As with terrestrial food crops, we are heavily reliant upon a very limited number of the 13 000 or so marine fish species. Herrings, sardines and anchovies made up 25 per cent of landings in 1989. Continuous growth in global catches throughout the 1980s was largely based on increased landings of these shoaling pelagic (free-swimming) species because most demersal (bottom-dwelling) stocks were, and still are, fully fished. This trend is worrying because shoaling pelagic species tend to be at lower trophic levels than demersal species, and although fishing down food webs in this way leads at first to higher catches, the increase eventually levels out and is then likely to decline. Hence the globally declining trend in the average trophic level of fish caught, identifiable over the period 1950-94, is unlikely to be a sustainable means of exploiting marine fish stocks (Pauly et al., 1998).