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.     

Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Based on data from Norwegian streams with sympatric populations of Atlantic salmon and brown trout, it is suggested that temporal segregation is the main mechanism segregating Atlantic salmon and brown trout during spawning. Peak spawning of trout was about 15 days earlier than that of salmon. Physical factors, such as water depth, water velocity and distance from the river banks segregate spawning sites of salmon and trout poorly. Gravel sizes of the redds of salmon and trout were significantly different, though with a considerable overlap, and mean egg depth of salmon and trout were 0.18 and 0.12 m, respectively, probably attributable to the different size of spawners of salmon and trout. None of the temporal or spatial parameters analysed segregate spawners of salmon and trout completely. Species determination of eggs and alevins from the redds showed no interspecific superimposition of redds. It is, therefore, concluded that low survival of hybrids after hatching does not explain the low frequency of hybrids observed in sympatric populations of salmon and trout.     

The post‐spawning movements of migratory brown trout Salmo trutta L.

The post spawning behaviour of sea trout Salmo trutta was studied over a 2 year period in the river and estuary of the River Fowey, south‐west England. Forty‐five sea trout kelts were trapped immediately after spawning in December and intraperitoneally tagged with miniature acoustic transmitters. The subsequent emigration into coastal waters was monitored using acoustic receivers deployed throughout the river catchment. The levels of gill Na⁺K⁺ATPase activity in sea trout kelts sampled at the same time as the tagged fish were within the range of 2·5 to 4·5 μmol Pi per mg protein per h indicating that the post‐spawning fish were not physiologically adapted to salt water. The tagged kelts were resident in fresh water between 4 and 70 days before entering the estuary. Sixty two per cent of the tagged kelts subsequently migrated successfully into coastal waters, with a higher success rate for male fish (75%) than females (58%). There was a significant size related difference in the run‐timing of the kelts with the larger fish moving more quickly into coastal waters after spawning than smaller fish. Seaward migration within fresh water was predominantly nocturnal and generally occurred in conjunction with increasing river discharge and rising water temperature. Migration through the estuary continued to be predominantly nocturnal and occurred during an ebbing tide. Residency within the estuary varied amongst individuals although it was invariably short, with most fish moving out into coastal waters within one to two tidal cycles. Five tagged kelts returned from the coastal zone and re‐entered fresh water during April and June. Marine residence time varied between 89 and 145 days (mean 118 days) and the minimum estimated marine survival was c. 18%. One of these sea trout was subsequently recaptured after successfully spawning in the vicinity where it had been previously tagged demonstrating a degree of spawning site fidelity.     

Movements of brown trout, Salmo trutta L., in the River Gwyddon, South Wales

Movements of native brown trout, Salmo trutta , 1 + and older, were limited with up to 93% of recaptured marked fish occupying sites where previously caught. Movements > 50 m were rare with the majority < 15 m. Between sampling occasions, the population could be separated into a static component and a smaller mobile component but there was no evidence of a permanently mobile group. The proportion of mobile native fish increased after stocking with hatchery-reared trout. Displaced native trout (1 + and older) showed the ability to home from 75 m upstream to 111 m downstream of a release site.     

Morphological variability among seven populations of brown trout, Salmo trutta L., in Greece

Morphological variation in five meristic and 21 morphometric characters was examined in the brown trout, Salmo trutta L., from seven distinct streams and rivers from N and NW Greece. Variation within each sample, estimated as the multivariate generalization of the coefficient of variation, did not vary substantially among the populations. The phenetic relationships of these populations were examined using the stepwise discriminant analysis. The taxonomic implications of these results are discussed.     

Characteristics of the spawning migrations of brown trout, Salmo trutta L., and rainbow trout, S. gairdneri Richardson, in Great Lake, Tasmania

Upstream spawning migrations of mature brown trout, S. trutta , and rainbow trout, S. gairdneri , were studied in Liawenee Canal, Great Lake from 1949 to 1985. Brown trout migrations normally occurred from early April to mid-May and rainbow trout from late August to early November. In 1983, 16 425 brown trout and 1338 rainbow trout passed through a fixed upstream diversion trap. Brown trout spawning migrations occurred predominantly over the temperature range 6–10° C, while rainbow trout migrated predominantly over the range 5–11° C. Migrations peaked at water temperatures of 7.6°C (males) and 7.8°C (females) for brown trout, and 8.3°C (males) and 9.6°C (females) for rainbow trout. Rainbow trout migrations occurred at high flow conditions and were positively correlated with canal flow increases, while brown trout migrated under low canal flow. Mean length, weight and condition of rainbow trout of both sexes decreased significantly during migrations. Female brown trout decreased in weight and condition but not in length; male brown trout did not change in condition despite decreases in both length and weight during migrations. Overall sex ratio was 2:1 (female:male) for both species, with the relative proportion of male fish decreasing as migrations progressed. Age composition changed during migrations; dominant age classes were 3 < 4 < 5 + years for both species. Comparison of length, weight, condition and age revealed minor changes during the 37-year period 1949–1985.     

Cortisol can increase the susceptibility of brown trout, Salmo trutta L., to disease without reducing the white blood cell count

Chronic elevation of plasma cortisol levels in the brown trout by means of slow-release, intraperitoneal implants increased the susceptibility of the fish to disease. Elevation from a mean basal level of 1–4ng ml⁻¹ to ∼10ngml⁻¹ for a period of 2–4 weeks was sufficient to increase the mortality rate due to furunculosis, Saprolegnia infection and bacterial fin-rot. This level of plasma cortisol is well within that capable of being produced by the fish under conditions of chronic stress. The increase in susceptibility to disease was not accompanied by a reduction in the number of circulating lymphocytes and it is suggested that under certain conditions chronically-elevated cortisol levels may be more sensitive, predictive indicators of reduced disease-resistance than are changes in blood cell counts.     

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.     

Mitochondrial DNA as a genetic marker for brown trout, Salmo trutta L., populations

Mitochondrial DNA was examined in natural and hatchery-reared stocks of brown trout, using different methods of restriction analysis. The methods included the development of a brown trout mt DNA hybridization probe through cloning part of the brown trout mitochondrial genome. In addition, fragments were analysed by ethidium bromide staining and end-labelling. The relative merits of each of these methods in assessing levels of genetic relatedness between the natural and hatchery-reared brown trout stocks were evaluated. In addition, the study revealed a diagnostic mtDNA restriction pattern which could be used as a genetic marker for the discrimination of these two groups of brown trout.     

Rates of gastric evacuation in brown trout, Salmo trutta L.

Brown trout of similar length and weight were fed a standard meal which contained a known number of food organisms of the same size-group and taxon (seven taxa were used). The weight of digestible organic matter in a trout stomach decreased exponentially with time. i.e. at a constant relative rate. At a particular water temperature, the food organisms were either evacuated from the stomach at similar rates (Group 1: Gammarus pulex, Baetis rhodani, Chironomidae, Oligochaetes) or at progressively slower rates (Group 2: Protonemura meyeri, Hydropsyche spp., Tenebrio molitor). Rates of gastric evacuation were not significantly different for food organisms of different size groups of the same taxon, or for different sized meals, or for different sizes of trout (range 20–30 cm), or for mixed and multiple meals (three meals over 16 h). Times are given for the gastric evacuation of 50%, 75%, 90% and 99% of the digestible organic matter in a meal. Starvation periods of 1, 2, 3,4 and 5 days prior to feeding did not affect evacuation rates. The rates were slightly, but not significantly, slower for starvation periods of 6 and 7 days, and were often significantly slower for starvation periods of 10, 15 and 20 days. Evacuation rates increased exponentially with increasing water temperature. It was possible to estimate both the rate and time for the gastric evacuation of different meals at water temperatures between 3–8°C and 19·1°C.     

Thermoregulatory behavior of brown trout,Salmo trutta

Brown trout, Salmo trutta, were allowed to thermoregulate individually in an electronic shuttlebox. Pooled data for 6 fish showed a diel pattern of preferred temperature, with a diurnal minimum of 10.3°C, an early nocturnal maximum of 13.7°C, a less pronounced mid-scotophase minimum of 11.7°C, and a secondary dawn maximum of 12.8°C, in a somewhat crepuscular pattern. The 24-hour mean preferendum was 12.2°C.     

Differences in the sensitivity of brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson, to physiological doses of cortisol

Interspecific differences in the stress response of fish may be due, in part, to differences in the sensitivity of target tissues to cortisol. The relative response of brown and rainbow trout to a standardized dose of cortisol was assessed by monitoring condition (K factor), the number of circulating lymphocytes and mortality due to disease, following cortisol treatment. Cortisol implantation resulted in a significant decline in K factor and number of circulating lymphocytes in immature brown trout, but not in immature rainbow trout, despite plasma cortisol levels being similar in both cases. Cortisol implantation in mature brown and rainbow trout significantly increased the mortality rate due to bacterial and fungal infection compared with control fish. Furthermore, the mortality rate due to disease was significantly greater in brown trout than rainbow trout, despite both groups receiving similar doses of steroid.     

Lymphocytopenia and interrenal activity during sexual maturation in the brown trout, Salmo trutta L.

In three different strains of brown trout a marked reduction in the number of circulating lymphocytes occurred in the blood of sexually mature male and female fish during the spawning season. This lymphocytopenia was closely correlated with elevated plasma cortisol levels in all three strains. It is well established that sexual maturation in salmonid fish is associated with an increase in susceptibility to disease: the present study provides strong evidence that this is mediated, at least in part, by a cortisol-induced suppression of lymphoid activity. These findings are discussed in relation to possible physiological roles of cortisol during the spawning period and to the evolution of semelparity in salmonid fish.     

Acclimation of the interrenal tissue of the brown trout, Salmo trutta L., to chronic crowding stress

Brown trout kept under chronically crowded conditions grew more slowly over a 110 day period than did uncrowded fish given similar rations. Crowding resulted in a significant elevation of plasma cortisol for at least 25 days but by day 39 the cortisol levels in both crowded and uncrowded fish were similar. After 110 days, plasma cortisol levels in both groups were low (<2ng ml⁻¹) and no significant differences in interrenal histology were detected. It is concluded that prolonged growth rate suppression as a result of crowding is not mediated by a chronic elevation of plasma corticosteroids. Under the conditions of the present study, water quality deterioration due to crowding was not sufficient to account for any of the changes in interrenal activity of the fish. These findings are discussed in relation to social interactions between saimonid fish maintained at unnaturally high stocking densities.     

Effects of hydropeaking on the spawning behaviour of Atlantic salmon Salmo salar and brown trout Salmo trutta

An in situ camera set‐up was used to study the spawning activity of Atlantic salmon Salmo salar and brown trout Salmo trutta throughout two consecutive seasons in a spawning area affected by hydropower‐related pulse flows due to hydropeaking. The purpose was to test whether the flow variation discouraged spawning in shallow areas or motivated spawning into areas with elevated risk of incubation mortality. There were more S. salar observed on the spawning ground during days with high discharge. The presence of S. salar in the spawning grounds was not affected by the hydropeaking cycles of the preceding night. Female S. salar were observed preparing nests within the first hour after water discharge had increased to levels suitable for spawning. In contrast, the number of S. trutta was not correlated with flow and nest preparation was also observed at a discharge corresponding to the lowest discharge levels during a hydropeaking cycle. Survival was generally high in nests excavated the following winter, with only 5·4% suffering mortality due to dewatering. The results suggest that S. salar may respond rapidly to variable‐flow conditions and utilize short windows with suitable flows for spawning. Smaller S. trutta may utilize low‐flow conditions to spawn in areas that are not habitable by larger S. salar during low flow.     

Phosphoglucose isomerase isozymes and allozymes of the brown trout, Salmo trutta L

1. In brown trout the Pgi-1 and Pgi-2 loci are predominantly expressed in white skeletal muscle; Pgi-3 being mainly expressed in most other tissues. 2. Total PGI activity determinations revealed that the allele formerly designated Pgi-2(65) is a null allele, Pgi-2(n). 3. Enzyme kinetic studies on the partially purified PGI homodimeric isozymes revealed that from 5 to 25 degrees C both PGI-1 and PGI-2 had significantly higher mean Km(F6P) values compared to PGI-3. 4. Distinct metabolic roles for the "muscle" (PGI-1, PGI-2) and "liver" (PGI-3) isozymes are proposed. 5. Significant Km (F6P) differences were found among the PGI-2 allozymes and among the PGI-3 allozymes.     

An aeromonad epidemic in the brown trout (Salmo trutta L.)

The pathology and bacteriology of an aeromonad epidemic in the spawning population of brown trout ( Salmo trutta L.) of the freshwater Loch Leven, Kinross, Scotland, is described, together with estimates of its quantitative effect on the stock of trout.     

Eubothrium crassum in migratory trout, Salmo trutta L., in the sea

The incidence and intensity of infection of various ages and conditions of migratory trout by Eubothrium crassum are described. A marine source of the worm seems likely and explanations for the characteristics of the parasite burden are given on this assumption. Within weeks of first going to sea the hosts accumulate large numbers of the parasite. One year suffices for the life cycle of Eubothrium but growth continues for at least two years and possibly for longer. The parasite burden decreases in older fish because of the stress imposed on the parasite by successive migrations of the host to and from fresh water and because older fish tend to feed earlier in the year.     

Recovery of the brown trout, Salmo trutta L., from acute handling stress: a time-course study

The response of the brown trout to a single, short ( c . 1 min) incidence of handling stress was monitored for a period of 1 month post-stress. Significant changes were found in feeding behaviour, in the levels of plasma cortisol, glucose and lactate, in the concentration of circulating lymphocytes and in the degree of epidermal mucification. No changes were detected in the growth rate and coefficient of condition, in the levels of plasma thyroxine, in the concentrations of circulating erythrocytes, neutrophils and thrombocytes or in the thickness of the epidermis. The time-course for recovery of each parameter was examined and from this it was concluded that a minimum of 2 weeks was required for complete recovery and a return to normality.     

Observations on the dietary habits of wild and stocked brown trout, Salmo trutta L., in Irish lakes

The dietary habits and feeding rates of wild and stocked brown trout were compared for populations in a number of Irish lakes. Wild trout and stocked fish, which had been present in a fishery for 12 months or longer, tend to feed on the same dietary items at similar rates. Stocked fish in their immediate post-planting period (1–14 days) ate less than both the wild trout and established planted fish. In some instances recently stocked fish appear to have a preference for surface food items. They also consumed stones and detritic material. Data indicate that stocked fish adopted a natural diet in less than 5 months. Results are discussed in relation to angling crops of wild and stocked fish and the comparative success of autumn and spring plantings of salmonids.