| Abstract |
Objective echoes identification is not possible nowadays when several species are present in an area. Mid-water trawl catches are usually used to split fish schools into species during acoustic surveys. Moreover it is weIl known that because of a lively behaviour of these types of species, the catchability may vary a lot from one species to another. Therefore, in order to verify the reliability of identification by trawling, 68 hauls carried out during 4 acoustic surveys (1991-1994) have been analysed. Five pelagic species (anchovy, mackerel, horse-mackerel, sprat and sardine), observed during these surveys in Bay of Biscay were considered. On the one hand, taking into account the vertical echo-sounder data and the vertical course of the trawl from nets onde data, a theoretical weight was calculated, and compared with the real catch. The results showed that catchability varied a lot between species and within species according to school structure. On the other hand, a vertical avoidance of fish was simulated by changing the vertical trawl position to approach the real catch and study specific behaviours which could bias the identification. As a result, it seems that trawl species compositions could be used with a better precision by applying a correcting factor according to specific catchabilities if completed by an aggregation classification. |
