Jörgen I. Johnsson

Growth hormone increases growth rate, appetite and dominance in juvenile rainbow trout, Oncorhynchus mykiss

Abstract Abstract. The hypothesis that exogenous growth hormone elevates feeding motivation, and thereby increases dominance status in juvenile rainbow trout was tested by comparing the feeding behaviour of growth hormone-treated and control (vehicle-injected) trout in two experimental series. Growth rate, food conversion and hypo-osmoregulatory ability of the two groups were also compared. During a 4-day treatment period, specific growth rate was significantly higher in growth hormone-injected trout than in control fish. Further, growth hormone-treated fish had significantly greater hypo-osmoregulatory ability than control fish. Satiation feeding experiments on individual trout revealed a considerably higher appetite in growth hormone-injected trout, which ate twice as much as control fish over the 2 days of feeding. Also, food conversion efficiency was higher in growth hormone-treated fish than in control fish. Feeding competition trials involving similar-sized matched pairs of growth hormone-treated and control trout revealed that growth hormone-treated juveniles were significantly dominant over control fish when the pairs were selected before the 4-day hormonal treatment but in a second dominance experiment, where growth hormone-treated and control fish were paired after the growth hormone treatment, no significant difference was found between the treatment groups. Overall, the results support the hypothesis that exogenous growth hormone can increase appetite and dominance in juvenile rainbow trout. It is suggested that the endogenous production of growth hormone in rainbow trout is limited by mortality costs associated with maximal growth rates.

Experience and social environment influence the ability of young brown trout to forage on live novel prey

Efficient feeding is crucial for the growth, survival and reproductive success of most animals. In artificial-rearing environments, however, animals are deprived of many stimuli normally experienced in the wild, which may alter feeding behaviour, and thus influence their survival and reproductive success upon release in nature. In a laboratory experiment, we investigated the effect of hatchery rearing on the ability of brown trout, Salmo trutta, to capture and consume a novel live prey item. Hatchery-reared and wild-caught trout, originating from the same river, were fed single black crickets, either in isolation or in visual and olfactory contact with another hatchery-reared or wild-caught fish. Total consumption, time to first bite and feeding efficiency were monitored. Wild-caught trout ate more, were quicker to attack, and consumed attacked prey more efficiently than hatchery-reared fish. Food consumption and efficiency increased in both wild and hatchery-reared trout during the experiment. We propose that the differences in feeding ability between wild-caught and hatchery-reared brown trout were mainly due to differences in previous experience of feeding on live prey. Wild-caught trout tended to eat more and sooner when in visual contact with another fish than when in isolation. This trend was not seen for the hatchery-reared fish, which may be due to environmental differences between the hatchery and the natural stream. The initial inability of hatchery-reared fish to forage on live prey may reduce their success when released in the wild, especially when in competition with resident wild fish. Copyright 2001 The Association for the Study of Animal Behaviour.

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.

Growth Hormone Increases Predation Exposure of Rainbow Trout

The energetic state of an animal strongly influences decisions that balances feeding against predation risk. Growth hormone increases the metabolic demands, which should elevate the feeding motivation of an animal. This, in turn, may increase the willingness to risk exposure to predators during feeding. To test this hypothesis, we studied the effect of growth hormone on the behavioural response of rainbow trout (Oncorhynchus mykiss) to simulated attacks from a model heron. After attacks, growth hormone treated trout foraged closer to the water surface, resumed feeding earlier, and ate more food than did control trout. Such behaviour should increase the susceptibility to aerial predation. Thus, predation may select against high endogenous growth hormone secretion in wild fish. Furthermore, genetic manipulations to increase growth hormone levels, intended to improve growth performance in aquaculture, may result in individuals with substantially altered behavioural patterns. In light of the increasing potential for interactions between farmed and wild fish, growth hormone transgenic fish may pose a threat to wild fish populations.

Growth hormone transgenic salmon pay for growth potential with increased predation mortality

Recent advances in gene technology have been applied to create fast–growing transgenic fish, which are of great commercial interest owing to their potential to shorten production cycles and increase food production. However, there is growing concern and speculation over the impact that escaped growth hormone (GH)–transgenic fish may have on the natural environment. To predict these risks it is crucial to obtain empirical data on the relative fitness of transgenic and non–transgenic fish under nature–like conditions. Using landscaped stream aquaria with live food and predators, we show that the predation mortality of newly hatched GH-transgenic coho salmon fry (Oncorhynchus kisutch) is much higher than in non–transgenic conspecifics, and that this difference is amplified when food abundance decreases. The growth rate of transgenic and non–transgenic fish is similar at high food levels, whereas transgenic fish grow more slowly than non-transgenic fish when food abundance is reduced. Our results suggest that the fitness of young GH–transgenic coho salmon in the wild will be determined by both predation pressure and food availability.

The cost of catching up: increased winter mortality following structural growth compensation in the wild.

Although laboratory and observational studies suggest that many animals are capable of compensatory growth after periods of food shortage, few field experiments have demonstrated structural growth compensation in the wild. Here, we addressed the hypotheses that (i) food restriction can induce structural compensatory growth in free-living animals, (ii) that compensation is proportional to the level of body size retardation and (iii) that compensation induces mortality costs. To test these, wild brown trout (Salmo trutta) yearlings were brought to the lab, tagged individually, subjected to four levels of food deprivation (including a control), released back into the native stream and recaptured after one, five and ten months. Brown trout fully restored condition and partially restored mass within a month, whereas compensation in structure (i.e. body length) was not evident until after five months, supporting hypothesis 1. As the level of growth compensation was similar among the three deprived groups, hypothesis 2 was not supported. A final recapture after winter revealed delayed mortality, apparently induced by the compensatory response in the deprived groups, which is consistent with hypothesis 3. To our knowledge, this is the first field experiment demonstrating structural compensatory growth and associated costs in a wild animal population.

Dominance, Nutritional State, and Growth Hormone Levels in Rainbow Trout (Oncorhynchus mykiss)

This study addressed three questions concerning interactions between physiology and dominance in juvenile rainbow trout: (1) the validity of a model predicting a time-dependent effect of fasting on competitive ability (i.e., the ability to obtain contested food items) was tested in a series of dominance trials between fed and progressively more fasted trout, as was (2) the association between fasting and plasma growth hormone levels. (3) The relationship between plasma growth hormone levels and the competitive ability of individual trout was also studied. The main results were as follows: (1) The predictions of the time-dependent model were supported by the fasting-dominance experiment. After 3 days, fasted fish were dominant over fed fish, whereas after 6 and 9 days, the competitive ability of fed and fasted fish was similar. After 12 days, there was a tendency for fed fish to be dominant over their fasted competitors. (2) Sampling of plasma from fed and fasted trout, after 3, 6, 9, and 12 days, demonstrated that plasma growth hormone levels increases in food-deprived rainbow trout after more than 6 days of fasting, which is consistent with previous work. (3) No difference in plasma growth hormone levels was found between paired dominant and subordinate trout. Possible interactions between nutritional state, growth hormone levels, and dominance, and their implications are discussed.

Investment in territorial defence depends on rearing environment in brown trout (Salmo trutta)

In many animals, territoriality will arise or cease depending on environmental factors such as intruder rate and resource availability. We investigated the effect of rearing environment on territorial behaviour in ~1.5-month-old brown trout. In the laboratory, wild-caught (reared at a low density) and hatchery-reared (high density) trout were allowed to defend a territory against a size-matched intruder reared in the same or the other environment. Because territorial behaviour should be relaxed at high-rearing densities, we hypothesized that hatchery-reared trout should value their territories less and therefore invest less in defence compared with wild-caught trout. However, in all cases, territory owners were more likely to win the contest and hatchery-reared trout were just as likely as wild-reared to win mixed contests. Furthermore, pairs of hatchery-reared trout initiated contests sooner, fought longer and were more aggressive during the contest compared with pairs of wild trout. When hatchery-reared owners met wild intruders, the contest ended sooner compared with when the roles were reversed. We conclude that territorial behaviour in brown trout is largely innate, but that the hatchery environment has promoted more aggressive individuals. These results suggest that hatchery-reared trout invest more time and energy to obtain the same contest success as wild trout. In conclusion, the lack of experience of territorial defence in a high-density rearing environment seems to reduce the efficiency of territorial behaviour. In turn, this may have negative consequences for the performance of released hatchery fish in the wild.

Habitat preference increases territorial defence in brown trout (Salmo trutta)

Abstract The striking ability of territory owners to repel intruders has generated a number of theoretical explanations as well as experimental studies in many animal species. However, effects of individual habitat preferences on territorial defence have rarely been studied. From the territory value hypothesis, we predicted that owners of preferred habitats should invest more resources in defence than owners of non-preferred habitats. We tested this prediction with young territorial brown trout in a two-stage experiment. First, trout were allowed to choose individually between gravel and a uniform bright substrate. As expected, they showed a significant (79%) preference for gravel. However, there was considerable variation between individuals in substrate preference, with a few fish preferring the bright substrate. Half of the tested fish were then transferred to a gravel substrate and the rest to a bright substrate, manipulating habitat type in relation to preference. Territory owners were then staged against size-matched intruders whereupon contest aggression was observed and the winner of each contest determined. Overall, owners won most of the contests. Satisfied owners won 86% and owners of less preferred territories, 74% of the contests. Furthermore, more satisfied owners attacked sooner and were more aggressive relative to the intruders. We conclude that brown trout show individual variation in habitat preference, which appears to be linked with their investment in territorial defence. These results suggest that understanding and modelling of animal contests could benefit from considering how territorial defence is influenced by individual habitat preference and specialisation.