Size‐dependent stress response in juvenile Arctic charr (Salvelinus alpinus) under prolonged predator conditioning

Predator conditioning can be used to improve post‐release antipredator recognition of hatchery‐reared salmonids. However, possible negative stress‐related effects of prolonged predator conditioning on juvenile fish physiology are poorly understood. We studied the effects of prolonged (91 days) predator odour exposure on whole‐body cortisol level and spleen size in six full‐sib families of juvenile hatchery‐bred Arctic charr (Salvelinus alpinus). Chemical cues from water containing charr‐fed pikeperch (Sander lucioperca) were used as the predator exposure stimuli and lakewater was used as a chemical control. Our study revealed that juvenile body cortisol levels post‐predator conditioning were affected by treatment, fish size and their interaction. Importantly, among the smaller (i.e. slowest growing) charr, the predator‐exposed fish had higher cortisol levels than control fish, while the opposite pattern was true for the larger fish. These results suggest that chemical cues from charr‐fed predators induce a prolonged stress response in juvenile charr. As prolonged predation exposure seems to elevate stress levels in a size‐dependent manner, the larger, faster growing fish could possibly have intrinsically lower stress responses to predation threats than smaller, slower growing fish. Possible coupling between stress sensitivity and growth requires further attention due to the likely implications for the management of unintended domestication among captive‐reared salmonids.     

Mianserin affects alarm reaction to conspecific chemical alarm cues in Nile tilapia

In this study, I show that mianserin, a chemical with serotonin and adrenoceptor antagonist activities, increases fish vulnerability to a potential predator threat, when prey fish must deal with this threat based on conspecific chemical alarm cues. For that, I evaluated whether mianserin, diluted in the water, influences the behavioral responses of Nile tilapia (Oreochromis niloticus) to conspecific skin extract (chemical alarm cues). I found that, while mianserin did not abolished antipredator responses, this drug mitigates some components of this defensive reaction. Thus, a potential decrease in serotonin and adrenergic activities reduces the ability of dealing with predators when perceiving conspecific chemical alarm cues.     

Learning about danger: chemical alarm cues and local risk assessment in prey fishes

An individual's behaviour patterns can be conceptualized as a series of threat‐sensitive trade‐offs between ambient predation pressure and a suite of fitness‐related activities, such as resource defence, foraging and mating. Individuals that can reliably assess local predation risk could increase their fitness potential by exhibiting predator avoidance behaviours only at appropriate times. However, such learned risk assessment requires reliable information regarding current predation risks. A diverse range of prey fishes are known to possess chemical alarm cues, which when detected by conspecifics and some heterospecifics, elicit a variety of overt and covert responses. These chemical cues, either alone or as a part of a predator's dietary odour, provide reliable information regarding local predation risk. In this review, I describe recent works examining the role of chemosensory information in: (i) acquired predator recognition, (ii) predator inspection behaviour and (iii) the use of conspecific and heterospecific cues as social information sources.     

Learned predator recognition and antipredator responses in fishes

Early research into the causes of geographical variation in antipredator behaviour in fishes revealed that population differences have an underlying genetic basis. However, evidence from a variety of fish species suggests that learning plays an important role in the development of antipredator responses. Here, we consider the opportunities for learning during the three main stages of a predator–prey interaction: detection, recognition and assessment, and attack avoidance. Much of the evidence for learning is based on the recognition and assessment stage of the predator–prey interaction, but this may reflect methodological biases. We also examine the relative roles of different sensory cues, in particular, vision and olfaction, and the importance of individual vs. social learning. We provide evidence that visual predator recognition skills are largely built on unlearned predispositions, whereas olfactory recognition typically involves experience with conspecific alarm cues. Populations display variation in their propensity to learn, and we predict that ecological factors are likely to mediate the balance between individual and social learning.     

Diet and growth of stocked and wild 0+ pikeperch, Stizostedion lucioperca (L.)

Wild and stocked 0+ pikeperch, Stizostedion lucioperca (L.), were sampled with a purse seine from Lake Oulujärvi, situated at the northern edge of the distribution range of the species. The stocked fry started immediate feeding on 0+ smelt, Osmerus eperlanus (L.) after their introduction into the lake in early August at about 50 mm length, whereas the smaller wild pikeperch fry continued to feed on zooplankton. Individual wet weight of the stocked pikeperch was about six-fold compared with the wild pikeperch throughout August. The length of the ingested smelt was as on average 50%, and maximum 66%, of the pikeperch length. Wild 0+ pikeperch were too small to forage on 0+ smelt. Predation by older conspecifics on 0+ pikeperch was observed.     

Vertical distribution of planktivorous 0+ pikeperch, Stizostedion lucioperca (L.), in relation to particulate or filter feeding

The main prey of 0+ pikeperch in the pelagic zone of the Sulejow Reservoir was Leptodora kindtii and copepods, which together constituted only about 20% of the zooplankton by number in the water. Leptodora kindtii predominated in pikeperch food close to the surface, and copepods in deeper water. A depth-dependent shift in feeding mode from large particle selection to filtering, as an adaptation to reduce intraspecific competition and avoid predatory pressure in the upper part of the water column, was suggested. Specimens with less stomach content were observed more frequently closer to the surface. They were exposed to heavy predation from adult conspecifics, which prevented them from foraging efficiently, although they were able to feed on larger zooplankton because of better penetration by moonlight.     

Latitudinal variation in growth and otolith‐inferred field metabolic rates of Canadian young‐of‐the‐year Arctic charr

Countergradient variation (CGV) is defined as genetic variation that counteracts the negative influences of the physical environment, minimising phenotypic variability along an environmental gradient. CGV is thought to have relevance in predicting the response of organisms to climate variability and change. To test the hypothesis that growth rate increased with latitude, consistent with CGV, young‐of‐the‐year (YOY) Arctic charr, Salvelinus alpinus, were examined along a ~27° latitudinal gradient in central and eastern Canada. Growth rates were estimated from fork lengths standardised by the thermal opportunity for growth based on experienced water temperatures derived using otolith oxygen stable isotopes. Results demonstrated patterns consistent with CGV, where northern populations demonstrated faster growth rates. A secondary aim was to test for similar geographical patterns in otolith‐inferred metabolic rates, which reflect the energetic costs of standard metabolic rate (SMR) and other processes such as feeding, locomotion, thermoregulation, reproduction and growth. Results demonstrated a significant, positive relationship between otolith‐inferred metabolic rate and latitude, which may reflect an increase in one, or a combination, of the above‐noted physiological processes. The similar latitudinal pattern in growth and otolith‐inferred metabolic rates suggests greater intake of food per unit of time by northern fish. The phenotypic variation in physiological traits observed here demonstrates the significant adaptability of Arctic charr to different thermal regimes with different growing season lengths. Determining the relative contributions of phenotypic plasticity and genetic variation to the observed latitudinal variation will be critical to predicting the responses of Arctic charr to climate change more accurately.     

Smolt characteristics of small first-time migrant, and resident Arctic charr, Salvelinus alpinus (L.), from a river system in northern Norway

Smolt characteristics were investigated in sympatric anadromous and resident Arctic charr. Salvelinus alpinus (L.), of a similar size (11-20 cm), A group of first-time migrant anadromous charr was caught while descending the Hals River and two groups of resident conspecifics were caught in Lake Storvatn. one before, and one after, the sea-run of anadromous fish had terminated. When sampled immediately after capture in fresh water the anadromous group had a higher proportion of fish visually classified as smolts. and these charr had higher gill Na⁺-K⁺ ATPase activity, higher densities of developed chloride cells in the gills, a lower condition factor, and lower plasma osmolality, than resident fish. When exposed to sea water (33%o S), only minor differences in mortality and plasma electrolyte levels were observed between anadromous and resident fish, average values of plasma osmolality and chloride concentrations being 377 mOsm and 169 mM and 387 mOsm and 174 mM, respectively. The results suggest that the first-time migrant Arctic charr had undergone some of the physiological changes that are considered typical for a parr-smolt transformation before they left fresh water. Such changes did not appear to have occurred in sympatric, resident, fish of a similar size. The parr-smolt transformation in the first-time migrants may, however, have been incomplete, because the fish did not appear to have the ability to rapidly re-establish osmo- and ionoregutatory homeostasis after direct transfer to sea water.     

Cichlids respond to conspecific sounds but females exhibit no phonotaxis without the presence of live males

Many fish species are able to produce sounds, which are often associated with courtship. In an earlier study, we showed for the Lake Victoria cichlid Pundamilia nyererei that females prefer to associate with a male with sound over a male without sound. As a follow‐up to this earlier finding, we here investigated whether playback of conspecific sounds is sufficient to attract females in the absence of a conspecific male. However, we did not find a phonotactic response for conspecific sounds in the absence of live males, using the same playback procedure as in our previous study. An additional playback test showed that both males and females discriminated between conspecific sounds and bursts of white noise. This suggests that the sounds may be recognised but that they seem only effective as attractant in the presence of visual and/or olfactory cues. These findings underline the multimodal complexity of fish communication and courtship and call for a more integrated study of the different modalities in future studies.     

Domestication effects on fin nipping, survival and growth in hatchery-reared arctic charr, Salvelinus alpinus (L.), in competition with wild conspecifics

When reared in captivity, first-generation hatchery-reared Arctic charr, Salvelinus alpinus (L.), from Møkkeland lake, northern Norway, differed from wild-caught conspecifics from the same source. Incidence of caudal fin damage (used as an indirect assessment of aggressive interactions) was high amongst the wild-caught charr, and there was considerable mortality amongst these fish. By contrast, hatchery-reared fish displayed little evidence of fin damage, and mortalities were low. When the charr were held in mixed groups of wild-caught and hatchery-reared fish, the hatchery-reared fish showed the most extensive caudal fin damage. Weight losses and decreases in condition were also greatest amongst the hatchery-reared charr that were exposed to competition with wild fish. The hatchery-reared charr held together with the wild fish also suffered high mortality. Thus, aggression may have been selectively directed towards hatchery-reared fish in mixed groups. Taken together, the results suggest that the energetic costs may be high for the recipients of aggression.     

Food resource partitioning of Arctic charr, Salvelinus alpinus (L.) and three-spined stickleback, Gasterosteus aculeatus L., in the littoral zone of lake Takvatn in northern Norway

The food resource partitioning of Arctic charr (Salvelinus alpinus L.) and three-spined stickleback (Gasterosteus aculeatus L.) were investigated in the littoral zone of lake Takvatn in northern Norway in the ice-free period June–November. Charr and sticklebacks had different feeding habits. Sticklebacks ate several small benthic prey items that were never eaten by charr, and the sticklebacks' diet were dominated by the benthic microcrustaceans Chydoridae and Ostracoda, chironomid larvae and stickleback eggs. Small charr (<17 cm) consumed a wide spectrum of chironomid pupae, terrestrial insects and zooplankton. Intermediate (17–20 cm) and small charr had quite similar feeding habits, while large charr (>20 cm) frequently ate both benthos, pelagic and terrestrial food. The diet overlap between small charr and sticklebacks was never larger than 0.6 (Schoener's index). The segregation in feeding habits indicates that small charr and sticklebacks are segregated in microhabitat when they are both in the littoral zone.     

An experimental test of the genetic component of the ontogenetic habitat shift in Arctic charr (Salvelinus alpinus)

Abstract— Fry of the Arctic charr, Salvelinus alpinus , were experimentally stocked into a small fish-free lake to test the hypothesis that the size-dependent habitat shift from the epibenthic to the pelagic habitat is genetically determined. The charr originated from a nearby lake inhabiting predatory brown trout Salmo trutta. The cohort of stocked charr was investigated for three years. The Arctic charr started to exploit the pelagic habitat in their first summer at a size of 7–9 cm in contrast to about 15 cm in the donor lake. In the next two summers, the pelagic fraction of the cohort increased. The main fraction lived in epibenthic areas, utilizing the same prey as pelagic charr. Water temperature moderated the habitat use of juveniles such that they avoided warm (>16°C) waters and resided in cool, deep areas. The result was consistent with the hypothesis of a tradeoff between feeding benefit and the predation risk producing spatial segregation of Arctic charr and demonstrated that the fish can facultatively respond to predation risk and adjust the size at which they migrate to the pelagic zone to feed on zooplankton.     

Phenotypic and genetic parameters of body and compositional traits in Arctic charr, Salvelinus alpinus (L.)

Abstract. Phenotypic and genetic parameters of body and compositional traits were estimated for a population consisting of 327 two-year-old Arctic charr, Salvelinus alpinus (L.), from 21 full-sib groups. Heritability estimates for body traits, such as fork length, gutted weight, condition factor and visceral weight, were all of intermediate magnitude (0·24–0·45). Heritabilities for carcass component traits, e.g. fat, protein, dry matter and astaxanthin contents, were low to medium (0·0–0·28). Genetic and phenotypic correlations were high between astaxanthin content and body traits, whereas they were medium to low between astaxanthin content and carcass compositional traits such as fat, dry matter and protein contents. The genetic variation in fat and dry matter contents was closely associated with variation in body weight. Selection for higher body weight at 2 years will probably lead to small, but positive, correlated changes in fat content. Our findings suggest that it should be possible to select for higher astaxanthin content in Artic charr to improve their flesh coloration.     

Effects of predator-induced visual and olfactory cues on 0+ perch (Perca fluviatilis L.) foraging behaviour

Abstract –  Foraging juvenile fish with relatively high food demands are usually vulnerable to various aquatic and avian predators. To compromise between foraging and antipredator activity, they need exact and reliable information about current predation risk. Among direct predator-induced cues, visual and olfactory signals are considered to be most important. Food intake rates and prey-size selectivity of laboratory-reared, naive young-of-the-year (YOY) perch, Perca fluviatilis , were studied in experiments with Daphnia magna of two size classes: 2.8 and 1.3 mm as prey and northern pike, Esox lucius , as predator. Neither total intake rate nor prey-size selectivity was modified by predator kairomones alone (water from an aquarium with a pike was pumped into the test aquaria) under daylight conditions. Visual presentation of pike reduced total food intake by perch. This effect was significantly more pronounced (synergistic) when visual and olfactory cues were presented simultaneously to foraging perch. Moreover, the combination of cues caused a significant shift in prey-size selection, expressed as a reduced proportion of large prey in the diet. Our observations demonstrate that predator-induced olfactory cues alone are less important modifiers of the feeding behaviour of naive YOY perch than visual cues under daylight conditions. However, pike odour acts as a modulatory stimulus enhancing the effects of visual cues, which trigger an innate response in perch.     

Factors affecting dam passage and upstream distribution of adult Pacific lamprey in the interior Columbia River basin

Low dam passage rates of adult Pacific lamprey (Entosphenus tridentatus) during upstream spawning migration have been implicated in the species' steep decline in the interior Columbia River basin. In this 2000–2010 study, we used radiotelemetry to evaluate potential predictors of lamprey passage success at McNary Dam, located ~469 river kilometres (rkm) from the Pacific Ocean. The tagged population included 276 lampreys collected at McNary Dam and 53 collected at Bonneville Dam (rkm 235) that migrated to McNary Dam. McNary Dam passage efficiency was similar for the two samples, with multiyear estimates of 0.65 and 0.75, respectively. Larger‐bodied lampreys and those with earlier migration timing were more likely to return to McNary Dam after release, to pass the dam and to be detected upstream from McNary reservoir. Far more lampreys entered the upper Columbia River than the Snake River, suggesting that environmental cues (e.g., water discharge, temperature) or conspecific cues (e.g., pheromone concentrations) affect lamprey distribution above this large confluence. Overall, results indicate that Pacific lamprey passage success at barriers depends on a combination of individual lamprey traits plus seasonal and site‐specific effects on behaviour.     

Size-dependent predator–prey relationships between pikeperch and their prey fish

Abstract –  Predator–prey interactions in aquatic food webs depend on the sizes of both predator and prey. In the present study, size-dependent interactions between >70 mm total length pikeperch Sander lucioperca (L.) and four different prey species in the biomanipulated Bautzen Reservoir were investigated. Gape widths of 597 pikeperch were measured, and the stomach contents of 806 specimens were analysed. Additionally, total lengths (TL) and body depths of 1448 prey fish were determined. The highest prey length to predator length ratio (PPR) was 0.63. Total lengths of piscivorous pikeperch and total lengths of prey fish [pikeperch, ruffe Gymnocephalus cernuus (L.) and roach Rutilus rutilus (L.)] were positively and linearly related. This was not the case for prey perch ( Perca fluviatilis L.) as all size groups of pikeperch fed strongly on age-0 perch. This study coupled with results of previous studies suggests that predation by pikeperch can have a major impact on the population dynamics of especially perch.     

Growth of five landlocked Arctic charr stocks under hatchery conditions

Growth dynamics of five stocks of Arctic charr, were compared from the fry stage in June 1986 until December 1987. Each stock originated from a different lake in Sweden: Hornavan; Näckten; Torrön; Rensjön and Visjön. Hornavan charr reached the highest final mean weight (237 g), but they were not significantly heavier than Visjön charr (206 g). Growth of fish of the three remaining stocks was similar but less rapid. The superior growth performance of Hornavan charr was most pronounced during autumn. Coefficients of variation for individual size within stocks was between 30 and 54%, with Hornavan and Näckten charr displaying least variability. Charr from Lake Visjön had a significantly lower proportion of mature males (6%) than the other stocks (19–29%) at the end of the experiment. The results demonstrate that differences in growth potential exist between stocks of charr, and these differences may be related to differential seasonal rhythms in growth. Variations in early growth, size distributions and sexual maturation seemed to be subordinate factors affecting overall growth performance.     

Failure of predator conditioning: an experimental study of predator avoidance in brown trout (Salmo trutta)

Many studies have documented that hatchery‐reared salmonids generally have inferior survival after being stocked compared with wild conspecifics, hatchery and wild salmonids have been observed to differ in their antipredator responses. The response of brown trout (Salmo trutta) juveniles (0+) of differing backgrounds to a live predator was compared in two experiments. First, the antipredator behaviour of predator‐naïve hatchery‐reared brown trout and wild‐exposed brown trout were assessed in behavioural trials which lasted for eight days. Second, predator‐naïve and predator‐conditioned hatchery‐reared brown trout were assessed in identical behavioural trials. Brown trout were ‘predator‐conditioned’ by being held in a stream‐water aquarium with adult Atlantic salmon (Salmo salar) and adult brown trout for two days prior to behavioural trials. Predator‐conditioned hatchery‐reared brown trout spent more time in shelters in the trial aquaria than predator‐naïve hatchery‐reared fish, but did not differ in time spent in the predator‐free area. Predator conditioning may account for the increased time spent in the shelter, but does not appear to have affected time spent in the predator‐free area. However, even if significant alteration in behaviour can be noted in the laboratory, the response might not be appropriate in the wild.     

The importance of egg size and egg energy density for early size patterns and performance of Arctic charr Salvelinus alpinus

The primary target of selection in most breeding programmes in aquaculture is to promote rapid growth while delaying maturation. In most cases, little is known about how such selection programmes may affect maternal provisioning of offspring and early development. Under favourable and predictable condition, which is typically the case in fish farming, females generally produce more but smaller eggs than wild females. By using a continuous range of egg size originating from females of a fourth generation of Icelandic domesticated Arctic charr Salvelinus alpinus (Linnaeus, 1758), we assessed the relative importance of maternal effects and individual egg size for energy density of individual egg and several offspring traits from fertilization until 138 days post fertilization (dpf). We found that egg size and related energy density varied more among than within females. Although egg size was not correlated with energy density (at 1 dpf) or survival (at hatching and 138 dpf), it was positively correlated with most offspring traits throughout ontogeny. We further did not find any temporal variation in the correlation with the offspring traits during ontogeny. Within clutch variation in growth, yolk sac volume and hatching time were under the influence of direct female effects (maternal or genetic effects). These results show how egg size relates to individual performance in Arctic charr and constitute a baseline for understanding how domestication may alter offspring phenotypes early in life. These findings are discussed with regard to their consequences for aquaculture of Arctic charr and salmonids in general.