INTRODUCTION

Following the recommendations of the International Consultation on Fishery Resources Allocation, (Vichy, France, 1980) and its own discussions during its Eleventh Session (Stavanger, Norway, May 1980), EIFAC felt that subsequent sessions should examine closely the current situation for managing stocks of fish in European inland waters. The subject was to be covered in two symposia. The first symposium on Stock Enhancement in the Management of Freshwater Fisheries, held in Budapest, Hungary, 31 May to 2 June 1982, in conjunction with the Twelfth Session of EIFAC, covered the biotic aspects of management. The abiotic aspects were covered in the present Symposium.

The objective of the Symposium was to focus attention on various activities which could have damaging effects on fisheries, and the way in which such effects could be avoided or ameliorated. The Symposium considered the specific aspects of the impact of habitat modification on freshwater fisheries in six sessions as follows:

The body of this report consists of the reviews presented at each of the sessions together with discussions that have arisen. Full text of the papers will be printed separately in the Proceedings of the Symposium, edited by J.S. Alabaster.

SESSION REVIEWS

An analysis of interaction of biotic and abiotic factors with riverine fish communities (Zalewski and Naiman) indicates that the following factors determine the rate of fish community response to changes in abiotic environment: (i) stream order, which determines the degree of environmental diversity and stabilization, (ii) temperature, which influences the physiological performance of the fish and ecosystem productivity, and (iii) the slope of the stream, which influences the above two factors.

In rivers dominated by abiotic regulation, the fish reproductive success fluctuates highly and there is a risk that introduced species will experience a high mortality. Fisheries management of such systems is difficult. On the other hand, in rivers with fish population regulated predominantly by biotic factors, management of fisheries is easier as it allows the application of measures such as, for example, reduction of predators, and introduction of insect or detritus feeding species.

To define the ecological value of environment for fish and fisheries purposes, a new approach using the morpho-dynamic character of the river and evaluating benthic invertebrate stock was proposed by Wasson, Pialot et al. The first use of this method for three managed rivers in France showed its applicability in predicting changes in fish population due to manipulation of the physical environment.

The discussion in this session stressed that whenever abiotic factors predominate and regulate the fish population, there is little scope for improving the species composition because the environment is too unpredictable. Better opportunities for fisheries management exist where biotic factors regulate the community. It has been generally agreed that there is a need for more research on typology of aquatic environment to assist fisheries management of unstable systems manipulated by abiotic factors.

Topics covered related to the isolation of an indigenous fish stock as a result of a pumped storage scheme (Rogers and Cane), the impact of water level regulation on fish-food organisms (Heikinheimo-Schmid) and fish stock development and exploitation (Huhmarniemi et al..; Kaatra and Simola; Mutenia) and the impact of introduced fish-food organisms (Furst et al.). Most papers focussed on waters of high latitude.

Changes in the hydrology of the upper Seiont river system, North Wales, consequent to the construction and operation of a pumped storage power station, have resulted in loss of salmonid spawning and rearing areas above a diversion tunnel which effectively isolated Lake Llyn Peris, converted now to the lower reservoir of the scheme (Rogers and Cane). Salmon and trout are now using a 2.2 km long diversion tunnel in spite of its complete darkness and very low water velocity, to move upstream to spawn. The Arctic char population, whose former migration route was affected by the scheme, is now using for spawning a deepened short river connecting two lakes. Thus, a combination of physical modification of a river channel (for chars) and the adaptability of salmon and trout to the new environment (migration through a tunnel) have assisted in overcoming a potentially negative impact on fish of the largest European pumped storage system.

Regulating water levels of hydro-electric power reservoirs in Finland often reduces the abundance of bottom fish-food organisms leading to severe competition between different whitefish populations. Fish with more flexible feeding habits (e.g. Coregonus wartmani) may adjust more easily to the change in environment and start sharing plankton food sources with other plankton feeders such as C. pallasi and vendace. A corrective measure which has been applied especially in Swedish lakes for some time is introducing crustacean fish-food organisms.

Introduction of Pallasea quadrispinosa as a crustacean fish-food organism is giving promising results in Sweden and seems to be preferable to Mysis, which has some undesirable impact on the ecosystem despite its selective predation on zooplankton (Furst et al.). Although benthic fish do feed on the introduced Mysis, pelagic fish avoid it and the result is a drastic decrease in fish yield. Mysis is however important as a fish-food organism during winter. As introduction of Mysis may result in decline of pelagic fish stocks and hence in the total fish production, new introductions of Mysis must be considered with utmost care.

Regulation of the oligotrophic Lake Pyhajarvi in Finland causes about 80% mortality of whitefish eggs during the winter. However, spawning of vendace (C. albula) appears unaffected as this fish lays eggs in 2–6 m depth (Huhmarniemi et al.). Stocking the lake with C. muksun and C. peled has more than compensated for the losses, resulting in a four-fold increase in whitefish catches. In the large regulated Lake Inari, the strong erosion of shores has caused reduction in the littoral flora and benthic fauna and has affected the fish stocks as well, resulting in a 50% decline in the number of professional fisherman (Mutenia). As a compensatory measure, the lake has been successfully stocked with Arctic char (Salvelinus alpinus) and lake char (Salvelinus namaycush. However, brown trout (Salmo trutta m. lacustris) and landlocked salmon (S.salar m. sebago) have mostly migrated out after being stocked into this lake (Kaatra and Simola). Further measures to improves fish stocks should concentrate on improving methods of breeding, increasing the size of the smolts, delay in spring stocking, identification of better locations for release of fingerlings to reduce the risk from predators and introduction of new fish-food organisms.

In spite of an amplitude of water level fluctuation of 5 m and 11 m respectively, two large Finland reservoirs (Lokka and Porttipahta) of over 10 years age have shown good annual catches (3.5 kg and 8.3 kg/ha/year respectively) in spite of being situated in the high latitude of 68°N. Commercial and recreational fisheries are based on pike and perch, and on the introduced Coregonus peled (Mutenia). The successful fish establishment can be explained not only in terms of a peak production characteristic of newly formed reservoirs, but also because pike and perch spawn in spring rather than during the winter.

The contributions have shown that careful measures can counterbalance the negative impacts of lake regulation such as on the migration of fish, fish-food organisms and winter spawning areas.

This session has considered the impacts of a number of habitat modifications. The review of stream channelization effects on fisheries (McCarthy) identified as the three most important impacts: habitat loss, addition of enormous quantities of suspended solids, and alteration of stream flow patterns. Channelization of marshes, swamps and floodplains may result in a total elimination of any aquatic biota due to drying out of their habitat, but another important negative impact is saltwater penetration into the formerly freshwater habitat. The positive impacts are substrate enhancement through the removal of peaty silt deposits, and alteration of stream-flow patterns by increasing the rate of run-off - both impacts of importance for salmonid fish. A diversity of remedial measures have been tested through their practical application in a number of the EIFAC member states. Some of them have cost-benefit estimates, and this information may already have been built into a scheme in the design stage. But it was felt that a wider use of currently available measures for land drainage, gravel extraction and other habitat changes would be beneficial for habitat enhancement. In-stream modifications by dredging in the past for various purposes, such as agriculture enhancement in Denmark (Hermansen and Krog) and England (Linfield), or for smoothing the passage for timber-floating in Finland (Jutila; Eronen and Shemeikka) led to impoverishment of fish habitats. In Denmark, composition of the gravel bed is among the most important factors determining the success of spawning of brown trout, Salmo trutta. Rehabilitation of rivers on a pilot scale has shown that in future the entire catchment must be considered, as spawning beds are controlled by the use of the entire system. Using the existing weirs in the construction of spawning beds in Danish lowland rivers to create a required slope for such beds is a promising approach to the problem and one which does not interfere with agricultural interests (Hermansen and Krog). Land drainage in Denmark frequently results in an increase in water of yellow ochre soil-rich in iron which is toxic to fish and aquatic invertebrates.

Since timber-floating has ceased in most rivers of Finland, restoration programmes applying the return of boulders back into the river have shown that reproduction of salmon is again possible in rehabilitated stretches of such rivers (Jutila). Large-scale restoration, if launched, will have a very favourable impact on the river fish yield. A compromise restoration approach to accommodate both the fisheries and boating requirements has been successful in Vaikkojoki River (Eronen and Shemeikka).

Lowland rivers of England have been progressively impounded during the past few centuries with the construction of weirs, sluices and locks for various purposes (Linfield). The more recent changes, frequently results of pollution, led to habitat degradation and decline in many fisheries. In past decades when fisheries problems were limited in extent and fish stocks were readily available, restocking was a feasible remedy and regularly practised. But a number of documented examples has demonstrated that such measures are invariably of insufficient benefit to justify the effort and expenditure involved. An approach involving habitat protection, together with schemes for promoting in-situ fish production, appears more feasible.

A scheme for boosting natural productivity by use of Off-River Supplementation Units shows promising results and may be applied in the future to support fish production in rivers of high angling importance (Linfield). Routine maintenance dredging on the River Thame reduces the standing crop of chub and roach between 31 and 64%. Eight years after engineering works on the River Cole, fish biomasses in some sections are less than 10% of their pre-scheme levels. A number of measures have been tested. Rehabilitation of dredged rivers with clay substrate by using crushed limestone and flint gravels has resulted in provision of cover for fish and enhancement of invertebrate and macrophyte colonisation. Other measures have involved provision of wooden hurdles, installment of stone gabion weirs, groyones and deflectors, to accelerate the recovery rate (Spillett and Armstrong).

Extraction of gravel and sand causes considerable damage to fish stocks, by destabilizing the substratum, increasing the turbidity of water, silting-up the channel bottom, and modifying the flow which in turn may result in erosion of the river channel (Rivier and Seguier). These alterations upset the composition and balance of aquatic organisms. Although a law may have been enacted to protect certain biotopes, it is frequently not enforced. In Ireland, the Arterial Drainage Act imposes on the drainage authority the obligation to do whatever the Minister for Fisheries and Forestry deems necessary to avoid injury to fisheries during the execution of a drainage scheme, or as a consequence of it. This requirement is conditional in that what is requested by the Minister for Fisheries and Forestry should not cause substantial detriment to the drainage work or substantially hinder its execution (McGrath). In executing arterial drainage the least damaging to salmon and trout fisheries in Ireland was the approach of limiting major works to one subcatchment per year. Such spacing of work gives an opportunity for recovery before other areas are dealt with. It was found that maintenance work is more disruptive, especially in small streams, than the original scheme of drainage. In the River Thames, maintenance takes place in 10 to 15-year cycles.

The general discussion stressed the need of including the measures into the planning stage. In the execution phase, when fisheries officers work together with the engineers and the construction workers on the spot, problems get solved more readily, as, for example, in the United Kingdom and Ireland.

Increasing demand for water by other users than fisheries has led to the need for identifying the minimum water requirements of salmonid migratory fish. These are a minimum quantity of water for fish to survive, and an appropriate availability at appropriate seasons to allow upstream migration of fish to the spawning areas. Data obtained in north west England by using automatic fish counters operating on the resistivity principle have confirmed that during summer months most fish movement occurs in the higher range of the available flow, but the migration flow range varies from year to year, depending on prevailing river levels (Cragg-Hine). Information on migration flow ranges has been used in conjunction with computer simulations of the effects of abstraction proposals on historical flows to assess the implications of these proposals for migratoy fish. Fifty to ninety per cent of important flow ranges should be maintained for the flow requirements of salmonids in the U.K.

Krogg and Hermansen attempted to quantify the variation in physical factors due to imposition of changes by drainage schemes in Denmark. Current velocity 7 cm above the stream bed was the physical factor best correlated with the number of trout greater than 15 cm. This is explained by the behaviour of brown trout which spends most of its time at the stream-bed. Poor correlation was found with bank cover. It was suggested that in Danish conditions self-shading of river banks may provide sufficient cover for brown trout, hence reducing the need for and importance of over-hanging vegetation. There was no correlation with area and volume of stream section. The results supported the hypothesis of interdependence between a stream's morphology and its hydraulic components. The practical implication is that lack of maintenance quickly leads to a change in stream morphology and hence to recovery of trout population.

While the present Symposium concentrated predominantly on impacts of engineering structures on water resources, the need to pay more attention to gradual cumulative impacts on fishery resources by other users of water was highlighted in the contribution considering outdoor recreation (Bninska). Intensive recreation on non-regulated lake shores, and water sports, such as boating may lead to littoral destruction which in turn may be followed up by a drastic decline in some fish species. In Poland, tench, crucian carp and pike decline first, because of the destruction of the littoral zone. The high demand for lakeside recreation was often in direct conflict with the need to protect fish habitats of lake margins, and the meeting felt that because of the common occurrence of such situations in EIFAC countries, there was an urgent need for more studies to find appropriate remedial measures. The issue was, however, complex and should be approached through comprehensive studies of environmental, including socio-economic, impacts on such threatened systems.

In a review paper by Egglishaw the most harmful impact of afforestation on fish was seen in the deposition on stream beds of soil eroded during land preparation for tree planting. A less important hazard was that of the use of biocides and fertilizers, and some harm was caused also through the increased acidity of soils due to collection of atmospheric pollutants by coniferous trees. The review of literature showed lack of information on the impact on fish of fertilizers used in forestry practices, and of vegetative impacts on fish of planting coniferous forests. It was concluded that afforestation appeared to have more beneficial impact on fish than deforestation. However, in countries with acid rain problems and the resulting acidification of soils, more attention should be paid to afforestation practices.

Freshwater crayfish was found to be very sensitive to habitat modifications, and as a result the current crayfish stocks in many European countries were low. The major negative impacts on crayfish from engineering schemes were the increase in suspended solids, increase in dissolved iron concentrations, lower pH, and lower dissolved oxygen content. The crayfish was as demanding as the salmonids in regard to oxygen concentration and it was very sensitive to acidification, specially in hypoxic conditions. The combined effect of simultaneous acidification and an increase in iron content often seemed lethal. Permanent or longterm alterations of crayfish habitat were often more detrimental than temporary changes of the construction period. Adverse effects could be reduced by proper timing and by selecting the most appropriate construction methods. Although the restoration and management of crayfish stocks in permanent and profoundly modified habitats were difficult, a variety of remedial measures were available, including the temporary removal of crayfish and creation of artificial niches. It was usually necessary to restore habitats before restocking crayfish. In many European countries populations were severely affected by plague and one solution was to import disease-resistant species. It was pointed out in discussion that the only chance to get a good production of crayfish was to stock plague-resistant species. It was suggested that there may be plague-resistant ones within the species amongst the Romanian Danube population, which would be worthwhile investigating.

The effect of damming and canalization of large areas was exemplified by the situations on the lower Danube, the Vistula and by the review of work on the Columbia River (U.S.A.). In the Lower Danube (Bacalbasa-Dobrovici) and Vistula (Backiel) habitat modifications were brought about by damming and consequent creation of reservoirs, reduction of floodplains by construction of levees, modifying river beds, abstracting water for irrigation and (for the lower Danube only) construction of off-river fish ponds. In both river systems these habitat modifications were reflected in changes in commercial fisheries. In the Danube, dam construction in the 1960's negatively affected fisheries of over 80% of the Romanian floodplains and about 25% of the Romanian Danube delta. Further damming was planned, which would further aggravate the capture fisheries. Fish ponds were constructed as compensatory measures in the Danube and thus the total fish production was maintained, with 24,400 tons currently being produced, of which 13,400 tons came from aquaculture-related fisheries. Expansion of fish ponds was envisaged to replace the losses to fisheries anticipated as the result of the completion of a second major dam on the lower Danube in the near future. The dams represented a definite barrier for migratory fish such as sturgeon and shad (Alosa sp.), but the latter was less affected as it spawns downstream. The discussion pointed out that satisfactory fish passes for shad (and clupeids in general) do not exist. In the Canadian St. John River (Bay of Fundy), shad was trapped, then trucked around a dam and released into reservoirs. It was suggested that homing might develop once a reservoir shad population established itself. Sturgeons cannot orientate themselves and lose sense of direction if there is insufficient current, which is a common feature of fish passes. Sturgeon catches in the Danube declined from about 1,000 tons/year in 1900, to 60–70 tons today.

In Poland, river basin development was believed to be responsible for the decline in anadromous fish and for changes in fish communities (Backiel). Fish passes failed to preserve anadromous fish, although in the Vistula River the disappearance of sturgeon coincided with canalisation of the lower course and estuary of the river long before any dams were created. Dams on some tributaries of the Vistula were considered responsible for the total elimination of Atlantic salmon from the system and the decline of sea-trout and Vimba vimba. Successful stocking was reported for pike-perch, sea-trout and eels. It was suggested that in the future fisheries managers should pay more attention to developing new fish communities in modified environments rather than spending effort on preservation of the old ones. Migratory fish in a system of the Vistula, with its 12 dams, can be maintained only by stocking.

In some circumstances hydraulic works may cause little damage or even be beneficial to fisheries. A regulated water discharge from an upland reservoir in England resulted in a significant increase in abundance of brown trout and bullhead (Cottus gobio L.) (Crisp). This improvement has been through an increase in population density rather than a marked increase in growth rate. There was also an upstream extension of bullhead into afferent streams after impoundment, with some evidence of seasonal movements between reservoir and streams. It was thought that negative impacts on downstream fish populations of a small hydroelectric dam in France could be minimized by increasing the reserved discharge by 20% (Demars). Such a managerial measure might be built into the operating schedule.

Under European conditions the creation of reservoirs was generally found to be beneficial to non-migratory species, although stocking with pike-perch and tench was only successful initially and the species fared less well in later years of the reservoir's existence. Indigenous fish population tended to adapt itself better to less pronounced changes in the abiotic habitat than to significant biotic changes. Management policies designed to utilize the new conditions had the best theoretical chance of success, although where the original fish population was rich in species, it was difficult to predict the most successful course of action.

The review of experience with ameliorative measures to enhance the salmonid passage in the Columbia River, U.S.A. (Ebel) stressed among others the need to focus further research on improvements of adult passage facilities, improvement of environmental conditions in the river at the entrance to fishway facilities, and on development of new technologies to provide effective juvenile passage facilities at all dams. Fishery enhancement has become an important research area primarily because the attempt to restore the declining fishery resources with hatchery production has not achieved the intended objectives.

The papers presented at the Symposium, and the subsequent discussions, reinforced the importance of man-made aquatic habitat changes as a major factor which can have an adverse effect on fish stocks. Various mechanisms are involved, but the most common factor appears to be reduced recruitment. In salmonid waters this arises from reduced availability of areas suitable for spawning and reduced survival of eggs in gravel substrates. For cyprinids, reduced survival of fry is probably the major factor. Reduction of areas with adequate depth or cover for adult fish is also an important consequence of dredging in some areas while in certain situations fish stock damage arising from direct disturbance from the wash and propulsion of boats has been recorded.

Damming affected fish migration adversely, and where fish passes were available, they were not utilised by clupeid fish. On the other hand, salmonids were found to migrate through tunnels of considerable length.

Water level fluctuations in reservoirs reduced the abundance of bottom fish-food organisms leading to severe competition between different white fish populations, the plankton-feeding species becoming dominant in nordic lakes. There were also destruction of eggs in winter and a decline in fish recruitment. Catches of indigenous whitefish generally declined largely because of the lowering of the water level in winter.

Invertebrate fish-food introduction exemplified by Mysis introduced in Sweden, showed that in competing with fish for zooplankton, the final result may be a shift in food base from zooplankton to phytoplankton. It was stressed that in lakes with valuable pelagic fish stocks Mysis should not be introduced. The use of Mysis would have most value in lakes with extensive shallows holding trout, whitefish and burbot.

The creation of reservoirs was generally found to be beneficial to non-migratory species. Local fish populations adapt better to less pronounced changes in the abiotic habitat than to profound changes. In the latter situations, management measures must be designed to adjust fish stocks to new conditions. Where the original fish population is rich in species, it is difficult to predict the most successful course of action.

Land drainage, gravel extraction and other similar habitat changes cause reduction in the standing crop or yield of fish stocks or both. For major engineering schemes, measures to minimize the impact on fisheries should be incorporated at the design stage. Remedial measures already tested in some EIFAC member states provide cost-benefit estimates. The percentage of total costs of land drainage and other schemes that can be realistically expected to prove acceptable for allocation to fisheries protection purposes has been quoted for England to be up to 10 per cent.

Afforestation was considered to be beneficial to water habitats in most member countries, but in those with acid rain problems, it may aggravate the situation. The problem should be reviewed from time to time and, if deemed necessary, more attention should be paid to it. However, siltation of rivers affected by afforestation and deforestation practices may be a major problem.

Crayfish were a special case because of their vulnerability to habitat disturbances such as dredging at times of low flow, high temperature and, particularly, during the moulting period. A variety of remedial measures were available including temporary removal and creation of artificial niches, such as pipes to shelter crayfish from siltation. The vulnerability of crayfish to water manipulation and the lack of full understanding of factors which cause the gradual disappearance of crayfish, call for continuation in biological and ecological studies. There is a need to focus attention on crayfish restoration in modified habitats. However, the cost of such restoration is considerable.

While the Symposium concentrated on engineering modifications of water resources, especially their regulation, it was felt that there is a need to pay more attention to gradual cumulative impacts on fishery resources by other users of water, such as intensive recreation on lake shores, and water sports. The impact of recreation on lakes, with special emphasis on the reduction of marginal vegetation around the lake, and the increased input of nitrogen and phosphorous, were identified as being in direct conflict with the need to protect fish habitats in lake margins. Because recreation forms a constitutional right in a number of EIFAC countries, one way of solving this problem would be to provide a free, but not unlimited access to such areas. Appropriate remedial actions include the alternation and adaptation of fish stocking and exploitation policy to match the changes in habitat and water quality characteristics, or, where nutrient inputs are considered to be excessive, the installation of improved facilities for the treatment and disposal of human waste.

The Symposium has drawn attention to the persisting difficulty with forecasting impacts on fish and fisheries of physical modifications of the habitat. Some Environment Impact Statements (EIS) have proved incorrect, and often there is no checking on their validity during the post-construction phase of the project. New problems may arise which are not considered in the original EIS. In spite of such shortcomings, the Symposium felt that the principles of EIS are valuable and EIS's should be prepared as far as practicable in connection with all future proposed work affecting fisheries. For major engineering schemes, measures to minimize the impact on fisheries should be incorporated at the design stage.

There were doubts expressed whether the formulation of Codes of Practice on a European scale for works with potential environmental impact was a practical proposition. It was felt, however, that collection of information on Codes of Practice and guidelines in member countries of EIFAC would be useful. It was therefore suggested that in the initial stage the legal situation in each member country should be examined as a preparatory step to assess whether formulation of a Code of Practice would be suitable.

The major conclusion of the Symposium was that in dealing with the habitat modification impacts on fisheries, there is a need to obtain quantitative data on specific cause and effect relationships, so that the need for remedial measures can be clearly stated at the outset of any activity which will result in a change in river or lake habitat. It is important that where such relationships have already been obtained in specific local situations, they should be tested in similar situations in other countries to see whether they have a wider application so that the extent to which they can be used can be determined. In this way, where works are planned which would have a direct or indirect impact on rivers and lakes, the need for modifications to ameliorate effects on fisheries can be more firmly based and a cost-benefit analysis incorporated into an environmental impact statement. The establishment of quantitative cause/effect relationships represent an important research need.

When man-made changes to the aquatic habitat are unavoidable, the only remedy may be changes in fishery management procedures, including stocking within new species where appropriate. There is a need to draw together case histories where stocking policies and new management procedures have been successful, as well as those which have failed.

The Symposium recommended that, in view of the increasing pressures on fish communities and the quality of fishery resources arising from multiple uses of freshwater ecosystems and associated catchment areas, member countries of EIFAC should give high priority in their research and management programmes to:

1. forecasting and monitoring the impact on fish of man-made habitat modifications and of any necessary remedial measures, and

2. testing and improving existing and new methods of predicting the impact of habitat modifications and of the effectiveness of remedial measures.