Catch rates, size structure and sex ratio of Melicertus kerathurus (Decapoda: Penaeidae) from an Aegean Sea trawl fishery

Otter-trawl catch rates and population structure of the prawn Melicertus kerathurus were studied at the fishing ground of Thermaikos Gulf (North Aegean Sea, Eastern Mediterranean), where the stock is exploited both by otter-trawlers and the artisanal local fleet. Sampling was conducted on board commercial trawlers on a monthly basis in two consequent open fishing seasons (October to May 2000–2001 and 2001–2002) resulting to a total of 70 hauls. Samples were also obtained through experimental trawling (24 hauls) during the closed season. The main trawling ground is located near the border between the outer and the inner gulf, and along the western and northeastern coasts of the outer gulf in depths ranging from 35 to 70 m (most frequently at 40–50 m). Catch per unit effort (CPUE) varied significantly with month but not with fishing season (mean CPUE 3.18 and 1.78 kg/h for the 2 successive years) or depth. Catches were at maximum (up to 7.5 kg/h) at the beginning of the open season in autumn and decreased sharply as the season proceeded (<2.5 kg/h), which is likely to be due to induced fishing mortality in the preceding months, to prawn dispersal and/or to its tendency for longer burrowing. Commercial fishing trips consisted of multiple, short efforts when catches were high, whereas low yields during winter–spring led to less hauls of longer duration: ln CPUE = 2.974 − 2.047 ln HD, r = −0.57. Prawn size ranged from 17 to 60 mm CL, showing a consistent difference in favor of females. A weak recruitment to the fishery was observed from autumn through winter, while, as shown by experimental trawling, the main recruitment pulse to the fishery occurs just prior to the season's opening. The delayed arrival of the prawns on the fishing ground during the second fishing season, accompanied by smaller prawn size and shallower exploitation depth, was possibly related to decreased autumn rainfall. In both years, sex ratio showed a temporal pattern, which seems to be the result of sex-related differences in fishing vulnerability, in timing of inshore migration and in mating mortality.