Puissances de pêche et relations avec les caractéristiques techniques des navires. Exemple de flottilles pratiquant les arts traînants en Manche Ouest, Atlantique et Méditerranée.

Three fleets of the French fishery are investigated : the dredge fleet of the Western part of the Channel, the bottom-trawl fishery of South-Britanny harbours, and the Mediterranean trawl fishery. Landing and effort data by species, vessel and trip lead to yields (catch per unit of effort) which are analysed by a multicplicative model with two or three factors. Individual fishing powers are then quantified for some given years, together with variation in relative abundance of the stock within area and/or period strata. Attempts to explain these fishing powers by vessels characteristics are made. After the study of the Mediterranean trawl-fleet, which takes Year into account in the model, two multi-annual series are analysed, and some conclusions on fishing power variations along time are obtained. The amount of total variance explained by the model depends on fisheries and stocks. The three factors retained for the South-Brittany study (Vessel, Month and Areas) account for 50 to 80% of the total variance. For the Mediterranean data, 15 to 50% of the total variability of yields are explained by a three-factor (Vessel, Month, Year) model. Whatever the model retained, the quality of the model mostly depends on data quality. Biased data of landings or effort usually leads to bad fits. It is also noted that the quality of the fit decreases when the stock abundance decreases. If the fishing powers (Vessel effect) mostly stand for the main explicative factor of the yield variability, the engine power of vessels does not appear as the main source of variability of vessel efficiencies. A multi-annual study of the yields of warty venus shows that sudden increases of individual efficiency seem to be explained by changes of skippers or changes of engines. For Saithe off the West Coast of Scotland, the 1983-1997 period has been divided in two in order to take into account the dramatic decrease of this stock abundance since 1990. The two main fleets involved in this fishery show different ways of variations : the most powerful vessels have lost part of their efficiencies, while the less powerful have increased their fishing powers. Engine powers seem to explain rather well the individual values of fishing powers. However, it is important to note that fitted models do not show any variation in the differences between fleets. Furthermore, differences in fishing power still exists between vessels within a fleet, despite the fact that they are quite similar in terms of technical characteristics. Any further study requires a good knowledge of any technological and human changes which have occured during each vessel life.