The alternating recruitment pattern in Ensis minor, an exploited bivalve in the Gulf of Trieste, Italy

Abstract

E. minor is a shallow water razor clam, actively exploited by hydraulic dredges on the sandy bottom between 2.5 and 4 m depth. The fishing season lasts 6 month, from October to March. Despite of the heavy exploitation exerted in the previous years the resource exhibited dramatic recovery in 1983, 1988 and 1993 mainly due to an exceptional recruitment, that sustained the fishing activity for 2 y after the settlement. In the recruitment years the catches are normally low, but increase dramatically the following year, when little settlement occurs, probably because of space and resources competition. The last one is also reflected in the lower growth rate of the recruits when abundant settlement occurs. Two years later, when the 2+ cohort is nearly extinct and the 1+ age class is not very abundant (as stated above), the new settlement can take place and the cycle restarts. The amount of the recruitment is not predictable, but every five years very abundant age classes were observed. Apparently this process started in ‘83, after heavy depletion and at least two recruitment failures. The population structure was completely modified, with 0+ class absolutely dominant and the absence of the older individuals, at least in the more exploited areas, where the young-of-the-year were specially abundant. The fishery activity have been assumed to be partially involved in the maintenance of the cyclic pattern. However, by examining the data from the early seventies, preceding the full development of mechanical harvesting, it seems that this species ‘naturally’ adopts alternative patterns in recruitment, as other molluscs appear to do. This phenomenon is amplified by the exploitation.