Morphometric examination was carried out on the gills and skin of wild and caged hatchery brown trout Salmo trutta fario in an acidic (pH 4.9 to 5.4; Al 203 to 250 µg l-1) and in a non-acidic (pH 6.7 to 7.0; Al 27 to 67 µg l-1) stream in the Vosges Mountains (NE France) to assess the sublethal effects of acidic water on the mucous cell response. The caged fish were randomly collected after 2, 4, 7 and 11 d and the wild fish were obtained by electrofishing. After 2 d, a reduction of both mucous cell (MC) number and size was observed in the gills of fish held in the acidic stream, suggesting a massive mucus discharge. Hyperplasia and hypertrophy of cells immediately followed this mucus secretion. In the same fish population, skin examination showed a slight and delayed decrease of MC number but a significant increase of cell size. The number of mucous cells of gills and skin was similar in both wild trout populations, whereas a significant MC hypertrophy was observed in the wild fish of the acidic stream. The present field experiment indicates that caged fish could be useful as early indicators of acidification. In addition, the examination of wild populations suggested the occurrence of adaptive mechanisms, information that might be of importance in the context of river recovery programs.
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