A genetic analysis of egg,alevin and fry mortality in salmon (Salmo salar), sea trout (Salmo trutta) and rainbow trout (Salmo gairdneri)

Heritability of mortality in eggs, alevins and fry were estimated using data from salmon, sea trout and rainbow trout. The highest value (average for all species and both estimation methods) based on the sire component was obtained for the eyed egg stage, h² = 0.08, followed by the alevin stage, h² = 0.05, while estimates for fry mortality did not differ significantly from zero.The maternal effect was rather large for uneyed eggs and eyed eggs, while it was low for alevin mortality. Maternal effects were also found to have a significant influence on the mortality of salmon fry. It was, however, not possible to separate maternal effects from tank effects.     

Acclimation of the brown trout, Salmo trutta L., to the stress of daily exposure to malachite green

Exposure of brown trout to a single dose of malachite green (2.27 ppm) caused a significant elevation of blood cortisol levels within 1 h. After 4 weeks of daily exposure to malachite green, however, this stress response was totally abolished. Daily treatment had no long-term effects on basal plasma cortisol levels, or on the numbers of circulating erythrocytes, thrombocytes and lymphocytes. Blood neutrophil numbers were significantly reduced in malachite green-treated fish, an effect thought to be due to a decreased ectoparasitic load.     

Application of an electrophoretically detectable genetic marker to ploidy testing in brown trout (Salmo trutta L.) triploidised by heat shock

A phosphoglucose isomerase (PGI) variant, of brown trout (Salmo trutta L.), expressed in adipose fin tissue, was incorporated into experimental batches through selective crossing of brood-stock having the PGI-3 (110) marker gene. PGI-3 (110) homozygous females were crossed with PGI-3 (100) homozygous males to yield all PGI-3 (110/100) progeny. These were subject to heat shock for 10 min at 28°C, commencing 10 min after insemination. Electrophoretic examination of fry indicated that diploids and triploids were separable on the basis of PGI-3 banding patterns, having staining ratios of 1:2:1 and 4:4:1, respectively. This confirmed the retention of an extra maternal PGI-3 (110) gene in triploid individuals. Potential applications of this marker in triploid studies in brown trout are discussed.     

Growth of three salmonid species in mono and double culture (Salmo salar L., S. trutta L. and S. gairdneri Rich.)

Juvenile Atlantic salmon, sea trout and rainbow trout were used in an experiment in which two species in each tank were compared with controls of each species separately. Specific growth rate was best in the experimental groups where the two species were reared together. The theory was developed that social facilitation or moderate interaction could cause the better growht. No starvation as a result of competition was discovered in any group.     

Early recognition of metabolic disturbances in trout (Salmo gairdneri Rich.) stocks

A method is proposed for detecting metabolic disturbances or diseases in trout stocks before typical symptoms appear. The method involves determination of erythrocyte numbers, hemoglobin contents, and hematocrit readings for about 10 animals. The measured parameters are tested for independence. The results permit the stocks tested to be classified as “normal” or “not normal”.     

Accumulation of cobalt during embryonic development of rainbow trout (Salmo gairdneri Rich.)

Studies on the accumulation of cobalt by fish eggs are described. γ-Energy measurement of ⁵⁷Co and atomic absorption with the flameless technique were used for the detection of the cobalt. The cobalt level of the eggs was found to be directly proportional to the cobalt content of the aquatic medium, and its uptake decreased with increasing calcium ion concentration in the water. The accumulation of bivalent cobalt ions was found to take place on the surface of the eggs, where they are reversibly bound by the binding force of chorion-mucopolysaccharide. The present study did not show any increase in cobalt content within the larvae that hatched from the eggs.     

Oxygen utilization by triploid landlocked Atlantic salmon (Salmo salar L.)

There was no difference between diploid and triploid Atlantic salmon in either oxygen consumption rate or PO₂ at asphyxiation, in spite of the increase in erythrocyte volume and lower blood haemoglobin content associated with triploidy. It is suggested that triploids should adapt to reduced oxygen levels as well as diploids.     

Artificial hatching substrate: Effect on yolk absorption,mortality and growth during first feeding of sea trout (Salmo trutta)

Groups of sea trout (Salmo trutta) eggs were hatched in a Californian hatching system with and without an astro-turf artificial substrate, and were later transferred to separate feeding units. Alevins reared in astro-turf absorbed their yolk faster and more efficiently than alevins reared on a flat screen, an effect which is probably caused by high activity stress in the flat-screened system. Probably due to higher yolk reserves, the fry hatched without astro-turf grew faster than the fry reared with astro-turf during the first periods of feeding. Later this was reversed, giving the astro-turf-reared alevins the highest growth rate. The flat-screen-reared alevins/fry suffered higher mortalities during the experiment and the mortalities were clearly size dependent. These results have consequences both for intensive culture and stocking programmes since the traditional hatching systems both reduce growth and the chance of survival of the fry.     

Feeding by hatchery-reared brown trout, Salmo trutta L. released in lakes

Abstract. Shortly after their release in two Norwegian lakes (3 hours to 3 weeks), hatchery-reared brown trout, Salamo trutta L., yearlings were recaptured, and their feeding was compared with that of wild brown trout. Investigations were conducted during three different periods (June, August and September). In all three periods most of the hatchery-reared fish started feeding immediately after their release. The amounts of exuviae from Ephemeroptera larvae and Chironomidae pupae found in the stomachs of hatchery-reared trout decreased with time, indicating that their feeding habits were influenced by a learning process. However, within one week of their release, hatchery trout appeared to be feeding on wild prey as well as did wild fish.     

Sarcoma of the swim bladder of Atlantic salmon (Salmo salar L.)

A neoplastic disease of the swim bladder of farmed Atlantic salmon (Salmo salar L.) was investigated. The tumour was classified as a leiomyosarcoma and had a detected prevalence of 4.6% of a population of 500 second-year sea fish.     

Response of resident brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown trout

Two strains of hatchery-reared adult brown trout, Salmo trutta L., [208–334 mm total length (TL); n =  591] were individually marked and released into a limestone stream. The estimated survival after one month (86%; n =  508) was comparable to that for resident brown trout and rainbow trout, Oncorhynchus mykiss (Walbaum), (89%; n =  771), but declined to 14% ( n =  83) after 8 months compared with 52% ( n =  451) for resident trout. The movement of resident trout out of stocked stretches was higher (14%) than from control sites (5%), but the population size in both individual sites and the overall study area were unaffected. The growth of resident brown trout was unaffected by stocking, but rainbow trout showed lower growth rates in stocked versus unstocked stretches both one and 8 months after stocking ( P <  0.002).     

Distribution of lipid droplets is an indicator for egg quality in brown trout, Salmo trutta fario

In the eggs of the brown trout, Salmo trutta fario, lipid droplets are clearly visible beneath the oolemma on the surface of the protein yolk. When examining the distribution of lipid vesicles in egg batches of different quality, four categories of lipid vesicle distribution could be distinguished: In the first category, the lipid droplets were evenly distributed throughout the egg. These egg batches had a high quality (percentage of eggs developing to eyed stage embryos: 96.2 ± 5.2). Some lipid droplets had the tendency to coalesce in one pole of the egg in the second category. This coalescing of lipid droplets was accompanied with a reduction in egg quality to about 50% of the first category (% of eyed stage embryos: 40.6 ± 15.2). In the last two categories, the lipid droplets were mostly coalesced in one or two poles of the egg and the egg quality was very low (% of eyed stage embryos: 3.3 ± 2.9 and zero, respectively). In conclusion, examination of lipid droplets distribution, in brown trout, can distinguish between high and low quality eggs in an easy and reliable way.     

Frequency and distribution patterns of Saprolegnia infection in wild and hatchery-reared brown trout Salmo trutta L. and char Salvelinus alpinus (L.)

Abstract. Fungal infection of sexually mature brown trout and char was associated with a particular type of Saprolegnia exhibiting a low degree of homothallic sexuality. Hatchery-reared brown trout were more severely infected (in terms of the % area of the body covered with the fungus) than were wild fish. The fins of hatchery-reared fish were particularly prone to Saprolegnia infection regardless of sex. In wild brown trout, a sexual difference in the pattern of infection was demonstrated. The flanks of the male fish appeared to be more prone to infection when compared with the female and there was a greater susceptibility of the caudal and ventral fin of the female when compared with the male. Evidence is presented which suggests that the incidence of infection in mature male salmonid fish prior to spawning is significantly greater than in the females. This difference may not be apparent in spent fish after spawning. These findings are discussed in relation to the background concentration of fungal spores in the water, the behavioural characteristics of spawning fish and differences in epidermal structure.     

Environment-induced gill disease of cultured rainbow trout (Salmo gairdneri)

Rainbow trout from eight North German freshwater culture facilities were investigated for chronic health damage resulting from captivity. The fishes were raised under a wide variety of conditions in net cages, circulation tanks, raceways, and earthen ponds.The most obvious non-contagious health problem was gill injury. Hyperplasia of the gill filaments, leading to lamellar fusion or the development of apical caps, as well as necrotic or advanced degenerative processes were observed. In addition, the gill filaments were frequently shortened, significantly reducing the functional surface of the organ.The lesions occurred in almost the same form in all of the culture units. It is likely that they are part of a syndrome induced by the environmental conditions typical of freshwater aquaculture facilities. These are characterized by an accumulation of nitrogenous excretion and decomposition products, extreme diurnal fluctuations in the concentration of dissolved gases, and intensive feeding involving the sudden addition of large amounts of artificial products. Under such conditions even small quantities of un-ionized ammonia can be toxic for epithelial tissues and disturb the normal elimination of protein metabolites across the gills. The pathological processes may be accelerated by secondary bacterial infections.     

High genetic introgression in alpine brown trout (Salmo trutta L.) populations from Hardangervidda, Norway

Abstract –  We studied the effect of stocking brown trout in three alpine lakes (Skavatn, Midtre Grøndalsvatn and Nordmannslågen) of the Hardangervidda mountain plateau by comparing microsatellite data from historical (1933 and 1967) and contemporary (2003) population samples. Historical and contemporary samples from a control lake (Krokavatn) and a contemporary sample from Gjuvsjå (suspected donor lake) were included as well. A total of 331 brown trout were genotyped with 11 microsatellites. Very low level of genetic diversity is observed in the historical samples from Nordmannslågen and Midtre Grøndalsvatn but not in the contemporary samples. In Skavatn historical and contemporary samples show similar genetic diversity as in the other contemporary samples. Admixture analyses with the structure program indicate high levels of introgression by nonnative brown trout in all three lakes. The stocking source for two lakes (i.e. from Gjuvsjå) is partially verifiable via our analyses. The high introgression in these lakes is discussed and the management issues are briefly addressed.     

Importance of life‐history and landscape characteristics for genetic structure and genetic diversity of brown trout (Salmo trutta L.)

Abstract – Investigating the influence of evolutionary forces on the genetic structure and genetic diversity remains a major challenge. Yet, it is of considerable interest for conservation and management of a species. This study investigates the influence of life‐history and landscape features, such as altitude, connectivity and habitat size, on genetic diversity and genetic structure of brown trout (Salmo trutta L.) with stream‐resident, lake‐dwelling and sea‐migrating life‐history in two river systems in northern Sweden. Using regression tree analysis including ecological and landscape characteristics, we show that life history is the most important variable explaining genetic diversity and population differentiation. Sea‐migrating populations show high diversity and low differentiation, and lake‐ and stream‐resident populations show low diversity and high population differentiation, among all samples. No overall genetic correlation with geographical distance was noted; however, among sea‐migrating populations within the River Vindelälven drainage, this pattern was observed. This study illustrates that life‐history and landscape features help to explain genetic structure and genetic variation. The information is important for conservation and management actions, such as fisheries regulations, habitat restorations, stocking of hatchery fish, defining management units and introducing genetic monitoring programmes.     

Migration performance of wild and hatchery sea trout (Salmo trutta L.) smolts—Implications for compensatory hatchery programs

Migration success of hatchery-reared and wild sea trout smolts through the lower stretches and the estuary of a Baltic Sea river were studied. During 3 years, wild and hatchery trout smolts were implanted with acoustic transmitters and released 14 km upstream from the river mouth. In order to monitor their out-migration pattern, acoustic receivers were deployed along the migratory route. Data on number of fish detected and date and time of detections were analysed and the migratory performance of wild and hatchery-reared fish was compared. A significantly higher proportion of wild fish (80%) successfully migrated to the coast compared to fish of hatchery origin (27.5%) and migration was faster in wild smolts. Hatchery fish were larger and had a higher condition factor and lipid concentrations, which are proposed as possible reasons for the poorer migratory performance of the hatchery-reared fish.     

Seawater adaptation in atlantic salmon (Salmo salar L.) at different experimental temperatures and photoperiods

A study was made of the effect of gradually increasing day length at different temperatures on the smolting process and on growth of Atlantic salmon during smolting.The experiment consisted of ten experimental groups: a combination of three photoperiod- with three temperature-groups and a tenth control group.Size was found to be the most important factor in seawater adaptation of underyearling Atlantic salmon. Optimal growth in the experimental temperature and photoperiod regimes occurred at 15°C and at the photoperiod with the longest period of increasing day length.At the time for seaward migration, the most pronounced effect of photoperiod on seawater adaptation occurred at 11°C. At this temperature there was a significantly higher percentage of survivors at the photoperiod with the longest period of increasing day length.     

Recapture of pond- and hatchery-reared brown trout, Salmo trutta L., released in small streams

Abstract. Two groups of 0+ brown trout, Salmo trutta L., one pond-reared and one hatchery-reared (fish length 32-80mm), were released in eight small streams in three different areas of Norway between 1984 and 1987. Hatchery and pond fish were reared at different sites, but releases were made both near the rearing site and in the home area of the other group. A total of 2550 fish were recaptured by electrofishing the year after stocking. We found great variations in the recapture rates of pond and hatchery fish in one stream between years, and between streams in different areas the same year. These results indicate that the rearing method is not essential to the recapture rate. Both pond and hatchery fish generally had higher recapture rates in streams near their rearing site than in the distant areas. Thus factors associated with transportation seem to influence the survival of stocked fish. The results also indicate that size at stocking may be an important factor for the recapture rate.     

Feeding by hatchery-reared and wild brown trout, Salmo trutta L., in a Norwegian stream

Abstract. Hatchery-reared brown trout, Salmo trutta L., yearlings were captured shortly (3h to one week) after their release in a Norwegian stream. The feeding of recaptured hatchery fish was compared with that of wild brown trout. The investigations were carried out during three different periods (May, July and October). Investigations of drift fauna indicated that food availability was best in May. Most hatchery-reared brown trout started feeding shortly after their release in all three periods. Hatchery fish went through a learning process with respect to feeding. This was most clearly demonstrated by the amounts of plant fragments in their stomachs, which were always greater in hatchery fish than in wild fish but which decreased with time after release in hatchery fish stomachs in all three periods. By about a week after release, hatchery trout appeared to be feeding on wild prey nearly as well as did wild fish, but they achieved this better in May than in October.