Productivity estimates for the Mediterranean: evidence of accelerating ecological change
Abstract This paper re-examines recent trends in fish landings in the Mediterranean and Black Sea basins separately for each GFCM (General Fisheries Council for the Mediterranean) statistical area and compares estimates of production in different basins using comparable units of measurement, namely the productivity in tonnes per surface area of continental shelf. Comparisons were based on Geographical Information System (GIS) databases, and maps of catches were created per type of fishery per statistical and continental shelf area. Despite generally high exploitation rates, the overall Mediterranean landings for all species continued to rise in the 1970s and 1980s (Fishery fleet statistics, 1970, 1975, 1980–1989. Bulletin of Fishery Statistics. FAO, Rome, 1991, Vol. 30, 344pp.). This is not easy to explain in terms of steady state fisheries theory, given that landings were expected to decline as effort further increased and stocks became depleted. An examination of human-induced environmental trends over the last few decades pointed to the likely importance of nutrient enrichment for increasing fishery productivity within the Mediterranean and Black Sea basins, seas where biological and fishery productivity were formerly considered to be strongly limited by low nutrient levels. It seems from the GFCM statistics that an increase in fishery production in the 1970s and early 1980s was particularly evident for semi-enclosed basins such as the Black Sea and Adriatic, where nutrient enrichment from land and river runoff has been more pronounced than for the generally nutrient-poor regions of the eastern Mediterranean and around Sardinia. The effects of large inputs of nutrients, as for the Aegean Sea (receiving nutrient-rich water from the Black Sea outflow), and the Gulf of Lions (from the Rhone river), also appear to be reflected in recent landing trends, as well as where there is a more diffuse discharge of nutrients from human activities on shore. The still largely oligotrophic nature of water masses in the southern Mediterranean appears to be reflected in the significantly lower productivities in terms of tonnes of fish production per shelf area.