Testing the assumptions of the ideal despotic distribution with an unpredictable food supply: experiments in juvenile salmon

Abstract

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.

Utilising spatial distribution in two-tank systems to investigate the level of aversiveness to crowding in farmed rainbow trout Oncorhynchus mykiss

“…This 302 distribution pattern resembled an Ideal Despotic Distribution (IDD), first described in birds, where 303 movement between patches was controlled by intraspecific competition (Fretwell, 1972). The IDD has 304 previously been described in laboratory situations in Salmonids, where dominant individuals excluded other 305 individuals from a favourable patch (Hakoyama and Iguchi, 2001;Maclean et al, 2005). In our study, 306 although behavioural quantifications of the individuals in the "dominant" tank were not carried out, 307 observation of the fish confirmed that they displayed agonistic behaviours towards other individuals in the 308 tank and fish attempting to enter the tank.…”

Mobility and home-range use of Atlantic salmon parr over short time scales

“…Armstrong et al (1999) suggested that dominant parr settle in restricted territories, which they defend from intruders whereas subdominant parr adjust their positions and movements accordingly. However, a recent experimental study suggested that parr can also be dominant and move frequently between feeding locations; but parr tend to leave rich patches less frequently than they leave poor patches (Maclean et al 2005). In the present study, social relations of parr could not be analyzed but it may be hypothesized that parr could have held to different social status.…”

Habitat Choice by Atlantic Salmon Parr in Relation to Turbulence at a Reach Scale

“…Furthermore, the variability observed in habitat use among individuals with respect to turbulence may be due to other habitat variables, which may override habitat selection based on turbulence (Heggenes 2002). Alternatively, there may be a strong preference for specific turbulence characteristics, but an individual may be forced by competition to use alternative habitat characterized by flow conditions other than those preferred (McLaughlin and Grant 1994;MacLean et al 2005;Kemp et al 2006). Whereas Atlantic salmon parr seem to react and respond to turbulence on a smaller microhabitat scale, the application of dynamic hydraulic variables on reach scale seem to not provide a useful tool for fisheries and habitat managers.…”

“…The proportion of time used for feeding decreases with the mean and SD of velocity (Enders et al 2005). However, on a local scale, parr may demonstrate only minor adjustments to habitat conditions as individual variation in habitat use HABITAT CHOICE BY ATLANTIC SALMON PARR is primarily affected by dominance status (MacLean et al 2005;Kemp et al 2006).…”