Trawl selectivity trials on the deep-water rose shrimp (Parapenaeus longirostris) in sicilian waters.

The selectivity of the traditional commercial bottom trawl net employed Sicily to catch the Mediterranean deep water rose shrimp, Parapenaeus longirostris, has been assessed. Two fishing campaigns were carried out in the Strait of Sicily and in the Southern Tyrrhenian Sea, using the covered cod end method (mesh 20 mm vs. 31 mm). Of a total catch of 11601 individuals, 23.4% escaped in the cover; the sample length structure from the Strait is unimodal, that from the Tyrrhenian is polymodal. A logistic curve, fitted with a maximum likelihood criterion, has been used to model the selectivity data, obtaining the parameters CLc50% (50% retention size), SR75% 25% (selection range) and SF (selection factor, i.e. CLc50%/mesh). The two sets of data, besides a larger selection range for the Strait sample (5.2 mm vs. 2.3 mm), produced very similar estimates (retention sizes of 13.0 mm vs. 12.8 mm CL), fitting the logistic curve well. Almost no shrimp larger than 20 mm does escape from the cod end; moreover, from the amount of damaged specimens found in the cover, even the evaded shrimps sustain a high fishing mortality. An increase of the present cod end mesh opening, even above the size required by the EU bylaws (at present, 40 mm stretched) seems necessary in managing the fishery.