No difference in survival, growth and morphology between offspring of wild-born, hatchery and hybrid brown trout (Salmo trutta)

Abstract –  We studied survival, growth and morphological characters in the offspring of native hatchery and wild-born anadromous brown trout ( Salmo trutta ) and their hybrids (wild-born female × hatchery male and wild-born male × hatchery female) in a 1-year field experiment. We also conducted laboratory studies where we examined social interactions between the offspring of the same hatchery and wild-born trout. All offspring were raised in a hatchery and nose tagged before being released into the stream. In total, 1125 individuals were released into the stream (1999) and a total of 614 individuals were recovered (2000). We found no differences in growth and survival between the offspring of hatchery, wild-born and hybrid trout. Morphology was also similar among groups, where only 38% females and 36% males were classified into the right category, which were only 12% better than random classification. In the laboratory experiment, we compared only the offspring of hatchery and wild-born trout with respect to growth, dominance, aggressiveness, feeding and activity. We found small differences between the offspring of hatchery and wild-born fish with respect to growth but this effect was not found in the field experiment. Our result suggests that the offspring of hatchery trout and hybrids between hatchery and wild-born trout performed equally well to the offspring of wild-born trout.     

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.     

Differentiation of gonad maturation in sibling precocious males of the sea trout (Salmo trutta m. trutta L.) in their first year of life

Age at maturity and dynamics of gonadal cycle vary among individuals in a standard population; the extent of that variability in a group of siblings is not known for any species. Therefore, the aim of the study was to find out how the differentiation of gonad development varies in a group of sibling fish. The study involved 176 seven-month-old sea trout parr (mean fork length of 11 cm) collected at a single time point during their first spawning season (late October) from the brook of Pniewa in Western Pomerania (Poland). Gonad maturation was determined from histological analysis which was interpreted according to specially developed criteria.

The stage and forms of gonad maturation were diverse in the sibling group studied. Gonads of 52.8% males were not maturing (Class I), with only spermatogonia A present. The remaining males (47.2%) had gonads differing widely in the development, from beginning of maturation to completion of the reproductive cycle (Classes from II to VIII). In addition to the classes described, some males were observed to feature attempted maturation. It is difficult to provide an unequivocal estimate of the proportion of precocious males (mature at a young age, prior to the first descent to the sea) in the first year of life because of difficulties in categorizing males in Class II. It was predicted that 32.4% of the population would complete the cycle during the first spawning season. The body length and weight of the maturing males was significantly smaller (P < 0.05) than that of the non-maturing males.

Three forms of gonad maturation were identified. Gonads of a large group of precocious males (70%) developed incompletely (only some germ cell developed during the reproductive cycle), some of those gonads assumed the form of attempted maturation and partial maturation. Testes of the remaining 30% of the males underwent complete maturation. Gonads that undergo attempted maturation do not spermiate.

It is only from microscope-aided analysis of the testes that the class of gonad development can be accurately evaluated and the extent of precocious maturation assessed, particularly in fish maturing for the first time. It is impossible to correctly identify maturing males of the underyearling sea trout on the basis of morphological examination of gonad appearance, abdominal massage to detect spermiation or gonadosomatic index value.     

Spatial and temporal variation in prey selection of brown trout in a cold Arkansas tailwater

Abstract –  We examined prey utilisation patterns of brown trout ( Salmo trutta Linnaeus) as related to available benthos in a cold tailwater (Little Red River, Arkansas) having low biodiversity of benthic macroinvertebrates and low availability of fish as prey. Benthic macroinvertebrates were sampled monthly in upstream and downstream sites over a 1-year period. Stomach content analysis was also performed on 30 brown trout (10 each of three size classes) collected monthly from each site. Benthic and diet diversities were low at both sites, as isopods ( Lirceus ) accounted for 67% and 51% of the numerical abundance upstream and downstream, respectively, and 80% and 70% of all prey consumed by upstream and downstream brown trout. Physid snails (upstream) and chironomid larvae (downstream) accounted for most of the remaining prey taxa consumed. Piscivory and consumption of terrestrial invertebrates were rare. There was no clear relation between diet diversity and trout size. Densities of benthic macroinvertebrates and prey consumption were significantly greater upstream than downstream. Benthic macroinvertebrate assemblages and prey consumption were seasonally constant at both sites. Generally, prey consumption was consistent with availability of each prey in the benthos; the lack of benthic diversity and overwhelming abundance of one taxon ( Lirceus) most likely contributed to food selection patterns. Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna because of the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major limitation to brown trout growth.     

Water level influences migratory patterns of anadromous brown trout in small streams

Habitats modify the population ecology of species. Here, we show how low water level influences abundance and size of adult anadromous brown trout (Salmo trutta) entering a small, South Norwegian stream for spawning. After smolting, the fish appear chiefly to feed within 10 km of the home stream. In the autumn, South Norwegian streams typically flood because of heavy rainfall, when the anadromous brown trout entered from the sea. Mean annual duration of the upstream migration period was 34 days and ended when the flooding ended and the water temperature dropped to below 4°C. During most of the migration period, on average two trout ascended the river per day. The sexes entered the spawning area concurrently, and the male:female ratio of the anadromous trout was 1.27. No fish entered when the water depth just downstream of the spawning area was below 5 cm, and mean number of fish increased with increasing water depth to ca. 30 cm, but not at higher flows when the ascent gradually decreased. Mean and maximum size of the entering spawners increased with water depth between 5 and 16 cm. Among those that had been to sea, most were recaptured in the home stream, 4% in other streams, but only two of the strays were caught close to spawning time. The present results illustrate that population traits of anadromous brown trout from a small stream differ from those in larger rivers, probably because of selection associated with water flow.     

Genetic characterization of Portuguese brown trout (Salmo trutta L.) and comparison with other European populations

Abstract– Allozyme and other protein loci were examined to study the genetic structure of Portuguese brown trout ( Salmo trutta ) populations. A total of 247 individuals from three tributaries of the Lima hydrological basin and a hatchery, all located in northern Portugal, were analyzed. Four of 22 protein coding loci were found to be polymorphic: CK-A1^*, GPI-A2^*, MPI-2^* and TF^*. A new allelc at the latter locus was found in Atlantic populations. The data obtained for Portuguese brown trout were compared with published data for 14 European populations and three hatchery stocks. Six polymorphic loci (CK-A1^*, GPI-A2^*, GPI-B2^*, LDH-C^*, ME^* and MPI-2^*) were used in a cluster analysis. This showed the similarity of Portuguese natural populations and northern Iberian populations and that Portuguese hatchery fish have an autochthonous origin, distinct from that of other Atlantic hatchery stocks.     

Acoustic characteristics of two feeding modes used by brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss) and turbot (Scophthalmus maximus)

Under conditions of intensive culture, the acoustic signals produced by fish during feeding depend on their feeding mode. Exclusive suction, as used by turbot, is characterised by a maximum acoustic energy in the frequency range 7–9 kHz and a sound duration of about one minute depending of time duration of pellet distribution. Suction feeding in conjunction with forward swimming, as employed by brown trout and rainbow trout, had a maximum acoustic energy in the frequency range 4–6 kHz and feeding sounds were measurable only for short periods (less than 1 s) in between two pellet distributions by hand. The brevity of these feeding sounds (ca. 1000 ms) requires adapting the turbot acoustic-detection systems to actively feeding fish for developing automated feed distribution systems feasible in trout aquaculture.     

Effect of habitat type on growth and diet of brown trout, Salmo trutta L., in stream enclosures

The effect of habitat on the growth and diet of brown trout, Salmo trutta L., stocked at the same densities in nine stream enclosures, comprising three habitat types of different quality, were tested. The habitats, which were created based on microhabitat preference data, were a shallow water habitat lacking cobbles (habitat 1), a deeper, mixed cobble-bottomed (128-384 mm diameter) habitat (habitat 2) and a large cobble-bottomed (256-384 mm) habitat of intermediate depth (habitat 3). Brown trout were found to have greater increases in total biomass in habitats 2 and 3 than in habitat 1. The pattern for length did not follow that of biomass as trout had greater increases in total length in habitat 2 than in the other two habitats. Biomass of food in trout diets reflected habitat-specific fish biomass changes, with a greater total biomass of prey as well a greater biomass of the leech, Erpobdella, in habitats 2 and 3 than in habitat 1. There were no habitat-specific differences in the biomass of benthic or drifting invertebrates in the enclosures, with the exception of a tendency for an effect of habitat on the biomass of Erpobdella. Although there may have been habitat-specific differences in food resources that were not detected, it is believed that the higher biomass growth in habitats 2 and 3 may have reflected differences in cover afforded by the deeper water and coarser substrates and/or improved foraging opportunities facilitated by the larger volumes of water in the deeper habitats in which the trout could search for prey.     

Recaptures and resource use of native and non-native brown trout, Salmo trutta L., released in a Norwegian lake

Abstract Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.     

Small-scale spatial variation in age and size at maturity of stream-dwelling brown trout, Salmo trutta

Abstract – This study documents substantial small-scale spatial variation in age and size at maturity of brown trout ( Salmo trutta ) found either in allopatry (above major waterfalls) or in sympatry (below waterfalls) with the Alpine bullhead ( Cottus poecilopus ) in forest streams in south-east Norway. Within two streams, female brown trout above waterfalls tended to delay the onset of sexual maturity, as compared with females from neighbouring sites below the waterfalls. Four additional streams were represented with either an allopatric or a sympatric site. There was considerable variation in age and size at maturity among these streams, but no consistent difference between allopatric and sympatric sites. It is suggested that the spatial variation in maturity responses is influenced by local opportunities for growth, and possibly also survival. Earlier studies in these streams have linked spatial variation in brown trout behaviour and demography to the presence or absence of the Alpine bullhead.     

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.     

Influence of environmental factors on the density and biomass of stocked brown trout, Salmo trutta L., parr in brooks affected by intensive forestry

Abstract The influence of environmental factors on the density and biomass of stocked brown trout, Salmo trutta L., parr was studied in brooks subjected to intensive forestry in the Isojoki river basin, western Finland. Multivariate regression analysis showed that 69% of the variation in the population density of parr was determined by five variables: (1) mean water depth; (2) the abundance of pools; (3) stony bottom substrates with stones sized between 2 and 10cm in diameter; (4) undercut banks; and (5) the percentage of shading by trees. Correspondingly, 57% of the variation in the biomass of parr was determined by three variables: (1) mean water depth; (2) the abundance of pools; and (3) benthic vegetation. Dredging of the brooks and forest ditching had the most harmful consequences for the nursery habitats of brown trout parr. Measures for the rehabilitation of brown trout production in these brooks are discussed.     

The residence time of stocked hatchery-reared brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in Lake Trawsfynydd, and their accumulation of radioactive caesium

Abstract Small amounts of radioactivity in liquid effluent are discharged under authorization into Lake Trawsfynydd in north Wales. MAFF inspectors from the Directorate of Fisheries Research (DFR) advise the Welsh Office on the terms of authorization and on the power station operators' compliance with them. DFR also has the responsibility for environmental monitoring, including fish caught for consumption. Trout angling is particularly popular in Lake Trawsfynydd, and because of angling pressure, additional brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), need to be introduced. An important factor in determining the concentration of radionuclides in these fish is the length of time that they spend at liberty. During the fishing season, samples of these stocked trout were tagged and released to assess the average residence time. This was found to be 6 days for rainbow trout and 10 days for brown trout. Less than 7% of the recaptured stocked trout of either species had a residence time of more than 20 days. Radiocaesium concentrations in recaptured trout were very low. Trout which avoided recapture and overwintered in the lake were found to have radiocaesium concentrations similar to those of indigenous trout sampled at the same time.     

Recovery of Hafnia alvei from diseased brown trout,Salmo trutta L., and healthy noble crayfish,Astacus astacus (L.), in Bulgaria

Hafnia alvei was isolated in Bulgaria from healthy noble crayfish, Astacus astacus (L.), and then from farmed diseased brown trout, Salmo trutta L., with signs of haemorrhagic septicaemia. The isolates were identified initially with conventional phenotyping and commercial Merlin Micronaut and API 20E rapid identification systems, followed by sequencing of the 16S rRNA gene. Hafnia alvei Bt1, Bt2 and Aa4 were of low virulence to rainbow trout and brown trout, although cytotoxicity was demonstrated by Bt1 and Bt2, but not by Aa4.     

Comparative efficacy of Pyceze® (bronopol) in controlling mortality of brown trout Salmo trutta eggs

The efficacy of Pyceze^® (Novartis Animal Vaccines) and Proxitane^® 0510 (Solvay Interox) in controlling the mortality of eggs was studied in brown trout Salmo trutta eggs under the usual incubation conditions in a hatchery affected by saprolegniosis. Eggs from eight spawnings and from two lines of brown trout were used. The cumulative mortality of eggs at the start of the eyed stage (M₁) and at hatching (M₂) was measured, as was the percentage of eggs with fungal infection at weekly intervals during the green stage. Mortality at M₂ with Pyceze^® ranged between 2.38% and 12.61% depending on the batch, with a mean of 6.10%. Mortality at M₂ with Proxitane^® varied between 5.83% and 43.86%, with a mean of 22.36%. Fungal colonization at the start of the eyed stage ranged between 0% and 0.15% when Pyceze^® was used and between 0.82% and 12.50% with Proxitane^®. Mortality rates were higher among those eggs left untreated. The results indicate that Pyceze^® (bronopol) is efficacious in controlling mortality caused by Saprolegnia spp. and other biological factors in fertilized brown trout eggs, as has been demonstrated previously in other salmonid species.     

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.     

A test for interaction between brown trout (Salmo trutta) and inanga (Galaxias maculatus) in an artificial stream

Abstract –  The interaction between brown trout ( Salmo trutta ; fork length (FL) range 255–390 mm) and inanga ( Galaxias maculatus ; FL range 55–115 mm) was tested during summer through autumn in an artificial stream consisting of a single run-riffle-pool sequence with a natural food supply. Each experimental trial lasted for 15 days, and consisted of two brown trout and 50 inanga collected fresh from a nearby stream, with each species given prior residence in four replicate tests, totalling eight trials in all. In addition, two control trials (each 10 days), with 50 inanga in each, were run. Brown trout almost exclusively occupied the pool, whereas inanga occupied all habitat types, although in different proportions, when tested with and without brown trout. The proportion of inanga in the pool was appreciably lower in the experimental trials with brown trout than in the control trials with no brown trout; prior residence had no significant effect on inanga habitat use. Mortality of inanga attributable to predation by brown trout ranged from 0 to 40% with a mean of 14.5 ± 4.7%. The results suggest that habitat use and survival of inanga populations in small streams can be adversely affected by brown trout.