Response Of Juvenile Atlantic Salmon Research Articles

Movement and habitat shift responses of juvenile Atlantic Salmon (Salmo salar) to annually permanent stream flooding.

Reservoir formation in a river system changes a lotic environment to more lacustrine conditions, with impacts throughout the ecosystem. In this study, a river reach containing typical salmonid riffle/run habitat was flooded to create a large, deep pool from June to September in each of 3 years. We test the hypothesis that juvenile Atlantic salmon (Salmo salar) with their preference for run/riffle habitats will respond to the transformation to a lentic environment by moving into adjacent lotic environments. Movements of juvenile Atlantic salmon were monitored using a combination of biotelemetry (radio- and passive integrated transponder-tagging) and electrofishing. Results showed that no tracked fish moved away from the created pool habitat. Mass-specific growth rates showed the created pool habitat resulted in net growth of juveniles. The results confirm that fish may not immediately (i.e., at least for an approximate 2 months) respond to rapid, large-scale habitat alterations by moving to find similar habitat conditions outside the altered habitat. This is most probably related to plasticity of behavior and habitat use, and no change in biological conditions to a point that would negatively impact fish growth and survival, for example food availability, competition, or predation. The results also support the hypothesis that the relative importance of physical habitat variables is not universal among streams and populations, therefore limiting the value of applying standard habitat suitability criteria and use.

Physiological and growth responses of juvenile Atlantic salmon (Salmo salar) transferred to seawater during different stages of smolt development

Developing a better understanding of the importance of transfer time/smolt window is critical for farmers of salmonids to maximise growth performance and minimise the poor growth/mortality that can be associated with SW transfer. This study examined physiological and growth responses of juvenile Atlantic salmon (Salmo salar) transferred to seawater (SW) after different lengths of exposure to a long-day photoperiod (LD 24:0). Four treatment groups were acclimated in experimental FW tanks for 8 weeks on a short-day photoperiod (LD 8:16), and then transferred to SW after exposure to LD 24:0 for a further 2 weeks (ETA – 196 degree days), 4 weeks (ETB – 392 degree days), 6 weeks (ETC – 588 degree days) and 8 weeks (ETD – 784 degree days). Fish from the ETA and ETB treatment groups exhibited reduced growth and condition factor after transfer to SW compared to ETC and ETD groups. Condition factor was significantly lower in fish from the ETA group after SW transfer compared to in FW, and to ETB, ETC and ETD treatment groups. ETA treatment had significantly higher pinhead numbers (CF < 0.8) compared to ETC in SW. Mortality was significantly related to treatment group, with higher mortality in ETA treatment (13%), followed by ETB (5.2%); ETC (1.4%) and ETD (0%). Pinheads were significantly varied in physiological profile to non-pinhead population and showed minimal growth over the 16-week experiment. Overall, pinheads were significantly lower in mass, fork length, condition factor and cholesterol levels than non-pinheads. Gill Na+/K+-ATPase activity was not significantly different between pinheads and non-pinheads in any treatment group. Osmolality was highly variable in ETA pinheads, but mean levels were not significantly different between pinheads and non-pinheads. This study demonstrated that at 14 °C, six weeks of long-day photoperiod is necessary for juvenile Atlantic salmon (~120 g) to minimise negative effects on growth, mortality and the occurrence of pinheading in the transferred population.

The effects of increasing dietary levels of amino acid-supplemented soy protein concentrate and constant dietary supplementation of phosphorus on growth, composition and immune responses of juvenile Atlantic salmon (Salmo salar L.).

Diets with 50 (SPC50), 65 (SPC65) and 80% (SPC80) substitution of prime fish meal (FM) with soy protein concentrate (SPC) were evaluated against a commercial type control feed with 35% FM replacement with SPC. Increases in dietary SPC were combined with appropriate increases in methionine, lysine and threonine supplementation, whereas added phosphorus was constant among treatments. Diets were administered to quadruplicate groups of 29g juvenile Atlantic salmon were exposed to constant light, for 97days. On Day 63 salmon were subjected to vaccination. Significant weight reductions in SPC65 and SPC80 compared with SPC35 salmon were observed by Day 97. Linear reductions in body cross-sectional ash, Ca/P ratios, and Ca, P, Mn and Zn were observed at Days 63 (prior vaccination) and 97 (34days post-vaccination), while Mg presented a decrease at Day 63, in salmon fed increasing dietary SPC. Significant reductions in Zn, Ca, P and Ca/P ratios persisted in SPC65 and SPC80 compared with SPC35 salmon at Day 97. Significant haematocrit reductions in SPC50, SPC65 and SPC80 salmon were observed at Days 63, 70 and 97. Enhanced plasma haemolytic activity, increased total IgM, and a rise in thrombocytes were demonstrated in SPC50 and SPC65 salmon on Day 97, while increased lysozyme activity was demonstrated for these groups on Days 63, 70 and 97. Leucocyte and lymphocyte counts revealed enhanced immunostimulation in salmon fed with increasing dietary SPC at Day 97. High SPC inclusion diets did not compromise the immune responses of salmon, while SPC50 diet also supported good growth without compromising elemental concentrations.

Effects of diel temperature fluctuation on the standard metabolic rate of juvenile Atlantic salmon (Salmo salar): influence of acclimation temperature and provenience

We assessed the metabolic response of juvenile Atlantic salmon (Salmo salar; JAS) originating from two rivers with different natural thermal regimes to different acclimation temperature (15 or 20 °C) and diel temperature fluctuation (constant: ±0.5 °C; fluctuating: ±2.5 °C). Diel temperature fluctuation (15 ± 2.5 °C) near the thermal optimum (16 °C) for the species did not influence standard metabolic rate (SMR) compared with JAS acclimated to a constant temperature of 15 °C. Diel temperature fluctuation at 20 ± 2.5 °C increased SMR of JAS from the warmer river by 33.7% compared with the same fish acclimated to a constant temperature of 20 °C. SMR of JAS from the cooler river held at fluctuating conditions had SMR that were 8% lower than SMR at constant conditions. The results suggest that the mean temperature to which JAS is exposed may affect their responses to diel temperature fluctuation and that this response may vary between populations originating from rivers with different natural thermal regimes. Results were used to develop the first empirical SMR model for JAS subjected to diel temperature fluctuation using fish mass (3–36 g wet) and temperature (12.5–22.5 °C) as explanatory variables.

UVB Radiation Variably Affects n‐3 Fatty Acids but Elevated Temperature Reduces n‐3 Fatty Acids in Juvenile Atlantic Salmon (Salmo salar)

Temperature and ultraviolet B radiation (UVB 290-320 nm) are inextricably linked to global climate change. These two variables may act separately, additively, or synergistically on specific aspects of fish biochemistry. We raised Atlantic Salmon (Salmo salar) parr for 54 days in outdoor tanks held at 12 and 19 °C and, at each temperature, we exposed them to three spectral treatments differing in UV radiation intensity. We quantified individual fatty acid (FA) mass fractions in four tissues (dorsal muscle, dorsal and ventral skin, and ocular tissue) at each temperature × UV combination. FA composition of dorsal muscle and dorsal and ventral skin was not affected by UV exposure. Mass fractions of 16:0, 18:0, and saturated fatty acids (SFA) were greater in dorsal muscle of warm-reared fish whereas 18:3n-3, 20:2, 20:4n-6, 22:5n-3, 22:6n-3, n-3, n-6, polyunsaturated fatty acids (PUFA), and total FA were significantly higher in cold-reared fish. Mass fractions of most of the FA were greater in the dorsal and ventral skin of warm-reared fish. Cold-reared salmon exposed to enhanced UVB had higher ocular tissue mass fractions of 20:2, 20:4n-6, 22:6n-3, n-3, n-6, and PUFA compared to fish in which UV had been removed. These observations forecast a host of ensuing physiological and ecological responses of juvenile Atlantic Salmon to increasing temperatures and UVB levels in native streams and rivers where they mature before smolting and returning to the sea.

The influence of water depths and inter‐specific interactions on cover responses of juvenile Atlantic salmon

Abstract – The responses of salmon parr, Salmo salar, to instream cover, related to several water depths, were tested in an ellipsoidal stream tank. Opaque plastic covers, most of which were 20 cm in length and 16 cm high, were randomly distributed through the tank, occupying about 3% of the bottom area. Six salmon parr were used for each experiment. In experiments with channel depths of 40 cm, the proportion of salmon under covers was 36.4%, at 30 cm 60.7%, and at 20 cm, 79.4%. Presence of similar‐sized brown trout, Salmo trutta, increased the use of covers in channels and greater use of the pool area by salmon. The salmon were completely dominated by the trout, making about four times more more aggressive than salmon. Our findings show that water depth itself provides cover for young salmon.

Response of juvenile Atlantic salmon, Salmo salar, to the introduction of salmon carcasses in upland streams

Abstract The role of nutrients in the production of Atlantic salmon, Salmo salar L., in oligotrophic upland catchments is poorly understood. Atlantic salmon carcasses were introduced to 12 upland streams in the River Conon catchment, Scotland, UK, to assess potential nutrient limitation and evaluate a potential role for migratory Atlantic salmon in nutrient import. The objective was to test the hypothesis that nutrient supplementation would increase juvenile Atlantic salmon biomass. Streams received one of six treatment levels up to a maximum of one carcass per 15 m2 of wetted area with duplicates at each level, and study areas were sampled to measure salmon biomass in treated and reference reaches. There was a significant positive relationship between treatment addition level and response of juvenile salmon biomass. At higher carcass addition levels, salmon biomass in treatment sections was approximately twice that in paired reference sections. The results are discussed in the light of management methods in upland regions and the likely role of cultural oligotrophication in suppressing current levels of salmon production.

1H-NMR and mass spectrometric characterization of the metabolic response of juvenile Atlantic salmon (Salmo salar) to long-term handling stress

Stressors of various kinds constantly affect fish both in the wild and in culture, examples being acute water temperature and quality changes, predation, handling, and confinement. Known physiological responses of fish to stress such as increases in plasma cortisol and glucose levels, are considered to be adaptive, allowing the animal to cope in the short term. Prolonged exposure to stressors however, has the potential to affect growth, immune function, and survival. Nonetheless, little is known about the mechanisms underlying the long-term stress response. We have investigated the metabolic response of juvenile Atlantic salmon (Salmo salar) to long-term handling stress by analyzing fish plasma via 1H nuclear magnetic resonance spectroscopy and ultra high performance liquid chromatography–mass spectrometry (UPLC–MS), and comparing results with controls. Analysis of NMR data indicated a difference in the metabolic profiles of control and stressed fish after 1 week of stress with a maximum difference observed after 2 weeks. These differences were associated with stress-induced increases in phosphatidyl choline, lactate, carbohydrates, alanine, valine and trimethylamine-N-oxide, and decreases in low density lipoprotein, very low density lipoprotein, and lipid. UPLC-MS data showed differences at week 2, associated with another set of compounds, tentatively identified on the basis of their mass/charge. Overall the results provided a multi-faceted view of the response of fish to long-term handling stress, indicating that the metabolic disparity between the control and stress groups increased to week 2, but declined by weeks 3 and 4, and revealed several new molecular indicators of long-term stress.

A synopsis of ‘hydropeaking’ studies on the response of juvenile Atlantic salmon to experimental flow alteration

In insular Newfoundland, Canada, studies were conducted from 1999 to 2003 on the effects of ‘simulated’ hydropeaking power generation on juvenile Atlantic salmon (Salmo salar). In 1999, Atlantic salmon parr were released into an experimental reach below a hydroelectric facility and flow was manipulated over a range of discharge (1.0–4.2 m3 s−1) during a series of ‘experiments’ simulating hydropeaking in both summer and fall. Fish were implanted with radio transmitters, manually tracked, precisely located (±1 m), habitat selection evaluated, and movement response determined. Experiments were continued in 2002 and 2003 to contrast response of salmon between summer and winter, the magnitude of flow changes were greater (0.7–5.2 m3 s−1) and changes were made more rapidly (instantly). As discharge was increased, velocity and depth use by parr increased, and fish adapted behaviourally by increased contact with the substrate. Salmon parr also exhibited two distinct movement patterns in the summer and fall of 1999 studies; high site fidelity or considerable movement during trials. Salmon were more mobile during both static and dynamic flow conditions and throughout the diel cycle in the summer of 2002 experiment, and 2 fish were stranded and died, the only time this happened in the four series of experiments. Within each experiment generally there were no differences between movements at static high and low flows for day and night movements, with one exception, and night time movements were always greatest, again with one exception. During dynamic flow changes, within each experiment, distances moved during down ramping and up ramping were not significantly different except in the summer of 1999. Overall, comparing between experiments for up and down ramping events, distances moved in the summer of 2002 were statistically higher than for all other experiments. Not surprisingly, the home ranges of fish in the summer of 2002 were also the greatest while the smallest home ranges were in the winter of 2003. Results suggest hydropeaking regimes may be energetically costly potentially affecting over-winter survival which is related to energy reserves obtained during summer. Collectively these studies provide comprehensive information on the response of juvenile Atlantic salmon parr to hydropeaking, on both diel and seasonal scales, and will assist hydro producers and regulators design and operate hydropeaking regimes to minimize ecological impact.

Context-dependent responses of juvenile Atlantic salmon (Salmo salar) to forestry activities at multiple spatial scales within a river basin

We used classification trees and regression trees to relate the incidence and density of juvenile Atlantic salmon (Salmo salar) to forestry activities measured at four spatial scales (subbasin and 8, 2, and 0.5 km radii upstream of study sites) and environmental features in 120 stream reaches of the Cascapedia River basin, Québec, Canada. At all scales, incidence increased with reach size and accessibility to the reach from the river mainstem. Incidence declined with areal coverage of logging at all scales, but only in larger reaches. The time horizon over which logging effects were detected increased with spatial scale. At all scales, density in salmon-bearing reaches increased with accessibility. Density in more accessible reaches was negatively related to logging over the preceding 9 years at the subbasin and 8 km scales, but no effects of logging on density were detected at the 2 and 0.5 km scales. Overall, apparent effects of logging activities on salmon incidence and density were mostly negative and strong, but were both markedly scale-dependent and conditional on environmental context.

Seasonal Effects of Suspended Sediment on the Behavior of Juvenile Atlantic Salmon

AbstractShort‐term increases in suspended sediment levels significantly influenced the behavior of juvenile Atlantic salmon in both fall and winter trials. The initial introduction of sediment (20 mg/L, or ≈15 nephelometric turbidity units [NTU]) increased foraging activity, which subsequently declined at sediment levels greater than 180 mg/L (≈35 NTU). Cover use also declined as fish emerged to forage. No fish returned to cover at sediment levels greater than 60 mg/L (≈22 NTU) in the fall trials, whereas some fish in the winter trials never emerged from cover. A rapid decline in territorial behavior and an avoidance response (i.e., alarm reaction) at sediment levels ranging from 60 to 180 mg/L (≈22‐42 NTU) were also noted in fall trials but not winter trials. Seasonal differences in the response of juvenile Atlantic salmon to suspended sediments may be explained by temperature‐related changes in diel activity patterns.

Seasonal Response of Juvenile Atlantic Salmon to Experimental Hydropeaking Power Generation in Newfoundland, Canada

AbstractVariable hydropower production leads to hydropeaking, which causes discharge fluctuations that are potentially harmful to aquatic organisms. In this study, an experimental approach was used to investigate hydropeaking effects and associated hydraulic and habitat conditions on the home range and movement of juvenile Atlantic salmon Salmo salar. Prior studies examined the responses of Atlantic salmon and brook trout Salvelinus fontinalis to experimental hydropeaking during summer and autumn. The present study focused on Atlantic salmon, involved more rapid and extreme discharge manipulation, and included winter experiments to reflect influences of reduced temperature, ice conditions, and seasonal differences in behavior and habitat selection. Experiments were conducted over a range in discharge (0.5–5.0 m3/s) that resulted in dramatic habitat changes in the wide, shallow, boulder‐strewn study reach. Experiments were repeated in summer and winter; however, the winter range in discharge was narrower due to constraints on water release. Fish response was monitored using manual telemetry in both seasons, and fixed telemetry was used to monitor fine‐scale diel winter movements. Atlantic salmon had larger home ranges and were more mobile during all flow conditions and over diel cycles in summer than in winter, and there was anecdotal evidence of stranding in isolated pools in summer. Stream morphology, in addition to the magnitude of discharge change, was an important determinant of the propensity to move. In our study, there were considerable refugia from increased velocity and dewatering, which may have reduced the need to move. In winter, fish remained relatively sedentary in comparison with the summer foraging period, and this behavior may increase the likelihood for dewatering, stranding, and freezing. A secondary concern with hydropeaking regimes is the energetic cost to fish of moving to find suitable habitats, and during summer this cost could affect stored energy reserves, which could, in turn, affect overwinter survival.

Survival and bioenergetic responses of juvenile Atlantic salmon along a perturbation gradient in a natural stream

Abstract – We tested the effects of a pollution gradient on the survival and energetic responses of Atlantic salmon fry stocked in Onondaga Creek, a historic but highly polluted nursery stream. A relatively unperturbed reference stream provided similar physical and thermal gradients that co‐varied longitudinally with the pollution gradient in Onondaga Creek, and macroinvertebrate communities provided indices of water quality degradation. Survival, cohort biomass, instantaneous growth rate and growth index of Atlantic salmon were inverse functions of water quality in the perturbed stream. Generally, survival and energetic parameters of Atlantic salmon responded differently to longitudinal gradients in the two streams, indicating that water quality, rather than habitat variation, exerted the strongest control over salmon. Effects were more pronounced during the early summer, possibly because tolerance to such perturbations increases with age or body size. Energetics‐related parameters were more sensitive to water quality than were survival‐related parameters, indicating that sub‐lethal effects of pollution may be detected and quantified by examining energy balances of individuals.

Behavioural responses of juvenile Atlantic salmon (Salmo salar) to presence of boulders

The influence of boulder presence on the behaviour of juvenile Atlantic salmon (Salmo salar) was investigated experimentally in an indoor flume fed by a 161s(-1) flow of natural river water. The flume was divided into 16 arenas (each 1 m2) that were landscaped with river gravel and standardized boulders, to represent relative 'complex' or 'simple' habitats. Each arena housed three wild-caught fish. In a three-week trial, the effects of landscape on aspects of individual behaviour were recorded. Food intake was highest in the simple landscape and directly related to social status and time spent in the water column. The fish in the complex chambers actively maintained station in the water column significantly more than fish in simple landscapes and therefore partially compensated for reduced foraging rates associated with complexity. Fish in simple chambers spent more time orientated upstream than those in complex landscapes. There was no evidence that habitat complexity influenced levels of aggression, average aggressive distance, 'constrained' territory size, or dominance. Activity and space use varied with social status. Overall, this study illustrates that addition of boulders can result in costs to Atlantic salmon parr, which can be expected to offset to some extent benefits brought about by increased stream complexity. Work is now needed to evaluate the balance of costs and benefits at different life stages and under various environmental conditions.

The behavioural response of juvenile Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) to experimental hydropeaking on a Newfoundland (Canada) river

AbstractDeregulation of the electric power market globally will lead to increased requirement for electricity on demand resulting in more emphasis on ‘hydropeaking’ generation. A research study was conducted on the regulated West Salmon River, Newfoundland, Canada, to examine habitat selection and movement of juvenile Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) in response to flow changes related to ‘experimental’ peaking flow power generation. Fish were surgically implanted with microscale radio transmitters, released into an experimental study, and discharge was experimentally manipulated simulating two scenarios: (i) water storage during the day and generation at night, with a 2 hour transition; and (ii) night‐time storage with generation during the day. Experiments were repeated in the summer and fall. Fish were tracked throughout the diurnal cycle of each manipulation and precisely positioned in two‐dimensional space. Atlantic salmon exhibited two distinct patterns to movement: fish that showed high site fidelity and those that moved considerably during trials. Both salmon and trout were more active during fall hydropeaking experiments. Fish generally did not move long distances and moved more in a longitudinal fashion than laterally. Salmon moved greater distances, on average, than trout under all experimental conditions and during both seasons but these differences were not statistically significant. Brook trout moved more in relation to dynamic events (up‐ and down‐ramping) than at steady state flows. Trout also moved more at night during these dynamic changes and under low flow conditions. These results will assist producers of hydroelectricity to reduce the impacts of hydropeaking operations on fish and fish habitat. Copyright © 2003 John Wiley &amp; Sons, Ltd.

How juvenile Atlantic salmon,Salmo salarL., respond to falling water levels: experiments in an artificial stream

The responses of juvenile Atlantic salmon,Salmo salarL., to an experimentally imposed drop in water level were studied in an artificial stream. In a pilot experiment, 20% of fish which had a feeding station in an area of riffle remained there throughout the period of low water levels. The remaining riffle‐dwelling fish moved into areas of deeper water, although not necessarily to the pool nearest their feeding station. Out of the fish which left the area, 89% moved in an upstream direction and 11% moved downstream. In a second experiment, which was designed to look in more detail at this response to de‐watering, 95% of riffle‐dwelling fish left when the water level dropped and moved into a pool, mostly within the 4‐h period after water levels started to fall. Seventy‐four per cent of fish which left moved upstream and the remainder moved downstream. There was a strong tendency to leave in an upstream direction as shallow areas began to dry out, and this tendency persisted irrespective of variability in fish size, prior social experience and the size of the home range prior to de‐watering. It is suggested that the prior opportunity to explore alternative habitat may be an important determinant of response of riffle‐dwelling salmon to a sudden drop in water level.

Characterization of the heat shock protein response of Atlantic salmon (Salmo salar)

The heat shock protein (hsp) response of juvenile Atlantic salmon, Salmo salar was investigated in isolated tissues subjected to various degrees of thermal shock. Distinct but overlapping arrays of proteins from the major hsp families (hsps 100, 90, 70, 60 and small hsps) were induced in branchial lamellae, hepatic tissue and erythrocytes. The two most prominent proteins induced by heat shock (MW ≅ 65 and 66 kDa) were found to be antigenic homologues of mammalian hsps72/73. A 2.6 kb transcript upregulated by the same conditions hybridized with cDNA probes to both human and salmon hsp70. Branchial lamellae exhibited the greatest degree of thermotolerance and mounted the most significant heat shock response. Moderate thermal shock induced more species of proteins in branchial lamellae than in hepatic tissue or erythrocytes, with the rate of hsp65/66 synthesis increased by as much as five fold. Thermal shock induced hsp65/66 eight fold in erythrocytes. In contrast, hepatic tissue which was least tolerant of thermal shock, lacked the inducible hsp65 and exhibited minimal induction of hsp66. Persistence of hsps was tested in erythrocytes, where elevated levels remained in the cells for at least 48 h after heat shock. The temporal pattern and magnitude of the hsp response in the stenothermal Atlantic salmon differed from that previously reported for eurythermal species. Also notable was the limited hsp response mounted by salmon tissues exposed to sodium arsenite, a known inducer of hsps. The characteristics of the hsp response to thermal shock support the significance of these proteins in adaptation of Atlantic salmon to environmental insult.

Ontogenesis of rheotactic and optomotor responses of juvenile Atlantic salmon

Rheotactic and optomotor responses have been used to describe the ontogenetic relationship between young Atlantic salmon and water velocity. Rheotaxis changes dynamically as the fish develop from alevins to parr and to smolts. Sensitivity to water velocity, and the ability to maintain station in a flow, increase as the fish develop, allowing the fish to occupy different microhabitats as the temperatures rise within 13–24°C during the summer. As the temperature drops below 10°C in the winter, the intensity of these responses decreases, and the fish hide in the substratum. Among parr, the rheotactic response results in local migration seasonally, and site retention in flowing habitats. At smolting, as the parr become pelagic, their rheotactic and optomotor sensitivities and their station holding abilities decrease, and with these decreases the tendency to group increases. The change in the rheotactic response results in the pelagic behaviour at smolting, and the successful downstream migration.

Growth Compensation in Juvenile Atlantic Salmon: Responses to Depressed Temperature and Food Availability

This study examines behavioral and physiological responses of juvenile Atlantic salmon (Salmo salar) adopting alternative life history patterns following a period of reduced growth. We manipulated the growth rates of premigratory and nonmigratory salmon by either reducing food availability or maintaining water at low temperature (4–6°C). A third group of fish was kept at ambient temperatures (12–14°C) and fed ad libitum to provide a control. Fish in both experimental groups exhibited compensatory growth after the manipulation period, even though the manipulations had slowed growth rather than caused mass loss. The timing and duration of compensatory growth were affected by the nature of the constraint and the developmental pathway adopted. Compensatory responses were more persistent and stronger among premigratory fish than among nonmigratory. Fish kept at low temperature did not accelerate growth immediately after transfer to ambient temperatures, but they subsequently grew faster than controls for up to 215 d after the end of the manipulation period. This mitigated the effects of the period of low temperatures, although by the end of the experiment they were still smaller than the controls. Fish on reduced rations showed no such time lag, and they grew significantly faster than controls immediately upon regaining access to full rations. These fish attained the same body size as controls by the end of the experiment (day 215). The manipulations caused fish to reduce their growth in mass more than their rate of skeletal growth, but all fish achieved “normal” mass for their length (as compared to controls) within a week of transfer to full feeding or ambient temperature. The main mechanism underlying compensatory growth rates was apparently the increase of intake rates, although this was insufficient to explain the strong compensation shown by temperature-manipulated fish in the presence of larger (and thus competitively superior) individuals. Instead these fish enhanced their growth rate by apparently increasing the duration of the daily feeding period, and avoiding aggressive interactions. We interpret the observed compensation for periods of slowed growth as indicating that growth rate is normally submaximal and can be increased if the animal has fallen below its expected trajectory; thus premigratory fish may have shown a greater compensation because survival rates during migration are strongly size-dependent.

Effects of location of food delivery and social status on foraging-site selection by juvenile Atlantic salmon

Experiments were conducted to investigate the response of juvenile Atlantic salmon,Salmo salar, to changes in the location at which food entered a stream tank. Experience with the location of food input into the system significantly influenced foraging-site selection by the dominant fish. Dominant fish changed location of their foraging site in response to a change in the location of food input, and occupied, aggressively defending, sites just downstream of the location of food introduction. In contrast, subordinate fish occupied foraging sites at the downstream end of the stream tank, regardless of location of food input. As a result of both site selection and social status, dominant fish captured significantly more prey than subordinates. Our results support the contention that salmonids choose foraging sites to maximize foraging opportunities. Our results also provide a possible explanation for the use of ‘atypical’ foraging sites by individual fish within their home range over the course of a single day, as observed in a number of salmonid species in the field.