Effects of fisheries on the Cantabrian Sea shelf ecosystem

Abstract

The Cantabrian Sea shelf ecosystem is described using a mass-balance model of trophic interactions, in order to understand the effects of the different fisheries that operate in this area. The study was based on a database of bottom trawl surveys, ICES stock assessment working groups, stomach analyses, fisheries research and was supplemented by published information. The model had 28 trophic groups corresponding to pelagic, demersal and benthic domains, also including detritus and fishery discards. The results indicated that the biomass and production of some groups would be unrealistic if they were independently estimated by single-species assessment approaches. Summaries are given to illustrate the flow distributions between groups. Strong relationships existed between the pelagic, demersal and benthic domains due to key groups, like zooplankton suprabenthic and horse mackerel, that transferred the flow from primary production to the upper trophic levels. Feeding pressure on phytoplankton was low and detritivorous species were an important component of the ecosystem. Estimations of the trophic level of the fisheries, transfer efficiency between trophic levels and mixed trophic impact analysis, that consider the fishery both as an impacting and as an impacted component, were also included. The results indicated a fisheries impact level in the Cantabrian Sea comparable to that in the most intensively exploited temperate shelf ecosystems of the world. The fishery was operating at a mean trophic level of 3.7. The importance of discards as food in the ecosystem was low, in comparison with detritus, primary producers or other low trophic levels. The negative trophic impact of trawling on the different groups of the system was high and much stronger than the other gears studied. All fishing gears, except the purse seine, had negative impact on fish feeders and elasmobranchs. The mean trophic level of Cantabrian Sea fisheries declined from 1983 to 1993 but has remained steady since then.