Distribution, growth, movements and homing behaviour of juvenile Atlantic salmon and brown trout in the Girnock Burn, Aberdeenshire
The biology of juvenile Atlantic salmon and brown trout was studied within a small section of the Girnock Burn, a tributary of the Aberseenshire Dee. Salmon parr and brown trout were captured, measured, weighed, individually marked, and repeatedly recaptured over the period 1985-1988. Physical resources were mapped in detail within the study section, and were found to be patchily distributed. Patch size was small, typically a few square metres. Significant differences in micro-habitat existed between young and old fish, between sympatric Atlantic salmon and brown trout, and between seasons. All fish avoided shallow waters and fine substrates. Sites that combined low bottom velocities with access to fast surface currents were significantly selected by both salmon parr and brown trout. These sites supported the highest local densities of fish, and held the largest individuals. As drift-feeders, salmon appear to choose sites during the growing season that maximise food intake and minimise energy expenditure. Growth of juvenile salmon was related to water temperature, being rapid from March to June, slowing down from July to September, and in some fish ceasing altogether from September to March. In the fall, salmon 0&'43 moved to parr habitat, resources became more limited, and some parr left the stream. At low temperature, juvenile salmon hid within the substrate in riffles, while brown trout overwintered around large rocks and in a spring-fed tributary. Habitat overlap between salmon 0&43 and older parr increased from summer to fall while habitat overlap between salmon and trout decreased. Juvenile salmon were spatially aggregated. Highest densities were found at favourable sites near surface boulders. The area used by fish and the proportion of fish found at unfavourable habitats increased with fish density. Habitat selection and the spatial distribution of the population appeared to be density-dependent. Site-fidelity was shown both by salmon parr and brown trout, though in general fidelity was stronger in salmon parr. The majority of recaptures of salmon parr and brown trout were close to previous capture locations in the stream. Long-term site-fidelity was also shown by salmon parr from one year to the next. Two groups of fish, differing in their degree of mobility, were apparent both for salmon and trout: a large, stationary group, and a smaller, more mobile one. The groups were not distinguished by size, season, or stage of sexual maturity. Movements out of the burn were confined to the fall and spring. Immature parr, mostly females, migrated downstream in the fall, but were not yet adapted to full-strength seawater. Mature males performed complex upstream and downstream movements in the fall, and some of these fish homed back to their summer sites after spawning. For many males, maturation in the fall was followed by smolting in the spring, but repeated maturation was also common. Homing success in displaced fish was related to fish size and to the direction of displacement. Homing was better among larger than smaller fish, and it was better among fish displaced downstream than those displaced upstream. Fish rendered anosmic showed a greater tendency to stay where released, and homed less successfully than intact fish. The home areas of salmon parr were smaller than those of brown trout. The home areas of individuals overlapped and were unrelated to fish size; they appeared to be inversely related to substrate size and to current velocity. By spending most of their lives in a small area of the stream, juvenile salmon and brown trout may only interact with a few conspecifics. The population appears to consist of a number of small social units, rather than a single freely mixing group. The fate of individuals may be determined by very localised environmental and social conditions.