Approche de la dynamique de stocks accessoires

For a few years now, the landings from the Lorient's industrial fleet have been continually increasing (32,000 tons in 1980, 56,000 tons in 1985). This fleet practices a multispecific exploitation of the international fishery of the west and north of Scotland. The rock salmon (Pollachius virens) is the fleets' target species, its catches of other gadidae are considered secondary. Two of the catches, those of the blue ling (Molva dypterygia) and the ling (Molva molva) have been advancing since the beginning of the decade and in 1985 represented 21% of the total amount in quantity and value landed by the industrial fleet. The objective of this study is to asses the status of these ling resources and examine whether their exploitation can, in the short term, postpone a possible decline in the rock salmon yield. Analysis of the structure of the industrial fleet's exploitation shows that in the global strategy carried out by this fleet, the rock salmon is a favoured species. Secondary species catches are done more than anything as a function of the yields obtained and the state of the market. Among these species, the two lings, the blue ling in winter and the ling from the beginning of spring to the end of fall play a fundamental role in the diversification of catches, and their products are substituted in a significant way with those of the rock salmon. The parameters necessary for the structural modelling of ling exploitation are assessed: A series of annual efforts since 1974, demographic structure of the catches and coefficients of mortality by fishing season. For the latter, a rectified analytical model of pseudo-cohorts extended to the case of exploitation by several trades was developed. The sensitivity of the results to the parameters entered into the model is tested. The predictions of the Thompson and Bell model (1934) show that the ling is at its maximum weight yield. Analysis of the trawler-boulter interaction as it relates to this resource shows that any planning of the trawlers' fishing effort improves the exploitation results of the trawlers but especially those of the boulters. The blue ling is underexploited by weight, but its exploitation has the handicap of a much higher production cost than that of the other gadidae. These results indicate therefore that the exploitation of lings cannot replace that of the rock salmon. Exploitation by the Lorient fleet of the lings, in particular, and the gadidae in general, fits in with the international context of Scotland's western and northern fisheries. Analysis of this fishery's current management policy shows that it is not taking into account competition among the fleets for the resource, and of the catch runs of certain species. The general state of the fishery's resources reveals that we must consider a decline in the total yield in the years to come. In order to keep their industrial fleet's exploitation profitable, the Lorient ship owners have available as solutions the reduction of production costs, enhancement of their landed products' value, and diversification of its yield to the pelagic species of the fishing zone.