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.     

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.     

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., smolts down a release ladder. 3. Reactions on exiting the ladder

Twenty-eight hatchery-reared Atlantic salmon ( Salmo salar L.) smolts were tracked as they left a release ladder and moved through the estuary of the Lussa River and out to sea. Smolt passage through the estuary was fastest at high water and slowest at low water; the reverse of the situation observed for passive objects floating in the surface water layer. At low tide the water channel was effectively a freshwater stream and the smolts behaved in it accordingly, attempting to maintain station. At high water smolts moved downstream with the current close to the water surface. The degree of variation in individual speed through the estuary was the main difference observed between immature S2, stripped mature male S2, unstripped mature male S2 and S1 smolts. Smolts moved directly out of the estuary with no apparent check in their progress. Sea contact times in the vicinity of the river mouth, for both passive objects and smolts, were longest around high water and shortest around low tide. In the sea no differences were observed in the behaviour of the four different smolt types. While passive objects showed no tendency to move off in a particular direction, smolts showed a significant inclination to head off on a southerly bearing.     

Thyroid hormone levels in Atlantic salmon (Salmo salar) during the return migration from the ocean to spawn

Serum thyroid hormone levels were determined in adult Atlantic salmon ( Salmo salar ) of both sexes caught in the ocean and at a sequence of locations on their return migration to spawn. Tri-iodothyronine (T₃) levels were greatest in fish caught in coastal or estuarine waters or in a river near head-of-tide. T₃ levels were lower in fish caught in rivers throughout the angling season and lowest in those captured entering a tributary near spawning. Thyroxine (T₄) levels were lowest in immature fish captured in the ocean in winter but raised in fish captured in spring; many of the latter group showed endocrine evidence of their becoming sexually mature. T₄ levels were greatest in fish captured in coastal waters and progressively lower in fish captured in an estuary, near head-of-tide and in rivers. T₄ levels in fish captured at tributary entry near spawning exceeded those in fish caught in rivers earlier in the year. In general, these data support the hypothesis that motor activity level in migrant fish is a determinant of thyroid status.     

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.     

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.     

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.     

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., down a release ladder. 2. Effect of fish developmental strategy on speed and pattern of movement

The behaviour of different types of Atlantic salmon smolt migrating down a release ladder were compared. Immature S2 smolts were faster through a release ladder, and their journey times less variable than S1 and mature male S2 fish. Journey time variability was less among stripped than among unstripped mature male smolts. The mean residence time of passive objects (oranges) in each pool of the ladder was correlated with pool surface area, whilst that of fish was correlated with pool volume. The general pattern of movement of smolts through the ladder was consistent with the hypothesis of non-directed passive displacement of fish near the water surface. In fact, fish spent most of their time nearer the bottom of the pools with the result that their migration speed was slower than that of passive objects. Pools towards the top of the ladder had the highest volume and surface area and therefore offered the greatest hindrance to passive displacement. Immature S2 smolts may have been more active in seeking a passage through this barrier. In 1986 and 1987 no significant change in ladder travel time or fish condition factor was detected over the period May to July. In 1986 fish condition factors and variance in ladder travel time increased significantly in August. Immature S2 smolt migration may have been less nocturnal. Mature male smolts were affected more by high daytime water temperatures than immature S2 and S1 smolts.     

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.