Spatial variation in brown trout production:: The role of environmental factors

Spatial variation in Spanish populations of brown trout Salmo trutta was studied in 10 streams of contrasting environmental and biological characteristics based on data compiled over 7 years (1992-1998). Three of the streams had soft water (mean alkalinity as CaCO3=19.3 mg/L) supplied by granite catchments at elevations around 1,250 in above sea level and had a low abundance of macroinvertebrates (mean density=598 individuals/m(2); mean biomass=0.63 g/m(2)). The remaining streams had hard water (mean alkalinity 253.6 mg/L) flowing over limestone at 850-1,400-m elevations and possessed a greater benthic faunal abundance (mean density=2,433 individuals/m(2); mean biomass=2.76 g/m(2)). Mean brown trout population characters varied significantly throughout the study area (density=1,567-5,594 fish/ha; biomass=56.6240.2 kg/ha; annual production=47.0-182.0 kg/ha, and the ratio of annual production to mean biomass=1.01-1.56). A stepwise multiple regression analysis revealed a significant relationship between brown trout production and chemical features indicative of high water productivity, which accounted for 61% of the variance explained by the model. A broader spatial analysis, based on a review of the available European work, corroborated that annual brown trout production in streams flowing over limestone bedrock was greater (mean=121.6 kg/ha; range=30.0-253.3 kg/ha) than that in streams flowing over siliceous bedrock (mean=76.6 kg/ha; range=3.5-234.0 kg/ha). Data from brown trout populations throughout Europe showed a significant positive correlation between production and alkalinity according to the model log(10)(production)=1.41+0.31-log(10)(alkalinity), which explained 53% of the variance. Our findings support previous evidence on the influence of water fertility on salmonid production and expand knowledge of the factors that influence brown trout production within the native range of the species.