| Summary: | Movement, at various spatial and temporal scales, plays a major role in shaping the ecology of animals at the individual and the population level. In this study, I applied recently developed tagging methods (visible implant fluorescent elastomers) to examine the space use of Atlantic salmon ( Salmo salar ) during their first growing season. First, I monitored 320 YOY salmon to test alternative views on the patterns (restricted vs. not-restricted), causes (competition vs. habitat use) and consequences (mobile fish of lower vs. equal fitness) of movement in stream fish. Most fish (mean = 63.8%) stayed in the study sites (10-120m) throughout their respective study season (28-74 days), and 61.8% of the re-sighted fish moved less than 1 m up- or downstream. Fish originally found in slow water moved farther than fish from fast water, and fish found at high population density were more likely to disappear than fish from low density. Mobile fish grew as fast, or faster, than more sedentary fish, suggesting that movement can be advantageous. Second, I mapped the daily territories of tagged YOY salmon to test if the typical single-central-place view of territoriality among YOY salmonids holds when fish are followed for longer periods, at low population densities. In contrast to earlier studies, YOY salmon visited several foraging stations (median = 12.5; range = 3-26) within their territories, and showed limited fidelity to any particular station. When mapped around several stations, rather than assuming one station as is customary, territories of YOY salmon were larger than previous reported (mean = 0.932 m 2 ), were less circular in shape, and were elongated along the stream length rather than the stream width. Although the study fish used large multiple-central-place territories, aggressive acts directed toward other YOY Atlantic salmon were usually found on the outskirts of these areas, suggesting these were fairly efficiently defended against conspecifics. A literature review suggested that YOY salmonids defend small territories from one central-place foraging station at high population density, but use several stations and large territories at low density. Third, as current velocity, prey abundance and prey mobility increase, stream-fish are predicted to become less mobile, use smaller home ranges, and become more aggressive as they switch from "cruising" to "sit-and-wait" foraging. I tested if these predictions held for YOY Atlantic salmon that specialize as sit-and-wait foragers, but vary in the number of foraging stations visited and the distance traveled within a territory (15.7-95.0 m/40min). As predicted, territory size decreased with increasing current velocity and prey availability. YOY salmon, however, were most mobile and attacked intruders more often at intermediate, optimal, current velocities. These findings, and the fact that fish in slow water do not feed more on benthic prey than fish in fast water, suggest that mobility in YOY Atlantic salmon reflects the patrolling of territories, rather than just the direct exploitation of other food resources than drifting prey
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