The feeding behaviour of juvenile Atlantic salmon in relation to turbulent flow

The feeding behaviour of juvenile Atlantic salmon Salmo salar in the Sainte‐Marguerite River, Quebec, Canada, varied with the characteristics of turbulent flow. Simulations indicated that juveniles would decrease their swimming costs during attacks by 19·8% in low and by 31·1% in high turbulent conditions by initiating movements in low‐speed flow events. The real swimming costs did not differ from the swimming costs estimated for a situation where fish initiate their movements at randomly selected flow velocities. The juvenile Atlantic salmon did not seem to prefer low‐speed flow events when initiating their movements. The proportion of time used for movements by fish decreased with an increase in the mean and the s . d . of the flow velocity.     

The influence of food distribution upon the development of aggressive and competitive behaviour in juvenile chum salmon, Oncorhynchus keta

Groups of juvenile chum salmon were reared on food that was either dispersed throughout, or localized in one area of, the rearing tank. Groups receiving localized food displayed more aggression than those receiving dispersed food. This led to differences in growth, with fish reared on localized food having greater individual growth variability, i.e. growth depensation. However, after several months of rearing in these different feeding/social environments, fish reared on dispersed food were just as aggressive when first exposed to localized food as were fish reared on localized food. Furthermore, in competitive contests between fish of the two rearing histories, those reared on dispersed food were just as likely to become dominant as those reared on localized food. These results suggest that the behavioural development of aggressiveness is not amenable to alteration by manipulation of food distribution.     

The stability of standard metabolic rate during a period of food deprivation in juvenile Atlantic salmon

Among juvenile Atlantic salmon Salmo salar either being fed ad libitum throughout a 3 month experiment or deprived of food in the middle month, food deprivation led to a decrease in SMR, which increased again once food was supplied ad libitum again. While the rank order of SMR among fish fed throughout remained relatively stable, that within the deprived group was inconsistent, suggesting that individual fish vary in their ability to reduce metabolic costs when food availability is low.     

Diel feeding rhythms and daily food consumption of juvenile Atlantic salmon in the River Alta, northern Norway

Atlantic salmon fry (0+) sampled from the River Alta exhibited only minor differences in stomach content weights and feeding rates throughout diel periods, but feeding rates were generally lowest at night. In contrast, salmon parr (1+ to 3+) had large diel fluctuations in stomach content weight, with the largest weights usually being recorded during the night and early morning. Accordingly, their feeding rates were highest at night. This nocturnal feeding pattern was consistent throughout all sampling occasions, and appeared to reflect a persistent feeding periodicity in the salmon parr. The daily food consumption rates of both fry and parr were highest during midsummer and decreased towards autumn.     

The effects of intraperitoneally implanted dummy acoustic transmitters on the behaviour and physiology of juvenile Atlantic salmon, Salmo salar L

The behavioural and physiological effects of surgical implantation of dummy miniature acoustic transmitters into the peritonealcavities ofjuvenile Atlantic salmon, Salmo salar L., were assessed. lntraperitoneal implantations had no significant effect on growth, feeding or swimming behaviour in either parr or smolts. Recovery from the surgical implantation was both rapid and total; infection was absent; and physiological processes such as smoltification and maturation of testes in precocious parr were unaffected. Expulsion of the transmitter through the body wall, not through the implantation wound, occurred in a number of fish but without adversely affecting the animals. The intraperitoneal implantation technique is discussed in relation to its use during biotelemetry studies.     

中华倒刺鲃和胭脂鱼游泳行为、应激和免疫能力对短期禁食的响应

近年来,长江流域鱼类资源急剧下降,而有关鱼类对环境变化的行为和生理响应对于评估环境变化对种群动态的影响具有重要意义,相关研究亟待开展。选取中华倒刺鲃(Spinibarbus sinensis)和胭脂鱼(Myxocyprinus asiaticus)幼鱼为实验对象,考察两种鱼类在1-2周禁食后的自发群体运动时间比、游泳速度、个体间距离、最大匀加速速度、溶菌酶含量、鱼体免疫球蛋白(IgM)水平、血清皮质醇水平和超氧化物歧化酶活性的响应。研究发现：(1)相比之下,胭脂鱼的自发游泳行为表现不太活跃,游泳能力和IgM水平更低,超氧化物歧化酶SOD水平更高;(2)胭脂鱼在正常摄食条件下比中华倒刺鲃生长更慢,但是在1-2周短期禁食条件下其体重下降更少;(3)1-2周的短期禁食结束后,两种实验鱼的SOD活性和游泳能力变化不明显,但是其IgM水平和皮质醇都提高,胭脂鱼表现不及中华倒刺鲃;(4)胭脂鱼的溶菌酶含量在1-2周短期禁食后明显提高,但是中华倒刺鲃的溶菌酶含量没有受到1-2周短期禁食的显著影响。研究表明：(1)相比于中华倒刺鲃,胭脂鱼不太活跃,免疫和运动能力也更低,当食物充足时生长优势不明显,但在食物短缺时,营养物质和能量损失更少;(2)两种实验鱼的生理机能(比如游泳行为和免疫功能)对短期禁食有不同的响应,胭脂鱼更加不敏感(溶菌酶除外)。总体而言,研究表明同样的食物资源波动可能导致同一水域不同鱼类不同的生态后果,更多鱼类的相关研究亟待开展。     

The effects of surgically-implanted dummy radio transmitters on the behaviour of wild Atlantic salmon smolts

The social behaviour of Atlantic salmon smolts was evaluated during their migration period under controlled conditions in an experimental stream tank. Agonistic behaviour, dominance, distance to nearest neighbour, and distance from substrate were examined pre- and post-surgical implantation of dummy radio transmitters (2.4–4.3% body weight). Smolts were able to quickly equilibrate after transmitter insertion. Social ranking changed in nine of the eleven trials with four fish, and in only one of the five trials with pairs. No significant differences were found (p>0.05) in any of the behavioural parameters measured. Overall, the only effect on smolts surgically implanted with radio transmitters was a large shift in dominance. The presence of an antenna also elicited aggressive attacks from other individuals.     

Size variability of juvenile Atlantic salmon: links to environmental conditions

Climate change models predict a 2 to 6° C increase in air temperature within the next 100 years in the Maritime Provinces of eastern Canada. Higher air temperatures are expected to contribute to increased water temperatures, alterations in stream flow conditions, and ultimately reductions in fish growth. Mean annual size-at-age of juvenile Atlantic salmon Salmo salar decreased in the Northwest Miramichi and Southwest Miramichi Rivers between 1971–1999. Lengths-at-age of juveniles were significantly correlated between the two rivers. For Atlantic salmon parr, stronger associations between inter-cohort fork length ( L _F) than intra-cohort L _F were observed, suggesting that environmental conditions in the current year of growth have the more significant effects on size of age 2 year parr than conditions encountered the previous year by age 1 year parr of the same cohort. Fork lengths of parr were significantly and negatively associated with spring air and water temperatures. In the Miramichi River, increases in air and water temperature as predicted from climate change models may adversely affect growth of juvenile Atlantic salmon parr, reducing the overall productivity of the Atlantic salmon populations in this region.     

Shelter availability affects behaviour, size-dependent and mean growth of juvenile Atlantic salmon

1. Anthropogenic disturbances of the physical habitat and corresponding effects on fish performance are key issues in stream conservation and restoration. Reduced habitat complexity because of increased sediment loadings and canalization is of particular importance, but it is not clear to what extent fish populations are influenced directly by changes in the physical environment, or indirectly through changes in the biotic environment affecting the food availability. 2. Here, we test for the direct effect of habitat complexity on the performance (growth) of juvenile Atlantic salmon by manipulating shelter availability (interstitial spaces in the substrate) across 20 semi‐natural stream channels without altering the substrate composition, and stocking each channel with a common density of fish. A simple method for measuring salmonid shelters using flexible PVC tubes was developed and tested. Daytime sheltering behaviour and growth rates were compared across the channels differing in shelter availability. 3. Measured shelter availability was strongly negatively correlated with observed number of fish not finding shelters and mass loss rates of the fish (growth performance) increased with decreasing number of measured shelters. Number and mean depth of interstitial spaces explained up to 68% and 24% of the among‐channel variation in sheltering behaviour and growth performance, respectively. Furthermore, negative effects of shelter reduction increased with fish body size. Thus, changes in habitat structure may even influence the size selection gradients. 4. Shelter availability is an easily measured variable, possibly affecting the population demographics and long‐term evolutionary processes, and is therefore a key habitat factor to be considered in stream restoration and habitat classification.     

Migratory and reproductive behaviour of female adult Atlantic salmon, Salmo salar L., in a spawning stream

Migration and spawning behaviour of eight Atlantic salmon adult females were analysed by radio-tracking in relation to the degree of sexual maturity in a spawning tributary of the R. Sélune. Six of them were grilse and four of them were ripe. All the fish migrated upstream until reaching their spawning site at a distance of 4–12 km from the trap. The daily migration rate up to this site was inversely correlated with the length of the female. Spawning occurred in all fish at the same time when the water temperature increased dramatically. Spawning lasted 1–10 days according to the fish. After spawning, females quickly moved downstream only small distances and then stayed in approximately the same location until death. This study concluded that ripeness did not influence behaviour on the spawning migration and describes certain characteristics of the reproductive phase in a spawning tributary.     

Physiological stress responses to confinement in diploid and triploid Atlantic salmon

Confinement of juvenile Atlantic salmon Salmo salar from four different populations (all–female diploids, all–female triploids, mixed sex diploids and mixed sex triploids) either before (FW parr) or after (SW smolts) transfer to sea elevated plasma cortisol and plasma lactate from control levels irrespective of ploidy status. Both before and after confinement, plasma cortisol levels in SW smolts (6–174 ng ml^–1) were higher than those in FW parr (4–58 ng ml^–1), which possibly reflected the physiological challenge of acclimation to SW. Mixed sex populations of SW smolts had higher cortisol levels than all–female populations. The duration of confinement (1, 3 or 6 h) affected the magnitude of the plasma cortisol and lactate responses in SW smolts. Plasma cortisol levels in diploid and triploid SW smolts subjected to 2 h of confinement decreased to pre–stress levels within 6 h post–confinement. Plasma lactate levels were not significantly different from pre–stress levels 48 h after confinement. As no difference exists in primary and secondary stress responses of Atlantic salmon of differing ploidy status, it is unlikely that differences in mortality rates between diploid and triploid populations under commercial conditions can be attributed to differences in their physiological responses to periods of stress lasting up to 6 h.     

Survival and behaviour of post‐smolt Atlantic salmon in coastal habitat with extreme tides

The early marine migration of 55 Atlantic salmon post‐smolts tagged with acoustic transmitters was automatically monitored using 13 to 25 km long arrays of receivers deployed inside the Bay of Fundy, a coastal system on the east coast of Canada. The survival of post‐smolts from groups with short‐ and long‐term transmitters in coastal habitat up to 10 km from the river was 92 to 100%, indicating a successful transition to salt water and departure. Migration for 68 to 77% of post‐smolts followed a direct route and it was rapid (transit time usually <12 h). Post‐smolts initially migrated in a south to south‐west direction ( i.e . orientation towards the mouth of the bay) and they were aggregated near the coast. Post‐smolts with long‐term transmitters were monitored 20 km from the river where they continued to be aggregated, moving near the coast through a 'common corridor', and their survival to that point was at least 84%. Post‐smolts from both groups travelled heading out of the coastal system during ebb tides. Flood tides interrupted migration, and they caused changes in travel direction and delays in departure for post‐smolts not leaving by a direct route. Monitoring of coastal habitat inside the Bay of Fundy intercepted 62% of migrating post‐smolts with long‐term transmitters returning after an initial absence of 2 to 22 days. Returning post‐smolts displayed a resident behaviour, using the habitat monitored inside the Bay of Fundy during July and August.     

Chondroid bone and secondary cartilage contribute to apical dentary growth in juvenile Atlantic salmon

The microstructure and tissue composition of the dentary bone in Atlantic salmon salmo salar parr were examined using a variety of histological and whole‐mount techniques. Proximally, the dentary is composed of typical cellular lamellar bone with Sharpey's fibres extending dorsally, proximally and ventrally. Meckel's cartilage is located medially through the entire length of the dentary, and degrades distally resulting in a short transitional zone between hyaline cartilage and connective tissue. At the distal tip of the dentary, isogenic clusters of chondrocytes of periosteal origin were observed secreting small amounts of pericellular cartilage matrix within the bone matrix. These characteristics are highly indicative of secondary chondrogenesis, and suggest that the apical part of the dentary bone in Atlantic salmon does not grow via 'pure' intramembranous ossification, but rather via a modified mode of periosteal ossification involving secondary cartilage and chondroid bone. Furthermore, the unusual mode of gender‐related dentary growth (kype formation) in adult male Atlantic salmon could be the continuation of a general mode of salmonid apical dentary growth.     

Adaptive winter survival strategies: defended energy levels in juvenile Atlantic salmon along a latitudinal gradient

Current knowledge suggests that patterns of energy storage and depletion in animals are governed by behavioural trade-offs between risks associated with feeding and future energy demands. However, the length of adverse periods varies over geographical or climatic gradients. To explore the potential for genotypic sources of variation in behavioural trade-offs, we compared the winter energy-depletion patterns among 13 wild populations of juvenile Atlantic salmon (Salmo salar L.) along a latitudinal gradient (58–70°N) and performed common-environment experiments of energy-state-dependent feeding. In the wild, winter lipid-depletion rates were lower for northern than for southern populations. The variation in spring lipid levels among the population was lower than autumn variation, with storage lipid levels clustered close to critical limits for survival. In semi-natural stream channels with natural food supply, hatchery-reared fish originating from northern populations showed a positive scaling of feeding activity with decreasing energy levels, whereas southern populations did not. In conclusion, juvenile Atlantic salmon from northern populations defend their energy levels more strongly than fish from southern populations. Adaptive variation in feeding activity appears important for this difference. Thus, the present study shows a link between geographical patterns in storage energy trajectories and adaptive differences in state-dependent feeding motivation.     

Temporal and spatial variation in growth of juvenile Atlantic salmon

Spatial and temporal variation in length‐at‐age and environmental factors affecting variation in growth rate of juvenile Atlantic salmon Salmo salar were studied using data from a long‐term study in the River Stjørdalselva, central Norway. Mean annual instantaneous growth rate among 1+ and 2+year juvenile Atlantic salmon varied between 0·59 and 1·50 g g⁻¹ year⁻¹ and mean instantaneous daily growth rate of young‐of‐the‐year (YOY) varied between 0·013 and 0·033 g g⁻¹ day⁻¹. Between year variation in growth was larger than the within year intra‐watercourse spatial variation. For YOY and 1+year Atlantic salmon, a major part of the observed between year variation in growth rates was explained by variation in mean daily water discharge and spring temperature. For 2+year juvenile Atlantic salmon, mean daily water discharge and cohort density were the only variables to significantly explain variation in growth rates. A large part of the within water‐course spatial variation could not be explained by temperature variations and juvenile Atlantic salmon in the uppermost areas of the river, experiencing the lowest ambient temperatures during the growth period, displayed the highest growth rates. Within the baselines set by temperature, biotic and abiotic factors connected to water flow regime and variation in food availability are suggested to be a major determinants of the temporal and spatial variation in juvenile Atlantic salmon growth rates.