Effect of an intensive mechanical removal effort on a population of non‐native rainbow trout Oncorhynchus mykiss in a South African headwater stream

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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.     

Micro‐scale distribution of brown trout: an opportunity for kin selection?

Abstract – Fourteen microsatellite loci were used to analyse the degree of relatedness between and within cohorts of stream-living brown trout ( Salmo trutta L.). Differences in degree of relatedness were found between cohorts with mature and older having the highest intracohort relatedness. The higher genetic relatedness among the mature and older trout is interpreted as indications of clustering of related mature and older trout prior to spawning in the vicinity of spawning grounds. Young-of-the-year trout showed a lower degree of association with related trout of the same cohort than did mature and older trout. It is argued that the clustering of mature and older trout may facilitate kin selection and that the low degree of association among related young-of-the-year trout could be explained by juvenile fish avoiding competition for territories with related individuals.     

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.     

A conceptual model for examining density dependence in the growth of stream trout

Determination of the occurrence and importance of densitydependent responses is central to understanding stream trout population dynamics. I propose a conceptual model of growth, based on a distribution of feeding site quality, that considers the effects of density on growth. The site quality model assumes that trout select the best feeding sites available and, as the number of trout increases, they will be forced to use less energetically profitable sites, resulting in decreased growth, but also an increase in variance of size or growth. Results from a 3-year study of a brown trout (Salmo trutta) population show that growth to age 1 was significantly reduced for the 1981 year class, which was about 4 times more dense than other year classes. The reduction in growth was not due to a decrease in the growth of all trout but rather an increase in the number of slower growing fish. These results are consistent with my site quality distribution model and suggest that individual fish growth, the distribution of growth, and the variance of growth rates should be considered in addition to mean size and growth.     

Exploring the relative influence of biotic interactions and environmental conditions on the abundance and distribution of exotic brown trout (Salmo trutta) in a high mountain stream

Abstract – In the Logan River, UT, USA, exotic brown trout demonstrate a strong allopatric distribution and occur at high densities at low‐elevation sites and in tributaries, and in low densities at native trout dominated, high‐elevation sites. Summer temperatures and discharge do not appear limiting for growth; adult growth rates were high overall and were greatest when fish were held experimentally at high elevation where they do not occur naturally. Brown trout are superior competitors; competition for space or food was stronger with their own con‐specifics than with other species. Evidence of density dependence was not observed at the juvenile life stage; no consistent relationships were detected between brown trout density and age‐1 condition or lagged, age‐0 weight (g). In contrast, adult brown trout demonstrated density‐dependent effects on condition and growth when reared experimentally. Field estimates of adult growth rates (g·day^?1), although variable, declined subtly with increasing density, and annual survival was significantly greater in the mainstem sites (mean = 52%) relative to a high‐density tributary site (mean = 22%). Annual predicted age‐0 brown trout growth potential was four‐times greater at the lowermost site, compared with the highest elevation site, although fish lost weight over winter months at all sites. While adult density dependence may influence population abundance at some sites, extreme spring–winter conditions may ultimately limit the upper elevational extent of brown trout in this system. With changing climatic conditions and the potential for habitat degradation in the future, these results have important implications for native fish conservation.     

Changes in the density and growth of brown trout (Salmo trutta) after intensive removal of sympatric Arctic charr (Salvelinus alpinus) in the sub‐Arctic lake Møkkelandsvatn,Norway

This study tests the basic hypothesis that the removal of charr, Salvelinus alpinus (L.), would cause an increase in both the growth and density of a sympatric trout population, Salmo trutta L. The charr population was characterised by slow‐growing individuals, with a high proportion of mature fish, that is typical for so‐called overpopulated populations. A total of 31,000 charr was removed from the lake in the period 1990–1992, and the density of younger trout (1+, 2+), but not older trout (3+, 4+), increased. The growth of older trout (3+, 4+) increased, but the evidence for similar growth increases of younger trout (1+, 2+) was limited. From 1989 to 1990, the proportion of trout increased from 30 to only 40% of the total catch, but from 1991 to 1994, it was significantly higher (60–80%) than that of charr. Total trout biomass increased to a maximum in 1992 and then decreased so that the biomass of 1994 was nearly similar to that of 1989, that is before the start of the charr removal. Back‐calculated lengths of trout from otoliths showed that 2+ and 3+ trout caught in the pelagic were growing consistently faster over previous years than those caught in the littoral, while this was not the case for the 4+ fish. Therefore, the hypothesis was partially supported; the growth rate of trout increased (age groups 1+ to 4+), while the density of juvenile trout (1+, 2+), but not the older trout (3+, 4+), increased after the removal of charr.     

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.     

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.     

A modelling approach to evaluate potential management actions designed to increase growth of white perch in a high‐density population

Abstract A deterministic, age‐structured population model was developed to simulate potential management scenarios designed to increase growth of individuals (quantified by maximum length) in a hypothetical population of white perch, Morone americana (Gmelin). Four scenarios were developed that included non‐selective mortality of adult white perch, increased mortality of age groups most influential on population growth, increased age‐0 mortality and inhibiting recruitment after spawning. The greatest increase in maximum length occurred with non‐selective adult mortality when population biomass was reduced by 97%; lesser increases in maximum length were achieved with the other management scenarios. Populations returned to their original state after control efforts ceased for each scenario.     

Biotic and abiotic regulation of a low‐density stream‐dwelling brown trout (Salmo trutta L.) population: effects on juvenile survival and growth

The effects of biotic (density‐dependent) and environmental (flow and temperature) factors on the apparent survival, mean length and size variation of a low‐density brown trout population in the juvenile stage, that is, from their first summer (0+) to the end of the second year (1+), were determined. Apparent survival was negatively related to the age class density during the three periods (first summer, first winter and second summer). A significant interaction between the mean flow and 0+ density highlighted a gradient towards strong density dependence acting on fish loss (i.e., mortality or migration) with decreasing summer flow. Conversely, no density dependence was reported at higher mean flows. The mean length was determined by density‐dependent and density‐independent (temperature and flow) factors throughout the study period. The negative relationship between fish length and intracohort density was highly significant during the three periods. The yearling (1+) density was negatively related to 0+ fish length measured after the first summer, suggesting intercohort effects. A positive effect of temperature on fish length was observed. Mean length after the summer seasons (0+ and 1+ fish) was also positively related to mean flow. Fish size variation around the mean measured with the coefficient of variation (CV) increased with increasing 0+ densities, both at the end of the first summer and the first winter. Results suggested that density‐dependent and density‐independent factors acted jointly on apparent survival and growth with a predominance of biotic processes. We discussed the potential implications of density‐dependent regulations on growth and survival for population resilience after catastrophic events.     

Swimming performance of brown trout and grayling show species‐specific responses to changes in temperature

Fishways have historically been constructed to restore and preserve the ecological connectivity for fish in fragmented rivers. However, the fishways are often selective on species due to different size and swimming capacity. As the proportion of dammed rivers is still increasing, there is a growing need for more information on wild fish and their migration potential. In this study, we compared the swimming capacity of wild caught brown trout (Salmo trutta) and European grayling (Thymallus thymallus) until the fish were exhausted in a critical swimming speed (U_crit) test, under three different naturally occurring stream temperatures in Norway: 1.7, 5.5 and 10°C. The results indicated that trout swim better at the warmer temperatures than at colder temperatures. The grayling showed consistent swimming patterns with little variation across all tested temperatures. The results therefore signify the need to have operational fishways already early in the spring when the grayling migration starts and highlight the need for more studies on fish migration abilities across a wider range of species and seasons.     

Use of first‐order tributaries by brown trout (Salmo trutta) as nursery habitat in a cold water stream network

Many investigators have examined the importance of suitable in‐stream habitat and flow regime to salmonid fishes. However, there is much less known about the use of small (<5 l·s^?1 discharge) first‐order streams within a larger stream network by salmonids. The purpose of this study was to evaluate the use of small headwater streams by juvenile brown trout Salmo trutta in the Emmons Creek stream network in Wisconsin, USA, and to determine whether abundance was related to habitat variables in these streams. Fishes in eight spring‐fed first‐order streams were sampled during a 7‐month period using a backpack electroshocker and measured for total length. Habitat variables assessed included stream discharge, water velocity, sediment composition and the abundance of cover items (woody debris and macrophytes). Densities of YOY trout ranged from 0 to 1 per m² over the course of the study and differed among first‐order streams. Stepwise multiple regression revealed discharge to be negatively associated with trout density in spring but not in summer. All other habitat variables were not significantly related to trout density. Our results demonstrate the viability of small first‐order streams as nursery habitat for brown trout and support the inclusion of headwater streams in conservation and stream restoration efforts.     

Total population density during the first year of life as a major determinant of lifetime body‐length trajectory in marble trout

Abstract – The conditions experienced early in life can strongly influence life‐history trajectories in a variety of animal species. Here, we use data from four isolated populations of the endangered stream‐dwelling salmonid marble trout (Salmo marmoratus Cuvier 1817) living in the Soca and Idrijca river basins (Slovenia) to explore the influence of the total density experienced during and after the first year of life by marble trout year‐classes on body length of marble trout through the lifetime. Analyses were performed by pooling together the stream‐specific datasets to cover a wider range of densities. Mean body length of marble trout year‐classes through the lifetime (from age 1+ to 5+) was negatively related to total density of marble trout during the first year of life. The relationship between density during the first growth period and body length through the lifetime was well described by negative power curves. Total population density after the first year of life was not correlated with body length, thus suggesting that body growth trajectories are heavily determined early in life. Given size‐dependent sexual maturity and egg production in marble trout, the relationship between density early in life and lifetime individual growth may have strong implications in terms of population dynamics and regulation of population size.     

Effects of total fish oil replacement to vegetable oils at two dietary lipid levels on the growth,body composition,haemato‐immunological and serum biochemical parameters in caspian brown trout (Salmo trutta caspius Kessler, 1877)

This research aimed to evaluate the effects of two dietary fat levels [low fat (LF) (10%), high fat (HF) (20%)] and sources [fish oil (FO), vegetable oil (VO)] on the growth and some physiological parameters of Caspian brown trout fingerlings for 60 days. Tuna oil or blends of canola and soybean oils (85:15) were added to diets to design four feeds namely LFFO, HFFO, LFVO and HFVO according to the fat levels and sources. The fish fed the LFFO diet had lower weight gain than the other fish (P<0.05). The total n‐6 fatty acids increased in fish fed diets with the blends of VO, while the total n‐3 fatty acids decreased in these fish (P<0.05). Serum lysozyme activity was higher in fish fed the HFVO diet than the other fish (P<0.05). Serum glucose, total cholesterol, triglyceride and very low‐density lipoprotein were lower in fish fed LFFO than the other fish (P<0.05). The present study demonstrates that in terms of fish growth, VOs can be used as an alternate source of dietary fat, whereas fish health and nutritional value are improved with the LFFO diet. According to these results, a partial substitution of FO by VO in high‐level fat diets is suggested for long‐term feeding of Caspian brown trout.     

LHRHa‐induced ovulation of the endangered‐Caspian brown trout (Salmo trutta caspius) and its effect on egg quality and two sex steroids: testosterone and 17α‐hydroxyprogestrone

To induce synchronized ovulation, migrating wild Caspian brown trout (Salmo trutta caspius) females were treated with two interperitoneal injections of Des‐Gly¹⁰, d ‐Ala⁶ LHRH (LHRHa), given 3 days apart. Two injections of 100 μg kg^?1 body weight of this hormone effectively induced ovulation. Within 27 days from the second injection, all fish injected with 100 μg kg^?1 LHRHa had ovulated compared with 54.5% of the controls. The mean time to ovulation was reduced significantly (P<0.05) from 31.67±4.84 days in control fish and 28.83±7.31 days in sham‐treated fish to 16.36±1.61 days in fish injected with 100 μg kg^?1 LHRHa. The fertilization rate in 50 and 100 μg kg^?1 LHRHa‐injected fish was significantly lower than that in the control fish (P<0.05). In fish injected with 50 and 100 μg kg^?1 LHRHa, significant (P<0.05) changes in testosterone (T) and 17α‐hydroxyprogestrone (OHP) levels were observed. After the second LHRHa injection, the fish injected with 100 μg kg^?1 showed the highest serum levels of testosterone and OHP. These results demonstrate that the use of LHRHa can effectively reduce the mean time to ovulation and induce synchronized ovulation in Caspian brown trout.     

Combination effects of dietary EPA and DHA plus alpha‐tocopherol: effects on performance and physiological status of Caspian brown trout (Salmo trutta caspius) fry

The effects of dietary n‐3 highly unsaturated fatty acids [eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA)] with α‐tocopherol on growth, non‐specific immune response and oxidative status were investigated in Caspian brown trout, Salmo trutta caspius, fry. Six experimental diets containing three different dietary levels of n‐3 HUFAs (low: 1 + 0.5% of total fatty acids, DHA+ EPA, medium: 2 + 1%, DHA + EPA, high: 4 + 2%, DHA + EPA) with two different levels of α‐tocopherol (low: 300 and high: 1000 mg kg^?1 diet) were prepared and named: LL, LH, ML, MH, HL and HH (HUFA/α‐tocopherol) groups, respectively. Diets were fed to triplicate groups of 60 fry with an initial weight of 600 ± 25 mg for 10 weeks. Results showed that increase in dietary DHA and EPA up to high level improved fry growth in terms of the body weight and specific growth rate, particularly when dietary α‐tocopherol levels were high, suggesting a higher antioxidant protection value when these fatty acids are high. At all dietary n‐3 HUFA levels, increase in α‐tocopherol from low to high level enhanced the alternative complement (ACH50) activity. Fry fed diets medium and high n‐3 HUFA displaying significantly higher lysozyme activity (P < 0.05). Moreover, fish fed medium or high levels of n‐3 HUFA had significantly lower prostaglandin E2 (PGE2) than those fed low n‐3 HUFA (P < 0.05). Significant differences in antioxidant enzymes (catalase, glutathione peroxidase, glutathione S‐transferase, glutathione reductase and superoxide dismutase) activity were also observed between groups, with higher activity in high levels of n‐3 HUFA (P < 0.05). Results of this study suggest that the effect of dietary n‐3 HUFA on examined non‐specific immunity parameters are not uniform; however, these impacts are closely related to the α‐tocopherol supplement and their interaction. In conclusion, increased dietary levels of n‐3 HUFA and α‐tocopherol would enhance growth performance and welfare of this species.     

Direct and indirect effects of riparian canopy on juvenile Atlantic salmon,Salmo salar,and brown trout,Salmo trutta,in south‐west Ireland

Abstract Dense riparian tree canopy is generally found to have a negative effect on salmonid populations. Canopy can affect fish both directly via trophic impacts and its effects as cover, and indirectly via its effects on the distribution of instream vegetation. This study examined the impact of riparian canopy on the ecology of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in south‐west Ireland. Riparian canopy reduced the density, length and gut contents of juvenile salmon, but not brown trout. The negative relationship between canopy cover and fish size and feeding was strongly influenced by the abundance of instream macrophytes, which in turn varied as a function of catchment water chemistry. As a management strategy, the reduction of dense riparian canopy is unlikely to have the same effect on juvenile salmonids on all streams within an ecoregion because of differences in catchment‐wide factors.     

Stocking impact,population structure and conservation of wild brown trout populations in inner Galicia (NW Spain), an unstable hydrologic region

1. Brown trout (Salmo trutta) is an important conservation resource in the Iberian Peninsula. The Atlantic is considered the most hydrologically stable region for the species, although inner Galicia (NW Spain) shows Mediterranean (unstable) climatic conditions. The Galician region, threatened by past releases of brown trout individuals from central European origin, harbours two native lineages, one of them endemic to the Iberian Peninsula. These populations are thus highly valuable for conservation, as well as being important for recreational fisheries. 2. In total, 546 individuals from 16 sampling sites (15 natural locations from inner Galicia and one from a central European hatchery stock) were genotyped for 11 nuclear markers (10 microsatellite loci and the LDH‐C* locus) to analyse genetic variability, population structure and introgression impact from stocking in order to assess the conservation status of brown trout in the region. Moreover, correlation among hatchery introgression and environmental variables relevant for species population dynamics was also investigated. 3. Genetic variability was within the range of Iberian brown trout (He = 0.500–0.600). Stocking impact was higher than previously reported values for the Atlantic region and was related to environmental instability. Highly significant native population differentiation was observed in the whole region (F_ST = 0.283), at least four main genetic groups being detected across the geographic distribution studied. 4. Conservation strategies at local level (including the creation of genetic refuges and temporal monitoring of genetic composition) are suggested to agencies and administrations for the sustainable management of brown trout.     

Diet of the Eurasian otter (Lutra lutra) on the River Gradac,Serbia: Predation in a brown trout‐dominated stream

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