The long-term decline of the Chamelea gallina L. (Bivalvia: Veneridae) clam fishery in the Adriatic Sea: is a synthesis possible?

Abstract

       Since the early 1970s a comparatively large clam fishery based on hydraulic dredgers has beendeveloping, though later declining, along the Italian coast of the Adriatic Sea (Central Mediterranean). In the late 1970s the fishery yielded 80,000-100,000 metric tons per year (at least according to the best available estimates, based on confidential interviews with fishermen, fish retailers as wellas examination of trade sheets) but later progressively decreased in most recent years to one sixth of the previous level.
       In Italy it was the first fishery to be controlled through licences whose number had been set at the national level to keep the fleet and the fishing effort nearly unchanged, but the number of authorise dvessels actually increased during the 1980s and technical improvements on the fishing gear allowed the boats to sweep greater ground areas per unit time. This explains why various measures were sought to reduce the fishing pressure on clam beds (such as withdrawal of vessels, imposition of closed seasons, larger open spaces in the sieves), and a limited reduction of the fleet was pursued inthe late 1990s after large mortality events were recorded during late summer-early autumn of 1996.
       At the same time the influence of unknown environmental factors was occasionally assumed for the fishery, and the greater abundance of clam beds as well as other fish and shellfish resources in the Adriatic in comparison with other Mediterranean areas was tied to the large freshwater inflow from the Po and other rivers.
       Because of such discrepancies in the literature we reviewed all available papers (or nearly so) on the Adriatic clam fishery (including national laws, reports on the eutrophication of the coastalareas of NE Italy, and others) in order to understand the role of environmental factors (i.e. unrelated to the fishing effort) vs. fishing pressure for determining causes of the clam decline. Although the low qualitative level of many data makes it difficult to disentangle the decline’s causes, indirect clues show that the progressive reduction of freshwater flow into the Adriatic Sea, as well as of its phosphate content, have been playing a relevant role in the matter.