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.     

Characteristics of mature male parr in the northernmost Atlantic salmon populations

The incidence, relative abundance and biological characteristics of mature male parr of Atlantic salmon Salmo salar were studied in two subarctic (69–70 N°) river systems in the northernmost part of Scandinavia. The proportion of mature males was low (0–25%) among ≥1 year‐old parr, and clearly lower than reported earlier in more southern rivers. The ages of mature males ranged from 1+ to 6 + year, the most abundant age group was 3 + year. Both age structure and size of the mature male parr varied between rivers and years. Mature male parr in the main stem were younger and also smaller than the parr of the same age in the tributaries.     

Compensatory sea growth of male Atlantic salmon, Salmo salar L., which previously mature as parr

The proportion of mature male parr in 11 families of Atlantic salmon, Sulrno sulur, reared under similar conditions in fresh water varied from 0–43%. The mature males were smaller than their siblings in December as 1 + and in late March. After individual tagging and transfer to a sea cage in early April. the previously mature males grew faster than previously immature salmon during the next 6 months. This compensatory growth resulted in almost equal size between the two groups. The results are discussed in relation to the different life strategies of salmon.     

Stimulatory and inhibitory effects of testosterone on testes in Atlantic salmon male parr

Immature 1-year-old Atlantic salmon Salmo salar parr were implanted with Silastic capsules of different sizes filled with testosterone (T). Testosterone had both positive and negative effects on testicular weights, spermatogenesis and steroidogenesis. The positive effects: higher incidence of males with enlarged gonads, spermiation, and high plasma levels of 11-ketotestosterone (11-KT) and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P), were most pronounced in males treated with small T capsules. The negative effects: suppression of gonadal development and depressed plasma levels of 11-KT and 17,20β-P compared with mature controls, were most evident in fish treated with large T capsules.     

Differences in growth between maturing and non-maturing male Atlantic salmon, Salmo salar L., parr

Sibling male Atlantic salmon parr that matured tended to be the larger fish in January, but their monthly specific growth rates between January and July did not differ from those of non-maturing fish. Maturing fish had lower condition factors in March, but greater increases in condition factor during April, exceeding those of non-maturing males by May. In maturing males, feeding rates between July and September, and specific growth rates in August and September, were lower than those of immature fish. Consequently, the mean size of immatures equalled or exceeded that of maturing males by October. Maturation rates were strongly correlated with increases in mean condition factor only during April.     

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.     

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.     

The effects of initial length and body curvature on shrinkage of juvenile Atlantic salmon during freezing

Fork length was measured in two groups of salmon parr (32–139 mm, frozen in a straight posture and frozen in a curved posture) before (L₁) and after (L₂) freezing and thawing. All the fish shrank. The decrease in length was significantly greater in the curved fish than the straight fish. The absolute reduction in length (L₁–L₂) was related directly to L₁, whereas the percentage reduction in length [(L₁–L₂)/L₁× 100] was related inversely to L₁.     

Competition between brown trout and Atlantic salmon parr over pool refuges during rapid dewatering

Variations in distributions and behaviours of Atlantic salmon Salmo salar in allopatry (homogeneous) and in sympatry with brown trout Salmo trutta (mixed) were observed before, during and after 2 day periods of dewatering in a large glass-sided indoor stream at densities typical of Scottish upland streams. Brown trout utilized pools more than Atlantic salmon at normal flows and in both species the majority of fishes moved into pools during dewatering. There was no significant effect of brown trout, which was the more dominant species, on the overall ability of Atlantic salmon to use pool habitat as a refuge during dewatering. Within mixed and homogeneous groups, average feeding levels decreased during dewatering. The highest ranking fish, which was always a brown trout in mixed groups, predominantly monopolized the pool and other individuals in pools adopted a more cryptic, stationary behaviour. Dewatering effectively increased local population density with the result that dominance status became much more important in maintaining food intake, and polarization between the top ranking fish and others increased. During the first day of dewatering, there was extreme behavioural polarization such that the dominant fish exhibited most aggression and least feeding within the group. Among dominant fish on the second day of dewatering, aggression had largely abated and feeding had returned to pretreatment levels despite the reduced average feeding within the group. The main difference between mixed and homogeneous groups was in the behaviour of the most dominant Atlantic salmon, which was near-despotic in allopatry and subordinate to brown trout in sympatry.     

A method for tracking the behaviour of mature and immature salmon parr around nests during spawning

A remote monitoring system was developed to provide information on the behaviour of mature and immature Atlantic salmon Salmo salar parr at nests during the spawning season. An octagonal passive integrated transponder (PIT) detector (0·865 m maximum diameter) designed to surround nests of Atlantic salmon was used to identify individual salmon parr present at 38 spawning events in three circular spawning channels. The range of the detector for PIT tags presented in the optimum orientation was 2·4 cm (range between tags 1·7–3·0 cm). Using a sub-sample of 20 spawnings, the mean efficiency of the detector (number of fish passes registered relative to number of passes observed on video) was 70·5% (range 32-100%). There were no significant effects of time from spawning, total number of registrations, body size or maturity status (mature or immature) on efficiency. However, fish were more likely to be detected entering nests than leaving, as departures were more rapid and higher in the water column. The PIT detector did not affect the numbers of parr at spawnings or between spawning intervals of females, and allowed for the individual identification of 65 of the 72 parr observed in nests during spawning. In all cases where certain identifications were not possible and the video was of satisfactory quality, this was due to obstruction of the camera view by anadromous fish. The remote monitoring system was thus effective in identifying behavioural differences, and only one of 20 immature parr was ever detected during the period encompassing 10 min before and after spawning compared with 30 or 40 mature parr.     

The relationship between first vertebra width and body length of Atlantic salmon differs between parr and smolts

The relationship between the width of the first vertebra ( W^V ) and the fork length ( L^F ) of Atlantic salmon differs between parr and smolts. The previous use of the first vertebra to reconstruct the diets of predators from the bone remains of ingested prey will have underestimated the size of smolts and probably also their frequency of occurrence in the diet. Using the ratio W^V:L^F provides potential for differentiating between intact parr, smolts and pre-smolts in the diet.     

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 influence of metabolic rate on otolith increment width in Atlantic salmon parr, Salmo salar L.

The influence of metabolic rate on otolith accretion in Atlantic salmon parr was investigated by comparing oxygen consumption rate and increment width in fast and slow growing individuals. Increment width was found to be positively correlated to mean daily oxygen consumption in both fast growing (S1) and slow growing (S2) parr. The results support previous suggestions that a process related to metabolic rate, rather than somatic growth, governs the rate of otolith accretion.     

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.     

Feeding chronology of Atlantic salmon parr in subarctic rivers: consistency of nocturnal feeding

During the ice-free season in three subarctic rivers in northern Norway and Finland, Atlantic salmon parr Salmo salar exhibited significant diel fluctuations in stomach content weights in August and September, the largest weights being recorded consistently at 0300 hours. Accordingly, the feeding rate of the parr was highest during the night and lowest in daytime, whereas during the other periods of the ice-free season, no pronounced diel fluctuations in food intake occurred. The consistent nocturnal feeding periodicity in early autumn in subarctic rivers may be associated with benthic feeding and an increased availability of benthic prey at night.     

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.     

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.     

Movements and habitat use by PIT-tagged Atlantic salmon parr in early winter: the influence of anchor ice

1. Movements and habitat use by Atlantic salmon parr in Catamaran Brook, New Brunswick, were studied using Passive Integrated Transponder technology. The fish were tagged in the summer of 1999, and a portable reading system was used to collect data on individual positions within a riffle‐pool sequence in the early winter of 1999. Two major freezing events occurred on November 11–12 (Ice 1) and November 18–19 (Ice 2) that generated significant accumulations of anchor ice in the riffle. 2. Individually tagged parr (fork length 8.4–12.6 cm, n = 15) were tracked from 8 to 24 November 1999. Over this period, emigration (40%) was higher from the pool than from the riffle. Of the nine parr that were consistently located, seven parr moved <5 m up‐ or downstream, and two parr moved more than 10 m (maximum 23 m). Parr moved significantly more by night than by day, and diel habitat shifts were more pronounced in the pool with some of the fish moving closer to the bank at night. 3. During Ice 2, there was relatively little movement by most of the parr in the riffle beneath anchor ice up to 10 cm in thickness. Water temperature was 0.16 °C above the freezing point beneath anchor ice, suggesting the existence of suitable habitats where salmon parr can avoid supercooling conditions and where they can have access to low velocity shelters. To our knowledge, these are the first data on habitat use by Atlantic salmon parr under anchor ice.     

A flat-bed passive integrated transponder antenna array for monitoring behaviour of Atlantic salmon parr and other fish

A system for monitoring automatically the movements of individually identified small fish in natural habitats is described. The system incorporates novel flat-bed antennae to detect the passage of miniature passive integrated transponders.