Johan Höjesjö

Coping with divided attention: the advantage of familiarity

The ability of an animal to perform a task successfully is limited by the amount of attention being simultaneously focused on other activities. One way in which individuals might reduce the cost of divided attention is by preferentially focusing on the most beneficial tasks. In territorial animals where aggression is lower among familiar individuals, the decision to associate preferentially with familiar conspecifics may therefore confer advantages by allowing attention to be switched from aggression to predator vigilance and feeding. Wild juvenile brown trout were used to test the prediction that familiar fishes respond more quickly than unfamiliar fishes to a simulated predator attack. Our results confirm this prediction by demonstrating that familiar trout respond 14% faster than unfamiliar individuals to a predator attack. The results also show that familiar fishes consume a greater number of food items, foraging at more than twice the rate of unfamiliar conspecifics. To the best of our knowledge, these results provide the first evidence that familiarity–biased association confers advantages through the immediate fitness benefits afforded by faster predator–evasion responses and the long–term benefits provided by increased feeding opportunities.

Habitat complexity reduces the growth of aggressive and dominant brown trout (Salmo trutta) relative to subordinates

Animals often prefer areas containing physical structure, and population density often increases with structural complexity, presumably because physical complexity in habitats may offer protection from predators and aggressive competitors. Consequently, increased habitat complexity often results in reduced territory size, lower aggression levels and reduced resource monopolisation by dominants. If behavioural plasticity is limited at early life stages, increased habitat complexity may reduce the relative fitness of aggressive, dominant strategies. Here we tested this hypothesis in an experiment on newly emerged brown trout (Salmo trutta) fry. We show, for the first time, that increased habitat complexity reduces the fitness (i.e. growth rate) of aggressive dominant individuals in relation to subordinates, and that this relation is reversed in simple habitats. Variation in environmental complexity may thus induce fluctuating selective pressures, maintaining behavioural variation in natural populations and allowing subordinate and dominant strategies to coexist.

Density‐dependent growth rate in an age‐structured population: a field study on stream‐dwelling brown trout Salmo trutta

A field experiment during autumn, winter and spring was performed in a small stream on the west coast of Sweden, aiming to examine the direct and indirect consequences of density-dependent intercohort competition in brown trout Salmo trutta. Individual growth rate, recapture rate and site fidelity were used as response variables in the young-of-the-year (YOY) age class, experiencing two different treatments: presence or absence of yearlings and over-yearlings (age > or = 1+ year individuals). YOY individuals in stream sections with reduced density of age > or = 1+ year individuals grew significantly faster than individuals experiencing natural cohort structure. In the latter, growth rate was negatively correlated with density and biomass of age > or = 1+ year individuals, which may induce indirect effects on year-class strength through, for example, reduced fecundity and survival. Movement of YOY individuals and turnover rate (i.e. proportion of untagged individuals) were used to demonstrate potential effects of intercohort competition on site fidelity. While YOY movement was remarkably restricted (83% recaptured within 50 m from the release points), turnover rate was higher in sections with reduced density of age > or = 1+ year individuals, suggesting that reduced density of age > or = 1+ year individuals may have released favourable microhabitats.

Sneaky feeding by salmon in sympatry with dominant brown trout

We compared the feeding performance and overt aggressive behaviour of two closely related salmonids occupying the same food patches. Foraging competition between pairs of Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in which trout were the larger and predicted to be the more dominant species, was investigated in a controlled stream channel environment. Trout held station by swimming actively in the central regions of food patches, whereas salmon occupied the margins, generally remaining stationary on the stream bed. There was little overt aggression, but the direction of attacks was consistent, with trout being dominant over salmon. Trout made significantly more foraging attempts (61.4% of food items presented) than salmon (28.9%). However, efficiency of food capture was higher in salmon (97%) than in trout (78%) and there was no significant difference between species in food intake. Salmon fed by briefly invading the space occupied for most of the time by trout. This behaviour has similarities to the ‘sneaky’ behaviours often shown by subordinate male salmonids during spawning, when gametes rather than food are the contested resource.

Effects of density on foraging success and aggression in age-structured groups of brown trout

The benefit of monopolizing a limited resource is influenced by competitor density and by the relative competitive ability of defenders and intruders. Nevertheless, few studies have investigated the effect of density on resource defence in groups with large asymmetries in competitive ability, as a consequence of, for example, age and/or body size. We used two age classes (i.e. size groups) of stream-living brown trout, Salmo trutta, to investigate this issue. While old (and large) trout are assumed to be superior during interference competition, younger individuals may be both numerically dominant and constitute more than half of the total population biomass. In this experiment, the ability of one yearling to monopolize a concentrated food source was observed at four densities of under-yearlings (zero, two, six and 12) in an indoor seminatural stream. We predicted that the success of defence would decrease with increasing under-yearling density and that the frequency of defence (i.e. aggression) would peak at an intermediate density. As predicted, yearlings made significantly more unsuccessful foraging attempts and adopted darker body coloration at high density of under-yearlings, suggesting increased stress levels. However, in contrast to our second prediction, the number of aggressive interactions increased progressively with density. These novel findings suggest that the cost of defence increases with under-yearling density, probably as a consequence of stress from interference with under-yearlings employing alternative competitive strategies. However, the difference in size seems to enable yearlings to defend the food resource at higher density of competitors than predicted from the resource defence theory.

Behavioural syndromes in juvenile brown trout (Salmo trutta); life history, family variation and performance in the wild

The main aims of this study were to investigate (1) the occurrence and strength of the shyness–boldness behavioural syndrome in brown trout fry, (2) whether this syndrome is associated with paternal migratory life history, (3) whether fry survival and growth in the wild is associated with paternal life history and/or boldness and (4) whether offspring performance showed maternal effects. Nine female migratory trout were each crossed with one migratory and one resident male and the offspring were raised in hatchery tanks until first feeding. The behavioural studies showed that fry that responded bold towards a novel object also accepted a novel food item earlier and responded more aggressively towards their own mirror image. Principal component analysis showed that this shyness–boldness syndrome (PC1) explained 34% of the behavioural variation. Offspring boldness was not affected by paternal migratory life history, but significant effects of the female parent suggested maternal and/or genetic effects. Deviations from this pattern (PC2), where some individuals behaved less aggressively and performed more approaches to food, explained an additional 17% of the variation in behaviour, and was significantly influenced by length and paternal migratory life history. Fry growth and survival in nature was not associated with boldness (PC1) or PC2, suggesting that alternative behavioural strategies can be successful in nature. However, female parent effects on variation in fry size persisted over the first growth period in the wild, suggesting that these effects may influence offspring fitness during early life when major selection occurs.

Habitat exclusion and reduced growth: a field experiment on the effects of inter-cohort competition in young-of-the-year brown trout

Competition during the juvenile phase is a key process for regulating density in organisms with high fecundity. Juvenile density-dependent bottlenecks may become even more pronounced if several cohorts compete, but this has received relatively limited attention in previous literature. We performed a manipulation experiment in seven coastal streams to investigate the presence of inter-cohort competition, using habitat selection, body-size and density of newly emerged (age-0) brown trout (Salmo trutta) as response variables. The trout population (age ≥1 fish) was estimated using electro-fishing prior to the emergence of fry (April–May) and was either removed (manipulated sections) or maintained (control sections). Age-0 habitat selection was examined in June while density and body-size was evaluated in October (end of the growth season). We found that age-0 trout selected habitats that were located further from riffles (nursery habitats) in the absence of age ≥1 trout, suggesting a niche overlap between cohorts in the habitat dimension and, hence, that both inter-cohort competitive interactions and ontogenetic preference may influence habitat utilisation in the wild. Furthermore, we also found age-0 body-size to be significantly larger in manipulated sections and negatively related to its own density. We argue that competition from older cohorts influence the availability of age-0 feeding territories at the critical phase of emergence with secondary negative effects on age-0 growth. These results not only have implications for understanding the mechanisms of density dependence but can also provide valuable knowledge to the management of salmonid populations and their habitats in the wild.

Inactive trout come out at night: behavioral variation, circadian activity, and fitness in the wild

Theory suggests that high activity levels in animals increase growth at the cost of increased mortality. This growth-mortality tradeoff has recently been incorporated into the wider framework of the pace-of-life syndrome (POLS) hypothesis. However, activity is often quantified only in the laboratory and on a diurnal basis, leaving open the possibility that animals manage predation risk and feeding efficiency in the wild by modulating their circadian activity rhythms. Here we investigate how laboratory activity in wild brown trout parr (Salmo trutta L.) associates with circadian activity, growth, and mortality in their natal stream. We found that individuals with high activity in the laboratory displayed high dispersal and cathemeral activity in their natal stream. In contrast, trout with low laboratory activity showed variation of activity in the wild, which was negatively related to the light intensity. Our results do not support the growth-mortality trade-off of the POLS hypothesis as highly active, fast-growing individuals showed higher survival than inactive conspecifics. These novel results show for the first time that active and inactive individuals, as scored in the lab, can show different circadian patterns of behavior in the wild driven by light intensity. This implies that studies conducted under a narrow range of light conditions can bias our understanding of individual behavioral variation and its fitness consequences in the wild.

Co‐existence with non‐native brook trout breaks down the integration of phenotypic traits in brown trout parr

Summary A phenotypic syndrome refers to complex patterns of integration among functionally related traits in an organism that defines how the organism interacts with its environment and sustains itself. Human-induced biological invasions have become important sources of environmental modifications. However, the extent to which invasive species affect the phenotypic syndromes of individuals in a native is currently unknown. Such knowledge has important implications for understanding ecological interactions and the management of biological invasions. Here, field monitoring in a natural stream were combined with standardized estimates of behavioral, physiological and morphological traits to address the hypothesis that coexistence with a non-native invader induces a novel environmental pressure that disrupts the adaptive integration among phenotypic traits of the native species. We compared the strength of integration among key phenotypic traits (i.e. aerobic scope, standard metabolic rate, body growth, activity, and body shape) and ecological niche traits (i.e. spring and summer diet, home range size, daily movements) of an allopatric group of native brown trout (Salmo trutta) with a group of brown trout living in sympatry with non-native brook trout (Salvelinus fontinalis). We found that the integration of phenotypic traits was substantially reduced in the sympatric brown trout and that allopatric and sympatric brown trout differed in key phenotypic and ecological niche traits. Brown trout living in sympatry with non-native brook trout consumed more terrestrial prey, had smaller home ranges, and a stouter body shape. Sympatric brown trout also had lower specific growth rate, suggesting a lower fitness. The results are generally in line with our hypothesis suggesting that the reduction in fitness observed in sympatric brown trout is caused by the breakdown of their adaptive phenotypic syndrome. This may be caused by differences in the plasticity of the response of phenotypic traits to the novel selection pressure induced by the non-native species. Our results may help explaining deleterious effects of non-native species reported in the absence of direct competition with the native species. A lay summary is available for this article.

Behavior and growth of juvenile brown trout (Salmo trutta) following upstream and downstream displacement

Juvenile brown trout were experimentally displaced 400 m both upstream and downstream, whereupon growth and homing behavior were examined for three months. No obvious signs of homing behavior were detected, no matter what the direction of displacement was, although an increased activity in displaced fish was detected. Our findings generally support the restricted movement paradigm, at least under favorable habitat conditions and low densities.