Parental effects on embryonic viability and growth in Arctic charr Salvelinus alpinus at two incubation temperatures.

The parental influences on three progeny traits (survival to eyed-embryo stage, post-hatching body length and yolk-sac volume) of Arctic charr Salvelinus alpinus were studied under two thermal conditions (2 and 7 degrees C) using a factorial mating design. The higher temperature resulted in elevated mortality rates and less advanced development at hatching. Survival was mostly attributable to maternal effects at both temperatures, but the variation among families was dependent on egg size only at the low temperature. No additive genetic variation (or pure sire effect) could be observed, whereas the non-additive genetic effects (parental combination) contributed to offspring viability at 2 degrees C. In contrast, any observable genetic variance in survival was lost at 7 degrees C, most likely due to the increased environmental variance. Irrespective of temperature, dam and sire-dam interaction contributed significantly to the phenotypic variation in both larval length and yolk size. A significant proportion of the variation in larval length was also due to the sire effect at 2 degrees C. Maternal effects were mediated partly through egg size, but as a whole, they decreased in importance at the high temperature, enabling a concomitant increase in non-additive genetic effects. For larval length, however, the additive component, like maternal effects, decreased at 7 degrees C. The present results suggest that an exposure to thermal stress during incubation can modify the genetic architecture of early developmental traits in S. alpinus and presumably constrain their short-term adaptive potential and evolvability by increasing the amount of environmentally induced variation.

Quantitative genetic architecture of parasite-induced cataract in rainbow trout, Oncorhynchus mykiss.

Parasites impose costs on their hosts. The capability to fight against them is of great advantage, but may also be traded off with other traits. Although often observed at the phenotypic level, our knowledge of the extent to which such trade-offs are genetically determined is relatively poor. We tested this possibility with a farmed rainbow trout population suffering from natural Diplostomum spp. infections that cause cataracts in fish. We estimated the heritability of cataract severity and examined phenotypic and genetic correlations between cataract and a set of performance traits measured three times during a 3-year rearing period. A cataract score was used as an indicator of the host's capability to resist and/or tolerate the parasite. Our results showed moderate heritability for the cataract. Nevertheless, we found no evidence for a genetic or phenotypic trade-off between parasite resistance/tolerance and the measured performance traits. Initial body weight was not correlated with the cataract score. Phenotypic and genetic correlations of cataract severity with body mass and condition measured in the second and third year were strongly negative, indicating reduced growth and condition in fish with a high cataract score. The reduced body size and condition in cataract-bearing fish were probably reflected in the phenotypic association between a high cataract score and delayed maturity age in females. Put together, our study did not provide evidence of genetic or phenotypic trade-offs between Diplostomum resistance/tolerance and a number of performance traits. Therefore, selection for lessened Diplostomum-caused cataracts is unlikely to have a negative impact on the studied performance traits.

Metabolic depression and spleen and liver enlargement in juvenile Arctic charr Salvelinus alpinus exposed to chronic parasite infection.

The present study on the connection between standard metabolic rate (R(S)) and chronic Diplostomum spp. infection resulted in a decrease in R(S), and an enlargement in spleen and liver sizes in the infected juvenile Arctic charr Salvelinus alpinus compared to control fish. As splenic enlargement observed in infected fish was not due to condition-related changes in the spleen, it could most probably be explained by increased leucocyte synthesis. The higher liver masses in infected S. alpinus may have been related to disorders in energetic function, which could have had major effects on biochemical regulation by the liver. The proposed metabolic syndrome with a possible reduction in insulin sensitivity in tissues results in ineffective glucose and lipid metabolism and thus it is suggested that chronic Diplostomum infection in S. alpinus might not impose direct energetic costs, but it may weaken the efficiency of energy metabolism and thus lead to lowered R(S).

Phenotypic variation in growth trajectories in the Arctic charr Salvelinus alpinus.

Animals with determinate growth have shown little variation in individual growth patterns, but similar analyses for animals with indeterminate growth have been lacking. We analysed the amount of phenotypic variation in growth patterns across ages among individuals of a hatchery-based population of Arctic charr, Salvelinus alpinus, Salmonidae, using the infinite-dimensional model and including the effects of group size structure. There was little phenotypic variation in growth trajectories: individuals that were small (in relation to the mean) early in life were among the smallest 2.5 years later. If the genetic variation reflects phenotypic variation, not much evolutionary change can be expected. Our results show that there are ecological conditions that determine the strong covariation of size across ages, most likely size-related dominance behaviour, which can mask the true variation of growth patterns. Thus, social interactions can have strong evolutionary effects on traits not directly involved in the behavioural interactions.

The social organization of fish shoals: a test of the predictive power of laboratory experiments for the field.

By contrast with a multitude of laboratory studies on the social organization of fish, relatively little is know about the size, composition and dynamics of free-ranging fish shoals. We give an overview of the available information on fish shoals and assess to what degree the predictions made from laboratory studies are consistent with field data. The section on shoal choice behaviour in the laboratory is structured so that the evidence for different shoaling preferences is discussed in the context of their mechanisms and functions. Predictions based on experiments in captivity regarding preferences for conspecifics, individuals of similar body length and unparasitized fish were highly consistent with field observations on free-ranging shoals whereas preferences for familiar conspecifics and kin remain to be conclusively demonstrated in the field. In general, there is a shortage of studies in which shoaling preferences have been investigated both in the laboratory and the field, and field studies have so far been largely descriptive revealing little about the underlying mechanisms of observed patterns. Given the great importance of fish shoals both in fundamental and applied research, an advancement of our knowledge of their social organization should significantly contribute to a better understanding of a whole range of topics including reciprocal altruism, group-living and self-organization.

Producers, scroungers and the price of a free meal

In social foraging, scroungers take a disproportionately large share of the food found relative to their own food-searching efforts, while producers find more food than they manage to monopolize. We present a model of social foraging acknowledging the finder's advantage and foraging role asymmetries among individuals but incorporating the possibility that producers and scroungers differ in vigilance level and in vulnerability to predators. This allows simultaneous examination of both foraging benefits and anti-predatory aspects of grouping behaviour. Instead of seeking for equal payoff conditions, we first look for groups in which foraging character combinations and anti-predatory properties of producers and scroungers minimize the phenotype-specific predation hazard over food-intake rate, Ri/Ii, that is, fixed phenotype Ri/Ii minima. In the second approach, we allow individuals to change their foraging status to achieve lower Ri/Ii and look for combinations where it no longer pays for either producers or scroungers to change their roles, that is, evolutionarily stable group compositions, ESS. Various character combinations allow the phenotype-specific minima. In most cases, however, producers' and scroungers' minima are achievable only in different group compositions. The ESS combinations of producers and scroungers deviate widely from those combinations yielding phenotype-specific minima of Ri/Ii. If individuals are allowed to be flexible in adopting either a producer or a scrounger role, ESS group compositions will emerge, even though they are more expensive for both producers and scroungers in terms of Ri/Ii than group compositions yielding the phenotype-specific Ri/Ii minima. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Size-assortative shoaling in fish: the effect of oddity on foraging behaviour

Current theory predicts that fish should show size-assortative shoaling in order to avoid increased predation risk by being the odd one out (oddity effect), or in order to minimize competition for food. I investigated with three-spined sticklebacks, Gasterosteus aculeatusthe importance of the oddity effect in promoting size-assortative shoaling. The greater an individual assesses its predation risk the less actively it is likely to forage. Hence, I examined with small and large fish whether an individual's foraging activity depends on its appearance (size) in relation to that of others in a shoal. The shoals were composed of three, six and 12 fish. Either one individual deviated in size from the rest of the shoal members or all the fish in a shoal were of similar size. When a stickleback was larger than others in the shoal its foraging activity was lower than that of large individuals in a shoal dominated by large fish or those in a size-assorted shoal. Small sticklebacks, however, did not change their foraging activity on the basis of their appearance in a shoal. These responses of individuals to their appearance did not depend on shoal size nor on the presence or absence of a predator. The results suggest that the oddity effect is likely to prevent larger sticklebacks from joining shoals of smaller individuals. They also suggest that factors other than the oddity effect, potentially food competition, may be more important in leading individuals to avoid the company of larger ones and prefer shoaling with matching conspecifics.