Amenity, Fisheries and Recreation Dept., Yorkshire Water Authority
Fisheries Department, Welsh Water Authority
Brecon, Powys, Wales
University of Wales Institute of Science and Technology
The success of stocking with hatchery-reared trout has been the subject of varied investigations for the past half century. Percentage returns and post-stocking movements are summarized and related to the size and species of trout stocked and to the season of stocking.
Factors influencing the movements, recapture and survival of hatchery-reared brown trout, Salmo trutta L., stocked into rivers in Wales, U.K., were investigated during the period 1975–78. Experiments involved stocking more than 10 000 individually tagged trout, many of which had been acclimated to specific conditions. Information regarding the subsequent distribution of stocked fish was obtained from electro-fishing surveys, trapping and tag returns by anglers.
The majority of stocked trout moved only short distances; generally less than 1 km and principally in a downstream direction. The degree of dispersion after stocking was largely dependent on the character of the receiving river; dispersal was most extensive in fast-flowing rivers and more limited in deeper, more slow-flowing waters. Acclimation to flowing water prior to release had little effect on the distribution of stocked fish but did result in their being recovered in greater numbers. The presence of a resident wild population affected neither stocked fish movement nor their percentage recovery. “Spot-and scatter-planting” caused no difference in post-stocking movements but “spot-planting” did result in a higher percentage recovery.
Tag returns from stocked trout ranged from 10 to 62 percent. More than 65 percent of the stocked fish caught were taken within five weeks of stocking and more than 50 percent by less than 20 percent of the anglers benefiting. Less than 1 percent of the fish stocked contributed to the catch in the season after stocking.
The majority of stocked trout did not begin feeding until several weeks after stocking. The food intake of these fish tended toward that of wild trout as their period of residence in the river increased, but most stocked trout still had a lower food consumption after 40 to 50 days in the river. Many fish lost condition, reducing their chance of survival over the winter period. Hatchery-reared trout retained their distinctive silvery appearance for 2–3 months after stocking, making them more conspicuous to predators.
The implications of these findings are discussed in relation to the expediency, efficiency and cost-effectiveness of future management strategies involving the stocking of hatchery-reared trout.
Le succès de l'empoissonnement avec des truites élevées en alevinière a fait l'objet de diverses recherches ces cinquante dernières années. On indique brièvement les rendements et les déplacements après la mise en liberté des alevins (en pourcentage) par dimension et espèce de truites et par saison de peuplement.
On a fait des recherches de 1975 à 1978 sur les facteurs qui influencent les déplacements, la recapture et la survie des truites brunes élevées en alevinières Salmo trutta L. qui peuplent les rivières du pays de Galles en Grande Bretagne. Les expériences comportaient le lâcher de plus de 10 000 truites marquées individuellement, dont un grande nombre avait été acclimatées à certaines conditions. Les renseignements concernant la distribution ultérieure du poisson d'alevinage ont été obtenus à partir d'enquêtes halieutiques à l'aide d'appareils électriques, de captures dans des nasses ou des marques renvoyées par les pêcheurs à la ligne.
La majeure partie des truites de repeuplement ne se déplacent que sur de courtes distances, en général moins d'un kilomètre et surtout en aval. Le degré de dispersion des alevins après l'empoissonnement dépend en grande partie du type du cours d'eau où le poisson est lâché. Cette dispersion est plus large dans les cours d'eau à débit rapide et plus limitée dans ceux où les eaux sont profondes et le débit lent. L'acclimatation aux eaux courantes avant la remise en liberté du poisson a peu d'effet sur la distribution des sujets mais a permis d'en récuperer un plus grand nombre. La présence d'une population sauvage n'a affecté ni les déplacements du poisson de repeuplement ni le pourcentage de récapture. L'ensemencement sur place et disséminé n'a causé aucune différence dans le déplacement du poisson après l'empoissonnement mais l'ensemencement sur place a permis de récuperer une plus grande proportion de poisson.
Le renvoi des marques des truites de repeuplement a variée entre 10 et 62 pour cent. Plus de 65 pour cent du poisson de repeuplement a été capturé dans les cinq semaines qui suivent sa mise en liberté et plus de 50 pour cent a été pêché par moins de 20 pour cent des pêcheurs à la ligne bénéficiaires. Moins d'un pour cent du poisson de repeuplement entre dans les captures pendant la saison qui a suivi l'empoissonnement.
La plupart des truites de repeuplement n'ont commencé à s'alimenter que plusieurs semaines après leur mise en liberté. Elles ont eu tendance à adopter la même alimentation que les truites sauvages au fur et à mesure que leur permanence dans le cours d'eau augmentait et la plupart d'entre elles avaient encore un niveau d'alimentation plus faible qui la normale 40 ou 50 jours après leur mise en liberté. Un grand nombre de poissons n'étaient plus en bonne santé, ce qui réduisait leur chance de survie pendant l'hiver. Les truites élevées en écloserie conservent leur apparence argentée caractéristique pendant deux à trois mois après leur mise en liberté ce qui permet aux prédateurs de les répêcher plus facilement.
On étudie les incidences de ces facteurs du point de vue de l'opportunité, de l'efficacité et du rapport coût-rentabilité, des futures stratégies d'aménagement concernant l'empoissonnement avec des truites élevées en alevinières.
In recent years, native brown trout Salmo trutta L. populations in many rivers have declined as a result of increased exploitation and environmental pressures. In an attempt to maintain the quality of river trout fishing, many fishery managers have become increasingly dependent upon restocking with hatchery-reared trout. With the expense involved it is important that stocking programmes and procedures be designed and implemented so as to ensure the best possible results in terms of angler catch and increased recreational opportunities.
The success of stocking with hatchery-reared trout has been the subject of varied investigations for the past half-century. This paper examines the reported percentage recaptures and post-stocking movements in relation to the species and size of trout stocked and the season of stocking. More recent investigations (Cresswell, 1979) into factors affecting the movements, recapture and survival of brown trout stocked into rivers in South Wales, U.K., are summarized.
Restocking may be achieved by releasing fish of takeable size (in the U.K., usually more than 20 cm) which are immediately available to the angler, or by releasing smaller fish which will only be available for legal capture after a period of growth. The latter option is made attractive by the lower purchase price of small fish and the possibility that they will take on some feral characteristics before attaining takeable size. However, investigations carried out predominantly in the U.S.A. (reviewed by Cresswell, 1981) show that the percentage recapture (in terms of takeable-sized fish) resulting from stockings of small trout (8–14 cm) is generally less than 3 percent. Hence, the effective cost of each trout captured may be increased as many as 30 times, counteracting the apparent economic benefit of this type of stocking policy.
Investigations into the success of stocking with takeable-sized fish of different species and at various times of the year are well documented. Cresswell (1981) pooled data from many of these investigations and showed that brook trout Salvelinus fontinalis Mitchill, brown trout and rainbow trout Salmo gairdneri Richardson stocked in the autumn were caught in considerably lower proportions (4, 14 and 10 percent, respectively) than those stocked during the spring or open (angling) season (37, 23, 32 percent, respectively). As with stockings of fingerling trout, autumn introduction necessitates the exposure of stocked fish to the risk of overwintering mortality before the population is subjected to angling pressure; it is likely that this is responsible for the lower percentage recaptures obtained from these two stocking policies. The overall percentage recaptures for spring, pre-season and in-season stockings of brook, brown and rainbow trout were broadly similar (range 21–39 percent). However, individual authors did obtain considerable differences which were often attributable to specific conditions pertaining to the receiving river at the time of stocking.
Literature on the post-stocking movements of hatchery-reared trout has been reviewed by Cresswell (1981) who concluded that:
90 percent of the reported catch of stocked brown trout were taken within 4.5 km of the stocking site, but that lower proportions of brook and rainbow trout were taken within these limits;
Brook and rainbow trout showed a greater propensity toward downstream movement than brown trout. A small proportion of this latter species were taken further than 4.5 km from the stocking area principally in a downstream direction;
Greater dispersion was exhibited by fish of all three species which had overwintered prior to capture or which had been stocked in the smaller upstream reaches of a river.
These conclusions seem to conflict with the assertion so often made by trout fishermen that a large proportion of hatchery-reared fish move off downstream soon after stocking. However, it should be noted that although 90 percent of the reported catch of stocked brown trout were taken within 4.5 km of the stocking site, the reported catch generally represented less than 40 percent of the total number of fish stocked, leaving a large proportion unaccounted for. In the period immediately following an introduction of fish, angling pressure is often concentrated in the vicinity of the stocking point. Under such circumstances the spatial distribution of the catch can be misleading and tends to indicate a lack of movement away from the stocking point. This apparent lack of movement results from a combination of anglers catching fish before they had an opportunity to disperse and a lack of angling effort in areas further from the stocking point (Cresswell and Williams, 1979).
Concern over the large number of stocked trout which remain uncaught and the uncertainty surrounding their fate led to a study on trout stocking carried out during the period 1975–78 (Cresswell, 1979).
The investigations were carried out on a variety of rivers in Wales (Fig. 1). These rivers have been described by Cresswell (1979) and a summary of their physical parameters is given in Table 1.
All fish stocked during the investigations were of takeable size and were tagged with either Floy fingerling tags, for individual recognition during electro-fishing surveys, or with message-bearing Floy spaghetti tags when a particular investigation relied on anglers' returns.
Investigations into factors affecting the movements and survival generally involved the simultaneous stockings of matched batches of fish, one of which had been acclimated to specific conditions. This procedure eliminated the effects of factors other than those being examined. Several of the investigations were carried out on the Clettwr, a manageable river in terms of electro-fishing and where traps recorded fish moving large distances downstream (Cresswell, 1977; Cresswell and Williams, 1983). A single upstream electro-fishing run was found to be the most economical use of electro-fishing time and was effective in locating large numbers of stocked fish. This method of locating the fish eliminated the problem of uneven fishing effort, thereby allowing the true distributions of fish to be assessed. On larger rivers where the investigations relied on anglers' returns, absolute distributions could not be determined, but comparisons were drawn between the distributions of each batch of fish stocked simultaneously in one river.
Hatchery-reared trout inevitably become stressed by transportation procedures associated with stocking (Cotter, 1976) and, in this condition, they are stocked into the relatively strong currents typical of many trout waters. The effects of (a) retaining fish caged in the river for 24 h before release and (b) a period of acclimation to flowing water (up to 0.24 m/sec) in tanks prior to stocking were investigated (Cresswell and Williams, 1983).
In-stream acclimation resulted in a higher percentage recapture and a more limited dispersion of the fish stocked under low river flow conditions, but had no effect on trout stocked into a river where higher water velocities were experienced.
Acclimation, in tanks, to a flow of 0.1 m/sec for 14 days led to higher percentage recaptures, whereas acclimation for only two days resulted in fewer returns than for unacclimated fish. No difference in fish distribution within the rivers could be attributed to these acclimation procedures.
Hatchery-reared trout stocked into rivers not only have to adjust to unfamiliar physical conditions but also have to compete for food and space with a resident wild population. To examine the effect of a resident wild population, trout were stocked into two similar stretches of river, one of which had its native population of brown trout removed (Cresswell and Williams, 1984).
Recapture rates ranged from 67 to 76 percent 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.
“Spot-planting”, almost invariably used in preference to the more laborious “scatter-planting” method, involves liberating large numbers of fish at a few sites. “Scatter-planting” entails releasing a few fish at each of many sites within a river. The relative merits of these two methods were compared (Cresswell and Williams, 1982).
Higher percentage recaptures were recorded for spot-planting (65 and 31 percent) than for “scatter-planting” (16 percent). 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 of planting.
Previous investigations have shown that higher returns are obtained from stockings made during the spring and open season than from those made in autumn. Spring stockings allow fish to become accustomed to their new environment before being subjected to angling pressure and are, thus, preferred to stockings made during the open season. A further investigation compared the results obtained from spring stockings made at one and four weeks prior to the start of the angling season (Cresswell and Williams, 1982).
Stockings made one week before the start of the angling season yielded better returns (17.1 percent) than those made three weeks earlier (2.2 percent). Most fish were caught in the area of stocking irrespective of the time of their introduction.
4.5.1 Distributions determined by electro-fishing
The post-stocking distributions of more than 3 000 fish stocked in six separate experiments on four different streams were determined by electro-fishing (Cresswell, 1979). Recoveries ranged from 35 to 88 percent; higher rates being achieved in the smaller streams where electro-fishing was more efficient. Percentage recovery decreased as the period between stocking and survey lengthened. Despite differences in pre-stocking treatments the distributions were strikingly similar - a prominent peak in the region of stocking and an extended tail downstream. In the majority of experimental stockings, more than 90 percent of recaptures were recorded within 600 m of the stocking site during surveys conducted 5–13 days after stocking. These results emphasize the limited movement shown by the stocked trout in these small streams.
4.5.2 Distributions determined from anglers' returns
Anglers' returns ranged from 10 percent on the Afon Taf in 1978 to 62 percent on the Western Cleddau in 1977.
The distributions of fish stocked into the Taf, Western Cleddau, Dysynni and Teifi are compared (Fig. 2). The extent of these distributions varies from “limited”, in the cases of Taf and Teifi, to “extensive” in the case of the Dysynni. In all cases, movements were principally in a downstream direction. Although it might be expected that rivers with high angling pressure would show a limited distribution due to the rapid capture of fish before they moved far from the stocking site, this was not apparent. The Western Cleddau had both the highest percentage recovery and an extensive stocked fish distribution.
The distributions of stocked fish were more limited in the larger, deeper and more slow-flowing rivers (e.g., Taf) and more extensive in faster-flowing rivers offering less cover to stocked fish (e.g., Dysynni). Despite the variability in extent of the stocked fish distributions, all but one exhibited a distinct peak at or close to the stocking point; the exception being the Afon Dysynni where the angling season was suspended for one week immediately after stocking. This suggests that early angling pressure in the vicinity of the stocking point results in a large number of fish being caught in that area, but does not affect the overall extent of the distribution. Hence the distributions shown in Fig. 2 reflect not only the behaviour of stocked fish in each river but also the spatial and temporal distribution of angling pressure.
A detailed analysis of data from the River Sirhowy indicated that the stocked fish distributions were affected by irregularities in both the spatial and temporal distributions of angling pressure (Cresswell and Williams, 1979). Although the overall extent of the stocked fish distributions seems principally dependent upon the characteristics of the receiving river, angling pressure can have a considerable effect on the shape of these distributions.
On all rivers studied, more than 65 percent of the total catch of stocked fish was taken within five weeks of stocking, and on some rivers (Western Cleddau, 1977; Taf, 1977, 1978) more than 80 percent was taken within three weeks (Fig. 3). Less than 1 percent of the fish stocked contributed to the catch in the season following stocking.
More than 50 percent of the declared catch was taken by less than 20 percent of the anglers who made returns (Fig. 4). On the Western Cleddau, 10 percent of the anglers took almost 70 percent of the declared catch.
The percentage of stocked fish captured by anglers rarely exceeded 50 percent in these studies and less than 1 percent were recaptured in the year following stocking. This tends to suggest that the survival of stocked fish, particularly over the winter period is poor. During the stocking investigations it was noted that many of the fish lost condition. An investigation was, therefore, carried out to assess the feeding behaviour of stocked fish (Cresswell, 1979).
The majority of stocked fish did not begin feeding until several weeks after stocking and many lost condition. Although the food intake of these fish tended toward that of wild trout as their period of residence in the river increased, most hatchery-reared trout still had a lower food consumption than wild trout 40–50 days after stocking.
Hatchery-reared brown trout are often silvery and without the bright red spots characteristic of most wild trout, and because of this they may be more conspicuous to predators. The silvery appearance of the hatchery fish persisted for 2–3 months after stocking. The relatively slow change to a more wild type coloration was probably related to the limited intake of natural food by the stocked hatchery fish.
Effective management is essential to ensure the best use of available trout rivers and where natural production of takeable-sized fish does not satisfy angling demand restocking is a likely solution. Natural production may need augmenting for various reasons:
- angling demand is particularly high, e.g., in areas of high population;
- natural production is low due to poor spawning success or slow growth rate, e.g., in industrial rivers;
- the native population is reduced by a pollution incident.
Restocking after a pollution incident or in rivers where spawning is poor but natural food plentiful may involve the release of fry or fingerlings. In such instances the stocked fry or fingerlings fill a vacant niche, optimum use is made of the natural resource, and the resultant fish have more feral characteristics hence providing better sport. The low percentage recapture, in terms of takeable-sized fish, resulting from stockings of small trout reduces the apparent economic benefit of this type of stocking policy.
Restocking with fish of takeable size is the more usual practice on rivers where the natural growth rate is slow, where a pollution incident has occurred, or where a plentiful supply of large fish is required to satisfy angling demand. Stocking with takeable-sized fish rarely has any long-term benefits; few stocked fish survive to contribute to the catch in the season after stocking, and in view of the loss in condition of many fish their contribution to natural recruitment is likely to be insignificant. Thus, greatest benefit is derived from an introduction of takeable-sized fish by maximizing the return in the season of stocking. This reduces the effective cost of each fish caught and is generally associated with a greater number of anglers contributing to the total catch.
As a result of the findings described in this paper the following proposals are made to improve the efficiency and cost effectiveness of future management strategies involving the stocking of hatchery-reared trout:
Restock with brown trout rather than rainbow or brook trout; rainbow and brook trout have a greater tendency to downstream displacement.
Fish reared in flowing water in raceway type environments are best suited for stocking in rivers; acclimation to flow confers a survival advantage.
Stocking should be carried out shortly before or during the early part of the open (angling) season to maximize percentage returns. Stocking early in the year also provides uncaptured fish with a greater opportunity to feed and regain condition before the winter period.
Fish should be stocked at 3–4 week intervals throughout the season rather than a single major stocking so as to provide a more continuous benefit; the majority of fish caught are taken within 3–4 weeks of stocking.
“Scatter-planting” yields lower returns than “spot-planting” and should only be used if natural dispersion is negligible, e.g., in deep, slow-flowing rivers. If artificial dispersion is necessary, fish should be stocked at intervals along the river in batches of not less than 50.
The dispersal of stocked fish is usually greatest in fast-flowing rivers with a paucity of cover. In such circumstances consideration should be given to the provision of artificial instream cover.
Stocking should be confined to stretches of river where it is economical in terms of the effective cost of captured fish. In regions where angling pressure is low and returns are small, the effective cost of captured fish may exceed their recreational value.
We would like to thank the fisheries staff of the Welsh Water Authority, riparian owners and anglers for their valued assistance throughout this study.
The research was based at the University of Wales Institute of Science and Technology and was carried out during the tenure of a CASE award financed by the Natural Environmental Research Council and the Welsh Water Authority.
Cotter, D., 1976 An investigation of some aspects of fish transportation. M.Sc. Thesis, University of Wales
Cresswell, R.C., 1977 A simple and inexpensive trap for catching downstream migrants. Fish.Manage., 8:43–6
Cresswell, R.C., 1979 Factors affecting the movements of stocked fish. Ph.D. Thesis, University of Wales.
Cresswell, R.C., 1981 Post-stocking movements and recapture of hatchery-reared trout released into flowing waters - a review. J.Fish Biol., 18:429–42
Cresswell, R.C. and R. Williams, 1979 Studies on trout stocking in an industrial river and their management implications. In Proceedings of the First British Freshwater Fisheries Conference. Liverpool, University of Liverpool, pp. 285–95
Cresswell, R.C. and R. Williams, 1982 Post-stocking movements and recapture of hatchery-reared trout released into flowing waters: effect of time and method of stocking. Fish.Manage., 13(3):97–103
Cresswell, R.C. and R. Williams, 1983 Post-stocking movements and recapture of hatchery-reared trout released into flowing waters: effect of prior acclimation to flow. J.Fish Biol., 23(3):265–76
Cresswell, R.C. and R. Williams, 1984 Post-stocking movements and recapture of hatchery-reared trout released into flowing waters: effect of a resident wild population. Fish.Manage., 15(1):9–14
Table 1 Physical parameters of study rivers
|River||Catchment-||Average Daily flow (cumecs)||Gauging Station (National Grid Reference)||Altitude (m)||Average Gradient (m/km)||Gradient at Stocking Points (m/km)|
|Length/Max. Width||at source||at mouth|
|Western Cleddau||197.6||1.4||5.15||SM 954177||115||0||4.0||4.5|
- Measured to gauging station
Fig. 1 Study area
Fig. 2 Distributions of stocked fish as determined by anglers' returns
Fig. 3 Rate of capture of stocked fish
Fig. 4 Cumulative percentage of declared catch plotted against the percentage of those anglers making returns