On the selectivity of Gill-net used for catching Pink shrim Penaeus brasiliensis Latreille, 1917 and bown shrimp Penaeus paulensis Perez Farfante, 1967, in the lagoon-estuarine region of Cananéia, São Paulo State

Abstract

Three species of Penaeus are found in the coastal waters of São Paulo State: P. brasiliensis, P. paulensis and P. schmitti. The youngs of these shrimps that grow up in the lagoon estuarine region of Cananéia have been the subject of several fishing tackles employed to catch them. One of those named "corrico" is a small drift gill net that can be operated on a small size boat with or without outboard motor. The gill net is tossed from the stern of the boat moving at low speed at right angle to the flow of the tide. Carried by the movement of the tide, the gill net parts slowly and drifts over the bottom. The fishing operation can be expected whenever there is an incoming full tide. A detailed study of the selective characteristics of various mesh sizes is introduced. For this purpose data were taken from catches on pink shrimp P. brasiliensis and brown shrimp P. paulensis, in the same area, from a drift gill net constructed to this aim. This fishing gear was composed of 5 small nylon netting, having each one 16 m wide and 2 m in height. For the analysis of the data it was used the method proposed by Gulland (1969) considering the relation between the length of the individuals and the mesh sizes. The gill net selectivity curve formula may be represented as being c*(L) = e^{-E(L í  hm)2} , where c*(L) represents relative frequency of retention of individuaIs with length L, E and h are constants and m is the internal perimeter of gill net mesh. The shrimp were measured from the tip of rostrum to distal tip of telson. With the average values from E and h, the selectivity curves, in which the relative frequency of retention is plotted against total length were estimated, resulting the following expressions:

 c*(L)= e^{-0.003(L-1.238m)²} and c*(L)= e^{-0.003(L-1.311m)²} , respectively to P. brasiliensis and P. paulensis.