Comparing trophic flows and fishing impacts of a NW Mediterranean ecosystem with coastal upwelling systems by means of standardized models and indicators

Abstract

The NW Mediterranean has a number of structural features in common with upwelling ecosystems. Therefore, an ecological model representing a NW Mediterranean exploited ecosystem was standardized and compared with four previously standardized models from coastal upwelling ecosystems: the Northern and Southern Humboldt (Chile and Peru upwelling systems) and the Northern and Southern Benguela (Namibia and South Africa upwelling systems). Results from biomasses, flows and trophic levels indicated important differences between ecosystems, mainly caused by differences in primary production, which was smallest in the NW Mediterranean Sea. However, principal component analysis (PCA) of biomasses and flows suggested a similar pattern between theNWMediterranean and the South African systems due to the inclusion of an important fraction of the continental shelf in both ecological models representing these areas. At the same time, diets of commercial species from theNWMediterranean were more similar to Benguela than Humboldt species. However, the relatively heavy fishing pressure in the NW Mediterranean ecosystem was highlighted relative to its primary production, and was evident from the large catches and small primary production, largest flows from TL 1 required to sustain the fishery (%PPR), the low trophic level of the catch (TLc), high exploitation rates (F/Z), largest values in the trophic spectra portraying catch: biomass ratio, the FIB index and the demersal: total catch ration. Comparisons of %PPR, the trophic level of the community (TLco), the biomass of consumers and F/Z ratios seemed to capture the ecosystem effects of fishing: large in the NW Mediterranean, Namibia and Peru upwelling systems. Small pelagic fish were the most important component of the fisheries in the NW Mediterranean and Peruvian systems. However, the smaller production and biomass ratios from the NW Mediterranean could be an indirect indicator of intense fishing pressure on small pelagic fish, also in line with results from consumption of small pelagic fish by the fishery, F/Z ratios and trophic spectra. Moreover, similarities between the NW Mediterranean and Namibian systems were found, mainly related to the demersal: total catch ratios, the FIB index, the relevance of gelatinous zooplankton in the consumption of production and the importance of pelagic-demersalcoupling, in remarkable contrast to the other ecosystems. These similarities should be interpreted in terms of dynamic trajectories that the Namibian system has shown due to the collapse of its pelagic ecosystem, partly due to fishing intensity, and the signs that the NW Mediterranean could follow suit in the future.