Stocking hatchery‐reared brown trout in different densities into a wild population – a comparison of growth and movement

In spring 2001 and 2002 a small stream was stocked with tagged hatchery‐reared yearling brown trout ( Salmo trutta ), in order to study their influence on the resident brown trout population. The stream was separated into six sections: two sections without stocking, two sections where stocking doubled the trout population and two sections where the fish population was quadrupled. The working hypothesis was that due to food limitation (competition) growth of the wild fish will be negatively influenced by stocking, and wild fish will be displaced by the (possibly more aggressive) hatchery fish. Surprisingly, growth rate of wild and stocked fish of the same age was similar and independent of stocking density. Two main reasons may be responsible for this finding: only a low percentage of the stocked fish remained in the stream, and food was not limited during summer. Only 12–19% of the stocked fish were recaptured after six months, in contrats to 40–70% of one‐year old and up to 100% of older wild trout. The wild fish were not displaced by hatchery‐reared fish: During summer the wild fish remained more or less stationary, whereas most of the stocked trout had left their release site. The results indicate that in a natural stream stocking of hatchery reared brown trout does not influence negatively growth and movement of the wild fish independent of stocking density.     

Recapture rate and growth of hatchery‐reared brown trout (Salmo trutta v. fario,L.) in Blanice River and the effect of stocking on wild brown trout and grayling (Thymallus thymallus,L.)

Hatchery‐reared adult brown trout, Salmo trutta v. fario L., [215–335 mm standard length (L_S), n = 82] were individually tagged and released into three sections of the Blanice River in May 2007. Wild populations of brown trout and grayling, Thymallus thymallus, L., in these sections and three non‐stocked control sections were also tagged. The recapture rate of hatchery‐reared adult brown trout after 6 months (18%, n = 15) was comparable to that of wild adult brown trout in stocked (15%, n = 14) and control (14%, n = 11) sections. The recapture rates of wild brown trout and grayling after 6 months were higher in control sections than in stocked sections, but the differences were not significant. The movement of recaptured large juvenile wild brown trout from stocked sections was significantly higher (36%) than from control sections (9%). Wild brown trout growth and grayling growth were unaffected by stocking with adult hatchery‐reared brown trout.     

Partial migration in a landlocked brown trout population

Population densities of landlocked lake‐migratory brown trout Salmo trutta were estimated in two distinct lotic sections, separated by a lentic segment, in the Greåna River, Sweden, and individual growth and habitat use were monitored for 835 tagged brown trout from September 1998 to June 2000. Residency dominated in the upstream section where density of 0+ and 1+ year brown trout was low and growth rate high. In contrast, >90% of the brown trout that migrated to the lake originated from the downstream section, where density was high and growth rate low. For ≥2+ year individuals, growth rate was similar between the two stream sections, but densities were higher in the upstream than in the downstream section. Lake‐migrants had higher growth rates than non‐migrants (residents) during the autumn of both years. From September to May, migrants increased their body mass by >35%, whereas non‐migrants increased by <5%. Approximately 70% of the brown trout moved <10 m and <2% moved between the two stream sections, indicating that the lentic habitat might function as a barrier for juveniles. Differences in migratory behaviour, density and growth between the upstream and the downstream section might indicate that environmental factors influence the decision to migrate. It cannot be excluded, however, that the observed differences are genetically programmed, selected by migration costs that favour migratory behaviour downstream and residency upstream.     

Performance of juvenile brown trout exposed to fluctuating water level and temperature

Individual daily food intake, mass‐specific growth rate and growth efficiency in groups of juvenile brown trout Salmo trutta were compared in tank experiments with three water level regimes (fluctuating, stable high and low water levels) and two temperature regimes (fluctuating between 10 and 14° C and constant 14° C) to simulate events during hydropeaking in regulated rivers. Fish exposed to high stable water level showed higher food intake and growth rate, and higher or similar growth efficiency than fish exposed to fluctuating or stable low water level. Both groups of slow‐growing and fast‐growing individuals fed less and grew slower at stable low and fluctuating water level than at stable high water level. Furthermore, growth and growth efficiency were lower in brown trout exposed to stable low water level and fluctuating temperature, particularly for groups of fish with slow growth. Temperature did not have any effect at high water level. For groups of fast‐growing fish, there was no difference in growth efficiency between treatments. It is concluded that fluctuating water level and temperature have a potentially detrimental effect on growth in juvenile brown trout and effects are more severe in slow‐ than fast‐growing fish.     

Density‐dependent growth in hatchery‐reared brown trout released into a natural stream

Hatchery‐reared brown trout Salmo trutta stocked in a natural stream in addition to resident wild brown trout grew more slowly than those stocked with an experimentally reduced density of brown wild trout. In both cases, hatchery‐reared brown trout grew more slowly than resident wild fish in control sections. Mortality and movements did not differ among the three categories of fish. The results showed that growth of stocked hatchery‐reared brown trout parr was density‐dependent, most likely as a consequence of increased competition. Thus, supplementary release of hatchery‐reared fish did not necessarily increase biomass.     

Habitat degradation and trout stocking can reinforce the impact of flash floods on headwater specialist Alpine bullhead Cottus poecilopus – A case study from the Carpathian Mountains

In freshwater streams, flooding is a typical source of natural disturbance that plays a key role in the dynamics of animal populations and communities. However, habitat degradation and fish stocking might increase the severity of its impact. We tested the effects of a flash flood on the abundance of three size classes of headwater dwelling Alpine bullhead, Cottus poecilopus, in the streams of the Carpathian Mountains in the Czech Republic, that are stocked with hatchery‐reared brown trout, Salmo trutta. We showed that the overall abundance of Alpine bullhead was highest at the sites with the least degraded habitat (i.e., natural habitat) and we caught almost no Alpine bullhead at the sites with the most degraded habitat. The flash flood had a strong negative effect on the abundance of the largest individuals of Alpine bullhead. Abundance of small and medium size Alpine bullhead was negatively affected by the abundance of adult stocked brown trout before as well as after the flash flood. However, negative effect of adult brown trout abundance on abundance of large Alpine bullhead was not significant before the flash flood, and it became significant after the flash flood. This could indicate an accumulation of negative impacts of trout stocking and flash flood on this size class. Overall, our results suggest that stocking of hatchery trout and habitat degradation can reinforce the impact of flash floods on the population of Alpine bullhead in the streams of the Carpathian Mountains.     

Genetic diversity of brown trout in central Italy

Genetic diversity was analysed in brown trout Salmo trutta populations living in an area of central Italy using RFLP analysis of two mtDNA segments and of the nuclear locus LDH‐C1 *. The data indicated a genetic structure profoundly altered by repeated stockings with allochthonous material of Atlantic origin. In fact, four and 11 of the haplotypes detected were, respectively, identical or genetically very close to haplotypes found in Danish populations, the putative source of stocked brown trout. Furthermore, the LDH‐C1 * 90 allele, typical of north‐western Europe, was widespread among the samples studied. Nonetheless, four populations are characterized by a high frequency of both putative autochthonous haplotypes and the LDH‐C1 * 100 allele, common in the Mediterranean basin. These populations, sampled in areas where S. trutta is documented historically, might represent a remnant of the species' indigenous biodiversity, showing the scope for improving the management of brown trout in central Italy.     

Genetic introgression between wild and stocked brown trout in the Douro River basin, Spain

The genetic diversity of Spanish brown trout is currently threatened by stocking with exogenous brown trout from Central and Northern Europe. In the Douro River basin 25% of the analysed populations in the present study showed introgression by genes of hatchery origin. The mean introgression estimated by the single locus approach ( S ) varied from 0 to 22% among populations, with a mean value of 3%. The hatchery allele markers were absent in populations where stocking ceased in 1993. However, the introgression effect was observed in all populations stocked until 1998. It seems that cessation of stocking is a good measure for restoring native populations. A thorough review of published and present data of genetic interactions between wild and stocked brown trout in Spanish rivers indicates different levels of introgression between basins. The absence of a clear geographical pattern in the introgression level suggests that ecological interactions and local stocking programmes may play an important role in stocking success. Finally, several guidelines are provided for conservation and management of native brown trout populations in Spanish rivers.     

Pre‐migratory differentiation of wild brown trout into migrant and resident individuals

In February to March, wild brown trout Salmo trutta were captured by electrofishing in a natural watercourse (tributaries of the River Lille Aa, Denmark), individually tagged (Passive Integrated Transponders), and released. Representatives of the tagged brown trout were recaptured on the release sites in April by electrofishing and eventually caught in downstream smolt traps ('migrants') placed in the main river or by electrofishing ('residents') on the initial sites in June. Upon each capture, smolt appearance and body size were evaluated, and a non‐lethal gill biopsy was taken and used for Na⁺,K⁺‐ATPase analysis. Based on repetitive gill enzyme analysis in individual fish, a retrospective analysis of the rate of development in individual brown trout ultimately classified as migrants or residents was performed. Two months prior to migration, a bimodal morphological and physiological (gill Na⁺,K⁺‐ATPase) development concurred and was related to the subsequent differentiation into resident and migratory fractions of each population. This differentiation was unrelated to growth rate and body size of individual fish but skewed in favour of migratory females. Individuals destined to become migrants developed a smolt‐like appearance before the onset of migration and had higher rate of change of gill Na⁺,K⁺‐ATPase activity than fish remaining residents. The rate of change of gill Na⁺,K⁺‐ATPase activity was independent of the distance migrated to the trap (3–28 km). Thus in bimodal wild brown trout populations a major increase in enzyme activity takes place before migration is initiated and is a characteristic of migratory individuals only.     

Evidence for selective angling of introduced trout and their hybrids in a stocked brown trout population

Comparisons of the genetic composition of brown trout Salmo trutta captured by anglers and by electrofishing based on three diagnostic microsatellite loci provided strong evidence that angling is selective in a stocked brown trout population. At two sites, anglers caught significantly younger trout and proportionally more introduced hatchery trout and hybrids than were observed in electrofishing surveys. Selective angling, in combination with a small legal catch size, may have considerably eliminated introduced trout and hybrids before spawning at the study sites, and thus may have reduced the introgression of alien genes into the local gene pool. Angling can be an important factor influencing the genetic structure of fish populations and should be taken into account in studies of introgressive hybridization in stocked fish populations and their management. In this study, demographic consequences of stocking were not assessed. Thus, even though the genetic consequences of stocking may be minimal or largely reversible through angling, resource competition between native and introduced trout, until they reach legal catch size, is expected to have a negative effect on the productivity of the indigenous trout population.     

Diet of brown trout in relation to variation in abundance and size of pelagic fish prey

The seasonal diet of a predator, brown trout Salmo trutta [total length ( L _T) 17–69 cm] and simultaneous density and size‐structure of prey populations, vendace Coregonus albula and smelt Osmerus eperlanus (4–16 cm L _T), in a large boreal lake were analysed and compared in 2001 and 2002. The upper L _T limit for consumed prey was c . 40% of the predator L _T. All brown trout, however, preferred small (<10 cm L _T) and avoided large (≥10 cm L _T) prey. The results also suggested that equal densities of similar‐sized (4–10 cm L _T) fish of the two prey species led to random foraging on these species by brown trout, but if either one of the prey species predominated (>50%) in the lake, brown trout shifted to foraging on this species almost exclusively. Brown trout diets thus reflected the density dynamics of the two alternative prey species.     

Mass-marking of brown trout (Salmo trutta L.) larvae by alizarin: method and evaluation of stocking

This study describes otolith marking of brown trout (Salmo trutta L.) larvae by immersion in different solutions of alizarin red S (ARS). The best results were obtained after marking with ARS at a concentration of 150 mg L^?1. To evaluate the efficiency of stocking with brown trout fry, 10 000 20‐day‐old larvae were marked in years 2002 and 2003 with ARS and released 2 weeks later into sections of a river with natural brown trout reproduction. Electro‐fishing surveys carried out 2 months after stocking in 2002 revealed that only 4.8% of all caught young‐of‐the‐year trout originated from stocking; in 2003 the percentage was 8.9%. Based on the substantial natural reproduction and the low ratio of stocked to wild trout, it was recommended to discontinue stocking.     

Movement and mortality of stocked brown trout in a stream

The movement and mortality of stocked brown trout Salmo trutta were investigated using radio telemetry. Four brown trout left the study area whereas the remaining fish were stationary. After 5 weeks, 13 out of 50 tagged brown trout were still alive in the stream. Surviving fish had a significantly lower mean movement per day than fish, which later either died or disappeared. This difference in behaviour was most pronounced 2 to 8 days after release. Predation by the otter Lutra lutra was probably the main cause of the observed mortality.     

Competitive habitat displacement of brown trout by Siberian sculpin: the role of size and density

Habitat competition in brown trout Salmo trutta and Siberian sculpin Cottus poecilopus was investigated by varying density, fish size, and species composition in stream channels providing areas of different substratum particle sizes. In allopatry, both small (52 ± 4 mm L _T) and large (86 ± 6 mm L _T) brown trout exhibited strong preference for the intermediate (8–11 cm diameter) and large (17–21 cm) gravel substrata. There was a tendency for more brown trout to occupy finer (2–4 cm) substrata with increasing density, in particular for large brown trout. Also, more small brown trout were observed on finer substrata when tested with large brown trout, suggesting interspecific competition for restricted space. Both small (56 ± 6 mm L _T) and large (88 ± 10 mm L _T) Siberian sculpin preferred the large gravel in all tests, and did not change their substratum preferences much with increasing densities, suggesting higher tolerance for 'crowding'. The large Siberian sculpin preferred the coarser substratum, and the largest individuals were consistently found on it. In sympatry with large Siberian sculpin, habitat displacement of brown trout occurred, indicative of interspecific competition. A higher proportion of small and large brown trout occupied the finer substrata than in allopatry. Habitat selection by large Siberian sculpin appeared to be unaffected by species composition and density. Small Siberian sculpin were displaced to finer substrata when tested with large Siberian sculpin, suggesting intraspecific competition. The results indicate that Siberian sculpin are potential habitat competitors for young brown trout.     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.     

Genetic changes caused by restocking and hydroelectric dams in demographically bottlenecked brown trout in a transnational subarctic riverine system

Habitat discontinuity, anthropogenic disturbance, and overharvesting have led to population fragmentation and decline worldwide. Preservation of remaining natural genetic diversity is crucial to avoid continued genetic erosion. Brown trout (Salmo trutta L.) is an ideal model species for studying anthropogenic influences on genetic integrity, as it has experienced significant genetic alterations throughout its natural distribution range due to habitat fragmentation, overexploitation, translocations, and stocking. The Pasvik River is a subarctic riverine system shared between Norway, Russia, and Finland, subdivided by seven hydroelectric power dams that destroyed about 70% of natural spawning and nursing areas. Stocking is applied in certain river parts to support the natural brown trout population. Adjacent river segments with different management strategies (stocked vs. not stocked) facilitated the simultaneous assessment of genetic impacts of dams and stocking based on analyses of 16 short tandem repeat loci. Dams were expected to increase genetic differentiation between and reduce genetic diversity within river sections. Contrastingly, stocking was predicted to promote genetic homogenization and diversity, but also potentially lead to loss of private alleles and to genetic erosion. Our results showed comparatively low heterozygosity and clear genetic differentiation between adjacent sections in nonstocked river parts, indicating that dams prevent migration and contribute to genetic isolation and loss of genetic diversity. Furthermore, genetic differentiation was low and heterozygosity relatively high across stocked sections. However, in stocked river sections, we found signatures of recent bottlenecks and reductions in private alleles, indicating that only a subset of individuals contributes to reproduction, potentially leading to divergence away from the natural genetic state. Taken together, these results indicate that stocking counteracts the negative fragmentation effects of dams, but also that stocking practices should be planned carefully in order to ensure long‐term preservation of natural genetic diversity and integrity in brown trout and other species in regulated river systems.     

A study of inadvertent domestication selection during start‐feeding of brown trout families

A study was undertaken to compare first‐feeding mortality among 10 brown trout families fed high (100%) and low (25%) rations in replicate mixed family tanks. Microsatellite DNA profiling was used to assign individual brown trout to family of origin. At the end of the 35 day experimental period, highly significant differences in overall mortality were observed between the two treatments, and within the treatments there were highly significant differences in family mortality. Both replicates displayed similar patterns of family mortality. Notably, the distribution of mortality among families differed significantly between the two ration treatments, although this was more distinct for some families than others. No correlation between mean family egg diameter and family mortality were observed within either feeding treatment, although a significant positive correlation between mean family egg diameter and timing of family mortality was observed. It is suggested that these data indicate the existence of a genotype x environment interaction relating to feeding level during the critical start‐feeding period.     

Use of micreosatellite markers for identification of indigenous brown trout in a geographical region heavily influenced by stocked domensticated trout

Based on estimates of genetic differentiation between populations, assignment tests and analysis of isolation by distance, stocked populations of brown trout Salmo trutta of Funen Island, Denmark, had been genetically affected by domesticated trout, whereas the stocking of wild exogenous trout into one of the rivers had little or no impact. At the same time, there were clear indications of remaining indigenous gene pools in the Funen populations. The management implications of these findings are discussed and changes in trout release activity are recommended to avoid further mixing of trout gene pools.     

Stocked trout have minimal effects on littoral invertebrate assemblages of productive fish‐bearing lakes: a whole‐lake BACI study

1. Rainbow Trout (Oncorhynchus mykiss [Walbaum]) is commonly stocked as a sport fish throughout the world but can have serious negative effects on native species, especially in headwater systems. Productive fish‐bearing lakes represent a frequently stocked yet infrequently studied system, and effects of trout in these systems may differ from those in headwater lakes. 2. We used a Before‐After Control‐Impact (BACI) design to determine how stocked trout affected assemblage‐level and taxon‐level biomass, abundance and average length of littoral invertebrates in a stocked lake relative to three unstocked control lakes in the boreal foothills of Alberta, Canada. Lakes were studied 1 year before and for 2 years after stocking. Because characteristics of productive fish‐bearing lakes should buffer impacts of introduced fish, we predicted that trout would not affect assemblage‐level structure of littoral invertebrates but might reduce the abundance or average length of large‐bodied taxa frequently consumed by trout. 3. Relative to the unstocked control lakes, biomass, but not abundance, of the littoral invertebrate assemblage was affected indirectly by trout through increases of some taxa after trout stocking. At the individual taxon‐level, trout stocking did not affect most (23 of the 27) taxa, with four taxa increasing in abundance or biomass after stocking. Only one taxon, Chironomidae, showed evidence of size‐selective predation by trout, being consumed frequently by trout and decreasing significantly in average length after stocking. 4. Our results contrast with the strong negative effects of trout stocking on invertebrate assemblages commonly reported from headwater lakes. A combination of factors, including large and robust native populations of forage fish, the generalised diet of trout, overwinter aeration, relatively high productivity and dense macrophyte beds, likely works in concert to reduce potentially negative effects of stocked trout in these systems. As such, productive, fish‐bearing lakes may represent a suitable system for trout stocking, especially where native sport fish populations are lacking.     

A bioenergetic assessment of the influence of stocking practices on rainbow trout (Oncorhynchus mykiss) growth and consumption in a New Zealand lake

Summary

• 1 To investigate the carrying capacity and factors affecting growth of rainbow trout in Lake Rotoiti, we employed a bioenergetics model to assess the influence of stocking rates, timing of releases and prey abundance on growth and prey consumption. We hypothesised that stocking rates and prey abundance would affect growth and prey consumption by influencing per‐capita prey availability, and that the environmental conditions encountered by fish at the time of stocking would affect growth and consumption.
• 2 Prey consumption of stocked rainbow trout was calculated with the Wisconsin bioenergetics model. We calculated growth trajectories of released trout based on data from stocked trout that were released in spring and autumn from 1993 to 2009 and then re‐captured by anglers. Diet, prey energy density, body mass lost during spawning and lake temperature were measured locally.
• 3 Stocking timing had no effect on return rates to anglers or length or weight of caught fish. Although trout released in autumn were smaller than those released in spring, autumn‐released trout grew at a faster rate and had similar lengths and weights to spring cohorts after 2 years of growth in the lake. Modelled consumption parameters were negatively correlated with trout population size, suggesting that stocking rates (347–809 fish ha^?1 year^?1) caused density‐dependent effects on growth. Although common smelt (Retropinna retropinna) accounted for 85% of total prey consumption, no significant relationship was found between prey consumption by individual trout and adult smelt abundance, possibly because trout are targeting smaller smelt that our abundance estimate did not account for.
• 4 Releasing trout in autumn appears to be advantageous for growth, possibly because (i) temperature is more suitable for growth in autumn–winter than in spring–summer and (ii) prey for small trout is abundant in autumn. Mild winter conditions appear to enhance overwinter survival and growth of rainbow trout in warm‐temperate lakes compared to higher latitudes. This implies that moderately productive warm‐temperate lake ecosystems are highly suitable for trout growth in winter, but less so in summer, when lake stratification and high nutrient levels may create conditions suitable for algal blooms and hypolimnetic deoxygenation. High growth rates of trout in warm‐temperate lakes can therefore be supported by timing releases to coincide with favourable winter conditions.