Post-spawning mortalities in brown trout Salmo trutta L.

Abstract. An 'oedema syndrome' caused mortality of 55·5% in brood brown trout approximately 2 months after spawning. The pathology and clinical biochemistry of the condition are described. It is suggested that the severe hypoproteinaemia and anaemia were due mainly to the traditional reduction in feed levels prior to spawning at a time when the water temperatures were abnormally high. It is also suggested, however, that the syndrome would not have occurred if the fish had not developed an aeromonad infection prior to stripping. Concurrent fungal infection and subsequent extremely low water temperatures are also thought to have played a role.     

Sea-ranching of brown trout, Salmo trutta L.

Abstract  In total, 8211 1-year-old and 14839 2-year-old hatchery-reared brown trout, Salmo trutta L., from 11 stocks were released at the mouth of the River Imsa, southwestern Norway. The recapture rates and total estimated yield were higher for 2- than 1-year-old trout, although recapture rates varied between years of release and stocks. The recapture rate increased with mean individual weight at time of release. Total estimated yield from the individual groups of 1+ trout ranged from 2 to 20 kg per 1000 trout released and for 2+ trout between 11 and 250 kg per 1000 fish released. In all cases, yields were lower than the economic break-even yield. Most fish were recaptured the year of release (89.2% of 2+ and 76.2% of 1+ trout). Almost 31% of the recaptures were caught at sea and 69% in fresh water; 95.8% of the latter were taken in the River Imsa trap.     

Cardiac disorders in farmed adult brown trout, Salmo trutta L.

During summer in Brittany, France, sea farmed brown trout, Salmo trutta L., regularly experience a high mortality rate which is associated, at least in part, with cardiac disorders (aneurysms and infarcts). The present study is preliminary to a more extensive research programme, the objective of which is to determine to what extent the physiological performance of the cardiovascular system of brown trout is affected by the environmental conditions the fish experience in farm cages. We conducted a 2-week in situ experiment during which the heart rate of eight sea water acclimatized individuals was telemetered using acoustic tags. During the experimental period, water temperature ranged from 16.0 to 17.6 °C. Water oxygen saturation was above 80% at all times and salinity was very high (35.5‰) but stable. Although they were unfed and not active, seven of out the eight tagged animals displayed near maximum heart beat frequencies, which ranged between 83 and 98 beats per minute (bpm). On the other hand, the eighth animal exhibited medium-range heart rates (50–70 bpm). Using phase delay maps, we established that the maximum heart rate of brown trout at 17 °C was in the range of 96–100 bpm. This result suggests that in our experimental conditions, the heart rate of most of our inactive fish was between 85 and 100% of maximum myocardial performance. We hypothesize that the cardiac failures observed in brown trout during summer are most likely a result of strenuous workloads imposed on the cardiovascular system by a combination of elevated temperature, high salinity and possibly season-related decreased hypo-osmoregulatory abilities.     

Androgen levels and erythrocytosis in maturing brown trout,Salmo trutta L.

The number of circulating erythrocytes was monitored in two strains of brown trout during the spawning season. Erythrocyte numbers were significantly elevated in mature male fish of both strains compared with mature females or immature fish. This period of erythrocytosis coincided with elevated plasma 11-ketotestosterone levels in mature male fish, was out of phase with the testosterone peak in males and was not observed in females despite high levels of plasma testosterone. The results are discussed with reference to the control of erythropoiesis in higher vertebrates.     

Saltwater performance of triploid Atlantic salmon Salmo salar L. brown trout Salmo trutta L. hybrids

Survival and growth in a saltwater net-pen of sexually immature triploid Atlantic salmon Salmo salar L. brown trout Salmo trutta L. hybrids was comparable to that of immature diploid Atlantic salmon. Following 17 months of freshwater rearing, the experimental fish were individually tagged and transferred to a saltwater net-pen where they were raised communally for 376 days. Initial and final average weights were 158 and 760 g per fish for the diploid Atlantic salmon and 209 and 1010 g per fish for the triploid hybrids; weights for the hybrids were significantly larger in both cases (α= 0.05). Survival from transfer to harvest was 43% for the Atlantic salmon and 48% for the hybrids. Average specific growth rate of fish which survived to harvest was 0.42% day^-1 for Atlantic salmon and 0.41% day^-1 for hybrids; these values were not significantly different. No significant differences were observed in average condition factor and dress-out percentage between crosses. Average gonadal weights and gonadosomatic indices were not significantly different for male diploid Atlantic salmon, female triploid hybrids and male triploid hybrids, but were significantly greater for female Atlantic salmon.     

Changes in serum parameters of Saprolegnia-infected brown trout, Salmo trutta L.

Abstract. Fungal infection of both wild and hatchery-reared sexually mature brown trout, Salmo trutta L., was caused by Saprolegnia diclina Humphrey Type 1. The major ions in the serum of infected fish were all significantly reduced in concentration and it is suggested that the primary cause of death was osmoregulatory breakdown resulting in a lethal haemodilution. Severe hypoproteinaemia and a significant reduction in the albumin/globulin ratio were reflected in the electro-phoretogram of the serum proteins from infected fish.     

Genetic effects of introduced hatchery stocks on indigenous brown trout (Salmo trutta L.) populations in Spain

Abstract– To assess the levels of gene introgression from cultured to wild brown trout populations, four officially stocked locations and four nonstocked locations were sampled for one to three consecutive years and compared to the hatchery strain used for stocking. Allozyme analysis for 25 loci included those previously described as providing allelic markers distinguishing hatchery stocks and native populations. Different levels of hybridization and introgression with hatchery índividuals were detected in stocked drainages as well as in protected locations. These findings indicate that new policies for stocking and monitoring hatchery fish are needed if gene pools of wild Spanish brown trout populations are to be preserved.     

Stocking of brown trout, Salmo trutta L.: effects of acclimatization

One-year-old hatchery brown trout, Salmo trutta L., ( n  = 16 520) from a sea-run, local brood stock were marked and released (scatter-planted) into the River Laisälven in northern Sweden. Eight different groups were created using Alcian blue and Visible Implant Elastomer tags. Half the fish were kept in small enclosures in four stocking areas for 6 days before release. The other half were released just after transportation. To evaluate the effect of acclimatization on post-stocking performance, the areas were electric fished 2 months later. During the electric fishing survey, a higher number of the acclimatized hatchery fish were recaptured than those released immediately. The growth rate of stocked fish differed significantly between stocking areas and fish held in enclosures grew more than those released directly. The rate of recapture of hatchery fish varied between stocking areas (6.4–17.4%). Movements of juveniles within and between the stocking areas were low, and only 3.6% of the recaptured fish were found in an area not originally stocked. These results showed that acclimatization of fish before release increases the number and size at recapture within a stocking area.     

Lake survival of hatchery and pre-stocked pond brown trout, Salmo trutta L.

Abstract. A comparison was made of lake survival, after 2 years, of hatchery and pre-stocked pond brown trout, Salmo trutta L., (age 0+) in two small mountain lakes in south-central Norway, one which contained a resident population of brown trout. There was a significantly higher recapture of pond fish in both lakes. The mortality rate for the stocked fish was significantly higher in the lake which contained a resident population of brown trout. The competitiveness of the stocked fish is discussed in relation to foraging success, predation and stress.     

Reproductive performance of cultured brown trout (Salmo trutta L.) in Chile

Data on reproductive traits in a cultured population of brown trout (Salmo trutta L.) in southern Chile were collected over three consecutive reproductive seasons: 1999, 2000 and 2001, corresponding to the first (at 3 years), second and third spawnings in females. Data were collected from individual females (n=238, 273 and 169 respectively). The reproductive season lasted mainly from June to August. The peak (July) tended to increase with each season (55.46%, 62.27% and 80.81% of total spawning fish respectively). Female body weight (470.8±102.5, 735.0±150 and 1263.9±263.4 g), total fecundity (1182±344, 1904±595 and 2744±605) and egg diameter (4.64±0.11, 4.67±0.27 and 5.24±0.12 mm) increased significantly (P<0.01) over successive reproductive seasons, particularly between the second and third spawnings. Relative fecundity, on the other hand, decreased significantly with each season (3577±471, 2591±900 and 2181±360). Following analysis of the variables over the three seasons, the correlation pattern of female body weight with total fecundity (r=0.91, n=458; P<0.001) and relative fecundity (r=?034, n=451; P<0.001) proved similar to that described in other salmonid females. Fertilization rate and survival to the eyed egg stage were also positively correlated (r=0.73, n=453; P<0.001). The systematically high values obtained for these latter variables over the seasons evaluated (consistently above 90%) are clearly greater than those we registered in other species of salmonids bred in Chile under similar conditions and suggest highly efficient biological variables that determine the gamete fertilization of this species. The brown trout is, therefore, an interesting potential aquaculture resource in Chile.     

Distribution of genetic variation in brown trout, Salmo trutta L., in Scotland

Abstract. A total of 1680 brown trout, Salmo trutta L., from 63 different localities in Scotland were sampled and examined for 34 loci. Thirteen loci were polymorphic in one or more populations. The mean observed heterozygosity for wild populations ranged from 0.0 to 8-9% (x=4.5%) in the same range as previous studies on this species. Within drainage studies and gene diversity analysis confirms that a large proportion (33%) of total gene diversity was distributed between populations. The Ldh-5(100) allele was common in Scottish populations and its distribution would fit the hypothesis that this allele was indicative of an ancestral trout population in Scotland before the invasion of trout characterized by the Ldh-5(90) allele. The results have important implications for the management of the brown trout genetic resource.     

Determinants of parr size variations within a population of brown trout Salmo trutta L.

Abstract – The size of 2-month old trout Salmo trutta parr differed between sites and between years along the River Esva catchment (Asturias, northwestern Spain). Such variation was in a direction opposite to the variation observed in parental size. Parr were smaller in sites where parents grew faster, whereas larger parr occurred in sites where parents grew less. Parr size of six cohorts (1990–1996, except 1995) at 14 sites along the River Esva was inversely related to the growth rate and length of parents and positively related to canopy, egg size, and water temperature. The latter acted similarly on all parr independently of egg size and the site where the egg originated. Covariation patterns among parr size, parental traits, and canopy suggest that a canopy-regulated, growth-determined trade-off between egg size and number, previously described for the Esva trout, also extends to alevin size. In forested, shaded sites, adult trout grew less and spawned fewer larger eggs that resulted in larger alevins, whereas in fully insolated, production-rich sites, trout grew faster and spawned higher numbers of smaller eggs that produced smaller parr. I hypothesize that the phenotypic plasticity illustrated by the environmentally induced trade-off between egg size and number further extended to alevin size may be evolutionarily advantageous because it relates the size of trout alevins to food availability, as predicted by the growth previously experienced by parents. ^NOTE     

Density-dependence of growth characteristics and maturation in stream-dwelling resident brown trout, Salmo trutta, in Serbia

Abstract  Breakpoints in both length and weight that represent the onset of first maturation, revealed that growth and maturation were correlated with density-related parameters in stream-dwelling resident brown trout, Salmo trutta L., from nine different rivers in Serbia. The determination of density dependence of growth is not simple, as both river width and depth, as single variables of habitat and relative biomass, and density were not significantly correlated with breakpoints. The significant relationship between breakpoints and the mean weight of brown trout ( x _p) suggests that differences in breakpoints were inversely related to brown trout density. The implications of density dependence for management of streams holding resident brown trout are discussed.     

Resource use, growth and effects of stocking in alpine brown trout, Salmo trutta L.

Abstract. Wild and stocked brown trout, Salmo trutta L., were sampled in the alpine Norwegian Lake Bjornesfjorden by standard net gangs and commercial fishery. Stocked fish comprised about 30% of the total stock. There was no significant difference in spatial distribution and diet between native and stocked fish. Both groups used the littoral zone, and Gammarus lacustris and Lepidurus arcticus were the main food items. Stocked brown trout reached a specific length one year younger than corresponding wild fish. Therefore stocked brown trout entered the commercial catch one year younger than the native fish.     

Effect of triploidy on paternal and maternal variance components in brown trout, Salmo trutta L.

The covariation between diploid and triploid progenies from common breeders and the effect of triploidy on the parental variances were investigated in brown trout (Salmo trutta L.) using two progeny testing experiments, sampling, sires and dams respectively, from the same population. The traits studied were body weight, growth, condition factor and red spotting of the skin. Triploidization generated some interactions with the parental breeding value, but their effect was minor (less than 20% of the genetic variance, in most cases) as compared with the amount of variation common to both ploidy levels. These interactions were mainly caused by a scale effect, triploidy reducing the variance attributable to sires and increasing the variance attributable to dams. Actual lack of correlation (genetic correlation significantly less than 1) between diploid and triploid familial performances was observed in a single instance out of 18. The modification of respective parental variance components by triploidy, already observed in the rainbow trout Oncorhynchus mykiss (Walbaum), appears as a logical consequence of the genetic make‐up of triploids, and should be taken into account in selective breeding.     

Growth and feeding responses of a hatchery population of brown trout (Salmo trutta L.) at low temperatures

Abstract— Feed intake, specific growth rate and changes in body composition were studied in age 1+ (140-190 g) brown trout ( Salmo trutta L.) reared at three temperatures (2, 4 and 6°C) under continuous light conditions. Feed in take increased from 35.7 kJ-kg⁻¹.day⁻¹ at 2°C to 95.7 kJ-kg⁻¹-day⁻¹ at 6°C, and the growth rate increased from 0.19%-day⁻¹ to 0.42%-day⁻¹ over the same temperature range. The estimated lower temperature limits for feeding and growth were slightly above 0°C. For all groups of fish, the majority (about 75%) of the weight gain comprised water and protein, but lipid deposition tended to increase with increasing temperature. The deposition of lipid accounted for about 50% of the body energy gain at all temperatures.     

Experimental production of triploid brown trout, Salmo trutta L., using heat shock

Abstract. This study was designed to investigate the potential of heat shock to produce triploidy in brown trout, Salmo trutta L., and to develop a methodof routinely identifying triploids in this species. Triploids were produced in all heat-shocked batches and were identified by the size of their erythrocyte nucleus, which had a volume ratio of 1:1-57 relative to diploid controls. Cytogenetic and flow cytometric analyses confirmed that trout with the larger nuclei were triploid. Heat shock of 28°Cof 10 min duration initiated 5-15 min post-insemination produced high rates of triploidy in experimental batches (88-2-100%), later shocks at 20-25 min producing lower rates (down to 60%). Reproducibilicy of tripioid rates was generally good, a maximum difference between replicates of 21.9% being observed, the majority of differences being considerably less. The highest triploid yield was produced with a heat shock of 28°C for 10 min initiated at 10 or 20 min post-insemination, the difference between replicates being due to variability in survival to hatch. Survival to hatch was generally lower in groups having higher rates of tripioidy.