Post-stocking Movements and Recapture of Hatchery-reared Trout Released into Flowing Waters - Effect of a Resident Wild Population

The post-stocking movements and survival of hatchery-reared brown trout (Salmo trutta L.) (length 18–19 cm) stocked into depopulated and control stretches of the Afon Clettwr, South Wales, were investigated. Data were obtained from electrofishing surveys and rewarded tag returns. Recapture rates ranged from 67–76% for both stretches. The resident population of wild brown trout had no significant effect on the dispersion of the stocked fish, the majority of which remained close to the point of stocking. No stocked fish were recovered from the experimental stretches in the year following their introduction. Within one year the depopulated stretch had been recolonized by wild trout. The implications upon restocking after a fish kill are discussed.     

Factors influencing the yield of brown trout, Salmo trutta m. lacustris L., in northern Finnish lakes

Abstract Lake-to-lake variation in brown trout, Salmo trutta m. lacustris L., yield from stocking was examined in 34 lakes in northern Finland. The trout were mainly stocked as 2–3-year-old fish. Catch statistics were compiled with information on water quality, water level fluctuations, fishing effort and lake geomorphology. Absolute brown trout yields (kgha^-1) increased with increasing stocking rate, but there was an indication of non-linearity at higher stocking densities. Relative yields (kg per thousand trout released) were highest at low stocking rates. Stepwise multiple regression analysis was used to determine the best predictive model for lake-to-lake variability in brown trout yields. Seventeen measured regressands were used initially, and then replaced with scores obtained in a principal component analysis of highly correlated water quality variables and species-specific fish yields. Three major determinants of brown trout yields in these lakes were found in both analyses: fish community, stocking rate and fishing effort. Brown trout yields from stocking were higher in lakes with proportionally high yields of vendace or vendace and whitefish and proportionally low yields of pike.     

Failure of predator conditioning: an experimental study of predator avoidance in brown trout (Salmo trutta)

Many studies have documented that hatchery‐reared salmonids generally have inferior survival after being stocked compared with wild conspecifics, hatchery and wild salmonids have been observed to differ in their antipredator responses. The response of brown trout (Salmo trutta) juveniles (0+) of differing backgrounds to a live predator was compared in two experiments. First, the antipredator behaviour of predator‐naïve hatchery‐reared brown trout and wild‐exposed brown trout were assessed in behavioural trials which lasted for eight days. Second, predator‐naïve and predator‐conditioned hatchery‐reared brown trout were assessed in identical behavioural trials. Brown trout were ‘predator‐conditioned’ by being held in a stream‐water aquarium with adult Atlantic salmon (Salmo salar) and adult brown trout for two days prior to behavioural trials. Predator‐conditioned hatchery‐reared brown trout spent more time in shelters in the trial aquaria than predator‐naïve hatchery‐reared fish, but did not differ in time spent in the predator‐free area. Predator conditioning may account for the increased time spent in the shelter, but does not appear to have affected time spent in the predator‐free area. However, even if significant alteration in behaviour can be noted in the laboratory, the response might not be appropriate in the wild.     

Effects of stocked trout on native fish communities in boreal foothills lakes

Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes.
Ecology of Freshwater Fish 2010: 19: 279–289. © 2010 John Wiley & Sons A/S Abstract – Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005–2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size‐limited predation. Dace were also more crepuscular and concentrated on the lake‐bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population‐level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes.     

Stocking of sea trout, Salmo trutta, in Lake Veere, south-west Netherlands

Lake Veere (1750–2050 ha), a brackish water lake in south‐west Netherlands, is a former branch of the Oosterschelde. The lake was closed off by the construction of two dams in 1961. Since the early 1970s Lake Veere has been regularly stocked for recreational and commercial fisheries with rainbow trout, Oncorhynchus mykiss (Walbaum), brown trout, Salmo trutta L., and glass eel, Anguilla anguilla (L.). Between May and September 1996 an experimental stocking of 18 054 trout of sea trout parentage (15–16 cm; 776 kg) was carried out to study their potential for recreational fisheries. The growth and mortality of the stocked trout were estimated from recaptures in eel fyke nets. The production and consumption of the stocked trout were estimated with a bioenergetics model. After 3.5 years (May 1996–November 1999) the stocked trout measured between 50 and 70 cm. The estimated annual total mortality was 84%. During the winter of 1996–1997, the biomass of the stocked trout reached a maximum of about 1800 kg. By November 1999 the biomass was estimated to be 100 kg. The maximum daily consumption by the trout population was 60 g ha^?1 in October 1996 and in June 1997. The total consumption of the stocked trout population over the 3.5‐year period was estimated as 54 244 kg. The analysis suggested that the stocked trout used about 0.2% of the average annual primary production of the lake system. Although the growth and initial production of the population are attractive from the perspective of a recreational fishery, the high mortality and infestation with the salmon louse, Lepeophtheirus salmonis (Krøyer), are serious drawbacks for a future stocking programme with trout in Lake Veere.     

Benefits of repeated stocking with adult,hatchery‐reared brown trout,Salmo trutta,to recreational fisheries?

Abstract  Recaptures of adult, hatchery-reared, brown trout, Salmo trutta L., and fishing time from anglers were used to evaluate the benefits of stocking programmes with repeated releases of adult brown trout. The recapture rate varied between 17% and 29%. The time between stocking and capture (referred to as residence time) varied between 1 and 160 days (median 3–49 days). Between 67% and 84% of trout caught in the river were recently released fish. Fishing effort increased after stocking, thereby increasing the impact of angling on wild stocks. Stocking with adult brown trout decreases the impact of angling on wild trout only if the time spent fishing by all anglers is kept stable. Furthermore, because of the short residence time of stocked trout, long-term impacts through competition for space and food, or genetic impact through introgression, are limited.     

Rainbow trout effects on zooplankton in the reproductive area of the critically endangered hooded grebe

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Survival and distribution of pond- and hatchery-reared 0+ brown trout, Salmo trutta L., released in a Swedish stream

During 1985-88, a total of 17500 under-yearling (0+) brown trout. salmo trutta L., were released in Låktabäcken Creek in Swedish Lapland. Of these, 15500 had been reared in a pond adjacent to the creek during their first summer, where they fed on natural prey. The other 2000 were conventionally reared hatchery fish fed dry food pellets. All fish were released in the autumn (size 6O-70mm) at the confluence of the pond outlet and the creek. Electrofishing revealed that the stocked fish gradually spread downstream from the point of release at the expense of the resident wild trout population. In 1989, stocked fish accounted for 70-90% and 30-50% of the trout population in the upper and lower stretches of the creek respectively. No long-term changes in total trout densities or standing crop occurred as a result of stocking. First-year survival of fish released in the creek varied between 15 and 30% over the 4 years. After 3 years, 5% of the stocked fish remained in the creek. Planted fish grew less rapidly than wild fish during the first year in the creek. Pond fish had a higher survival rate than hatchery fish and showed a greater propensity to disperse from the point of release.     

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.     

Observations on the Contribution of Planted Brown Trout (Salmo trutta L.) to Spawning Stocks in Four Irish Lakes

The relative proportions of mature wild and stocked brown trout in four Irish lakes were compared with their subsequent occurrence in spawning runs. Data indicate that stocked trout, of two strains, did not run the streams to spawn in significant numbers. Considerable stocks of introduced fish, which had shed their milt or eggs, were present in these lakes in springtime. The failure of brown trout stocked in lakes to run spawning streams subsequently has not been recorded previously.     

Density of brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr after point and scatter stocking of fry

Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., fry were point and scatter stocked in the early part of June at densities of 63–263 fry 100 m⁻² per species in the River Viantienjoki, a small river in northern Finland, and their population densities were assessed in late summer. Both species were always stocked together in similar quantities. Point stocking was used in the first 2 years and scatter stocking in the following 2 years. In point stocking, there was no correlation between the distance from the stocking sites (maximum = 250 m) and parr density in census sites ( r = −0.013 and 0.019 for brown trout and Atlantic salmon, respectively). The stocking density of fry did not influence parr density in August by either method or by species. Stocking density explained only from 11% to 23% of the parr survival depending on the species or stocking method. The mean densities of Atlantic salmon and brown trout parr did not differ significantly from each other at any fishing site ( P > 0.05). Both point and scatter stocking appear to be suitable methods for use in small rivers. The parr densities depend more on the other factors (e.g. habitat quality) than the stocking method, and the choice between methods could be based on the time and labour available.     

Post-stocking Movements and Recapture of Hatchery-reared Trout Released into Flowing Waters: Effect of Time and Method of Stocking

Hatchery-reared brown trout Salmo trutta L. (length 23–26 cm) were stocked in the Afon Taf, South Wales. The effects of ‘spot-’ and ‘scatter-planting’ and of stocking at 1 and 4 weeks prior to the start of the angling season were investigated. Data on the percentage recapture and post-stocking movements of the trout were obtained from tag returns. Higher percentage recaptures were recorded for ‘spot-plantings’ (65% and 31%) than for ‘scatter-planting’ (16%). Stockings made 1 week before the start of the angling season yielded better returns (17.1%) than those made 3 weeks earlier (2.2%). Neither ‘spot-’ nor ‘scatter-planting’ resulted in stocked fish contributing to the catch for an appreciably longer period of time. The majority of trout stocked were caught in the area of stocking irrespective of the method or time of planting.     

Efficiency of Northern pike (Esox lucius) stocking in metropolitan France at large spatial and temporal scales

Stocking and electrofishing occurrence and abundance data for northern pike Esox lucius L. in >3800 km of French rivers across 7 years were compared to assess the effect of recreational fisheries stocking programmes on wild pike populations. A positive relationship was found between the additive effect of stocking and the size of the stocked pike. However, the stocking programmes implemented in France by recreational fishery managers from 2008 to 2013 increased the probability of pike occurring in the river network, without increasing abundance in well-established pike populations, because pike stocked in their early-life stages were used in most of the stocking programmes.     

Resource use of native and stocked brown trout Salmo trutta L., in a subarctic lake

Habitat use, food composition and growth of stocked and native brown trout, Salmo trutta L., were studied in the subarctic Lake Muddusjärvi in northern Finland. Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout were stocked in October. In June stocked trout fed primarily on invertebrates while native fish were piscivorous. From July onwards the composition of the diet of both stocked and native trout was similar and consisted almost entirely of small‐sized whitefish. Brown trout were already piscivorous at a length of about 20 cm. The mean length of prey consumed was about 12 cm. Mean length‐at‐age was similar from the second year in the lake despite of the larger size of stocked fish during the first year in the lake.     

Parallelism in thermal growth response in otoliths and scales of brown trout (Salmo trutta L.) from alpine lakes independent of genetic background

Low density in natural populations of salmonids has predominantly been managed by stocking of non‐native conspecifics. Due partly to domestication, introduced non‐native fish may be maladapted under natural conditions. Interbreeding between introduced and wild individuals may therefore impair local adaptation and potentially population viability. Brown trout (Salmo trutta L.) from three headwaters (with stocked fish) and three interconnected lakes (with native fish) on the Hardangervidda mountain plateau, southern Norway, were tested for differences in thermal effects on scale and otolith growth. Otolith and scale annuli widths from immature brown trout showed positive correlation with mean annual summer temperature for all six sampled populations. In mature individuals, a similar positive thermal correlation was evident for the otoliths only. Interannuli width measurements from scales indicate a halt in somatic growth for brown trout in this alpine environment when reaching ages between 7 and 9 winters, coinciding with age at maturity. Our study indicates that otolith growth follows summer temperature even when individuals do not respond with somatic growth in these populations and that introduced brown trout and introgressed populations have similar thermal growth responses. Due to the continued otolith growth after stagnation in somatic growth and the impact of fluctuations in summer temperature, the utilisation of otolith annuli widths for back calculation of length at age should be treated with caution.     

Quantitative estimates of freshwater fish stocking practices by recreational angling clubs in France

Although freshwater fish stocking is widely used by managers, quantitative assessments of stocking practices are lacking in many countries. The general objective of the present study was to determine the quantity and characteristics of fish stocking in metropolitan France. Using a survey-based approach, stocking practices for 2013 by recreational angling clubs in France were quantified, which represented the bulk of fish stocking undertaken in that year. Stocking was found to be practiced by 88.6% of angling clubs in France, representing, on average, 65% of their annual budget. Overall, 22 species were stocked, including 13 native and nine non-native species, with strong variations among species in terms of life stages and body sizes used for stocking. Using Bayesian modelling, a total biomass of 2.029 t, representing approximately 90 million fishes, was estimated to be stocked in France in 2013. In terms of biomass, the most widely stocked species were rainbow trout Oncorhynchus mykiss (Walbaum), brown trout Salmo trutta L., roach Rutilus rutilus (L.), common carp Cyprinus carpio L. and northern pike Esox lucius L. A stocking volume of approximately 60 fishes or 1.5 kg of fish biomass per angler per year seems commonplace in industrialised countries for which data are available.     

Survival of stocked hatchery-reared brown trout, Salmo trutta L., fry in relation to the carrying capacity of a trout nursery stream

Abstract. During the period June 1982-84 hatchery-reared brown trout, Salmo trutta L., fry were Stocked into stretches of the Owendoher. a trout nursery stream on the east coast of Ireland. These experiments were designed to examine the survival of stocked fry and to estimate the carrying capacity of the system. During the first year fry were stocked into sectors already supptorting wild fish at densities normal for the system. In the following year fry numbers were artificially reduced prior to stocking with the hatchery-reared fish. Mortality of the stocked fry was high after release with less than 33% of the fish surviving beyond the first 3 weeks. No stocked fish survived after October 1982. In the second year, however, 2-9% of the fish survived. The best survival rates were achieved where wild fry numbers were lowest. Regardless of the initial stocking density the various experiments yielded autumn fry densities (0.07-0.7 fish/m²) similar to those at unstocked sites (0.1-0.62 fish/m²).
Stocking did not increase recruitment to the 1+ group and again 1+ densities (0.15-0.35 fish/m²) similar to unstocked sites (0.07-0.39 fish/m²) were obtained at the end of each year. These results suggest that spawning and recruitment in the Owendoher yield population densities approaching the maximum carrying capacity of the stream. The system appears to support a maximum summer fry density in the region of 1 fish/m² and a maximum autumn density of 0.7 fish/m².     

Do triploid Salmo trutta stocked into a chalk stream in the spring prey on wild Salmo salar smolts?

The predation impact of recently stocked triploid brown trout, Salmo trutta L., on migrating wild Atlantic salmon, Salmo salar L., smolts was investigated in two field‐based experiments. The first experiment employed a unique experimental facility to monitor 57 wild S. salar smolts through an enclosure containing a known density of stocked S. trutta to determine a predation rate. None of these smolts were preyed upon by the stocked S. trutta. The second experiment investigated the diet of free‐ranging stocked triploid S. trutta in a chalk stream during the spring. Although at least 6% of the free‐ranging stocked triploid S. trutta became piscivorous on Phoxinus phoxinus (L.), the results suggest that large, recently stocked, triploid S. trutta with a high condition factor do not represent a predation threat to wild S. salar smolts. However, it is recommended that a precautionary approach is maintained and the findings are not generalised until further investigation permits explicit management advice to be developed, and that the stocking of large triploid S. trutta is avoided before May/ June (regional variations to apply) each calendar year, where this can reasonably be achieved.     

Hatchery‐reared endangered Colorado pikeminnow (Ptychocheilus lucius) undergo a gradual transition to piscivory after introduction to the wild

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Angling catch returns as indicators of the status of upland trout lakes

Abstract. Over the period 1966–1983 angling catches from the Penrhyncoch trout, Salmo trutta L., lakes in mid-Wales showed a steady decline while catches from the nearby Trisant lakes steadily increased. By the early-mid 1970s some of the Penrhyncoch lakes had ceased to exist as viable fisheries. The declining catches from many of the Penrhyncoch lakes appeared to be attributable to the recent acidification of the lakes which has led to a severe decline in the size of the native trout population and a high mortality among stocked hatchery-reared trout, Salmo trutta L. The increase in catches from the Trisant lakes appears to be due to a shift in angling pressure but the relatively high catches are a real reflection of high trout population densities in lakes unaffected by acidification. The average annual angling catch from the Penrhyncoch and Trisant lakes over 1966–1983 was 1·2 and 6·7 kg ha^?l respectively. Catches per unit area from the Trisant lakes compare favourably with other much larger upland reservoirs.