Impact of demersal fishery and evidence of the Volterra principle to the extreme in the Adriatic Sea

Published comparison of data from two sets of cruises in the Adriatic Sea: the first during 1948–1949 and the second during 1996–1998, has shown a significant decrease of demersal fish stocks during 50 years of fishery. Furthermore, it has been shown previously that the stock of Chondrichthyes has decreased while the stock of Osteichthyes has increased relative to Chondrichthyes and, for several species, the increase is in absolute values too.By using Schaefer's model (logistic population under proportional harvesting), the overfished categories are identified. It is also shown that observed changes in Chondrichthyes and Osteichthyes are in agreement with the Volterra principle down to the extinction of predators. Furthermore, by using the Gause competition model under an increasing fishing effort it is shown that an initially dominant competitor may become a marginal competitor, in agreement with observations.Improvements in the management of fish stocks in the Adriatic are proposed. Given the fact that no one so far has determined the optimum harvesting effort of any species set within a marine ecosystem, improvements apply for other seas too.