Nina Peuhkuri

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 known 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 p is discussed in the context of their mechanisms and functions. Predictions based on experiments in captivity regarding p for conspecifics, individuals of similar body length and unparasitized fish were highly consistent with field observations on free‐ranging shoals whereas p for familiar conspecifics and kin remain to be conclusively demonstrated in the field. In general, there is a shortage of studies in which shoaling p 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.

Significance of memory properties in prey choice decisions

Abstract To forage efficiently in a spatially and temporally heterogeneous environment requires that an individual’s information from the immediate past is combined with information from the more distant past to track environmental change. We made use of a model involving exponentially devaluating weights for past events to emulate behaviour of the individual’s memory. As the devaluation rate increases, more weight is given to the most recent events. First, performance of individuals with different memory properties was tested in simulations in which two prey types with different profitabilities were available in different proportions. In a structurally stable prey environment a low memory devaluation rate gave better estimation of prey proportions than a high memory devaluation rate. In a highly variable environment, on the contrary, individuals with high devaluation rate could more quickly correct their estimates as prey availability changed, although this was achieved with the cost of high error rate of the estimate. Second, the ability to reliably assess relative abundances of the prey types proved to increase an individual’s success in prey choice (according to the decision rules by the optimal prey choice model). Third, in further simulations individuals were allowed to adjust their memory devaluation rate according to experience from their success in prey choice decisions in previous patches. We found that there was no need to adjust a high devaluation memory in a highly variable environment, but foragers starting with low devaluation value rather rapidly shifted to high devaluation rates. In a relatively stable environment the situation was reversed and finally all foragers used low devaluation rates. These results imply that the variation in estimation efficacy of prey availability may be critical in terms of optimal prey choice and thus memory properties should be included in examinations of prey choice. Including individual variation in foraging performance in individual-based models could increase our understanding of the consequences of these differences at the population and community levels.

Producers, scroungers and foraging group structure

The producer–scrounger model of Barnard & Sibly (1981,Anim. Behav.,29,543–550) predicts that at a certain ratio of producers and scroungers in a foraging group the payoff curves for individuals of the two phenotypes intercept. At this point all the group members obtain equal payoffs. In this paper, the original model is elaborated further by allowing individuals to differ not only in their food-finding abilities but also in their abilities to compete for the food once it is found. As in Barnard & Siblys original model foraging individuals are assumed to share information about the whereabouts of food patches and to benefit from patches found by others. Here it is determined under which combinations of searching and sharing characters food intake rate in mixed-phenotype groups would be equal for producing and scrounging individuals. In the present model the payoffs are much affected by both the finders advantage (the fraction of prey available only to the finder of the food-patch) and the combination of the foraging characters of the two phenotypes. Generally the intake rate of producers increases with their presence in the group and with increases in the finders advantage. Our model suggests that the equal payoffs arise only when the relative food-searching and competitive abilities of producers and scroungers fall within a narrow range of options acknowledging also the finders advantage. Moreover, such combinations are predicted to be unstable if only foraging benefits are called for in group formation, this being a common feature of social foraging models incorporating foraging role asymmetries.

A theoretical exploration of antipredatory and foraging factors promoting phenotype-assorted fish schools

AbstractWe explore factors promoting phenotype-assorted schooling in fish. Since predator avoidance and food-finding are frequently stated as the evolutionary keys for group-living they shall serve...

Aggressiveness is associated with genetic diversity in landlocked salmon (Salmo salar)

The amount of intraindividual genetic variation has often been found to have profound effects on life history traits. However, studies concerning the relationship between behaviour and genetic diversity are scarce. Aggressiveness is an important component of competitive ability in juvenile salmonids affecting their later performance and survival. In this study, we used an experimental approach to test the prediction that juveniles with low estimated genetic diversity should be less aggressive than juveniles with high estimated genetic diversity in fry from a highly endangered population of land‐locked salmon (Salmo salar). This was achieved by using a method enabling the accurate estimation of offspring genetic diversity based on parental microsatellite genotype data. This allowed us to create two groups of offspring expected to have high or low genetic diversity in which aggressive behaviour could be compared. Salmon fry with low estimated genetic diversity were significantly less aggressive than fry with high estimated genetic diversity. Closer analysis of the data suggested that this difference was due to differences in more costly acts of aggression. Our result may reflect a direct effect of genetic variation on a fitness‐related trait; however, we cannot rule out an alternative explanation of allele‐specific phenotype matching, where lowered aggression is expressed towards genetically more similar individuals.

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 individuals 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.

Female and male contribution to egg size in salmonids

Egg size contributes to other life history traits of an individual. It is traditionally considered as a maternally determined characteristic to which the male does not have any direct contribution. However, a recent finding in insects suggests that males can affect egg size also directly. In fish, the male effect could take place only during egg swelling, as the final egg size is reached after that. We studied egg size in four freshwater salmonid species (the land-locked Atlantic salmon, the brown trout, the Arctic charr and the lake trout) right after fertilisation (initial egg size) and after the swelling phase (final egg size). The results showed that the final egg size is affected not only by the initial egg size but also by both the female and the male through the process of egg swelling. This study suggests that paternal contribution may form a previously largely ignored source of variation in early life history traits in salmonid fish.

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).

The effect of captive breeding on aggressive behaviour of european grayling, Thymallus thymallus, in different contexts

Hatchery rearing is known to influence the aggressive behaviour of fish and to weaken antipredator responses. However, behavioural traits of an individual may not be free to evolve independently and can be correlated with one another. A positive link between aggressiveness and risk taking, for example, has been observed. We previously found second-generation hatchery grayling to be less aggressive than their wild conspecifics. In the present study, we examined both the level of aggression and the propensity to resume aggressiveness after a predator stimulus (risk taking) in young-of-the-year grayling from hatchery and wild strains of two populations. We were also interested in the effects of hatchery rearing on the link between the behaviours at the individual level and the consistency of this relation between populations. In contrast to our earlier findings, hatchery grayling appeared on average more aggressive than their wild conspecifics from the same population, whereas hatchery fish were more cautious after the threat stimulus in only one of the two populations. At the individual level, the behavioural association was, however, evident and similar in both strains of the two studied populations, such that the initially more aggressive fish also resumed aggressiveness faster after the threat than their less aggressive conspecifics. Consequently, while hatchery rearing for one generation has the potential to influence the expression of single behavioural traits, it may not necessarily result in changes in the link between them, that is, in the behavioural syndrome.

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.