Microgeographical variation in brown trout reproductive traits: Possible effects of biotic interactions

This study documents substantial variation in reproductive traits among populations of stream-dwelling brown trout (Salmo trutta L.) at a very small geographic scale. Within two streams, we found a parallel pattern of variation, where females living above major waterfalls produced fewer and larger eggs than conspecifics from below the waterfalls. Four additional streams were represented with either a below-waterfall site (n=2) or an above-waterfall site (n=2). When these streams were included in the analyses, there was no consistent difference in reproductive traits between females from above- and below-waterfall sites. There was no significant difference in total reproductive investment among sites within streams, but considerable variation among streams. Female first-year growth rates was estimated from scales, and differed significantly among populations. Within streams, females from below waterfalls experienced higher first-year growth rates as compared to females from above the waterfalls. Within seven out of eight populations, egg size increased significantly with increasing female body length. Within three populations, we found evidence for a trade-off between offspring size and offspring number, as a negative association between fecundity and egg size independently of adult body size. Within three populations egg size decreased significantly with increasing maternal first-year growth, independently of adult body size. We suggest that the within-stream differences in offspring size/number strategies are influenced by population density and growth effects. Earlier, we have shown that population densities are consistently lower below the waterfalls in these streams. The Alpine bullhead (Cottus poecilopus) is found only below the waterfalls and could influence brown trout demography

Interspecific competition between stream-dwelling brown trout and Alpine bullhead

Density and composition of benthic invertebrates and the diet of brown trout Salmo trutta and Alpine bullhead Cottus poecilopus were studied at two sites in one Norwegian stream. The sites were separated by an impassable waterfall, and brown trout density was five to 10 times higher at the upper, allopatric site than downstream where it lived in sympatry with the Alpine bullhead. Benthic invertebrate communities did not differ between sites; however, the size distribution of chironomids and trichopterans were skewed towards lighter individuals at the sympatric site. Diet composition suggested that sympatric brown trout foraged more on invertebrate drift and from the surface than allopatric brown trout. Alpine bullhead diet did not differ significantly from brown trout diet, except that the Alpine bullhead fed on heavier individual prey within a few taxa and did not consume chironomid pupae or surface insects. The collected data support the hypothesis that brown trout living in sympatry with Alpine bullhead feed at locations with higher predation risk, which is a probable explanation for their lower population density

First-year survival of brown trout in three Norwegian streams

Monthly survival rates during the first year of life were estimated for the 1999 cohort of stream-resident brown trout Salmo trutta in three Norwegian streams, using capture-mark-recapture methods and the Cormack-Jolly-Seber model. It was hypothesized that reduced survival would occur during the winter. For one of the study populations, the data did support seasonal variation in survival, with monthly survival rates being lower during winter than during summer (0.65 v. 0.99). For the remaining two populations, there was no evidence for seasonal variations in monthly survival rates, but monthly survival rates were significantly different (0.87 v. 0.99). No evidence was found for size-dependent winter survival. Some marked individuals emigrated from the study sites, suggesting that survival rates were underestimated and that different survival rates among populations were partly due to different emigration rates. Net immigration of brown trout was evident at all three study sites