Effects of food availability on temporal activity patterns and growth of Atlantic salmon

1. Patterns of sheltering and activity are of fundamental importance in the ecology of animals and in determining interactions among predators and prey. Balancing decreased mortality risk when sheltering with increased feeding rate when exposed is believed to be a key determinant of diel patterns of sheltering in many animals. 2. Despite lower foraging efficiency at night than during the day, Atlantic salmon Salmo salar parr are nocturnal during winter and at low summer temperatures. Nocturnal activity also occurs at warm water temperatures during summer, but little is known about the functional significance of this behaviour. 3. This study aimed to determine: (1) the preferred activity and shelter pattern of Atlantic salmon parr during warm summer months, and (2) their response to variations in food availability when balancing growth rate (G) and mortality risk (M), as expressed through time out of shelter. We differentiated among four potential responses to reduced food availability: (1) no response; (2) G decreases but M remains constant; (3) G remains constant but M increases; and (4) G decreases and M increases. 4. Time exposed from shelter was inversely related to food availability. Fish subject to high food availability were significantly less active during the day than those with restricted rations. However, food availability had no significant effect on the extent to which fish were active at night. There was no evidence of variation in growth rate with food availability. 5. Salmon were predominantly nocturnal at high ration levels, consistent with their previously reported behaviour during winter. Rather than switching to diurnal behaviour at high temperatures per se, as previously was supposed, it appears that the fish are diurnal only to the extent needed to sustain a growth rate, and this extent depends on food availability. 6. Atlantic salmon parr modulate the amount of time they are active rather than growth when responding to variations in food availability over an order of magnitude.     

Metabolic recovery in Atlantic salmon fry and parr following forced activity

Atlantic salmon Salmo salar fry and parr were subjected to 5 min of forced activity and the subsequent changes in oxygen consumption and ammonia excretion rates were evaluated over a 24 h period. In a second experiment, individual Atlantic salmon fry and parr were freeze‐clamped in liquid nitrogen, before, immediately following a 5 min activity period, or after periods of recovery up to 2 h. Samples were analysed for whole body phosphocreatine (PCr), ATP and lactate. Five minutes of forced activity resulted in significant increases in both oxygen consumption and ammonia excretion rates. Changes in the oxygen consumption rates were greater in the parr compared with the fry. In contrast, the post‐exercise ammonia excretion rates were nearly twice as high for the fry compared with the parr. Exercise also caused a marked decrease in PCr levels (c. 47 and 65% in fry and parr, respectively), no change in ATP levels and a significant increase in lactate levels in Atlantic salmon fry and parr. Recovery of PCr occurred quickly (between 15 and 30 min) in fry and parr. Although the post‐activity levels of lactate were lower in fry (c. 3 μmol g^?1) compared with parr (c. 14 μmol g^?1), lactate levels returned to control levels within 60 min in fry, but it took >2 h for this metabolite to recover in parr. Compared with parr, these findings show that Atlantic salmon fry possess a reduced anaerobic capacity, and these results are consistent with the theoretical and experimental evidence that smaller fish support burst swimming through aerobic processes.     

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.     

Summer temperature variation and implications for juvenile Atlantic salmon

Temperature is important to fish in determining their geographic distribution. For cool- and cold-water fish, thermal regimes are especially critical at the southern end of a species’ range. Although temperature is an easy variable to measure, biological interpretation is difficult. Thus, how to determine what temperatures are meaningful to fish in the field is a challenge. Herein, we used the Connecticut River as a model system and Atlantic salmon (Salmo salar) as a model species with which to assess the effects of summer temperatures on the density of age 0 parr. Specifically, we asked: (1) What are the spatial and temporal temperature patterns in the Connecticut River during summer? (2) What metrics might detect effects of high temperatures? and (3) How is temperature variability related to density of Atlantic salmon during their first summer? Although the most southern site was the warmest, some northern sites were also warm, and some southern sites were moderately cool. This suggests localized, within basin variation in temperature. Daily and hourly means showed extreme values not apparent in the seasonal means. We observed significant relationships between age 0 parr density and days at potentially stressful, warm temperatures (≥23°C). Based on these results, we propose that useful field reference points need to incorporate the synergistic effect of other stressors that fish encounter in the field as well as the complexity associated with cycling temperatures and thermal refuges. Understanding the effects of temperature may aid conservation efforts for Atlantic salmon in the Connecticut River and other North Atlantic systems.     

17 alpha,20 beta-dihydroxy-4-pregnen-3-one 20-sulphate is a potent odorant in precocious male Atlantic salmon (Salmo salar L.) parr which have been pre-exposed to the urine of ovulated females.

Electrophysiological recordings from the olfactory epithelium have shown that 17 alpha,20 beta-dihydroxy-4-pregnen-3-one 20-sulphate (17,20 beta-P-sulphate; a conjugate of the oocyte-maturation-inducing steroid in teleosts) is a potent odorant in precocious male Atlantic salmon (Salmo salar L.) parr. However, the olfactory epithelium of these fish only appeared to be responsive to the steroid after stimulation with the urine of ovulated female Atlantic salmon. Immature fish did not respond at any time. Stimulation with urine from immature and precocious male Atlantic salmon parr did not make the olfactory epithelium of precocious male salmon parr responsive to the steroid. 17,20 beta-P-sulphate was found in the urines of ovulated females, precocious male parr and mature male Atlantic salmon. The findings are discussed in relation to the possible role of 17,20 beta-P-sulphate in the physiology of Atlantic salmon.     

Pathogenesis and immune response in Atlantic salmon (Salmo salar L.) parr experimentally infected with salmon pancreas disease virus (SPDV)

Atlantic salmon parr were injected intraperitoneally with salmon pancreas disease virus (SPDV) grown on CHSE-214 cells. The viraemia, the histopathological changes in target organs and some immune parameters were taken at intervals up to 30 days post-infection (dpi). The earliest kind of lesion was necrosis of exocrine pancreas, appearing as soon as 2 dpi. It progressed towards complete tissue breakdown at 9 dpi before resolving gradually. Concurrent to this necrosis, a strong inflammatory response was in evidence from 9 dpi in the pancreatic area for a majority of fish. A necrosis of the myocardial cells of the ventricle occurred in infected fish mainly at 16 dpi and it faded thereafter. The monitoring of the plasma viral load showed a rapid haematogenous spreading of SPDV, peaking at 4 dpi, but also the absence of a secondary viraemia. No interferon (IFN) was detected following the infection of parr with SPDV, probably owing to an IFN activity in Atlantic salmon below the detection level of the technique. Neutralising antibodies against SPDV were in evidence from 16 dpi and they showed a time-related increasing titre and prevalence. The phagocytic activity in head-kidney leucocytes was always significantly higher in the infected fish than in the control fish, being particularly high by 9 dpi. Lysozyme and complement levels were both increased and they peaked significantly in the infected fish at 9 and 16 dpi respectively. These results demonstrated that an experimental infection of Atlantic salmon parr with SPDV provoked a stimulation of both specific and non-specific immunity with regards to the viraemia and the histopathology.     

Differential expression of Atlantic salmon thyrotropin β subunit mRNA and its cDNA sequence

To study the regulation of the thyroid system, an Atlantic salmon Salmo salar cDNA clone was isolated for thyroid stimulating hormone (TSH) β subunit gene. A cDNA (866 bp) was isolated from an adult Atlantic salmon pituitary cDNA library, this clone was sequenced and shown to be highly conserved when compared to other teleost β TSH subunit sequences. The cDNA was used as a probe for Northern blot analysis of total pituitary RNA from the different life cycle stages of Atlantic salmon. Northern blot analysis demonstrated that β TSH mRNA is expressed at all life cycle stages studied, including parr, smolt, immature fish at sea and sexually mature male fish. Densitometry of Northern blots showed that sexually mature male salmon had low levels of salmon β TSH mRNA compared to non-mature fish. Stunts, fish performing poorly in salt water, were shown to have elevated levels of β TSH mRNA when compared to healthy fish.     

Replacement of fish oil with sunflower oil in feeds for Atlantic salmon (Salmo salar L.): effect on growth performance,tissue fatty acid composition and disease resistance

Dietary sunflower oil (SFO) was used to gradually replace fish oil (FO) in six diets (which also contained fish meal) for Atlantic salmon parr (initial mass: 21.7 g). The effect on growth performance, tissue fatty acid profiles and disease resistance was monitored after 63 days. At the conclusion of the trial, no significant differences were detected in growth between any of the feeds. Fatty acid composition of whole carcass, dorsal muscle and liver generally reflected that of the diets. Forty percent of the FO could be replaced by SFO before tissue 22:6n-3 was significantly reduced, although other essential and non-essential fatty acids were more susceptible to change. Significant differences were detected in cumulative mortality of Atlantic salmon challenged with Vibrio anguillarum at the trials conclusion, although this was not correlated to the inclusion level of SFO. Despite the changes observed to the tissue fatty acid profile, there was no significant effect on growth suggesting that SFO is a suitable alternative to FO in diets for Atlantic salmon parr when fish meal is also included.     

The effect of ultrasonic tags on the growth rates of Atlantic salmon, Salmo salar L., parr of varying size just prior to smolting

The attachment of dummy, ultrasonic, acoustic tags, of the Stirling Mk. VI design, to pre-smolt Atlantic salmon parr significantly affected fish growth rates. Fish less than 160 mm in length lost weight and showed no change in length. Fish over 160 mm in length put on weight, though at a rate significantly lower than that of untagged controls. For fish between 160 and 180mm in length, the increase in length was significantly smaller in tagged fish than in untagged controls. Over 180 mm in length, no difference could be detected in the length gains of tagged and untagged fish.
Smaller tagged fish gave up maintaining station and showed signs of distress at an earlier stage in the experiment than larger tagged fish. Tag-related mortality was inversely related to fish size. All but two of the control fish survived the experiment and continued to maintain station throughout.
The results suggest that the minimum length of Atlantic salmon parr on which this design of acoustic tag could be employed is 160 mm, and that caution is needed in the interpretation of track lengths longer than l0 daysof parr in the size range 160–180 mm.     

The involvement of gonadotrophin and sex steroids in the control of reproduction in the parr and adults of Atlantic salmon, Salmo salar L.

Early sexual maturity occurred in the majority of male Atlantic salmon parr. Levels of the plasma androgens testosterone and 11-ketotestosterone rose steadily as the male parr matured, and decreased as the testes regressed. No such progressive changes were observed in the plasma gonadotrophin (GTH) levels, although the pituitary GTH levels were much higher in mature than in immature male parr; reasons for this, incluiding the possibility that the GTH radioimmunoassay employed is inadequate, are discussed. All female parr remained immature throughout the year, although the gonadosomatic index did show an annual cycle. Adult salmon had higher GTH and sex steroid levels than parr at the same stage of sexual maturity, the levels corresponding to the degree of sexual development of the adult fish.     

Infection of Atlantic salmon, Salmo salar L., by Gyrodactylus salaris, Malmberg 1957, in the River Lakselva, Misvær in northern Norway

Gyrodacrylus salaris was most probably introduced to the River Lakselva in 1975 through stocking of Atlantic salmon from an infected hatchery. The parasite population grew rapidly, and the parasite spread throughout the entire watercourse during the summer of 1976. This epidemic situation led to mortality among the young Atlantic salmon, and the density of salmon parr was heavily reduced from 1976 to 1977. The density of salmon parr has remained close to zero since then, while there are no apparent trends toward decrease or increase in the density of brown trout. In spite of the reduced density of young salmon, a new epidemic has developed each year among the few young 0+ and 1+ Atlantic salmon present in the river. Results from successive sampling during the summer of 1987, 1988 and 1989 indicate that most of the presmolt salmon are attacked during their first summer or autumn of life. The infection develops into an epidemic during the first autumn, winter or the next summer. The build-up of the parasite burden on the fish leads in turn to mortality. Norwegian Atlantic salmon probably have no resistance against G. salaris, since the parasite has recently been introduced to Norwegian rivers.     

Social Status and Feeding in Atlantic Salmon Salmo salar Parr: The Effect of Visual Exposure to a Dominant

Groups of four Atlantic salmon parr were established and the dominant individual identified. Feeding responses were then observed in the remaining (subordinate) fish housed in one of three conditions: in the same compartment as the dominant fish (full exposure), separated from it by a transparent partition (visual exposure) or behind an opaque partition (isolated). Subordinate parr in the full exposure condition responded to and ate fewer food pellets than did the dominant fish. However, subordinate parr in the visual exposure condition and in the isolation condition fed actively, indicating that neither visual intimidation nor long-term stress-induced reduction of appetite were responsible for the reduced food intake of subordinate parr in direct contact with dominants. Dominant parr and subordinates in the visual exposure condition, but not subordinates in the exposed condition, moved further from their feeding station when attacking food items that were contested by a competitor compared with the distance moved for uncontested items. The presence of a competitor therefore forced both dominant and visually exposed subordinates to pay higher costs while foraging.     

Impact of Beaver Dams on Abundance and Distribution of Anadromous Salmonids in Two Lowland Streams in Lithuania

European beaver dams impeded movements of anadromous salmonids as it was established by fishing survey, fish tagging and redd counts in two lowland streams in Lithuania. Significant differences in abundancies of other litophilic fish species and evenness of representation by species in the community were detected upstream and downstream of the beaver dams. Sea trout parr marked with RFID tags passed through several successive beaver dams in upstream direction, but no tagged fish were detected above the uppermost dam. Increase in abundances of salmonid parr in the stream between the beaver dams and decrease below the dams were recorded in November, at the time of spawning of Atlantic salmon and sea trout, but no significant changes were detected in the sections upstream of the dams. After construction of several additional beaver dams in the downstream sections of the studied streams, abundance of Atlantic salmon parr downstream of the dams decreased considerably in comparison with that estimated before construction.     

Daily feeding rhythms in Atlantic salmon I: feeding and aggression in parr under ambient environmental conditions

Daily feeding patterns of tank-reared Atlantic salmon Salmo salar parr under ambient conditions and their relationship with patterns of aggression were investigated during spring and autumn. A single, early afternoon feeding peak was observed during spring but no consistent pattern emerged from the autumn trial (fish did tend to feed in synchrony, even though the population's feeding pattern varied between days). Aggression was highest at first light during both spring and autumn, reducing thereafter throughout the day. Feeding and aggression were out of phase during spring, with the level of feeding increasing as aggression decreased. It is suggested that the high levels of early morning aggression are associated with re-establishment of the social order that breaks down after a period of darkness; in the spring this may have inhibited feeding.     

Daily growth increments in the otoliths of Atlantic salmon parr, Salmo salar L., and the influence of environmental factors on their periodicity

Daily increments were demonstrated in the sagitta otoliths of fast- and slow- growing Atlantic salmon parr, Salmo salar L., when held under natural photoperiod and temperature. Otolith increments continued to be deposited at a daily rate when fish were held under constant light and/or temperature and on single or multiple feeding regimes. However abnormally short photoperiods of 6L: 6D induced two increments per day. The results suggest that an endogenous rhythm, synchronized lo light/dark transitions within a 24 h period, controls otolith increment deposition.     

The effect of vaccination and sea water entry on immunocompetence and susceptibility to Kudoa thyrsites in Atlantic salmon (Salmo salar L.)

The effect of intraperitoneal (IP) vaccination and sea water entry (SWE) on the immunocompetence of Cascade Atlantic salmon was investigated. Smolts were IP injected with Aqua Health's Forte trade mark vaccine (Listonella (Vibrio) anguillarum, Listonella ordalii, Vibrio salmonicida and Aeromonas salmonicida) at four times (42, 238, 433 and 630 degree days, DD) prior to SWE and were examined for immunocompetence. Immune response measurements included mitogen-driven proliferation of peripheral blood leukocytes (PBL), head kidney leukocyte respiratory burst activity and alternative complement hemolytic titres and were measured 24h prior to SWE, 72 h post-SWE and again 3.5 weeks post-SWE. A 50% reduction in the number of PBL was observed 3 days post-vaccination. At this time LPS-driven proliferation was low (stimulation index, SI, 1.5-2.9) in all groups prior to SWE compared with that of PBL from freshwater-reared Atlantic salmon parr (6.7). By 72 h and 3.5 weeks post-SWE, the LPS-driven SI from unvaccinated salmon and those vaccinated 630 and 433 DD prior to SWE increased 3-fold. In contrast, SI from salmon vaccinated 42 and 238 DD prior to SWE remained low. A similar pattern was observed for cultured PBL stimulated with PHA, although unlike LPS-stimulated PBL, the SI of cells from parr and unvaccinated control smolts remained low following SWE but increased in fish vaccinated 433 and 630 DD prior to SWE. The respiratory burst activity of head kidney leukocytes was not affected by SWE but showed a transient 50% depression 3 days post-vaccination. The alternative complement activity (ACH50) was similar for all treatment groups prior to and at 72h post-SWE. By 3.5 weeks post-SWE, ACH50 values in salmon vaccinated 42 and 238 DD prior to SWE doubled to 874 and 860 U/ml, respectively. The prevalence and severity of Kudoa thyrsites infections, detected in all treatment groups approximately 2400 DD following SWE, were not significantly different among groups. Atlantic salmon parr should be IP vaccinated no earlier than 433 DD before SWE to avoid an enhanced risk of acquiring pathogens because of transient depression in some immune mechanisms.     

Determination of body shape variation in Irish hatchery‐reared and wild Atlantic salmon

Discriminant function analysis was used to distinguish morphologically between samples of parr, smolts and adult Atlantic salmon Salmo salar from several hatchery and river systems in Ireland. The effect of habitat shift was investigated in Atlantic salmon parr. Parr grown from the eyed‐egg stage with a non‐sibling group in a hatchery environment, came to resemble the mean body shape of their host hatchery Atlantic salmon stock more closely than that of a full sibling group grown at their natal hatchery. Wild Atlantic salmon smolts differed in shape from hatchery‐reared smolts. This difference was less pronounced, but still statistically significant when wild adults were compared with hatchery‐reared adults captured in the coastal drift‐net fishery after a year spent at sea. Rearing conditions had a significant impact on the production and growth of fish body shape. This in turn may have affected adaptability and survivorship of ranched Atlantic salmon in the marine environment.     

Changes in plasma protein patterns in smolting Atlantic salmon, Salmo salar L., are not dependent on changed growth rates

Juvenile Atlantic salmon, Salmo salar L., were raised under simulated natural photoperiod (SNP) and constant light (CL). SNP fish displayed typical characteristics of smolts in the spring including changes in colouration, elevated gill Na⁺/K⁺ ATPase activity and ability to pass a seawater challenge. CL fish grew at rates comparable to SNP fish, but did not demonstrate any of these molt characteristics.
Electrophoretic analyses of plasma proteins from both groups of fish revealed that SNP fish displayed reproducible qualitative changes in protein patterns from February to May with May patterns being the most complex. CL fish displayed patterns similar to SNP fish. The timing of the appearance of patterns was influenced by photoperiod as indicated by the fact that CL fish had a progression of pattern changes retarded by at least one month relative to SNP fish. The changes in plasma protein patterns in smolting Atlantic salmon were not dependent on changed growth rates.     

Is Atlantic salmon production limited by number of territories?

Thirty Atlantic salmon Salmo salar parr (mean 135 mm total length, L _T) in the River Alta, Norway, were radio‐tagged and tracked during a 11–13 day period. The parr stayed within defined home ranges with a 95% probability of localization within an average area of 1286 m²(241–3484 m²). On average, each parr had overlapping home ranges with 7·7 other parr, and the overlap between pairs of fish covered on average 24% of their home range areas. Mean length of the river stretch used was 90 m (22–383 m). On average, during 38%(16–77%) of the tracking surveys, the fish had moved >10 m since the previous survey. Mean total distance moved during the whole study was 402 m (208–862 m). The Atlantic salmon were most often recorded in riffles, but 27% of the parr alternated between riffles and pools. The extensive movements, flexible habitat utilization and relatively large home ranges, together with the fact that all the parr had home ranges partly overlapping with other radio‐tagged parr, indicate a flexible and variable behaviour and, thereby, a more complex social structure than a rigid defence of a fixed territory. Thus, Atlantic salmon production seems not to be limited by the highest potential number of territories for large parr in a given area.     

Social dominance and body size in Atlantic salmon parr, Salmo solar L.

Dominance relationships between pairs of Atlantic salmon parr of known size were assessed at various times during their first year of life. In tests conducted between first feeding and early July, the larger of two fish was dominant in only 54% of pairs, regardless of the magnitude of the size difference between the fish. In September, there was a stronger association between size and status, especially in pairs with a large size differential, where the dominant was larger in 72% of cases. In groups of parr tested in April of the following year, there was no relationship between size and status, the larger of two fish being dominant in 48% of cases, regardless of the magnitude of the size differential. This result suggests that status in early social interactions may depend on behavioural properties rather than size and that the larger size of dominant fish reported in a number of salmonids might be a consequence and not a cause of high status.