Population density of migratory and resident brown trout (Salmo trutta) in relation to altitude:: evidence for a migration cost

1. For anadromous salmonids, the positive relations found in previous studies between adult size/age and stream characteristics suggest that the migration cost increases with stream length, water discharge and the altitude of the spawning site. In this study we hypothesized that the altitude of the spawning site is positively related to the migration effort. 2. Life-history theory predicts (i) that the equlibrium egg density, which is a fitness measure, thereby will decline more rapidly with altitude in migratory than in stream-resident populations, and therefore (ii) that residency will be selected for at large enough altitudes. 3, As the density of juveniles is a function of egg density, we hypothesized that (a) altitude has stronger effect on juvenile density in migratory than in resident populations, and (b) juvenile density is larger in migratory than in resident populations. 4, We tested (a) and (b) using multivariate methods for electrofishing data from 164 sea-migratory and 167 stream-resident populations of brown trout Salmo trutta L. in Sweden. Both predictions were supported; the juvenile density was larger and declined more rapidly with altitude in migratory than in resident populations. 5. The results are further evidence that migration costs reduce fitness in anadromous salmonids.