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

Functional response and size‐dependent foraging on aquatic and terrestrial prey by brown trout (Salmo trutta L.)

“…Intraspecific size-class competition and size-dependent predation risk may also affect the diets of trout. Whatever the reason, the fact remains that large fish forage more in pools (Bremset & Berg 1997;Heggenes et al 1999Heggenes et al , 2002, where surface feeding is presumably favoured, whereas small fish forage in riffles (Heggenes 1996;Mäki-Petäys et al 1997), where benthic foraging should be favoured (Nakano 1994;Dineen et al 2007a). Even predation risk has been invoked to explain this differential habitat use by large and small fish (Power 1987;Schlosser 1987;Lima & Dill 1990;Greenberg 1994;Greenberg et al 1997).…”

Assessing the effect of visual isolation on the population density of Atlantic salmon (Salmo salar) using GIS

“…The parameters used in the viewshed analysis were an observer (i.e. fish) height of 0.05 m (see Heggenes et al, 2002), a field of view of 2008 and a visual range of 1.5 m. A field of view of 2008 was chosen to reflect where most aggressive attempts are directed (Keeley and Grant, 1995). The visual range of 1.5 m represents the maximum chase radius of age 2þ Atlantic salmon in Catamaran Brook (Keeley and Grant, 1995).…”

“…Current velocity data (AE0.01 m s À1 ) were recorded for each salmon, as well as averaged for each quadrat, using a one-dimensional Marsh-McBirney flow meter , oriented into the current. Current velocity for each individual Atlantic salmon (hereafter referred to as fish velocity) was recorded following each survey at each flagged position, at 0.05 m above the substrate to estimate snout velocity (Heggenes et al, 2002) and at 40% of the water column depth to estimate mean velocity (Hynes, 1970). Both snout and mean velocity for each fish were averaged per quadrat for each of 2002 and 2003 (n ¼ 8).…”

Forest-Stream Linkages: Effects of Terrestrial Invertebrate Input and Light on Diet and Growth of Brown Trout (Salmo trutta) in a Boreal Forest Stream

“…This is because numerous field studies have shown that there is a positive correlation between body size and the relative contribution of surface-drifting prey to the diet of salmonids, [40]–[42], [61]. Moreover, large, dominant salmonids inhabit pools [62]–[64] and forage in the water column, where they encounter and use generally large and conspicuous surface-drifting terrestrial invertebrates [19], [22], [43]. In contrast small fish spend most of their time in riffles [61], [65]–[67], where they forage mainly on benthic prey [42], [68].…”