Relative ventricular weight of wild Atlantic salmon parr in relation to sex, gonad maturation and migratory activity

Relative ventricular weight (RVW, ventricular weight/body weight · 100) was measured for five groups of Atlantic salmon parr from the Girnock Burn in Scotland. The mean RVW was: mature males, 0.087; immature resident males, 0.054; immature resident females, 0.056; immature migrant males, 0.056; immature migrant females, 0.053. For a group of mature male parr from the R. Nith, Galloway, Scotland, the mean RVW was 0.108. There were no significant differences in the RVW between groups of immature fish. The RVW of each group of mature male parr was significantly greater than those of each, group of immature parr. Condition indices (weight/length³) were closely similar between all groups of parr. The functional significance of plasticity in ventricular weight is discussed in relation to life histories of Atlantic salmon.     

The use of genetic marking to assess the reproductive success of mature male Atlantic salmon parr (Salmo salar, L.) under natural spawning conditions

The reproductive success of mature male Atlantic salmon parr ( Salmo salar ) under natural spawning conditions was estimated using the polymorphism at the MEP - 2 * locus as a genetic marker. The percentage of eggs per redd fertilized by parr varied considerably over the five redds examined (0·9–27.7%, mean 10·8%), but a gametic contribution from mature parr was detected in each case. Parr reproductive success has important implications for the population structure and evolution of the Atlantic salmon through its effect on gene flow.     

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.     

Occurrence of Atlantic salmon parr in redds at spawning time

Several Atlantic salmon parr of different sizes were found alive in salmon redds under a 10–30 cm layer of stones at spawning time. Parr, both mature and immature, remain inactive in redds during daylight but show nocturnal activity.     

The downstream migration of precociously mature Atlantic salmon, Salmo salar L, parr in autumn; its relation to the spawning migration of mature adult fish

The autumn emigration of juvenile Atlantic salmon, Sulmo salur L. from the Girnock Burn, Aberdeenshire, is shown to be comprised of two components. Precociously mature males and sexually immature fish differ in their dependence on seasonal and biological cues for the release of migratory behaviour. The emigration of ripe mature male parr is shown to follow the entry of mature adult salmon to the stream; when adult fish are prevented from entering the stream, the emigration is induced by seasonal factors. Spent male parr tend not to participate in the autumn emigration. The emigration is considered to be principally an appetitive phenomenon which maximizes the number of precociously mature males attaining sexual contact with adults.     

An electrophysiological study on the effects of pH on olfaction in mature male Atlantic salmon (Salmo salar) parr

The olfactory responses of mature male Atlantic salmon ( Salmo salar L.) parr to known odorants were studied after exposure of the olfactory epithelia to water of varying pH. Electrophysiological recordings from the olfactory epithelia indicated that the responses of fish to both testosterone and urine from ovulated female Atlantic salmon were significantly reduced at pH 5·5 and 4·5 and abolished at pH 3·5. Concentration response studies indicated that at pH 5·5 and 6·5 significantly higher concentrations of testosterone and urine were required to produce the same amplitude responses as controls. Both testosterone and urine have previously been shown to be important chemical cues eliciting behavioural and physiological responses in Atlantic salmon. The results are therefore discussed in relation to the possible sublethal effects of acidification on reproduction and behaviour of Atlantic salmon and the effects on salmonid stocks.     

Multiple paternity and variance in male fertilization success within Atlantic salmon Salmo salar redds in a naturally spawning population

The incidence and magnitude of multiple paternity were estimated for a natural, unmanipulated spawning population of Atlantic salmon Salmo salar. Egg nests were surveyed in the autumn and sub‐samples were excavated the following spring. Parentage data derived from microsatellite DNA revealed an unexpectedly high level of multiple paternity. Within a single redd, females may mate with as many as 16 different males, including small mature male parr and large anadromous males. Multiple paternity was most pronounced in areas of highest redd density, corresponding with increased abundances of mature male parr. In addition, there was considerable variation in success among males, although this variability did not depend upon the number of males participating in spawning. This work underscores the value of undertaking genetic studies on the mating systems of fishes in unmanipulated, natural environments.     

Effect of stripping on the length, condition and gonadal state of mature male Atlantic salmon, Salmo salar L., parr during autumn and winter

Hatchery-reared mature male Atlantic salmon parr were shorter than immature parr of the same age through the winter period and this difference increased prior to smolting in spring. Stripping had no effect on growth. In autumn mature male parr had a higher condition factor (CF) than immature parr, but from early January no difference was observed between unstripped mature males and immature parr. Stripping reduced the CF of male parr but the difference had disappeared by the time of smolting when the CFs of both mature groups were lower than that of immature fish. The quantity of expressible milt in stripped males decreased from c . 3% body weight in November and December to <0.05% by 2 March, after which no further milt expression was recorded. Milt expression in previously unstripped males was about 1% of body weight on 2 March, similar to that of stripped males 1 month earlier.     

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.     

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.     

Allozyme heterozygosity, date of first feeding and life history strategy in Atlantic salmon

The relations between allozyme heterozygosity, relative date of first feeding and life history strategy in juvenile Atlantic salmon Salmo salar were examined using eggs obtained from a 400 family cross (20 male × 20 female adult Atlantic salmon). Multilocus heterozygosity, through its positive associations with the timing of first feeding and growth rate, was correlated with life history strategy in juvenile Atlantic salmon, albeit under genotype × environmental (temperature, food availability) regulation. Under hatchery conditions, a 10 day difference was observed in the relative date of first feeding between early and late first feeding Atlantic salmon. Early first feeding Atlantic salmon exhibited a significantly higher mean heterozygosity, grew faster at ambient water temperature (April to November) and a significantly higher proportion adopted the early freshwater maturation (age 0+ years, male fish) or early migrant (age 1+ years, mainly female fish) strategies compared to late first feeding Atlantic salmon. Elevated water temperatures over the winter (December to April, >10·5° C) provided additional growth opportunity allowing previously mature male parr (mainly early first feeders) and lower modal group parr (mainly late first feeders) to adopt the early migrant strategy by the following spring.     

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.     

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.     

Inter- and intra-population morphological differences between wild and farmed Atlantic salmon juveniles

Whether population-specific morphological differences were detectable in small (26–52 mm) Atlantic salmon Salmo salar parr reared under similar conditions was tested. Discrimination based on morphological characters was total (100%) between the fish of farmed origin (from AquaGen) and four wild fish populations. Between the four wild populations the corresponding discrimination was 59·8–86·3%. The inter-population variation in morphological characters was larger than the intra-population variation. The fish originating from the local populations at Driva and Innfjord were narrower in body form, whereas fish from the AquaGen and Innfjord populations had smaller and less pointed heads with smaller eyes. The Driva population fish had the smallest mouth while the longest pectoral fin was found in the Bjoreio population, the river that also has the largest fall gradient. Population-specific morphological characters were thus detectable among Atlantic salmon parr relatively rapidly after yolk absorption.     

Maturation of male age-0 Atlantic salmon following a massive, localized flood

Maturation of male age-0 Atlantic salmon Salmo salar parr in New England, U.S.A. streams is rare (˜5%), but age-0 parr maturation was high (74%) by autumn in the Sawmill River following a massive, localized flood. Maturation was low in two other study streams (3, 7%) in the same year as the flood, and in the Sawmill River (6%) and the other rivers (5%) in the subsequent year, suggesting that high maturation rates were related to the flood. The high age-0 maturation rates appear to have been the result of greater growth opportunity following the flood. Masses of fish in October were two-fold greater in the Sawmill River (13·2 g) than in the other rivers (6·5, 6·9 g). Mechanisms contributing to the fast growth may include community reorganization following the flood and water temperature differences among rivers. The flood caused an age-0 year-class failure for brook trout Salvelinus fontinalis and brown trout Salmo trutta and a large reduction (69%) in the number of salmon compared to the other rivers, possibly reducing competition or agonistic interactions among remaining fish. Average water temperatures were slightly warmer in the Sawmill River (17·0° C) than in the other rivers (15·5, 14·9° C). By influencing community structure and growth of remaining fish, it appears that a strong environmental disturbance can also alter the direction and timing of life histories in Atlantic salmon.     

Using stable isotope analysis with telemetry or mark-recapture data to identify fish movement and foraging

Information about animal movements has often been inferred from stable isotope analysis (SIA), but is dependent on animals assimilating site-specific isotopic signatures via diet. This potential weakness in ecological interpretation can be overcome by using other investigative tools that provide precise information about individual movement patterns. In this paper, we demonstrate the value of combining SIA with telemetry or mark-recapture data from trapping, electrofishing and remote detection of individuals to study the movement and feeding ecology of fishes in different habitats. In a fjord lake system in Newfoundland, Canada, juvenile Atlantic salmon delayed downstream migration (smolts) or actively moved into a large lake (parr) where they foraged for periods reflecting different life history strategies. In the Miramichi River (New Brunswick, Canada), SIA provided evidence of distinct foraging habitats (tributary versus large river). By tracking fish implanted with passive integrated transponder (PIT) tags, we distinguished between movements related to foraging versus seeking cool water refugia during high temperature events. Finally, site fidelity and limited mobility of slimy sculpin, a small benthic fish, was established where δ¹³C in muscle tissue showed a progressive enrichment downstream and where a median displacement of <10 m was estimated for sculpin tagged with PIT tags. Technological improvements have permitted non-destructive tissue sampling of wild fishes for SIA, and the tagging and remote detection of animals smaller than was previously possible. These advancements and the combination of investigative tools promise new insights into animal ecology.     

Relative production of Atlantic salmon from fluvial and lacustrine habitats estimated from analyses of scale characteristics

Empirical and back-calculated growth of Atlantic salmon parr were compared between fish reared in fluvial and lacustrine habitats of Conne River, Newfoundland. Length at age was significantly higher for lacustrine parr. Various classification and maximum likelihood estimators indicated that 75% or more of the fish used lakes for rearing. Lacustrine use is another aspect of the inherent variability and plasticity of Atlantic salmon life-history traits. As most Newfoundland river systems include lakes, estimates of regional spawning targets and potential smolt production will need to take lake habitat into account.     

Interhabitat and instream movements of large Atlantic salmon parr in a Newfoundland watershed in winter

Radio‐telemetry was used to investigate movement of large, mainly mature male (80%) Atlantic salmon Salmo salar parr in Stoney River, Newfoundland during early winter (November; water temperature 6·0 ± 0·1° C) and mid‐winter (January to February; 0·8 ± 0·0° C). Site fidelity of parr in early winter was low. Parr moved between fluvial and lacustrine habitats and were active throughout the diel cycle. Parr caught in fluvial habitats in mid‐winter were smaller and younger than parr caught in early winter. Site fidelity of parr in mid‐winter was greater than in early winter. Parr in mid‐winter moved between fluvial and adjacent small lacustrine habitats, but avoided a larger pond inhabited by large piscivorous fishes. Instream movement rates in mid‐winter were lower than in early winter and occurred primarily during hours of darkness (dawn, dusk and night). Fluvial habitats were relatively stable and ice‐free throughout the study periods. These results suggested that large Atlantic salmon parr utilize a variety of habitats and remain active throughout the winter, even under stable environmental conditions.     

Effects of surgically implanted transmitters on swimming performance, food consumption and growth of wild Atlantic salmon parr

Experiments were conducted on wild Atlantic salmon Salmo salar parr to determine the effect of surgically implanted dummy transmitters on swimming performance, food consumption and growth. Swimming performance of tagged fish (tag 1·7–3·7% of fish mass) was similar to that of control fish 1, 5 and 10 days after surgery. Negative effects on growth, however, were found up to day 36 of a 45 day experiment (tag 0·9–2·6% of fish mass). Consumption rates were similar between tagged and control fish and did not explain differences in growth.     

Seasonal changes in androgen levels in stream- and hatchery-reared Atlantic salmon parr and their relationship to smolting

In stream-reared Atlantic salmon Salmo salar , plasma androgens were significantly greater in mature male parr than immature males and females in October, but had declined by January and did not differ significantly from immature fish throughout the spring. Immature fish in March were significantly larger and had greater gill Na⁺, K⁺-ATPase activity than their previously mature counterparts. Bimodal growth distribution was seen in hatchery-reared Atlantic salmon and a proportion of the male fish in the lower mode matured. Plasma testosterone (T) and 11-ketotestosterone (11-KT) were significantly elevated from September to December in mature male (1+ year) parr. In January, plasma androgens had declined in mature males and did not differ significantly from immature fish. By May all the hatchery fish were large enough to smolt and a proportion of the previously mature males had increased gill Na⁺, K⁺-ATPase activity. Therefore elevated androgens in the previous autumn do not prevent smolting. Parr with higher plasma T and 11-KT in April and May, that are presumably beginning to mature, had lower gill Na⁺, K⁺-ATPase activity, indicating that future maturation and associated increases in androgens may inhibit smolting.