Application of a fry (0+;) abundance index, based on semi-quantitative electrofishing, to predict Atlantic salmon smolt runs in the River Bush, Northern Ireland

An abundance index for 0+ Atlantic salmon was based on semi-quantitative electrofishing estimates at 137 sites in typical juvenile habitat throughout the River Bush (N. Ireland). 0 + abundance was linearly related to total smolt numbers migrating through a downstream trap in subsequent years ( r ²= 0.716, P <0.001), suggesting a high degree of predictive ability. This predictive ability decreased as smolt age increased, suggesting influence of density-independent mortality. Potential application of the technique to estimate smolt production from rivers having no trapping facilities is discussed, together with sources of variability potentially affecting estimates.     

Effects of body size on sympatric shelter use in over‐wintering juvenile salmonids

The study tests two hypotheses: (1) the degree of shelter dominance in Atlantic salmon Salmo salar and brown trout Salmo trutta increases progressively with increasing size differential between heterospecific fish in a pair and (2) shelter dominance, standardized to size differential, correlates with aggression. The results support the first but not the second hypothesis, suggesting that the fitness consequences of high growth performance during the summer are likely to become evident during winter. At this time of year, when mortality is high among both Atlantic salmon and brown trout, shelter dominance may increase the chances of survival.     

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.     

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.     

Introgression in the genus Salmo via allotriploids

Hybridization between sympatric species is not uncommon in the wild. Wild allotriploids (individuals with two chromosome sets from a species + one chromosome set from another species) are generally the result of a backcross between interspecific hybrids that produce unreduced gametes and one of the parental species. In animals, allotriploids are commonly sterile, except for some vertebrate species complexes in which allotriploids reproduce by parthenogenesis, gynogenesis and/or hybridogenesis, producing generally clonal or hemiclonal gametes; nuclear DNA introgression between hybridizing species is considered to be extremely rare. Employing species-specific molecular markers, we show genetic introgression between the chromosomally well-differentiated salmonids Atlantic salmon (2n = 58) and brown trout (2n = 80) through spontaneous bisexual reproduction of allotriploids leading to salmon-like offspring bearing some brown trout genes. Although introgression between these Salmo species can occur via allotriploids, we hypothesize that extinction of parental species can be discarded based on very low survival of allotriploid offspring.     

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.     

Avoidance responses to low frequency sound in downstream migrating Atlantic salmon smolt, Salmo salar

The possibility of using intense sound as an acoustic barrier for downstream migrating smolt of the Atlantic salmon ( Salmo salar ) was studied by observing, the reactions of smolt to 10 and 150 Hz sounds in a small river. At the observation site the river branched into a main course and a minor channel, the latter rejoining the main stream after 30 m. The sound sources were positioned at the lower end of the channel. The number of smolt re-entering the mam stream at the lower end of the channel was recorded during alternating periods with and without sound. Intense 150 Hz sound had no observable effects on the smolt, even at intensities 114 dB above the hearing threshold at this frequency. At intensities above 1.0. 10⁻²ms⁻² the 10 Hz sound was an effective deterrent for the smolt, which turned and left the channel at the upstream branching point.     

The relationship between river flow and entry to the Aberdeenshire Dee by returning adult Atlantic salmon

One hundred and nine returning adult salmon were radio-tagged in the estuary of the Aberdeenshire Dee, Scotland between February and August 1985 to 1989 and the times when 62 fish entered the river were recorded. Elapsed times between tagging and river entry were significantly greater during periods of lower than average river flows in all months where there were sufficient data to allow statistical comparison. The degree of association between river entry and particular levels of river discharge rate varied seasonally. The proportion of days associated with river entry declined at the lower end of the range of flows available to tagged fish in the summer months. Absolute levels of river discharge played a significant role in modifying the response of salmon to changing flows. During periods of lower than average seasonal flow, river entry was closely associated with days when flow had increased since the previous day. During periods of higher than average flow, river entry was not significantly associated with such periods of increased flow. The results suggest that models which relate river entry by salmon to absolute discharge rates alone are unlikely to be generally reliable.     

Predator‐induced hyperventilation in wild and hatchery Atlantic salmon fry

Following exposure to a predator stimulus (a brown trout Salmo trutta ), the opercular rate of Atlantic salmon Salmo salar fry increased by 35·3 ± 11·0%(mean ± 95% CI). The time taken for opercular rate to decline to baseline levels depended upon the occurrence of any associated locomotory activity. Opercular rates of fish that dashed when exposed remained elevated for 38·2 ± 20·6 min, whereas those of individuals that did not move ('freezers') recovered within 7·2 ± 2·9 min. The duration that opercular rate remained elevated was positively correlated with the magnitude of the elevation, which was higher in 'dashers' than freezers. The maximum opercular rate in 'freezers' was similar between wild fry and hatchery‐reared fry (from wild parents). There was a significant delay, however, in hatchery compared with wild fry in the time until peak ventilatory response and onset in the decline phase. This difference in opercular response suggests that hatchery fish were slower to realize fully the potential danger from the predator. Any delay in response could be directly attributed to the effect of hatchery‐rearing environment, rather than domestication or hatchery selection effects.     

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.     

Migration speeds and orientation of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system

We recorded the observed and actual swimming speeds of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system, and initiated studies on the orientation mechanisms of the post-smolts. We tracked Atlantic salmon and sea trout with acoustic transmitters for up to 14 h after release. The actual swimming speed and direction of a fish relative to the ground is the vector sum of the observed movements of the fish and the movements of the water. We determined actual swimming speeds and directions of the post-smolts, which reflect their real swimming capacities and orientation, by corrections for the speed and direction of the water current. The post-smolts were actively swimming. The observed direction of movement was dependent on the actual movement of the fish and not the water current. Water currents were not systematically used as an orientation cue either in Atlantic salmon or sea trout, as the actual movements were random compared to the direction of the water current. The actual movement of sea trout were in all compass directions, with no systematic pattern. The Atlantic salmon also moved in all compass directions, but with the lowest frequency of actual movement towards the fjord.     

Triploid progeny from a female Atlantic salmon × brown trout hybrid backcrossed to a male brown trout

A female Atlantic salmon × brown trout hybrid was backcrossed to a male brown trout. Electrophoretic analysis of diagnostic enzymes showed that the progeny were triploid. However, a few individuals were partially diploid.     

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.