The balancing act of captive breeding programmes: salmon stocking and angler catch statistics

The debate over Atlantic salmon, Salmo salar L., stocking in Britain centres on the trade‐off between enhancing rod fisheries and harming wild populations. This article informs the debate by quantifying the relationship between stocking and angler catch statistics for 62 rivers over 15 years. After controlling for environmental factors affecting adult abundance, the 42 rivers with stocking had non‐significantly lower mean catch statistics than the 20 rivers without stocking. This difference increased with the age of stocked fish. Among stocked rivers, weak relationships between mean stocking effort and catch statistics also became more negative with the age of stocked fish. For stocked rivers, there was no evidence for a generally positive relationship between annual stocking efforts and catch statistics. Those rivers for which stocking appeared to improve annual rod catches tended to have lower than expected mean rod catches. The results suggest the damage inflicted on wild salmon populations by stocking is not balanced by detectable benefits to rod fisheries.     

Economic,ecological and genetic impacts of marine stock enhancement and sea ranching: A systematic review

Hatchery release is one of the most popular management tools in fisheries, forestry and wild life management, while its negative impacts on wild populations are a global concern. Research and monitoring of its impacts are generally lacking, and the usefulness of hatchery release for fisheries and conservation objectives is unclear. Here, I evaluated positive and negative impacts of worldwide marine stock enhancement and sea ranching programmes in a systematic review associated with meta‐analyses with the goal of reducing bias of the review. Vast numbers of individuals of more than 180 species are released into the wild each year, but most studies are at experimental stages to assess its potential, and empirical studies are sparse for evaluating the impact on fishery production. Most cases are economically unprofitable except for a few successful cases or unevaluated. The effects of releasing juveniles can be dwarfed by the magnitude of natural recruitment when the spawning stock produces much larger recruitment than released juveniles. Density‐dependent growth caused by competition of food can be substantial, and growth rates of hatchery and wild fish and other competitive species can simultaneously be reduced when stocking exceeded the carrying capacity. Relative reproductive success can vary depending on the species, seed quality and environmental factors. Empirical studies show evidence of substantial gene flow from hatcheries, but fitness reduction in stocked populations has not been reported. The results represent the current state of worldwide marine stock enhancement and sea ranching activity and provide key information for growing fields of artificial propagation and conservation.     

Survival and growth of hatchery and wild brown trout (Salmo trutta) parr in three Austrian headwater streams

Brown trout populations of three headwater streams in the Northern Limestone Alps of Austria were supplemented by three‐month‐old hatchery‐reared parr from a wild and locally adapted strain and a nonresident domesticated hatchery strain. Growth and survival were monitored with three surveys over a period of 16 months after stocking. Fish descending from the wild reared origin strain demonstrated higher survival rates than the hatchery strain. Differences in growth were found among the investigated streams but not among the investigated strains. The differing temperature regimes of the streams were considered as the primary factor causing those disparities. We conclude that stocking measures had little or no additive effect on successful natural reproduction, as the resident wild brown trout performed significantly better than the stocked fish.     

Effect of supplementary stocking of juvenile brown trout,Salmo trutta,on yield in a Norwegian mountain reservoir

Abstract The effect of supplementary stocking of juvenile (age 0+), hatchery‐reared, brown trout, Salmo trutta L., on annual yields was assessed in a Norwegian mountain reservoir between 1979 and 2007. Fishing was mainly carried out by local fishermen with benthic gillnets. During the study period, annual stocking ranged from 0 to 52 500 fish (19.8 ha^?1). No stocking has been carried out since 1997. Annual yield varied from 1650 to 5653 kg, corresponding to 0.62–2.13 kg ha^?1. Exploitation rate in terms of number of gillnets and mean weight of 6+ fish (age when catchable size was reached) explained 64% of the variability in catches. Stocked fish contributed very little to the yield or catch‐per‐unit‐effort, exhibiting no positive correlation with stocking density. The lack of contribution from stocked fish was probably caused by a competitive bottleneck in the eroded epibenthic zone, causing high juvenile mortality. If stocking continues, it is recommended that fish with body lengths >15–20 cm are used.     

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.     

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.     

Current status of natural spawning of chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> in rivers with or without hatchery stocking on the Japan Sea side of northern Honshu,Japan

Information on the status of natural spawning is needed on the Japan Sea side of northern Honshu, Japan for ecosystem-based sustainable management of chum salmon resources. We conducted on-site visual surveys in October–December of 2015 and 2016 that targeted spawning chum salmon redds in all rivers?>?5 km long (total 94 rivers) in Akita, Yamagata, Niigata (including Sado Island), and Toyama prefectures. The ratio of rivers found to host natural reproduction to the total number of surveyed rivers was 93.6% (44/47) in stocked rivers and 74.5% (35/47) in non-stocked rivers. These results show that there is a wide occurrence of natural reproduction of chum salmon in these rivers, regardless of the history of hatchery stocking. The density of spawning redds (number of redds/1000 m²) as an indicator of chum salmon escapements did not differ (P?=?0.54) between stocked rivers (mean 3.5, N ?=?49) and non-stocked rivers (mean 2.4, N? =?36),when rivers where no redds were observed were excluded from the analysis. These results suggest that chum salmon escapements into non-stocked rivers may not be negligible. Conservation measures for wild fish are needed in stocked and non-stocked rivers to promote enhancement programs based on natural reproduction.     

Identification of stocked muskellunge and potential for distinguishing hatchery‐origin and wild fish using pelvic fin ray microchemistry

The effectiveness of pelvic fin ray microchemistry of muskellunge, Esox masquinongy Mitchill, to identify stocked individuals along with the potential to identify naturally reproduced fish were evaluated. Fish and water samples were obtained from one hatchery and seven lakes with natural differences in water Sr:Ca to determine whether location‐specific environmental signatures were recorded in sectioned muskellunge pelvic fin rays, including fish of known environmental history. Water and fin ray Sr:Ca were strongly correlated. Six lakes in Illinois possessed Sr:Ca signatures that were distinct from the hatchery where muskellunge were raised, resulting in pronounced shifts in Sr:Ca across sectioned fin rays of stocked fish. Hatchery and lake‐specific Sr:Ca signatures were stable across years. Sixteen of 19 individual fish known to have been stocked based on PIT tags implanted at stocking were correctly identified as hatchery‐origin fish using fin ray core Sr:Ca. Results also indicated that the hatchery Sr:Ca signal can be retained for at least 7 years in fin rays of stocked fish. Fin ray microchemistry is a non‐lethal approach for determining environmental history of muskellunge that could be used to assess movement patterns in lake and river systems and the degree to which muskellunge populations are supported by natural reproduction and stocking.     

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.     

The effects of stocking Atlantic salmon, Salmo salar, in a Norwegian regulated river

Abstract  Stocking is undertaken in the River Suldalslågen, western Norway, to compensate for an estimated annual loss of 20 000 Atlantic salmon smolts, Salmo salar L., caused by regulating the river for hydropower production. The annual contribution to angling catches from stocked hatchery fish varied from 7 to 334 kg, or <15% of the total number caught. Between 160 000 and 250 000 one-summer old fish were stocked, but only between 6 and 10 (<0.005%) were recaptured as adults in the river. Recaptured stocked fish never exceeded 0.03% by number, despite smolts dominating the stocking material in recent years. It is not certain whether the slight increase in catches comes in addition to or at the expense of natural reproduction. In most years more adults were used as parent stock than were caught as offspring. The lack of positive response to stocking is possibly due to lesser age, smaller size and later migration of hatchery smolts, and that seawater tolerance of hatchery smolts is poorly developed, all factors increasing mortality at sea.     

Otter (Lutra lutra) predation on stocked brown trout (Salmo trutta) in two Danish lowland rivers

Abstract –  This study aimed to evaluate otter predation on stocked trout. Large hatchery-reared trout (16–30 cm) were stocked into two Danish rivers with different fish populations. Otter diet before and after trout stocking was determined by analysing 685 spraints, collected regularly during the 35-day study period. Fish composition in the rivers before stocking was assessed by electrofishing. In River Trend, a typical trout river, the proportion of trout in the otter diet increased from 8% before stocking to 33% a few days after stocking. Moreover, trout lengths in the diet changed significantly towards the lengths of stocked trout, indicating that newly stocked trout were preferred to wild trout. In River Skals, dominated by cyprinids, there was no change in otter diet after stocking of hatchery trout, i.e., these were ignored by otter. Otter predation should be taken into account together with fish and bird predation, when stocking is used as a measure for conserving endangered salmonid populations.     

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.     

Forecasting the genetic influences of red drum stock enhancement in South Carolina

The South Carolina Department of Natural Resources (SCDNR) began stocking red drum, Sciaenops ocellatus (Linnaeus), in 1989 to augment the abundance of juveniles available for recreational harvest in South Carolina estuaries. While stock enhancement can help supplement wild populations under high fishing pressure, releasing hatchery‐raised fish into the wild also presents the risk of decreased genetic diversity. An individual‐based model (IBM) was developed to forecast the genetic influences of stocking on the wild spawning population to inform responsible stocking strategies. Model results indicated the SCDNR red drum stock enhancement programme should maintain mean contributions of stocked fish no greater than 30% per year class over a 45‐year stocking period, coupled with at least 10 effective breeders in the hatchery replaced annually, to maintain current levels of genetic diversity estimated in the wild population. The IBM is a useful tool for hatchery managers to guide responsible stock enhancement.     

Factors contributing to the outcome of stocking programmes in Cuban reserviors

Abstract In less-developed countries, fisheries enhancement using stocking is a common practice to manage reservoirs for biomass production. In many cases, there appears to be little control over whether the stocking activity is appropriate or necessary. Cuban reservoir fisheries have been usually managed to maximize fish biomass. Blue tilapia have been introduced and supplemented in reservoirs, and Chinese carp are introduced and maintained in reservoirs throughout the entire island. Supplemental stocking with introduced fish species is a usual practice to manage large and medium-sized reservoirs. Small and very small reservoirs have been managed as semi-intensively enhanced fisheries with carp and tilapia stocking. The main purposes of the present paper are to study the factors contributing to the outcome of stocking programmes in reservoirs and to test the hypotheses that supplementary stocking for self-sustained fish populations usually cannot contribute to increase fish yield. Yield and stocking data by groups of fish species were available for Cuban reservoirs. Total fish yield was only slightly related to total fish stocking density for the complete set of reservoirs. However, this result was mainly because of the relationship between total yield and cyprinid stocking for semi-intensively-used reservoirs. Tilapia yield was not significantly related to tilapia stocking for both extensively and semi-intensively-used reservoirs. However, yield and stocking for cyprinids were highly related for both reservoir subsets. Therefore, cyprinid stocking was effective in certain reservoirs, but tilapia stocking was not effective in any reservoir. Moreover, tilapia stocking was a superfluous activity for reservoirs where tilapia have an adequate natural reproduction cycle.     

Genetic refuges for a self‐sustained fishery: experience in wild brown trout populations in the eastern Pyrenees

Abstract – Management policies balancing harvest and conservation of natural populations of fish are difficult to establish, both scientifically and politically. This issue is particularly difficult when those populations represent native genetic resources. Since 1997, several brown trout populations in the eastern Pyrenees Mountains (Spain) were designated as ‘genetic refuges’ under varying fishing regulations, where releases of hatchery‐origin fish are not permitted. We analysed genetic variation in samples of brown trout from six of those refuge populations and four non‐refuge populations within the same region. Each population was sampled in four separate years: 1993, 1999, 2004 and 2006. Our analyses were based on a diagnostic allele (LDH‐C*90) that distinguishes native and exogenous hatchery populations. Comparisons were based on stocking histories before and after refuge designations and on three management strategies: fished, unfished and catch‐and‐release. Overall, we detected significant genetic introgression resulting from past stocking practices despite the current restriction of hatchery releases imposed by the recent genetic refuge policy. However, this new policy has prevented detectable introgression from increasing throughout the region and together with additional measures on length and number of captured fish is contributing to self‐sustained fisheries that are achieving conservation goals. Quick acceptance of ‘genetic refuges’ by anglers in one particular river, the Ter River basin, has been a key factor in protecting native gene pools compared with the Segre River basins where refuges were not readily accepted.     

Predation on wild and hatchery salmon by non‐native brown trout (Salmo trutta) in the Trinity River,California

Non‐native predators may interfere with conservation efforts for native species. For example, fisheries managers have recently become concerned that non‐native brown trout may impede efforts to restore native salmon and trout in California's Trinity River. However, the extent of brown trout predation on these species is unknown. We quantified brown trout predation on wild and hatchery‐produced salmon and trout in the Trinity River in 2015. We first estimated the total biomass of prey consumed annually by brown trout using a bioenergetics model and measurements of brown trout growth and abundance over a 64‐km study reach. Then, we used stable isotope analysis and gastric lavage to allocate total consumption to specific prey taxa. Although hatchery‐produced fish are primarily released in the spring, hatchery fish accounted for most of the annual consumption by large, piscivorous brown trout (>40 cm long). In all, the 1579 (95% CI 1,279–1,878) brown trout >20 cm long in the study reach ate 5,930 kg (95% CI 3,800–8,805 kg) of hatchery fish in 2015. Brown trout predation on hatchery fish was ca. 7% of the total biomass released from the hatchery. Brown trout only ate 924 kg (95% CI 60–3,526 kg) of wild fish in 2015, but this was potentially a large proportion of wild salmon production because wild fish were relatively small. As large brown trout rely heavily on hatchery‐produced fish, modifying hatchery practices to minimise predation may enhance survival of hatchery fish and potentially reduce the abundance of predatory brown trout.     

Effects of stocking density and natural food availability on the extensive cage culture of pejerrey (Odontesthes bonariensis) in a shallow Pampean lake in Argentina

An experiment was conducted for 80 days at La Salada de Monasterio Lake (Buenos Aires, Argentina) to assess the effect of stocking density and natural food availability on the growth and production of zooplanktivorous juveniles of pejerrey (Odontesthes bonariensis) in extensive cage culture. Ten cages were installed and stocked with 33‐day‐old fish, at three density treatments: 25, 50 and 75 ind. m^?3. Zooplankton was analysed in terms of abundance, biomass and diversity considering three size classes. Caged pejerrey diet was assessed regularly. The pejerrey gut contents composition was clearly different from cage zooplankton, showing a trend to contain bigger components. Reared fish exhibited a tendency to diversification of the diet and to change the mean prey size depending on fish length and stocking density. Regression models showed a positive and direct effect of the bigger zooplankton biomass on fish growth rates, which were also inversely affected by the availability of smaller zooplankton. The results demonstrate that stocking density and available zooplankton, in both quantity and quality, are key factors in regulating extensive cage culture of pejerrey. Handling stocking densities in a dynamic way it is possible to maximize fish growth, biomass harvest or fish number according to the goals of production.     

Comparison of the distribution and homing of hatchery-reared and wild Atlantic salmon,Salmo salar L., from north-east England

Hatchery-reared Atlantic salmon, Salmo salar L., parr and wild smolts which were microtagged and released into rivers in north-east England were recaptured by the fisheries at West Greenland, Faroes and on the Irish west coast, by the homewater net fisheries and by rod fisheries in rivers in north-east England. No significant differences were observed in the patterns of exploitation of hatchery-reared and wild fish in the distant water fisheries. The distribution of tag recoveries in coastal waters was strongly influenced by the pattern of fishing effort which was concentrated in the middle of the fishery area, but recaptures tended to be biased towards the river of release. There were significant differences in the distribution of recaptures of fish released in different rivers and between hatchery-reared and wild fish from the River Wear. There were also differences in the timing of recaptures of hatchery and wild fish from this river in the coastal net fishery. Very few fish were recaptured in rivers other than the one in which they were released, and there was no significant difference between straying rates for hatchery and wild fish. The relative numbers of recaptures in the coastal fishery and in the home river was the same for translocated hatchery fish and wild fish, thus refuting the suggestion that hatchery-reared fish have a reduced ability to home. The distribution of recaptures within fresh water provides clear evidence of tributary-specific homing of hatchery-reared fish.     

Effects of Feeding Diets with and without Fish Meal on Production of Channel Catfish,Ictalurus punctatus,Stocked at Varying Densities

ABSTRACT

Animal protein, generally fish meal, has traditionally been used in the diet of channel catfish. However, our previous research indicates that animal protein is not needed for growing stocker-size catfish to food fish when the fish are stocked at densities typical of those used in commercial catfish culture. Whether this holds when fish are stocked at high densities is not known; thus, we conducted an experiment to evaluate the effect of feeding diets with and without fish meal to channel catfish stocked in earthen ponds at different densities. Two 32% protein-practical diets containing 0% or 6% menhaden fish meal were compared for pond-raised channel catfish, Ictalurus punctatus, stocked at densities of 14,820, 29,640, or 44,460 fish/ha. Fingerling channel catfish with average initial weight of 48 g/fish were stocked into 30 0.04-ha ponds. Five ponds were randomly allotted for each fish meal level?×?stocking density combination. Fish were fed once daily to satiation for two growing seasons. There was a significant interaction between stocking density and fish meal for net production; net production increased in fish fed a diet containing fish meal compared with those fed an all-plant diet at the highest stocking density, but not at the two lower stocking densities. Net production of fish fed diets with and without fish meal increased as stocking density increased. Viewing the main effect means, weight gain decreased and feed conversion ratio increased for fish stocked at the two highest densities, and survival was significantly lower at the highest stocking density. Visceral fat decreased in fish at the two highest stocking densities. Body composition data were largely unaffected by experimental treatment except for a reduction in percentage filet fat in fish at the highest stocking density, and fish that were fed diets containing fish meal had a lower percentage fillet protein and a higher percentage fillet fat. It appears that at stocking densities two to three times higher than generally used, animal protein (fish meal) may be beneficial in the diet of channel catfish. In regard to stocking densities, high stocking results in higher overall production, but the average fish size decreased as stocking density increased.     

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.