Growth variability analysis of a brown trout (Salmo trutta L.) population in a Pyrenean mountain stream.

The brown trout population of the Neste d'Oueil, a Pyrenean tributary of th e Garonne river, was studied between July 1996 and March 1999. During the p eriod of two years and nine months, nine sampling experiments were carried out in February-March, July and September on two sectors of the river. Thus , we could follow the seasonal variation of growth, total density and total biomass of brown trout. Moreover, 652 brown trouts of age ranging between 1+ and 6+ years, for size s ranging between 100 and 290 mm, were marked individually by visible impla nts in March 1998. The recapture rates observed varied from 19.5 to 74%, fo llowing the season considered. Growth rates showed a strong seasonal variability, validated by individual tagging experiments. Because of reproduction and rigorous temperatures expe rienced by trout during autumn and winter, growth rates observed were almos t null and individuals >1+ lost weight during this period. In spite of theo retically non-optimal thermal conditions, growth occurred almost exclusivel y in spring. Poor growth rates observed during summer are not due to therma l factor which was theoretically optimal for growth in this season and coul d be attributed to the sexual maturation, to the saturation of the carrying capacity of physical habitat and to the strong intraspecific competition c aused by i) likely arrivals of 1+ and 2+ individuals emigrating from tribut aries and ii) reduction in "quantity and quality" of physical habitat at lo w-flow limiting accessibility and availability of energetically profitable prey. The role of growth in the dynamics of the brown trout population of the Nes te d'Oueil River is discussed.