Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Norwegian river

Seasonal microhabitat selection by sympatric young Atlantic salmon and brown trout was studied by diving. Both species, especially Atlantic salmon, showed seasonal variation with respect to surface and mean water velocities and depth. This variation is partly attributed to varying water flows and water temperatures. In winter the fish sought shelter in the substratum. A spatial variation in habitat use along the river due to different habitat availabilities was observed. Both species occupied habitats within the ranges of the microhabitat variables, rather than selecting narrow optima. It is hypothesized that the genetic basis allows a certain range to the behavioural response. Microhabitat segregation between the two species was pronounced, with brown trout inhabiting the more slow-flowing and partly more shallow stream areas. Atlantic salmon tolerated a wider range of water velocities and depths. Habitat suitability curves were produced from both species. It is suggested that habitat suitability curves that are based on observations of fish occupancy of habitat at median or base flow may not be suitable in habitat simulation models, where available habitat is projected at substantially greater water flows.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

Microhabitat use by brown trout, Salmo trutta L. and Atlantic salmon, S. salar L., in a stream: a comparative study of underwater and river bank observations

Two methods, visual observation from the river bank and visual observation underwater by diving, were compared for microhabitat studies in young brown trout and Atlantic salmon in a stream. A wide range of habitat conditions were surveyed. Each method yielded different results with respect to microhabitat use. River bank observations missed small fish under surface turbulence and in deeper waters. Underwater observations missed small fish in shallow areas.     

The growth of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Scottish river system subject to cooling-water discharge

The River Fiddich, a tributary of the R. Spey in north-east Scotland, is a spawning river for both Atlantic salmon and brown trout. Warm cooling water effluent is discharged from several distilleries at different points in the lower reaches and raises the temperature of the river 1–3°C above ambient for most of the year. Salmon and trout grow more rapidly in this region than further upstream, and juvenile salmon generally migrate a year earlier, as 2 + smolts. Available data were too few to determine whether there was a similar difference for trout. Similar studies on the R. Dullan, a tributary of the Fiddich, and on the Cromdale Burn in the same area, confirmed that the growth rate of fish is faster downstream from distillery discharge points. It is suggested that increased invertebrate production may influence the growth rate.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Otolith shape discriminates between juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Otoliths of Atlantic salmon, Salmo salar L., are more slender than the otoliths of brown trout, Salmo trutta L. Discriminant analysis on otolith measurements of juvenile Atlantic salmon and brown trout from four river systems revealed a discriminant function which distinguished more than 94% of the cases. This function was tested by using data from a fifth river with cohabiting Atlantic salmon and brown trout: all Atlantic salmon and 91 % of the brown trout were correctly classified.     

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.     

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.     

Home range of juvenile Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in a Norwegian stream

SUMMARY. 1. The sizes of home ranges of juvenile Atlantic salmon (age 1 +) and brown trout (age 2+ to 9+) in a Norwegian coastal stream were estimated by local movements of batch-marked fish from 12.5 and 25 m long sections. Only recoveries made in the release section and in up-and downstream neighbouring sections were considered.
2. There was no significant difference in the average percentage of recaptures of salmon and trout between 12.5 and 25 m sections; a stream area of about 40–50 m² defines the size of home range for stocks of both species.
3. The fraction of brown trout recaptured in release sections increased with increasing fish densities, indicating a smaller home range under these conditions.     

An aerial method of assessing spawning activity of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Norwegian streams

A method using light aircraft to observe spawning activity of Atlantic salmon and brown trout in some Norwegian streams was tested between the years 1981 and 83. From the air, spawning redds of these species appear as light, oval spots in the river bottom. The method was successfully applied to most of the rivers studied, and information about numbers of redds, distribution of redds and spawning times was obtained. However, it has several limitations, the most important being that it proved to be unsuccessful in a deep river with high water turbidity. Also, during periods with high precipitation and high water level, the method cannot be used. Several quantitative and qualitative aspects of the results obtained by this technique are discussed.     

Wild adult hybrids of Salmo salar L. and Salmo trutta L.

Ovarian development was impaired in three adult Salmo salar L. × S. trutta L. hybrids identified among adult salmonids in Scottish fisheries. Species-specific variation at enzyme loci indicated that the fish were F1 hybrids and mitochondrial DNA analysis showed them to be the progeny of S. salar females.     

Development of length–bimodality and smolting in wild stocks of Atlantic salmon, Salmo salar L., under different growth conditions

Growth dynamics of juvenile Atlantic salmon, Salmo salar L., from two sections of the Narcea River and one of the Esva River (Northern Spain) were examined in relation to the development of bimodality in their size–frequency distributions. Size–bimodality was clearer under intermediate growth (section A) than under relatively fast or slow growth. The proportion of fish entering the upper modal group increase with growth intensity. Composition of upper and lower modal groups became fixed prior to December, and at this time both groups separated on the 90–95 mm interval. Fish exhibiting smolt appearance in late March (larger than 130 mm) had already been upper group fish in December, while parr-like fish and those that remained in the river by May (potential 2-year-old smolts) had formed the lower modal group. Anadromous salmon catch in the Narcea River was mostly of previously 1-year-old smolts (97.6%), of which 94% were larger than 100 mm by their first winter. In the Esva River, slow growth of juveniles is consistent with a large proportion of 2-year-old smolts (47.9%) among anadromous salmon. Both juvenile samples and scale analysis of anadromous salmon indicate that 2-year-old smolts were larger than 1-year-olds. Early disappearance of the former (before March) is, at least, partially related to earlier migration of large fish, since sexual maturity of parr does not provide a complete explanation. The Narcea stock have a minimum length at smolting of about 130 mm and an optimum smolt size in the 155–175 mm interval. Mean smolt length did not vary although the winter length changed between years.     

Hyperplasia and hypertrophy in the growth of skeletal muscle in juvenile Atlantic salmon, Salmo salar L.

Both red and white muscle fibre numbers in juvenile Atlantic salmon increased gradually with fish length throughout the freshwater growth period. Mean fibre area increased as fish grew to 6.5 cm f.l. , but thereafter was unrelated to fish length. Hyperplasia was most obvious when fish were growing fastest, and was the dominant growth process in fish over 6.5 cm f.l. Hypertrophy was most important when growth was slow, as in autumn and winter.
Mean white fibre area was significantly smaller in deep muscle than at medial and superficial sites. Total cross-sectional area of red, white and total trunk muscle increased with fish length. The ratio of red: white cross-sectional area increased with fish length to a plateau at about 10% after 6.5 cm f.l.     

Patterns of growth and smolting in autumn migrants from a Scottish population of Atlantic salmon, Salmo salar L.

In late November 1990 salmon parr, Salmo salar L., from the Girnock Burn in northern Scotland were either caught on their feeding territories (n=25) or trapped during downstream migration (n= 18). They were then housed in a laboratory rearing tank and their food intake and growth rates were tracked, until their smolting status was ascertained in the following May. Female fish were predominant in both groups; although the range of ages was the same, the total age of migrants was 2+ while that of residents was 1+. In November, compared to resident fish of the same year class, migrants were larger, heavier and in better condition. Although growth rates dropped during the winter in both groups before increasing in spring, migrants ate more and consistently grew faster than residents. In seawater tolerance tests conducted in May, more residents than migrants failed to adapt. These results confirm the suggestion that autumn migrants smolt in the following spring and suggest that they represent the faster-growing component of their cohort.     

High frequency of natural hybrids between Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in a Swedish river

Among 332 parr from the Swedish River Grönån examined by electrophoresis, 44 (13%) were hybrids between Atlantic salmon and brown trout. The hybrid frequencies in three sections of Grönån were significantly different (23. 8 and 2%). All hybrids are evidently of natural origin. and possible factors promoting hybridization are irregular overlapping spawning times. lack of separate spawning grounds, and involvement of sneakers.     

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., smolts down a release ladder. 1. Environmental effects on migratory activity

The effects of light intensity, water temperature and river spate conditions on the rate of migration of hatchery-reared Atlantic salmon smolts down a release ladder were examined. Low light intensity and high day time water temperatures raised smolt migration rates: water temperature had littleor no effect at night. Thediel patternofdown-ladder movement wasdetermined by these two environmental stimuli. Smolts demonstrated a threshold response to both light and water temperature: a fall in light intensity, or increase in water temperature, below or above their respective thresholds elicited no further response. As fish progressed through the ladder their movement became increasingly nocturnal, and most entry into the estuary occurred at night. Heavy rainhll caused rapid migration of smolts through the ladder. As ladder discharge remained constant, smolts must have been responding to some other change in the water conditions. Smolts responded to spate conditions irrespective of water temperature and light intensity, and they responded to light intensity irrespective of water temperature. A hierarchy of environmental cues, responsible for triggering migratory behaviour on a day to day basis during the smolt run, of spate> light intensity > water temperature is therefore suggested.