Essai d'evaluation du stock de géniteurs d'alose Alosa alosa du bassin versant gironde-garonne-dordogne sur la periode 1987-2001 et comparaison de differents indicateurs d'abondance

Abstract

Three monitoring tools, the fishery using drift net, the fish pass of Golfech (Garonne) and Tuilieres (Dordogne) hydroelectric powerplants located on the middle parts of the basin and the spawning areas located downstream from these obstacles have permitted to estimate the stock of Alosa alosa of the Garonne basin. From 1987 to 2001, 7 600 000 shads migrated on the Garonne basin, i.e. 505 000 fish per year. The jump of the abundance of Alosa alosa observed in 1994, despite the important (annual mean of 61 %) and stable fishery mortality, can be related to the construction of fish pass at the end of the 80's and to the biological cycle of shad, most of fish returning in rivers after 5 years spent in ocean. However, the population seems to be stable over the period 1994-2001. If the reproductive stocks of 2002 and 2003 is taken into account, the stock show a decreasing tendency over the period 1994-2003. Thus, it appears necessary to look after the population, the reproduction activity and efficiency to ensure an abundance level compatible with a relative stable fishing mortality. On the study period, more than 63% of shad migrate each year on the Garonne river. This result can be related to the higher flows in this river than in the Dordogne River during the migratory period. However, flows in Garonne and Dordogne seem to have no influence on the repartition of fish on the two axes. Water temperature monitoring at the confluence of the rivers should be useful to study more precisely the influence of environmental factors on this repartition. The proportion of shads migrating on the Dordogne river increases during the study period: 25% of fish migrate on the Dordogne river before 1993 vs. 44% over the period 1993-2001. The reproductive stock (fish pass + spawning areas) appears to be a good abundance indicator on the Garonne basin because of the relative stable fishing mortality. The CPUE provided by the fishery appear to be significantly linked to the basin population and to the numbers of fish migrating on the two axes, thus revealing that they are also good abundance indicators. However, the CPUE cannot really indicate the status and the evolution of the reproductive stock, which is basic for the population management.