelagic fishes caught throughout the world. Based on this, we suggest that using mean TL as ?Marine Trophic Index? (MTI), as endorsed by the Convention on Biological Diversity always be done with an explicitly stated cutoff TL (i.e., cutMTI), chosen (as is the case with our proposed value of 3.25) such as to emphasize changes in the relative abundance of the more threatened, high-TL fishes. We also point out the need to improve the taxonomic resolution, completeness and accuracy of the national and international fisheries catch data series upon which the cutMTI is to be based.