Shelter selection in juvenile Atlantic salmon,or why do salmon seek shelter in winter?

During winter, juvenile Atlantic salmon Salmo salar become nocturnal and seek refuge during the day in the stream bed gravel interstitial spaces. The function of this behaviour is unclear, but two major types of hypothesis have been proposed. One is that the fish are hiding from something (e.g. a predator) and the other is that the fish are seeking shelter from the water current. These hypotheses were tested by examining the selection of juvenile salmon for refuges that offered different degrees of concealment or shelter. The fish clearly preferred refuges that allowed them to hide (i.e. they were dark and opaque) but offered little shelter from the current. Therefore, it can be assumed that the primary function of this nocturnal behaviour during winter is most likely to hide from diurnal predators.     

Diel activity patterns of juvenile Atlantic salmon in rivers with summer water temperature near the temperature‐dependent suppression of diurnal activity

Direct day and night underwater observations of juvenile Atlantic salmon Salmo salar during summer and autumn showed a duality in response to temperature between 7 and 11° C for young‐of‐the‐year (YOY) Atlantic salmon. They were predominantly diurnal in early summer and nocturnal in late summer although water temperatures were similar. Post‐YOY Atlantic salmon did not show a strong response to temperature as they were mostly nocturnal during the study period. It is suggested that the difference in activity patterns between YOY and post‐YOY Atlantic salmon can be explained by size‐dependent trade‐off between growth and predation risk.     

Life-history patterns in a southern population of Atlantic salmon

Juvenile Atlantic salmon Salmo salar in the river Esva, Asturias, north-west Spain, developed a bimodal growth pattern during their first growing season. Segregation between the two modes was apparent by late autumn. All fish in the upper modal group (UMG) grew throughout the year and migrated downstream in the following spring (by April) at the age of 14 months. Some lower modal group (LMG) fish (56% of the main stem, 31% and 50% of the two tributaries) silvered like smolts and apparently migrated downstream the same spring, although c. 1·5 months after UMG fish. Larger LMG fish appeared more likely than smaller ones to migrate. LMG fish did not grow in winter, but they grew fast between March and May. Maturing male parr were detected first at 5 months old in July, and they occurred initially among the faster growers, >6·8 cm long. In spite of this, length of maturing males did not change from late summer and throughout the winter, whereas that of non-maturing UMG and LMG fishes continued to diverge from September onwards. These findings suggest that favourable conditions for growth (very early start of a long growing season) at the southern limit of the species' range may influence the life-history pattern of this population.     

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 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.     

Mating success of alternative maturation phenotypes in male Atlantic salmon,Salmo salar

Summary Mature male parr successfully fertilized eggs of anadromous female Atlantic salmon, Salmo salar, in competition with anadromous males under simulated natural conditions. Mating situations were created in which mature male parr and anadromous males competing for the opportunity to spawn with an anadromous female differed in allelic forms of the same enzyme. Females deposited eggs into a sequence of 3 or 4 eggs nests. The mean proportion of eggs in a redd fertilized by parr increased with increasing numbers of parr present at a redd, reaching 23% at male parr: anadromous male ratios of 20:1. Single male parr fertilized, on average, 5% of the eggs in a redd. The proportion of eggs in an egg nest fertilized by parr also depended upon the order of egg nest construction, such that parr mating success was highest at the initial nest constructed and lowest at the final nest. Parr have relatively high fertilization success for their size when compared with the smaller maturation phenotype of other salmonids.     

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.     

Clonal Atlantic salmon × brown trout hybrids produced by gynogenesis

Clonal full-sib progeny groups of Atlantic salmon Salmo salar × brown trout Salmo trutta hybrids were produced by gynogenesis. Eggs obtained from two 3-year-old Atlantic salmon (female) × brown trout (male) F₁ hybrids were activated with UV-irradiated rainbow trout Oncorhynchus mykiss sperm. Fecundity, percentage egg activation and percentage survival to completion of yolk-sac absorption were similar for the two females, and averaged 800 eggs kg⁻¹, 90 and 65%, respectively. Flow cytometric and protein electrophoretic analyses confirmed the progeny to be diploid hybrids. Isogenicity within progeny groups and to the maternal parent was indicated by identical DNA fingerprint patterns detected with multilocus oligonucleotide probes–GATA₍₅₎ and ACTG_(n). Isogenicity was also observed in the gynogenetic progeny of a third female spawned the following year. It appeared that a large portion of the oocytes in females of this hybrid underwent a premeiotic chromosome doubling, or possibly a complete suppression of meiosis. The result was ovulation of diploid eggs, each possessing a full set of both Atlantic salmon and brown trout chromosomes identical to those in the maternal somatic cells. Lines of clonal hybrids could therefore be perpetuated by gynogenesis and would have potential both as experimental animals and in commercial aquaculture.     

Effects of sexual maturation, castration, and androgen implants on growth in one- and two-year-old parr in a Baltic Atlantic salmon (Salmo salar L.) stock

The effects of maturation, castration, and androgen implants on growth in 1- and 2-year-old Baltic salmon ( Salmo salar L.) were studied in a stock originating from the Umeälven (Ume river) in northern Sweden. 1 + male parr that later matured were larger than non-maturing parr in the spring. Later in the summer, during the period of gonadal growth, the growth rate of maturing parr was depressed compared to that of non-maturing males. Implantation of capsules with 11-ketoandrostenedione (OA) or testosterone (T) in sexually immature 1-year-old fish, accelerated the specific growth rate in spring and early summer, particularly considering growth in weight, resulting in an increase in the condition factor. In contrast, the growth rate was depressed from late June onwards, the period when the gonads normally develop in 1 + parr. T, but not OA, also promoted sexual maturation. Castration or OA treatment did not affect total summer growth in 2-year-old salmon. This study suggests a causal relationship between androgens and the growth pattern observed in maturing 1 + male parr.     

Marine post-smolt growth and age at maturity of Atlantic salmon

The annual variation in sea-age of maturation for a hatchery dependent stock of Atlantic salmon was compared to variation in post-smolt growth as evidenced by circuli spacing patterns. The proportion of returns of 1-seawinter (1 SW) and 2 SW salmon and the fraction of the smolt year class or cohort that maturated as 1 SW fish, were compared to seasonal growth indices determined from circuli spacing on the scales of smolt class survivors returning as 1 SW and 2 SW spawners. Using image processing techniques, we extracted inter-circuli distances from scales from 2244 recaptured fish. Spacing data for the first year at sea were collected and then expressed as seasonal growth indices for the spring period, when post-smolts first enter the ocean; the summer, when growth appears maximal; and winter, when growth appears to be at a minimum. In general, circuli spacings were wider for 1 SW than for the 2 SW returns of the same smolt cohort. The 1 SW fraction was significantly and positively correlated with late summer growth, suggesting that growth during this season is pivotal in determining the proportion of a smolt class that matures early.     

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.     

Sperm motility and fertilization time span in Atlantic salmon and brown trout—the effect of water temperature

The effect of water temperature on the duration of sperm motility, the time lapse after activation by fresh water and the fertility of eggs was studied in Atlantic salmon and brown trout. Eggs of both species were fully fertile in fresh water after 512 s. No interspecific differences were noted in egg fertility at the lower water temperatures, but the brown trout eggs showed a higher resistance to high temperatures, indicating a better physiological thermotolerance. A highly significant effect of temperature on the overall duration of sperm motility was found, with a marked peak at 3−4°C for salmon and a weaker one for trout. After freshwater activation the eggs of both species remained fertile for a longer time than the sperm were mobile.     

Muscle development in Atlantic salmon (Salmo salar) embryos and the effect of temperature on muscle cellularity

Salmon eggs were incubated at 5, 8 or 11° C from fertilization to hatching. At Gorodilov stages 25, 27, 29, 31 and 33 transverse sections of whole embryos (at somite level 10–15) were prepared for histochemistry and electron microscopy. At every stage up to hatching, cross–sectional areas of the embryos were not different between temperatures, and from stage 27 onwards there was also no difference in the ratio of white to red muscle. However, there were more muscle fibres but of smaller average diameter in both the red and white muscle for the colder temperature embryos. At hatching there were also more nuclei (per cross–section) in the colder embryos but more nuclei per muscle fibre in the warmer embryos. In all cases the 8° C embryos were intermediate between 5 and 11° C embryos in their muscle parameters. Fast and slow muscle fibres could only be distinguished in the embryos by alkali–stable ATPase reactions. Succinic dehydrogenase activity was low in embryonic fish. No differences between the temperature groups were detected in the histochemical reactions for either ATPase or succinic dehydrogenase activities.     

Size and age of maturity of Atlantic salmon correlate with the North Atlantic Oscillation Index (NAOI)

Sea‐age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one‐sea‐winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.     

Multiple paternity increases effective size of southern Atlantic salmon populations

Genetic analyses were performed on the progeny of Atlantic salmon (Salmo salar L.) sampled in natural redds of three rivers flowing into the Bay of Biscay, the Nivelle, the Mandeo and the Sella. These rivers are at the southern limit of the European distribution of the species and their populations are small and endangered by human activities. Nine variable number of tandem repeat (VNTR) loci (five minisatellites and four microsatellites) were used for parentage analysis. Multiple male participation was recognized in the fertilization of eggs. A large proportion was fertilized by precociously mature parr. We demonstrate that multiple paternity derived from mature parr is crucial for the conservation of genetic variability in small populations of Atlantic salmon.     

Counterintuitive migration patterns by Atlantic salmon Salmo salar smolts in a large lake

What little is known about the seaward migration of Salmo salar smolt migration through standing waters indicates that it is both slow and results in high mortality rates, compared with riverine migration. This may be partly because smolts in lakes need to swim more actively and require more complex directional cues than they do in rivers. In this telemetry study of smolt migration through Loch Lomond, S. salar smolts made repeated movements in directions away from the outflowing river, which considerably increased migration time.     

Movement of adult Atlantic salmon in the Usk estuary, Wales

A total of 56 salmon was tagged in the Usk estuary using combined acoustic and radio tags. Those fish migrating within the estuary oscillated with the tide over c . 10 km, being towards the seaward end at low water and moving upstream on the flood tide. Fish migrating through the estuary moved upstream on the flood tide and stemmed displacement downstream during the ebb. These findings, together with information on the hydrodynamics of the estuary, indicate that the fish utilize tidal currents to migrate passively in their preferred direction.     

Return migration of one‐sea‐winter Atlantic salmon in the River Tana

Fifty‐three one‐sea‐winter Atlantic salmon Salmo salar (45–63 cm L _T) were radio‐tagged in the Tana fjord, Barents Sea, in 1995. Thirty‐seven fish (70%) entered the freshwater zone of the River Tana in an average of 3 days after release in the fjord. The migration speeds in the lowest river section below the first riffle area were significantly higher than in the subsequent river section below the second riffle area. Similarly, the observed time spent in the first riffle area was significantly lower than in the next riffle area. The majority of Atlantic salmon entered the river during the hours of high tide and the subsequent ebb tide. In addition, most river entries were recorded around midnight. No effects of river flow on the river entry or migration speed were detected, but the migration speed of Atlantic salmon in both river sections examined was greater at lower temperatures. Twenty‐eight fish (72%) were recaptured in the river, 71% of them with weirs and gillnets, and 29% by rod and line. Over half of the Atlantic salmon (54%) were recaptured within 3 weeks following river entry, and within the first 100 km of the river (56%). The results are discussed in relation to earlier studies on multi‐sea‐winter Atlantic salmon in the River Tana.