No abstract

Juvenile Atlantic salmon Salmo salar subjected to three weeks of cooler temperatures were 8·5% smaller than controls at the end of the temperature manipulation, but had caught up in size 20 weeks later. The behavioural means is examined by which this catch-up or compensatory growth is achieved. While on average compensating fish did not spend more time feeding, dominant fish within each group gained more exclusive access to the feeding area during periods of catch-up growth. Therefore the extent to which compensatory growth could be achieved was dependent on both the social status of the individual and the dominants' ability to monopolize the food patch. 2001 The Fisheries Society of the British Isles

Summary1. Models linking the behaviours of individual animals, their positions within socially complex groups and spatio-temporal variation in resource distribution offer a promising base for predicting population responses to changing environments. The ideal free and despotic distributions and their derivatives are particularly influential in this regard. 2. Due to the difficulties of conducting work in the wild, for some groups of animals such models are often based on observations of animals in small-scale systems under conditions that are well controlled, but unnaturally simple. 3. Using an experimental system based on field observations of home range size and variation in food availability, the present study tested whether models derived using small-scale laboratory observations are valid for juvenile Atlantic salmon in more natural conditions. 4. Contrary to predictions, we found no differences in behaviour between the control fish (which experienced consistently rich feeding patches) and the experimental fish (which experienced unpredictable 10-fold changes in patch quality). 5. Also contrary to predictions, in the variable condition, salmon used high quality patches (which were an order of magnitude better than low quality patches) only marginally (5%) more than would be expected if they were to forage at random. There was significant variation in foraging strategies between individual fish, with 28% of the population making non-random use of foraging patches. 6. The only apparent systematic relationship between social rank and use of foraging patches was that fish that were both dominant and made many moves between feeding locations tended to leave rich patches less frequently than they left poor patches. 7. Despite the low correlation between patch quality and movement, there was substantial movement of fish among patches. Forty-four per cent of moves followed aggressive interactions and most others were spontaneous, with no obvious motivating factor apparent. 8. The study exposes a discrepancy between expectations derived from the basic concepts of patch choice theory and the behaviour of Atlantic salmon in the conditions pertaining in the present study. 9. It is suggested that this discrepancy may arise both from the fact that applicability of patch choice models may be very sensitive to the stability of differences in patch quality and from uncertainties about the costs of habitat sampling.

Juvenile plaice Pleuronectes platessa (n ¼ 1281) were tagged and released at two locations 300 m apart on a 1 km long sandy beach. Most (>90%) of the fish were recaptured within 100 m of the release site (shown by the colour of the tag), with very few caught >200 m distance after 6 weeks. The changing spatial distribution of marked fish was adequately reproduced by a simple dispersal model with a single parameter: a 78% probability of remaining in a 100 m wide zone from one day to the next, with a 22% chance that fish move into an adjacent zone. In a subsequent similar study at the same beach, fish were either released at the point of capture (n ¼ 881) or transplanted to the alternate site (n ¼ 910) 100 m distant. After 6 weeks, transplanted fish moved along the shore towards their sites of original capture. Fish replaced at the point of capture showed no such movement along the shore. Further modification of the dispersal model to allow for a distinction between dispersal from home sites and from sites away from the original point of collection, was sufficient to reproduce the behaviour of the populations of both transplanted and control treatment groups. The likelihood of dispersal from home sites was much less than that seen at sites away from home. Juvenile plaice thus have a degree of long-shore site fidelity not expected of a fish with strong depth-related migration behaviour in a relatively homogenous habitat. # 2004 The Fisheries Society of the British Isles

The influences of a light: dark cycle and a persistent endogenous rhythm of activity on foraging (on the bivalve Donax vittatus) and avoiding a predator (juvenile cod, Gadus morhua) were investigated in freshly‐caught juvenile plaice, Pleuronectes plalessa. Time lapse video recordings were made of fish in the presence and absence of prey and predators in laboratory tanks over 24‐hour periods between the times of successive daytime low waters. Endogenous rhythms of activity were seen in all experimental treatments. Swimming both close to the bottom and in the water column showed a strong circatidal rhythm, with most activity 2 to 3 h after the predicted time of high water. Swimming in the water column was more frequent at night than by day. In the presence of a population of Donax, whose siphon tips could be eaten as food, swimming close to the bottom became more frequent. This increase in benthic swimming was independent of the endogenous cycle of activity and was correlated with the frequency of attacks on siphons. The presence of the cod predator delayed the onset of foraging activity, producing a foraging/predator avoidance trade‐off. The independence of foraging from light and endogenous rhythms suggests that this trade‐off may be similarly independent. The cod also greatly reduced swimming in the water column in darkness, behaviour apparently unrelated to foraging.