J. D. Armstrong

Habitat requirements of Atlantic salmon and brown trout in rivers and streams

The distributions and abundances of trout and salmon are strongly influenced by their habitat. The habitat includes both abiotic and biotic factors, which interact in complex webs. Habitat probably has strongest effects during population bottlenecks, when the standing stock approaches the carrying capacity of the environment. Various approaches to modelling interactions between habitat and population density and mean weight have been explored, but further work is needed in this area of investigation. The importance of depth, current, substrate, cover, and to a lesser extent, temperature and oxygen availability to the various stages of the life cycles of salmon and trout are briefly reviewed. By drawing on published data, it is possible to define broad ranges of acceptable conditions for the life stages of each species. However, it is not possible to partition this variation into between-population differences, within-population preferences, within-population tolerances, and effects of interactions between habitat variables. To pursue this important issue further, a structured approach using experimentation both in the field and in suitable laboratory systems is recommended. There is abundant evidence that habitat requirements of salmon and trout overlap. Trout tend to out-compete salmon except often in areas of particularly fast flows and, perhaps, remote from the river bank. The habitat requirements of year classes of salmon and trout overlap and therefore, there is scope for interactions between them depending on the spatial arrangement of habitats and the occurrence of bottlenecks. It is particularly important to understand where the bottlenecks to production lie and to focus on these in the first instance. Otherwise, there is a risk of manipulating habitat that is already in excess, or increasing numbers of a population that will subsequently be constrained, e.g., by over-wintering habitat. For this reason, it is prudent to accept that although manipulations of habitat may appear to be beneficial when considered locally, they should be measured and assessed where possible in terms of the production of returning adults and/or high quality smolts. Because of the complexity of interactions between salmon, trout, and the animals that eat them, it is at present difficult, or impossible, to derive good predictive models of the effects of manipulating habitats under many circumstances.

What causes intraspecific variation in resting metabolic rate and what are its ecological consequences

Individual differences in the energy cost of self-maintenance (resting metabolic rate, RMR) are substantial and the focus of an emerging research area. These differences may influence fitness because self-maintenance is considered as a life-history component along with growth and reproduction. In this review, we ask why do some individuals have two to three times the ‘maintenance costs’ of conspecifics, and what are the fitness consequences? Using evidence from a range of species, we demonstrate that diverse factors, such as genotypes, maternal effects, early developmental conditions and personality differences contribute to variation in individual RMR. We review evidence that RMR is linked with fitness, showing correlations with traits such as growth and survival. However, these relationships are modulated by environmental conditions (e.g. food supply), suggesting that the fitness consequences of a given RMR may be context-dependent. Then, using empirical examples, we discuss broad-scale reasons why variation in RMR might persist in natural populations, including the role of both spatial and temporal variation in selection pressures and trans-generational effects. To conclude, we discuss experimental approaches that will enable more rigorous examination of the causes and consequences of individual variation in this key physiological trait.

Spatial strategies of wild Atlantic salmon parr: Exploration and settlement in unfamiliar areas

1. Relationships between distributions of animals and the resources they use can be expected to depend critically on the mobility of individual animals within populations. However, there is little information on the movements of individuals within populations of animals in most natural aquatic systems, so it is difficult to model accurately the processes that underlie their distributions. 2. Aspects of the processes involved in the colonization of vacant areas by streamresident Atlantic salmon Salmo salar (69-114 mm length) were measured under nearnatural conditions. In five separate trials over summer months, groups of salmon were introduced into enclosed 30-m long sections of stream, each comprising three distinct 10-m long regions of habitat. The subsequent movements of each individual fish within the enclosures were monitored remotely and continuously using a passive integrated transponder (PIT) tracking system. 3. Considerable variation was observed between the activity of different individual salmon. Some of the fish (range between trials, 3-33%) settled into localized home ranges without moving between regions, 10-38 % of the fish moved within two regions, and 37-87% of the salmon moved through all three regions of their enclosure. A fraction (0-20%) of some of the populations was particularly mobile and never settled but continued to move throughout all three regions of the enclosure. Within the scale of this current study, it would appear that, for territorial animals such as salmon parr, certain members of the population will settle in new territories after very little exploration of their new environment. 4. The propensity to explore was independent of habitat type, but was directly proportional to the size of the fish. 5. The time from release until 50% of fish in populations settled (excluding the mobile fraction) ranged from 0.3 to to 2.4 days. Activity levels were particularly high and initial movements by fish were directed upstream in trial 1, early in the summer, perhaps reflecting upstream migration by salmon parr within the population of the burn at this time.

Juvenile salmon with high standard metabolic rates have higher energy costs but can process meals faster

Basal or standard metabolic rate (SMR) has been found to exhibit substantial intraspecific variation in a range of taxa, but the consequences of this variation are little understood. Here we explore how SMR is related to the energy cost of processing food, known as apparent specific dynamic action or the heat increment of feeding. Using juvenile Atlantic salmon Salmo salar, we show that fishes with a higher SMR had a higher peak and a greater total energy expenditure when digesting a given size of meal. However, the duration over which their metabolism was elevated after consuming the meal was shorter. The greater energy costs they incur for processing food may be related to their assimilation efficiency. These relationships are likely to have implications for feeding strategies and growth rates, since individuals with a higher SMR have higher routine costs of living but recover more quickly following feeding and so may have a greater potential for processing food.

Self-thinning in juvenile sea trout and other salmonid fishes revisited

1. Self-thinning, the reduction in density (N) as a consequence of the increase in the mean weight (W) of individuals, occurs widely in populations of plants when their growth is constrained at high densities, but has only recently been reported to occur within populations of wild mobile animals. It has been suggested that a self-thinning relationship in which the gradient (Δlog(W)/Δlog(N) approximates to -1.33 describes concomitant changes in Wand N within cohorts of a population of anadromous brown trout (sea trout) Salmo trutta throughout their lives as juveniles in fresh water. This apparent self-thinning occurs with no evidence of density-dependent growth or mortality (as measured by key-factor analysis) after the critical period. 2. Here, the relationship between weights and densities of trout has been examined further. A linear model incorporating log(N), year class and time of year explained 99.5% of the variation in log(W), giving a significantly better fit than incorporating only log(N) and year class. Variation in log(N) explained 85% of the variation in log(W); the addition of year class increased this value by 6% and the addition of monitoring period increased it by a further 8.5%. 3. The gradients for the relationships between log( W) and log(N) within years varied significantly with time of the year. Pooled thinning trajectories for the first winter of the life of trout differed significantly from a gradient of -1.33, while those for the second summer were highly variable and sometimes positive (suggesting that immigration could exceed mortality). Thinning trajectories over the first summer of life were related inversely to the densities of trout at the start of the monitoring period in that summer (N s ), but this relation appeared to approach an asymptote at high values of N s . The mean of the thinning trajectories for cohorts with high N s (exceeding 300 fish 60 m -2 ) was not significantly different from -1.33. However, these slopes were shown to result primarily from the presence of high numbers of failed non-territorial fish early in the summer rather than true thinning throughout the summer. 4. The apparent absence of a simple consistent thinning relationship after a critical period in the first couple of months of the lives of cohorts can be explained by three factors. First, the total availability of limiting resources, such as space and food, is not constant but changes with the mean size of the trout. Secondly, there is a winter cessation of growth. Thirdly, trout switch to use different habitats during their development. The implications for interpreting thinning relationships for salmonid fishes in general are discussed.

Functional and aggregative responses of harbour seals to changes in salmonid abundance

There is intense debate over the potential impact of seal predation on declining salmon stocks in both the Pacific and Atlantic oceans. However, efforts to model such interactions have been constrained by a lack of data on the functional and numerical responses of these predators. Based upon theory, and data from small-scale terrestrial and freshwater systems, a type 3 functional response is expected to best describe predation by generalist pinnipeds. Similarly, theory also predicts that seal numbers should increase with salmon density in rivers following an aggregative response of predator to prey. We tested these predictions by studying the diet and local density of harbour seals in relation to seasonal variations in the abundance of salmonid in a Scottish river system. As predicted, the abundance of seals in the river was directly related to the abundance of returning salmon, and dietary data supported the type 3 functional response to changes in salmonid abundance. These studies provide empirical support for the use of type 3 response in modelling studies.

The benefits of genetic diversity outweigh those of kin association in a territorial animal

The theories of kin selection and heterogeneous advantage have been central to studies of altruistic behaviour and the evolution of sex over the last 35 years. Yet they predict diametrically opposite effects of genetic diversity on population density. Close relatives gain inclusive fitness advantages by preferentially associating with and behaving altruistically towards one another. However, heterogeneous advantage, which predicts competition to be highest when genetic diversity is low, suggests that benefits will be greater for individuals in groups of non–kin. Here we test how these two processes balance and affect the productivity of populations of animals in natural habitats. We report from a study of juvenile Atlantic salmon in the wild that heterogeneous advantage outweighs the benefits of kin–biased behaviour, resulting in a 1.8–fold higher population biomass and significantly better condition of individual fish.

DNA-based identification of salmonid prey species in seal faeces

Assessment of pinniped predation most often relies on analysis of the hard, undigested prey remains evident in faecal (scat) samples. For many prey species this method can yield valuable information on predator–prey interactions. For some genera, however, species diagnostic characteristics are lost during the process of prey digestion, thereby preventing morphological identification of fish prey species. Here, the feasibility of using faecal DNA to detect the presence of salmonids in pinniped scat samples and to distinguish reliably between sea trout Salmo trutta and Atlantic salmon S. salar was assessed. Novel salmonid mitochondrial DNA (mtDNA) primers were designed to amplify 162 bp of the 16S rDNA and a 327 bp section of the cytochrome b gene. Species-specific banding patterns were obtained by digestion of the cytochrome b PCR product with the restriction endonuclease AluI, and confirmed by the species-specific amplification of the 16S rDNA fragment from Atlantic salmon. Scats collected from captive grey seals Halichoerus grypus fed on known monospecific diets used to validate the PCR-RFLP assay indicated a probability of at least 95.8% (23 of 24 faecal extracts) of detecting salmonids using DNA extracted from the scat matrix. Implemented alongside conventional prey remains analyses, this technique presents a promising new method for examining prey composition and assessing pinniped predation on salmonids.

Can juvenile Atlantic salmon use multiple cue systems in spatial learning

An ability to form a map or spatial representation of a home range should facilitate efficient foraging and promote the use of effective escape routes to shelter. Although a few experiments have shown some species of fish are capable of simple spatial behaviour, little is known about the behavioural mechanisms they use to orient as they forage. Whether juvenile Atlantic salmon,Salmo salar, have the capacity to use conspicuous visual landmarks to help them track a moving resource was investigated. The experiment was then modified to determine whether the salmon could continue to track the resource in the absence of conspicuous visual cues. The salmon followed a moveable food source in the presence and, after retraining, in the absence of distinct coloured landmarks. It is suggested that juvenile salmon may be capable of using multiple forms of cue to help them solve spatial tasks such as tracking a moving resource.

Sympatric association influences within-species dominance relations among juvenile Atlantic salmon and brown trout

Abstract Size and aggressiveness are determinants of social dominance in many vertebrate species, including juvenile stream-dwelling salmonids. We used seminatural stream channels, landscaped to provide a range of depths, to test whether the factors influencing the formation of social hierarchies are similar in single- and mixed-species groups of Atlantic salmon, Salmo salar , and brown trout, Salmo trutta . We also tested whether these species have similar feeding rhythms during the day and whether dominant and subordinate individuals feed at different times, under both allopatric and sympatric conditions. Size appeared to be a good predictor of feeding success of both species in allopatry, but not when they were in direct competition. In contrast, rate of aggression was positively correlated with feeding success in both allopatry and sympatry. However, the timing and rate of feeding of dominants and subordinates differed significantly only in the allopatric trials, with subordinate individuals in the sympatric trials being able to adopt a nonaggressive alternative strategy and continue feeding. These results highlight the behavioural plasticity of juvenile salmonids, which can adapt their feeding behaviour to the social environment, and suggest that the presence of another species with similar, but not identical, ecological requirements may increase the opportunity for the expression of alternative behavioural strategies. We conclude that the advantages of social dominance may to a large extent be specific to the species assemblage. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .