Changes in the trophic level of Portuguese landings and fish market price variation in the last decades

Abstract

Fishing can alter the structure of marine food web by selective removal of some species. Changes in marine food webs in Portuguese waters were evaluated based on estimates of the annual mean trophic levels (TLm) and fishing-in-balance index (FiB) of mainland, Azores, Madeira and total Portuguese marine fisheries landings for the period between 1970 and 2006. The log-relative-price-index (LRPI), that translates the relationship between the logarithm of prices of low and high trophic level species, was also calculated for each year. TLm of mainland landings showed a decreasing trend, reflecting changes in the structure of marine food webs, whereas in Azores and Madeira TLm increased. FiB index also showed a downward trend and negative values in mainland waters, which may be associated with unbalanced fisheries. In the period studied LRPI increased, indicating that high trophic level species had become more valuable in relation to species feeding at lower trophic levels. It is likely that the persistence of present trends will compromise the sustainability of fisheries. A better management is needed in order to reverse this decreasing trend in TLm in the long-term using an ecosystem-based approach.