ANALYSE DE LA TENDANCE DE L’ABONDANCE DE L’ALOSE ALOSA ALOSA EN GIRONDE À PARTIR DE L’ESTIMATION D’INDICATEURS HALIEUTIQUES SUR LA PÉRIODE 1977-1998.

Abstract

The population of the allis shad A l os the last twenty years as declining, endangered, increasing, in a satisfactory status. The a alos a has been considered successively in objective of this study is to estimate the most appropriate fishing indicators which can allow us to state on the abundance trend of A l os a alos a during the 1977-1998 period. These fishing indicators are CPUE, total catch and total fishing effort. The non-random sample of data provided by a group of cooperative fishermen has been stratified with fishing techniques, fishing area, fortnight periods of fishing. The estimate of total catches and total efforts is based on the classical theory of stratified sampling. We used the General Linear Model method to obtain CPUE from a log-linear model, which limits the effects of the variations of capturability in space and time. These CPUE are supposed to be representative of the inter-annual variation of abundance. The model explains the CPUE as the addition of the main effects of the factors zone, year and fortnight. The trend of the abundance indexes is modeled by a step function with an auto- regressive error model. The plateaus appear to be very highly significant for the Gironde basin as well as for the sections Estuaire, Garonne, Dordogne. The analysis of the three fishing indicators and the comparison of their evolution lead to the conclusion that the abundance of A l os a alos a is at least stable and has certainly increased in the Gironde basin. The jump of the CPUE observed in 1994 can be related to the construction of fishways in the upper parts of the Garonne and Dordogne rivers. These encouraging results should however not dissuade managers to adopt a precautionary approach of the shad population in the Gironde basin, according to the status of this specie in Europe