Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Abstract

Direct underwater observation of micro-habitat use by 1838 young Atlantic salmon Salmo salar [mean L T 7·9 3·1(..) cm, range 3-19] and 1227 brown trout Salmo trutta (L T 10·9 5·0 cm, range 3-56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro-habitat variables snout water velocity and height above bottom, but with differences between size-classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size-classes. On a meso-habitat scale, brown trout were more frequently observed in slow pool-glide habitats, while young Atlantic salmon favoured the faster high-gradient meso-habitats. Small juveniles <7 cm of both species were observed most frequently in riffle-chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species-specific habitat overlap between streams may be complete. However, between-species habitat partitioning remains similar.2002 The Fisheries Society of the British Isles

Atlantic salmonSalmo salarL., brown troutSalmo truttaL. and Arctic charrSalvelinus alpinus(L.): a review of aspects of their life histories

“…During their first year in the river, the young dwell largely in shallow areas (<30 cm deep), often located along the river bank with moderately fast flowing water (0.2-0.5 m Á s À1 ) (Roussel & Bardonnet 1999). As they grow older and larger, they prefer deeper, more slowly flowing parts of the stream (Heggenes 2002), but their habitat use in nature is flexibly dependent on habitat availability and time of the day (Elso & Greenberg 2001;Greenberg et al 2001;Heggenes et al 2002). They are more inclined to exploit pools in the river during night than during day time.…”

Selective segregation in intraspecific competition between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

“…In several previous studies of Atlantic salmon living in sympatry with other salmonids, interactive segregation has been reported (Kalleberg 1958;Kennedy & Strange 1986;Heggenes et al 1999Heggenes et al , 2002Stradmeyer et al 2008). Selective segregation has also been reported: in a study of Atlantic salmon and rainbow trout (Oncorhynchus mykiss) in Vermont, small differences in habitat use in early stages were indicating selective segregation for the 0 + , while interactive segregation appeared to be the mechanism in competition among yearlings (Hearn & Kynard 1986).…”

Intra- and Interspecific Competition in Hatchery Landlocked Salmon and Brown Trout in Semi-Natural Streams

“…The water velocity range was well within the preferred range for juveniles of both species, while the depth range is in the lower end of preferred depths for both species (e.g. Heggenes et al 2002). However, there is no indication that the available depth range would give competitive advantage to either of the species.…”