Stocking strategies

Abbreviated version of the publication by I,G. COWX, Fisheries Management and Ecology 1994, 1. 15-30.                                 Back to Project profile


Abstract Stocking, transfer and introduction of fish are commonly used to mitigate loss of stocks, enhance recreational or commercial catches, restore fisheries or to create new fisheries, However, many stocking programmes are carried out without definition of objectives or evaluation of the potential or actual success of the exercise. This paper describes a strategic approach to stocking aimed at maximizing the potential benefits, A protocol is discussed which reviews factors such as source of fish, stocking density, age and size of fish at stocking, timing of stocking and mechanism of stocking, The potential genetic, ecological and environmental impacts of stocking are described.



Stocking, transfer or introduction of fish is frequently used by fisheries owners, managers and scientists in the belief they will improve the quantity or quality of catches and have long-term beneficial effects on fish stocks. Over the past 50 years, large-scale movements of fish have occurred, including a total of 1354 introductions of 237 species into 140 countries. Furthermore, many thousands of stocking events, involving millions of individual fish, take place annually in managed fisheries.

Although large sums of money have been invested in stocking activities, relatively few programmes have been properly evaluated and the evidence suggests stocking exercises rarely lead to any long-term tai benefit. This appears to be the result of indiscriminate stocking, without well-defined objectives or prior appraisal of the like likelihood of the success of the exercise. However, if stocking programmes are designed and implemented to satisfy defined goals it should be possible to improve the success rate.

More recently, concerns have also been expressed about the potential risks associated with the stocking and introduction of fish, and the subsequent interactions with wild stocks. These include the loss of genetic integrity in indigenous stocks and ecological imbalance and consequent shift in community structure.

Thus them is a need for fisheries managers to be more aware of the possible impactof stocking, both in terms of the effects on wild populations and the likelihood of improvements in stocks. Of particular importance is the need to develop a strategic approach to stocking which defines the objectives of the exercise and orients the implementation phase towards meeting these goals.


Objectives of stocking

The reasons for stocking are many and varied but generally fall into four main categories which are related to the states of the wild stocks, the impact of anthropogenic activities and the ease with which factors limiting natural production can be removed or ameliorated.


Stocking for mitigation

This encompassess stocking with fish carried out as a voluntary exercise or statutory function for fishery protection schemes, such as reservoir dam construction, land drainage works or similar habitat perturbation. Mitigation stocking has been considered the simplest way to compensate for such activities. However, stocked fish may be released into unaffected parts of the river catchment or lake, and the impact on the wild stocks in these areas must be considered.


Stocking for enhancement

Enhancement stocking is the principal method used to maintain or improve stocks where production is actually or perceived to be less than the water body could potentially sustain, but where reasons for the poor stocks cannot be identified. This type of stocking is used where fishermen express dissatisfaction with the quality of fishing, or to enhance stocks in sections of river where access is restricted by natural barriers, or in the operation of commercial or put-and-take fisheries where the production of exploited species needs enhancing. It also includes activities carried out to strengthen quality and quantity of spawning stock of a given species so as to improve natural reproduction potential.

The majority of stocking in the past probably falls into this category and it is driven by fishermen complaining about the status of the fishing. However, in many cases the fishermen’s and fisheries managers’ assessments of the state of the stock have probably been unduly pessimistic, resulting from natural fluctuations which can have a profound effect on sonic fish populations, or merely the estimates of the potential production have been unrealistically high.

If production is already limited or driven by natural population cycles, it is unlikely that stocking will have a beneficial long-term effect.


Stocking for restoration

Stocking for restoration relates to that which is carried out after a limiting factor to stock recovery or improvement has been removed or reduced, e.g. water quality improvement, habitat restoration or the easing of passage for migratory fish. Re establish of fisheries which have previously been eliminated by poor water quality or habitat degradation are in this category. Stocking in this case is justifiable because he underlying problems limiting production have been tackled and long-term benefits arc likely to accrue.


Creation of new fisheries

This category includes attempts to establish a new stock of fish in a river, lake or reservoir which has not previously held that stock because of natural barriers, evolutionary isolation, or where new (exotic) species are introduced into existing fisheries in an attempt to increase species diversity, improve fish yield or fill an apparent vacant niche. This is the most controversial stocking procedure and has led to considerable contention in the past. e.g. the introduction of Nile perch into Lake Victoria. However, if the new species is able to occupy a vacant niche the impact on the indigenous species may be negligible and the introduced species may become a valuable addition to the fishery. For example, barbel, introduced into the River Severn, England have become a valuable component of the recreational fishery. Introduction of species which will compete with indigenous species at some stage of their life history, for example mixing chub, dace, roach, or bream may lead to the failure of the stocked species to produce a self-sustaining population and/or a reduction in the biomass of one or all the indigenous species. Consequently, stocking of this type needs careful planning to avoid potential catastrophic effects on the natural fish populations.

Protocol for pre-stocking appraisal


Identify need for stock improvement

In view of the many concerns that exist about stocking, a responsible attitude towards the activity is essential. A step—by-step approach to planning, assessing and implementing a stocking programme is shown in Fig. 1. It is based on the ‘project approach’ to management activities. Essentially, the activities can be broken down into the following phases: identification, preparation, appraisal, implementation and evaluation.

The first step when considering any stock improvement activity must be to ensure proper clarification of the management policy and objectives. It is only then that he project proposal can be properly formulated to achieve the desired effects. Part of this exercise includes establishing whether the stock is below optimum production level or whether the quality of the stock (e.g. in terms of age or size distribution) could be improved. This requires not only an assessment of the status of existing stocks, but an appraisal of the condition of the water body and the natural and artificial factors that may limit production. These assessments must be based on firm evidence from scientific studies and not hearsay or unsubstantiated complaints.

To aid the decision-making process, a technique commonly employed in development project formulation. This approach is useful in setting out the design of the stocking programme in a clear and logical way so that any weaknesses that exist can be addressed at an early stage, or if these are insuperable, the stocking can be aborted.


Establish objectives of stocking

If the fishery is of the desired quality, the need for any stock improvement must be questioned. Stocking generally does not tend to improve the catches in waters where there is adequate natural recruitment. Under these circumstances alternative improvement strategies should be considered or the ‘do nothing’ approach adopted. If production is considered to be below the potential of the system it is important to try to identify the constraints and resolve them before stocking is carried out. If no apparent cause can be identified, enhancement stocking could be considered, but there is probably little sense in stocking a water if it is not capable of supporting a sustainable population. In this case it is probably worth while considering alternative improvement strategies or just leave the system alone and concentrate resources on rivers or lakes which can possibly be restored. This does not, however, exclude put-and-take fisheries,which are stocked to provide catchable-sized fish for rapid exploitation by anglers and do not take into account sustainability through natural recruitment.

Where the limiting factor(s) can be isolated, efforts should be made to resolve the problems before resorting to stocking. If remedial action cannot be taken, a in the case of a dam, for example, then mitigation stocking could be considered. This will probably not lead to a sustainable population and stocking may have to be on a continuous basis.

Alternatively, it mitigation stocking is not a viable or statutory option, considerations could be given to creating a new fishery based on species found elsewhere in the catchment or exotic species. If the system has not previously held the species to be stocked, it is important to establish whether it is suitable for the proposed introduction and whether there is likely to be an impact on other indigenous species.

Finally, if it is possible to remove or minimize the cause of the decline in the fishery, this course of action should be taken. The fishery may then recover without stocking. Habitat improvement is the most desirable option because it should lead to long-term sustainable improvement with minimal deleterious ecological impact. It is also an efficient use of resources because it may have greater long-term benefits than enhancement stocking and also have other conservation and ecological benefits, e.g. improved primary or secondary production. In cases where natural recovery may be ineffective, because, for example, the spawning stock has been reduced to an apparently critically low level, restoration stocking may be appropriate to promote stock recruitment.


Development of stocking strategy

If the stocking appears to be worth while then implementation of the proposal must be carefully planned to minimize potential problems and risks (Fig. 2)

This formulation phase should assess the resources available to undertake the project, including availability of stocking material, labour, transport and finance, and identify any constraints which might jeopardize the successful implementation of the stocking programme. The protocol to be adopted to accrue the maximum benefit from the exercise should also be defined. For example, the source of fish, species mix. stocking density, size or age of fish to be stocked, timing and mechanism of release, should all be defined. Details of the factors that need to be considered in devising the stocking strategy are discussed later


Assessment of proposal

All the ecological factors and consequences must be reviewed, with all the known or predicted advantages and disadvantages being accurate identified, The stocking proposal must also be assessed for its possible impacts on the potential yield and stability of both the introduced stock and the resident species. Details of the genetic risks and possible fishery and environmental effects associated with stocking activities must be discussed.

The socio-economic influence of any change can also be crucial and must, therefore, be evaluated, particularly in terms of justifying costs. Estimation of the costs should include all capital and recurrent expenditure and be evaluated against the predicted benefits of the proposal, for example in terms of improved catches. The assessment of benefits should not include the public relations value of the programme, although in many situations this may provide a major incentive for undertaking the work. However, where benefits are real and substantial, the opportunity for public relations should not be foregone.

Finally, a complete review of all the accumulated information should be carried out by an independent, impartial authority, and only then should a decision, be made whether to implement or reject the original proposal.


Post-stocking evaluation

One component of stocking exercises which has been largely neglected is an evaluation of the programme and an audit of the degree of success of the work. Such evaluations should be considered an integral part of the programme, and stocking should not be approved without building in a post-project evaluation. The evaluation should assess the efficiency and long-term benefits of the various stocking practices and regimes, and attempt to identify the factors contributing to their success or failure, it should be noted that demonstrating improved catches alone does not necessarily signal a successful stocking exercise, as it may not lead to a good spawning stock or sustainable recruitment. This information will provide a feedback mechanism (Fig. 1) to improve the formulation of realistic management objectives where stocking is a fundamental activity; to in stocking strategies and techniques; and to provide a database of experiences against which the risks and the feasibility of new proposals can be appraised.


Potential risks from stocking

Genetic interactions

Recently it has been recognized that fish stocks (particularly salmon) from different catchments, and to a lesser extent from within the same catchment, exhibit genetic variation. These geographic, genetic differences may manifest themselves in, for example. growth potential, age at maturity, fecundity, or, in salmon, season of return to fresh water. They may also have some adaptive significance with respect to the performance of the stocks in a particular environment. Consequently, stock from non-indigenous sources may be less well adapted to the riverine or lacustrine environment into which they are released, and stocking may be less successful than expected. In addition, any fish surviving to reproduce with indigenous individuals may also confer a reduced adaptation upon some or all of their offspring. Several examples of this problem exist in the literature particularly with respect to returning adult salmon, where pure donor stock have a lower return rate than natural stock, and hybrids exhibit an intermediate rate

Consequently, when embarking on any kind of stocking programme, consideration must be given to maintaining the genetic integrity of the indigenous stocks. Unfortunately, the intensity and long-term nature of many stocking programmes will make it difficult to rectify the situation once genetic integrity has been disrupted. However, where possible, stocking should be restricted to those using fish derived from local populations, or failing that habitats which are similar, or fish that have not been held in captivity for more than one generation.

Ecological interactions

Carrying capacity of the target habitat. Perhaps one of the greatest concerns with stocking programmes is that they rarely take into account the capacity of the recipient system to support the enhanced stocks. If too many fish are present, increased mortality rates, through predation and starvation, reduced growth rates and increased dispersion generally follow. Thus, while stocking may produce large increases in fish numbers at certain times or in localized areas, no more fish will survive than the habitat will allow.

In the worst case scenario, overstocking can lead to a reduction in the performance of the fishery, below that prior to the introduction. For example, when the spawning stock of salmon exceeds an optimal level, the number of smolts may decrease.

Species interaction. Stocking of one species may have undesirable effects on endemic stocks either through predation or competition. For example is a reduction found in the survival of stocked salmon fry due to competition from resident trout fry and salmon parr. Conversely, changes in trout populations have similarly been recorded following salmon parr stocking. In this case the salmon do not affect trout fry survival, but the presence of the salmon parr does cause a reduction in trout stocks.


Disease control. With the transfer of stocks between water bodies, there is an obvious risk of disease transmission. In any countries various legislation controls the movement of fish, but this is frequently violated with consequences that are to difficult to reverse. In the ideal situation all fish should be certified disease-free before stocking. This should be possible for fish originating from farms, and must become a statutory function attached to all consent applications.

However, when fish are transferred from one river or lake to another it is improbable that disease-free status can be guaranteed In this case the disease status of both the donor and recipient stacks should be assessed and stocking only allowed if no new pathogen is being introduced and the stocked fish are healthy and have a low parasite/pathogen loading. If any possibility of disease transfer exists, the fish should be held it quarantine until the risk has been assessed.

Conversely, it is possible that fish introduced from one system to another may not be resistant to an endemic disease or parasite and the stocking exercise may be unsuccessful. Although this may appear to have little impact on the receiving stock, there is a danger that the introduced fish may act as a reservoir for the proliferation of the disease.


Stocking strategies

When undertaking a stocking programme there are many procedures which should be considered during the implementation phase. It is therefore necessary to plan the stocking exercise to ensure its success, Several of these procedures have been discussed already and will be dealt with only superficially whilst others, which relate to implementation strategy, will be discussed in morel detail. Unfortunately, many of the practical aspects have to be considered in a descriptive way because relatively little information is available about the effect of various procedures on the success of stocking. A schematic approach to the planning exercise, to take on board these points, is suggested in Fig. 2.


Source of fish

There is an increasing awareness of the importance of maintaining genetic integrity of fish stocks. Consideration should therefore be given to minimizing the dilution of genetic variation by indiscriminate stocking policies with fish of unknown origin. Before implementing a stocking programme, a number of options relating to the source of fish should be considered

Options for systems where species is extinct

(1)   Donor stock with the same biological characteristics as the recipient system.

(2)   Stock chosen from a lake or part of a river with a similar environment (e.g. size of stream, gradient, water temperature, flow regime, altitude, profile).

(3)   . Artificial propagation based on stock from (1) or (2) (sufficient fish should be used as brootistoclc to avoid reducing genetic variability of the species).

(4)    Presume genetic differences of little adaptive significance and obtain stock from anywhere they are cheaply available, ideally from a number of sources, to maximize range of genetic material.



Options for depleted or relict stocks

(1) Build-up of stock by hatchery production based entirely on local stock and return brood stock to home system.

(2) Redistribution of adults from elsewhere in the catchment (may be unsuited for introduction to other parts with different prevailing conditions).

(3) Choose stock from a system with a similar environment.


Options for rivers where new species to be introduced

(1) Farm-reared fish, certified disease-free.

(2) Stock from a lake or part of a river with a similar environment which have been quarantined and certified clear of parasites and diseases alien to recipient system.

(3) No obvious ecological problems likely to be caused — as with introductions of predators.

Stocked fish should not have been reared in captivity for more than one generation in order to limit the possible effects of selection within the hatchery, thus particular care must be taken when obtaining fish from hatcheries.


Preconditioning and acclimatization

There is a growing body of evidence to suggest that fish should be preconditioned to survive the prevailing conditions in the receiving water body. For example, fish which have been reared or are to be transferred from still water to a river should be exposed to running water conditions for an extended period before their release. This exercising builds up the red muscle tissue in the stocked fish, thus increasing their ability for sustained swimming.

Acclimatization to temperature is also thought to be important. Prior to stocking barbel into the River Mehaigas they lowered the temperature of the water in which the fish were held over an extended period (10—11 days) until it approximated to that of the receiving water.

Handling and transportation of stock

Handling and transportation inevitably cause stress and possibly damage to fish, which can subsequently affect post survival, As a result, procedures which minimize handling time or frequency should be adopted from the time of capture of the donor fish to planting into the recipient system.

The techniques employed to capture the fish in the first instance should cause min damage; seine netting and ‘controlled’ electric fishing are the preferred techniques. During collection and transportation handling should be avoided where possible. Fish should be stored at low density and provided with an ample supply of oxygen.

All fish should be starved for at least 24h prior to transportation to reduce oxygen demand, due to increased respiration rates during digestion, and minimize ammonia production. If the fish are to be transported long distances consideration should be given to reducing the effective toxicity of un-ionized ammonia by lowering the temperature and pa

Finally, there is no point in introducing fish that are in poor condition or health, as this will affect the success of the exercise.


Stocking density

As previously discussed, a thorough assessment of the receiving water body should be carried out to determine the optimal stocking density. In lakes a relationship exists between shore line development, depth and predicted fish biomass. However, no definitive relationship is available for calculating stocking density of different species in rivers; it is generally based on the experience of the managers. If possible, a similar, appropriate relationship should be determined for river fisheries.

Alternatively, a database of experiences should be set up to provide guidance on stocking densities which maximize the benefits in terms of improving stocks. This should be based on measured success of stocking at different densities. Such data are more readily available for salmon, e.g see summary in Table 1, but rarely for other species. Thus, effort should be made to construct tables to indicate the success of stocking of all species at different densities. This can only be achieved if the outcome of stocking programmes is evaluated and reported.

When calculating the stocking density, consideration must be given to the existing stock biomass and allowances should be made for migration/dispersal, predation and predicted survival of stocked fish. Values of between 10 and 80% annual mortality are given in the literature, so compensatory densities will be difficult to determine The most important issue is that overstocking is avoided.


Size or age of stock

There has been much debate over the most appropriate size or age of fish for stocking. Many of these arguments can be removed by drawing up tables which illustrate the success of stocking of different a groups. Table 1 gives a summary example for salmon. This suggests that fed fry is probably the most effective life history stage to stock as the return rates are greatest. However, this information should be evaluated in relation to the advantages and disadvantages of obtaining sufficient fish of the appropriate age or size (Table 2). When this comparison is made, eyed ova or unfed fry are probably a more cost-effective life history stage for stocking.


Timing of stocking

There is a considerable volume of literature on the most appropriate time for stocking of salmonids. The general conclusion is that stocking in spring is more efficient (4—12 times) than winter. Very little similar information is available for other species but a common sense approach probably defines the timing reasonably well. Fish should be stocked when the flow rates and water temperature are generally low, to minimize displacement of fish and stress respectively. The stocking should preferably take place when the productivity of the receiving water is high, but not during the spawning period as the stocked fish may interfere with natural reproduction processes. Stocking early in the summer, when natural food availability is good and to allow the fish to adjust to the conditions in the receiving water before overwintering, is preferable.




Mechanism of release

Three mechanisms for releasing fish are used:

(1) Spot planting - introducing all the fish into the receiving waters ‘at the same site;

(2) Scatter planting — introducing fish into several sites in the same region:

(3) Trickle planting — introducing fish into the same region over a period of time.

Spot planting can lead to competition among the stocked fish, or with natural stocks, and in rives is often associated with considerable downstream displacement to reduce population interactions. Scatter stocking gives a wider dispersal at the outset and minimizes competitive pressures. Trickle stocking similarly removes competition but is often constrained by lack of labour, finance and available stock.

Evidence suggests that scatter and trickle stocking are more successful than spot stocking, but the latter is generally carried out because it is easier to undertake.


Resource problems

The previous sections have described many of the issues that must be addressed when designing the stocking strategy. 1-lowever, they are of little importance if the resources for implementing the programme are not available when required. A recurring problem is that the fish for stocking may not be readily available at the ideal time or in the numbers required. Access to the target zone may also be problematic, making scatter stocking difficult. All these circumstances must be addressed, and a compromise strategy for implementation must be drawn up.



Stocking is an important tool in the management of fisheries, albeit for commercial, recreational or conservation purposes. However, the management rationale and implications of stocking activities have not received the attention desired to support such a commonly used tool. It is recommended that a strategic approach to stocking is adopted. As part of this approach a number of aspects should be addressed:


• Whenever, stocking of fish is to be considered, the aims, and specific objectives of the exercise must be clearly defined and adhered to.

• When evaluating stocking as a possible management tool, the relative benefits and cost of all options should be considered. ‘The ‘do nothing’ option should not be disregarded but should be considered as fully as any of the other options under discussion, despite possible public pressure to stock.

• The strategy for any programme of stocking, transfer or introduction should becarefully tailored to suit the species in question, taking into account its entire suite of ecological prerequisites, so as to maximize the chances of success.

• The potential adverse impacts of stocking in terms of environmental, genetic and ecological interactions should be considered fully and the ‘precautionary principle’ adopted if any adverse impacts are foreseen.

• All projects should have in place the methodology to enable adequate monitoring of progress, and ultimately, success or failure. This post-stocking appraisal should include a mechanism o disseminating the outcome to minimize the risk of any unforeseen adverse effects in future exercises.

• A series of guidelines should be produced for all species which are stocked or introduced, clearly defining the most effective protocol for deciding whether stocking should take place, how it should be implemented and the potential impacts of such activities.

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