Abundance and spatial distribution of the Mediterranean scallop, Pecten jacobaeus, in a marine lake

Pecten jacobaeus is an exploitable bivalve, whose populations have greatly declined in the main Mediterranean fisheries since the mid-1980s. A substantial population of the species was studied in the marine Lake Vouliagmeni (Korinthiakos Gulf, Greece). The population density and the spatial distribution of P. jacobaeus were estimated in the lake with line transect sampling by SCUBA diving, a method not previously used for scallop density estimations. Individuals of the first size class (small) were mainly restricted to shallow waters (4-8 m), while individuals of the second size class (large) were mainly restricted to deeper waters (>12 m). Possible reasons for this size-separation were: (1) the most appropriate settlement substrate for pediveligers was in shallow waters, (2) the strong summer thermocline in the lake may have induced the selection of the shallow and warm bottoms of the lake as settlement areas, (3) intraspecific competition between adults and pediveligers, and (4) the high fishing mortality of large individuals in shallow waters. On the hypothesis that successful recruitment of P. jacobaeus occurred mostly in the shallow areas of the lake, it was deduced that P. jacobaeus gradually migrated, as they grew, from the shallow settlement fields of the lake to the deeper areas. The density of large individuals was associated with depth and the degree of scatter (QDφ) of the granule size frequencies. Higher densities of large individuals were found deeper and in poorly sorted sediments (QDφ > 1.0). The size of large individuals was positively correlated with depth. The size of P. jacobaeus population in Lake Vouliagmeni was estimated.