Escapement from the main body of the bottom trawl used for the Mediterranean international trawl survey (MEDITS)

Escapement through the body or under the footrope of the sampling trawl use d for the Mediterranean international trawl survey programme (MEDITS) was a ssessed i) by means of visual observations performed with a towed operating vehicle equipped with a high-sensitivity video camera, and ii) by attachin g small bags to the outside of the trawl body and under the footrope. Due t o the small size of most of the individuals present in the study area and t o the towing speed, fish behaviour in front of the trawl could not be frequ ently observed. In contrast, their behaviour was easily observed inside the net, especially upstream and inside the codend. Some species, such as hors e mackerel (trachurus sp.), showed greater swimming endurance compared with others such as annular seabream (Diplodus annularis). In three fishing tri ps conducted in different seasons in 1996 and 1997 to obtain data on severa l species in different areas of the Adriatic, six small net bags were mount ed outside the body of the MEDITS trawl to quantify escapement through the trawl belly and under the footrope. Significant escapement of many species occurred in the near-bottom part of the side panels. Norway lobster (Nephro ps norvegicus) had the highest rate (64%), mainly represented by small-size individuals. The escapement of greater forkbeard (Phycis blennoides) was a lso high (53%). For hake (Merluccius merluccius), common pandora (Pagellus erythrinus) and poor cod (Trisopterus minutus capelanus), rates ranged from 10 to 16%..The escapement of red mullet (Mullus barbatus) was very low des pite their great abundance. Blue whiting (Micromesistius poutassou) escaped mainly through the mid-height part of the side panel. This great variabili ty of escapement values could affect the proportion rates of the species sa mpled during a standard survey. Different escape behaviours among size clas ses were observed for M. poutassou, I: minutus capelanus and P. erythrinus, with smaller individuals tending to escape towards the bottom and larger o nes towards the higher parts of the trawl body. In the species where the es cape rate was size-dependent, therefore, the size-frequency distributions o btained from the codend catch may not reflect actual size-frequency distrib utions. (C) Ifremer/Cnrs/Inra/Ird/Cemagref/Elsevier, Paris.