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

Abstract

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 streamresident 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.

Migration costs drive convergence of threshold traits for migratory tactics

“…By contrast, the fitness of resident individuals is expected to be unrelated to distance from the sea, because they remain their entire life in their natal stream. Bohlin et al [32] showed that the fitness of resident brown trout, Salmo trutta, did not change with altitude, whereas the fitness of anadromous brown trout tended to decline with altitude, an indicator of migration cost in that study. Therefore, the threshold size at maturity in male salmonid parr is expected to decrease with distance from the sea, an indicator of migration cost, to promote residency.…”

Length Variation in Age‐0 Westslope Cutthroat Trout at Multiple Spatial Scales

“…Age-0 fish at lowerelevation sites within our study streams emerged 9-15 d earlier than those at upper-elevation sites within the same streams (K.E.M., unpublished data). Third, the spawning areas used by larger, migratory adults within individual streams may be lower than the areas used by smaller, resident adults (Averett and MacPhee 1971;Bohlin et al 2001). Because larger females tend to have larger eggs, they are also likely to produce larger offspring (Einum and Fleming 1999).…”

“…More importantly, it may promote diversification of life history strategies within and among populations (Jonsson et al 1988;Conover and Schultz 1995;Bohlin et al 2001) and contribute to population stability and long-term survival in temporally variable environments (den Boer 1968; Chandler and Bjornn 1988;Good et al 2001;Kendall and Fox 2002).…”

Environmental variability and population dynamics of juvenile brown trout (Salmo trutta) in an upstream and downstream reach of a small New Zealand river

“…However, mid-summer densities in the downstream reach were relatively low (0. (Kelly & Dick 2005), and 1.18 fish m~2 in a Swedish stream (Bohlin et al 2001). Space limitation is thought to occur at much higher densities (Grant & Kramer 1990).…”