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Downstream fish passage technologies are much less advanced than those concerning upstream fish passage facilities and are the areas most in need of research. This is obviously partly due to the fact that efforts towards re-establishing free movement for migrating fish began with the construction of upstream fish passage facilities and that downstream migration problems have only more recently been addressed. This is also because the development of effective facilities for downstream migration is much more difficult and complex. As yet, no country has found a satisfactory solution to downstream migration problems, especially where large installations are involved (EPRI, 1994). As a general rule, problems concerning downstream migration have been thoroughly examined in Europe and North America with regard to anadromous species, and more particularly to salmonids. Comparatively little information is available on other migratory species.

A large number of systems exist to prevent fish from being entrained into water intakes, although they are by no means equally effective. They may take the form of physical barriers which physically exclude fish from turbine intake or behavioural barriers which attract or repell them by means of applying sensori stimuli to elicit behavioural responses. Both types are associated with bypasses for downstream passage.

The design of effective facilities for assisting the downstream passage of fish must take into account the limited swimming ability and behaviour of the target species, and the physical and hydraulic conditions at the water intake.

4.1 Physical Barriers

One solution to prevent fish from passing through the turbines involves stopping them physically at water intakes using screens which must have a sufficiently small mesh to physically prevent fish from passing through. These screens have to guide fish towards a bypass, which is done most effectively by placing them diagonally to the flow, with the bypass in the downstream part of the screen.

Sufficient screen area must be provided to create low flow velocities to avoid fish impingement. The velocity of the flow towards the screen should be adapted to suit the swimming capacities of the species and stages concerned. Physical screens can be made of various materials: perforated plates, metal bars, wedgewire, plastic or metal mesh. Uniform velocities and eddy-free currents upstream of screens must be provided to effectively guide fish towards the bypass (ASCE, 1995; Larinier and Travade, 1999).

Photo 23: Impingement of downstream migrating adult American shad at a powerhouse intake. (Photo Larinier)

Photo 24: Fine mesh screen at an hydroelectric power plant intake on the Loch Ness in Scotland. (Photo Travade)

4.2 Behavioural Barriers

Visual, auditory, electrical, and hydrodynamic stimuli have resulted in a large number of experimental barriers: bubble screens, sound screens, fixed and movable chain screens, attractive or repellent light screens, electrical screens and hydrodynamic (`louver') screens.

Results obtained in particular cases with various screens (visible chain, light and sound screens) have not been of any great use because of their specificity (efficiency as a function of species and size), low reliability and their susceptibility to local conditions (water turbidity, hydraulic conditions).

The hydrodynamic or `louver' screen consists of an array of vertical slats aligned across the canal intake at a specified angle to the flow direction (ASCE, 1995) and guide fish towards a bypass. It has been used with some success in several sites, namely on the east coast of the USA: Louvers may be considered for sites with relatively high approach velocities, uniform flow and relatively shallow depths. The efficiency is highly dependent on the flow pattern in the canal intake (ASCE, 1995). The first louvers were installed over the full depth of the approach channel. However, more recently, several `partial-depth' louver systems have been installed in the USA, based on the observation that migrating Atlantic salmon smolts and juveniles clupeids remained in the upper portion of the water column. The partial-depth system recently installed in the intake channel of the Holyoke hydroelectric power station on the Connecticut River has an efficiency of 86% for juvenile clupeids and 97% for Atlantic salmon smolts (Odeh and Orvis, 1998).

The use of behavioural barriers, which are still experimental, must be considered with caution (OTA, 1995).

4.3 Surface Bypasses Associated to Surface Bar Racks or Deep Intakes

Surface bypasses associated with existing conventional trashracks or angled bar racks with relatively close spacing have become one of the most frequently prescribed fish protection systems for small hydroelectric power projects, particularly in the Northeast of the USA and in France. These structural guidance devices act as physical barriers for larger fish (downstream migrating adults) and behavioural barriers for juveniles. The efficiency is closely related to fish length to spacing ratio and to fish response to hydraulic conditions around the front of the structure and at the bypass entrance. Tests showed that under optimal conditions, efficiency can reach 60-85% (Larinier and Travade, 1999). Flow discharge in the bypass has also been proven to be critical. The design criteria currently applied in the USA and France call for a minimum discharge of 2% to more than 5% of the turbine discharge (Odeh and Orvis, 1998; Larinier and Travade, 1998).

In the Columbia River Basin, there is a major effort under way to develop surface bypasses associated with relatively deep water intakes. Various design configurations are being evaluated. The volume of bypass flow required to be sufficiently attractive is thought to lie in the 5% to 10% range. The design goal of theses bypasses is to guide at least 80% of the juvenile fish (Ferguson et al., 1998).

4.4 Eels

The problem of the downstream migration of eels (Anguilla spp.) at hydroelectric power stations is critical in the light of their size and the numerous fatalities which result. No specific solution has been implemented in North America or Europe due to the relatively recent awareness of eel migration. Only physical barriers are likely to work, but their installation would mean redesigning most water intakes (increase in the surface area of the filter due to smaller grid spacing). Due to the demersal behaviour of the species, there is no certainty that the approach used for juvenile salmonids with surface bypasses combined with existing trashracks would be efficient. Experiments on bottom bypasses need to be undertaken, although it must be recognised that even if this technique were to prove efficient, there would be a considerable challenge to design facilities that did not create significant maintenace problems. The principle of behavioural light screens appears promising, taking into account the species repulsion to light (Hadderingh et al., 1992). Stopping turbines during downstream migration is a solution already envisaged, as is the capture of individuals upstream of the obstacles for Anguilla rostrata in the USA (Euston et al., 1998) and Anguilla dieffenbachi in New Zealand (Mitchell, 1995). However, these solutions assume that the downstream migration period is both predictable and sufficiently short, which does not appear to be the case for the European eel (Anguilla anguilla) if we consider downstream migration monitoring (Larinier and Travade, 1999).


The following review is not considered exhaustive. It aims to explore the current use of fish passes throughout the world, the target species, the state of technology and the current philosophy. Some countries are not mentioned because the state of the art is poorly documented or unscientific and some because they are of no great interest within the framework of this limited document.

5.1 North America

There are about 76 000 dams in the USA, about 2 350 operating hydroelectric projects and only 1 825 are non-federal projects licensed by the FERC (Federal Energy Regulatory Commission) (Cada, 1998). Upstream facilities and downstream passage technologies are respectively in use at 9.5 and 13 percent of the FERC-licensed hydropower plants (OTA, 1995). Fish passage requirements are most common along the Pacific and Atantic coast which support the most important anadromous fisheries and in the Rocky Mountains which have valuable recreational fisheries.

The main advances in upstream passage technology have come from the west coast of USA and Canada, where fish passage facilities have gradually become more sophisticated over the years since the building of the first dam (Bonneville dam) about 60 years ago on the Columbia river (OTA, 1995). Currently, about 40 large-scale hydro developments are in place on the Columbia river. Upstream passage technologies are considered to be well-developed and understood for the main anadromous species including salmonids (Pacific salmon and steelhead trout), and clupeids (American shad, alewife and blueback herring, Alosa spp.), as well as striped bass (Morone saxitilis). Upstream passage fish facilities have not been specifically designed for potamodromous species, although some of these fish will use them (carp, northern squawfish, suckers, shiner, whitefish, chub, dace, crappie, catfish, trout...). Most of these fish passes are pool-type fish passes with lateral notches and orifices (Ice Harbor type pool fish pass), or vertical slot pool fish passes where it is necessary to accommodate higher upstream and downstream variations in water levels (Clay, 1995).

For smaller facilities, vertical slot fish passes are the most frequent type of design in British Columbia and pool and weir fish passes in Washington and Oregon (Walburn and Gillis, 1985). The Denil fish pass is not widely used in the West coast, except in Alaska for salmon (Oncorrhinchus spp.) where its light weight and mobility when constructed of aluminium, have proven useful for installations at natural obstructions that are inaccessible except by helicopter (Ziemer, 1962; Clay, 1995).

On the East coast of the USA and Canada, the advances in fish pass design are more recent, since anadromous species restoration programs on the main rivers of New England (Connecticut, Merrimack, Penobscot, St Croix river ) were launched in the sixties of the last century. Fish passes of all types have been used to pass the following target species, Atlantic salmon (Salmo salar), shad (Alosa sapidissima), alewife (Alosa pseudoharengus), striped bass (Morone saxatilis), smelt (Osmerus mordax) and sea-run brook trout (Salvelinus fontinalis). Fish lifts have been successfully used to pass large populations of shad on the Connecticut, Merrimack and Susquehanna rivers. Denil fish passes have been used in Maine, namely for salmon and alewife. Fish pass development in the Maritimes appears to have followed the Maine experience closely with the exception that Denil fish passes were not widely constructed (Washburn and Gillis, 1985). For the same species, pool and weir fish passes are preferred, with drops varying from 0.15 m for smelt and up to 0.60 m drop for salmon (Conrad and Jansen, 1983). In the East coast of Canada, Clay (1995) reported there are 240 fish passes.

Photo 25: Ice Harbor-type pool fish pass at Turner Falls on the Connecticut River. (Photo Larinier)

For central Canada and the USA, Clay (1995) lists 40 fish passes used by potamodromous species as catostomids, cyprinids, ictalurids, esocids , gadids and percids, as well as salmonids such as Salvelinus, Coregonus, Thymallus (Schwallme, 1985).

Francfort et al. (1994) completed a detailed study of the benefits and costs of measures used to enhance upstream and downstream fish passage at dams using operational monitoring studies data from 16 cases study projects across the USA which represent the measures most commonly used in the USA. At least six of the case study projects have successfully increased the upstream passage rates or downstream passage survivals of anadromous species. The most significant success are the two fish lifts at the Conowingo dam which are an essential part of the Susquehanna river shad restoration programme: adult shad numbers below the dam increased from 4 000 to over 80 000 between 1984 and 1992 (Cada, 1998). Although all of the projects had conducted some degree of performance monitoring of their fish passage mitigation measures, there were substantial differences in the extent and rigour of the studies : for some projects monitoring was limited to studies during a single season or based only on visual observations. For most case study projects benefits could be expressed only in terms of the increased numbers of fish transported around the dam. The influence of these increased numbers on the subsequent size of the fish populations was rarely known (Cada, 1998).

5.2 Europe

In England and Wales, a recent inventory suggests that there are approximately 380 fish passes. More than 100 have been built since 1989 (Cowx, 1998). For many years fish passes have been built almost exclusively for Atlantic salmon and sea-run brown trout. The awareness of the need for the passage of potamodromous species (`coarse' fish) and other non-salmonid diadromous species such as shad (allis and twaite) or eel is more recent. The most commonly used fish pass is the pool-type fish pass (Beach, 1984) in England and Wales, and more recently floor baffle Denil fish passes (Armstrong, 1996). In Scotland, submerged orifice fish passes, pool and weir passes and fish locks were used in the fifties of the last century.

In France, recent legislation, adopted in 1984, requires that free passage must be assured through all obstructions situated on designated `migratory fish' rivers. The diadromous species considered are Atlantic salmon, sea-run brown trout, sea lamprey, Allis shad, and eel. The only potamodromous species taken into account by the law are brown trout, northern pike and European grayling. Consequently, more than 500 fish passes have been built or retrofitted over the last 17 years. As a result of experience gained, in particular from experiments with hydraulic models, and on-site monitoring, certain advances have been made in the choice and design criteria for upstream fish facilities. Denil fish passes are only used for Atlantic salmon, sea-run brown trout and sea lamprey on small rivers. Fish lifts or large pool-type passes with large and deep passages (vertical slot or deep notches) are used for shad. When several species must be taken into account, the recommended fish pass is the pool type (Larinier, 1998) .

Photo 26: Submerged orifice fish passes at Clunie dam in Scotland. (Photo Larinier)

Photo 27: Due to the fish lift at the Tuilières Dam on the Dordogne River (France), migratory species such as salmon and Allis shad can now pass the obstacle and reach spawning grounds that are situated further upstream. (Photo Larinier)

In Germany and Austria, design and construction of fish passes has also been very active over the last 15 years. Fish pass design tends to take into consideration many of the potamodromous species (brown trout, cyprinids, percids, etc.). The most common fish pass used is the natural-like bypass channel (Parasiewicz et al., 1998). However, where land is limited, more conventional pool and weir fish passes are used (DVWK, 1996).

Pavlov (1989) reviewed fish passes in the former USSR. Conventional pool and weir fish passes are used for salmonids. He describes fish facilities built in the Caspian basin, Azov and Black seas, and in particular on the Volga, Don and Kuban rivers where target species were Acipenseridae, Clupeidae, Cyprinidae, namely Vimba vimba, Percidae and Siluridae. Very large fish locks, fish sluice, fish lifts and mobile devices for fish collection and transport have been designed for these species.

5.3 Asia

There are probably about 10 000 fish passes installed on Japanese rivers (Nakamura and Yotsukura, 1987). They are mainly designed for anadromous salmonids (Oncorhyncus spp.), Japanese eel, gobies (Rhinogobius spp.), and the ayu (Plecoglossus altivelis) which is a very valuable amphidromous species whose juveniles (50-60 mm long) migrate upstream. Recently, riverine species have also been selected as target species (Nakamura, 1993). Over 95 percent of fish passes are conventional pool and weir fish passes, the others are vertical slot and Denil type. Most of the first fish passes designed for ayu were not efficient because they were imitations of European designs which were only suitable for larger fish (Nakamura et al., 1991). Following the two Symposia on fish passes held in Gifu in 1990 and 1995, a large effort is being made to improve and adapt fish pass design to Japanese species: `the improvement of fish passes is progressing so rapidly that it is known as a fishway revolution' (Nakamura, 1993).

As noted by Wang (1990) and Clay (1995), China has a vast system of reservoirs (about 86 000) and the fisheries of these reservoirs are intensively exploited and maintained by stocking from hatcheries, so that little need has been felt for fish passes.

The first fish passes are only 40 years old (Wang, 1990) and around 60 to 80 fish passes have been built (Nakamura, 1993). The main target species are potamodromous species, mainly four species of carp, and catadromous species, mainly Japanese eel. Most fish passes are pool-type.

Zhili et al., (1990) describe the Yangtang fishway on the Mishui river, which pass 45 species and more than 580 000 fish per year. The fish pass effectiveness was fairly well monitored (5 000 hours of observation annually). The effect of the fish pass seems to be significant, statistics of fish harvest showed that the annual fish output in the upstream part of the Mishui river increased to 3.5 times compared with that in the years before the fishway building. This fish pass has been specifically designed to pass very small fish, with very low turbulence in pools and low drops (about 0.05 m) between pools. The attraction flow (16 m3/s) and the collection gallery above the turbines are considered to play an essential role in the effectiveness of the facility. This fish pass is one of the few examples of a well designed fish pass, adapted to native species and well monitored in developing countries.

5.4 Africa

Africa has over 2 000 known species of indigenous freshwater fishes. The construction of dams has multiplied since the 50s for both irrigation and hydroelectric power generation.

Shad populations are present in North African rivers, namely in Morocco, but the existing and (for some of them) recent fish passes seem not adapted to this species. Shad disappeared from the Oum-er-Rbia after the construction of the Sidi-Saïd dam, equipped with a Denil-type fish pass (Chapuis, 1963). The fish pass planned in 1991 on the Garde dam on the Oued Sebou was neither adapted to shad, nor to the dam and was clearly bounded to fail (Larinier, pers. comm., 1991).

Apart from shad in North Africa, no anadromous species are known. As noted in Daget et al. (1988), dams are only likely to hinder potamodrous species such as large Labeo, Barbus, Alestes, Distichodus and Citharinus which migrate long distances up and down rivers in relation to their breeding cycle and seasonal flooding. The impact of dams is perhaps more obvious in the disappearance of biotopes for some rheophilic species located in areas where there are rapids, gorges or rocky ground, all of which are areas likely to be chosen for dam building.

In South Africa, the need for fish passes has become apparent only in recent years. This country has a low diversity of freshwater fish. In the coastal streams there are only six catadromous species: striped mullet, freshwater mullet and four species of eels (Mallen-Cooper, 1996). In the more inland rivers of the Transvaal, there are potamodromous species, mainly cyprinids, with both juveniles and adult migrating upstream. The few existing fish passes (only 7 in 1990, Bok, 1990), have been based on existing European and North American designs for salmonids and do not meet the needs of native species.

5.5 Australia

In temperate south-eastern Australia, there are approximately 66 indigenous freshwater species; over 40% of these make large-scale movements or migrations that are essential for the completion of their life histories (Mallen-Cooper and Harris, 1990). Coastal streams have many migratory fishes that are catadromous or amphidromous, with both juveniles and adults migrating upstream. In the second major drainage system, the Murray-Darling river system, most migrating species are potamodromous with adults migrating upstream. About 50 fish passes have been recorded (Mallen-Cooper and Harris, 1990). Most of them are pool-type fish passes and were judged ineffective because inadequate maintenance and inappropriate design characteristics, i.e. steep slopes, velocities and turbulence were not adapted to native species.

In New South Wales, up to mid-1980's salmonid pool-type designs (submerged orifice and pool-and-weir) with salmonid design criteria were used. Recent laboratory studies on native fish using experimental vertical-slot fishways showed successful. Field studies on these vertical-slot fishways (with reduced head losses between pools and reduced turbulence compared with salmonid fishways) have confirmed effectiveness for native fishes (Mallen Cooper, pers. comm., 2000). Rock ramps and nature-like bypass channels with very low slope (1:20 to 1:30) are used on smaller barriers. Their use is still experimental. They have had some initial success in passing fish and assessment in most cases is continuing (Mallen Cooper, pers. comm., 2000).

In the state of Queensland, a tropical and sub-tropical region of Australia, about 22 fish passes were built prior to 1970, most of them on tidal dams (Barry, 1990). Early designs were based on fish passes used for salmon and trout in the northern hemisphere. The majority of these fish passes were judged to be ineffective in providing native fish passage, mainly striped mullet (Mugil cephalus) and barramundi (Lates calcarifer) (Beitz, 1997) which support important commercial fisheries.

Under the guidance of a Fish Pass Coordinating Committee, Queensland has begun a programme of fish pass design, construction and monitoring which better reflects the requirements of native fish. A major programme of retrofitting existing fish passes has been launched (Jackson, 1997). The actual philosophy in Queensland is to use locks where dam heights exceed 6 metres and vertical slot fish passes elsewhere with 0.08 to 0.15 m drop heights between pools (Beitz, pers. comm. 1999).

5.6 New Zealand

Of the currently recognised 35 indigenous freshwater fish species in New Zealand, 18 are diadromous. The species that require passage to and from the sea are the three eel species (Anguilla spp.), one lamprey (Geotria australis), five galaxiids (Galaxias spp.), two smelts (Retropinna spp.), four bullies (Gobiomorphus spp.), the torrentfish (Cheimarricthys fosteri), grey mullet (Mugil cephalus) and black flounder (Rhombosolea retiaria). There is also one shrimp (Paratya curvirostris) which require passage, and numerous marine wanderers have been affected by structures built in the lower reaches of waterways. Of the diadromous species, galaxiids (whitebait) and eels support important commercial, recreational and traditional fisheries. In addition to the indigenous species there is at least one species of the introduced salmonids that that do require passage to and from the sea. Other introduced species that have formed land locked populations, notably the introduced brown and rainbow trout and can also undertake extensive migrations within river systems (Boubée, pers. comm., 2000).

The 1947 fish pass regulation gave fisheries authorities the right to require a fish pass on any dam or weir built on rivers where trout or salmon did or could exist. No provision was made for passage of indigenous species, quite the contrary. Indeed, fisheries managers at that time advocated exclusion of elvers as beneficial to upstream population of introduced trout. By the early1980s, only around eight fish passes had been built at the 33 or so major power, water and flood control dams scattered around the country. All eight passes had been constructed for salmon which, although introduced, were considered the most economically valuable fish species (Jowett, 1987).

Only with the introduction of the 1983 Freshwater Fisheries regulation did passage of indigenous fish species become a requirement for new structures.

Although several fish passes have been built since the 1980s, numerous migration barriers remain, not only at high dams but also at weirs, culverts and flood gates. Passage upstream for the climbing native species has been aided by placing pipes or ramps lined with gravel or brushes over the barrier (Mitchell, 1990; 1995). Although some success has been achieved at high dams these type of passes have proven to be far more effective at low head structures. More successful for high structures are catch and haul operations where elvers, climbing galaxiids and bullies are collected via short ramps into holding bins, and transported upstream by road. Such operations have been particularly valuable in systems with one or more dam or where passage or access would be limited because of flow diversion (Boubée, pers. comm., 2000).

Photo 28: Elver trap at Karapiro dam (New Zealand). This floating trap is periodically raised to pass the small eels upstream. (Photo Larinier)

With the increasing success of fish passes and of transfer operations, downstream passage, notably of adult eels now needs to be adressed. So far there are no downstream passage facilities installed at any of the power dams.

5.7 Latin America

As noted by Northcote (1998), with possibly some 5000 species of freshwater fishes in South America and probably more than 1300 in the Amazon Basin (Petrere, 1989), the potential for fish passage problems at dams is enormous. Fish communities in the large rivers comprise mainly potamodromous characins and siluroids. Among the characins, prochilodids of the genera Semaprochilodus and Prochilodus make up a large proportion of the catches. The siluroids include Pimelodus, Brachyplatystoma, Pseudoplatystoma and Plecostomus. Fish can migrate distances from 200 to 1 000 km (Welcomme, 1985).

Hydroelectric impoundments are seen as potentially the most dangerous human threat to Amazonian fisheries (Bayley and Petrere, 1989). In Brazil, Petrere (1989) recorded about 1 100 dams, which include only dams owned and managed by the Central Government. Dam construction in the upper reaches of rivers appears to lead to the disappearance of migratory stocks in reservoirs and in the river upstream. Most dams have no facilities for fish passage (Quiros, 1989). He listed for the whole of Latin America only 46 fish passes with another 7 planned or under construction. Itaipu Dam, on the Parana River, was built without fish facilities for upstream migration, except for an experimental model, which was installed to obtain more precise information on the biology of the migratory species attracted to the structure. The attracting flow was only 0.3 m3/s when the average river flow-rate during the experiment was 11 800 m3/s (Borghetti et al., 1994). The recently built Petit Saut dam on the Sinnamary (French Guiana) has no fish passage facilities.

The first fish passes built were pool and weir types, used in the northern hemisphere for passing salmonids. More recently, fish locks and mechanical fish lifts based on Russian experience described by Pavlov (1989) have been built for obstacles over 20 m in height.

Very few fish passes have been evaluated and they seem to function with varying degrees of success. Quiros (1989) mentions 3 ineffective passes in Argentina. Godinho et al. (1991) captured in a fish pass 34 of the 41 species present in the region of the Salto do Morais dam. However, the fish pass seemed selective, there were few individuals of each species and only 2% of them reached the upper section of the fish pass. They mentioned another fish pass at Emas Falls on a low dam which seems to be more efficient.

As noted by Clay (1995), Latin American experience seems to be following that of other parts of the world, with limited success, because of lack of knowledge of the species involved and lack of application of the criteria needed for good fish pass design.


Fish passes have been developed mainly in North America and Europe for a very limited number of target species present in these countries, mainly salmonids and clupeids. These species are the only ones for which reliable, quantitative data exists on the effectiveness of passes. The data is gathered from sources such as control station monitoring (trapping or video surveillance), or some marking/recapture or telemetry methods.

We may consider that the design technique for such passes is relatively well developed for these species. By respecting a certain number of design criteria on the pass itself, its location, the position of its intakes and flow, it is possible to design relatively effective passes in terms of percentage of the population able to pass and migration delay.

For other species, and particularly potamodromous or catadromous species such as eels, we have much less data on pass effectiveness. While we know how to design passes for such species, i.e. passes whose hydraulic conditions appear suited to the swimming abilities and behaviour of these species, we only have at best counts which are exhaustive to a greater or lesser degree. It is often difficult to assess the real efficiency of such equipment in so far as we do not know the migration needs and the size of population likely to use the pass.

Re-establishing fish passage upstream is only one of the aspects of dam-induced problems: there is also damage caused when the downstream migration and indirect effects linked to changes in water flow rates, water quality, the increase in predation and more particularly the loss or deterioration of upstream habitat.

Photo 29: The 15 m high Kernansquillec dam was decommisionned and destroyed in 1996 to restore the ecological quality, particularly the salmon population, on the Leguer river (Britanny, France). (Photo Larinier)

An accumulation of these factors, especially for high dams or a series of dams, may compromise the balance, and even the survival, of migrating fish populations. This remark is in keeping with the trend in both North America and Europe to demolish dams of limited usefulness or those considered to have a major impact on the environment. Three dams have been destroyed in France on rivers whose migratory population was the subject of a restoration programme. In the USA, Elwha and Glines Canyon dams, and four dams on the lower Snake river have been proposed for removal to restore the native salmon fisheries.

In countries with advanced fish pass technology for a very limited number of species, we may consider the passes to be an effective means of mitigation for obstacles not drastically modifying either the habitat conditions (by their height or their number in the case of series of dams), or water flow and quality. On the other hand, no quantitative data is available on the effectiveness of passes for most other species, particularly potamodromous or catadromous species.

The situation is very different in other countries, in particular in South America, Asia and Oceania. There are many migratory species whose biology, periods and stages of migration are little - or even unknown. Fish passes must accommodate species of very different sizes, swimming ability and migratory behaviour, especially small catadromous species with limited swimming abilities.

Fish pass design has in the main been based on American or European experience with salmonids, and most often with less-than-optimal design criteria. Passes are generally unsuitable for the species concerned. They are often undersized and not particularly well-suited to the rivers concerned. The attraction aspect of the passes has rarely been considered. Not only is the position of the fish pass entrance open to discussion but the flow rate inside it is insufficient and not usually in keeping with the scale of the river in question.

To resume, for such countries (most of which are developing countries), the maintenance of fish passages has almost never been correctly considered if indeed it has been considered at all. The effectiveness of such passes has very rarely been assessed and in such conditions it is not surprising that the situation may be considered catastrophic.

As noted by Quiros (1989) when discussing passes in South America, "the fact that almost nothing is known of the swimming ability and migration behavior of the native species in developing countries, coupled with the lack of available data on their behaviour means that it is impossible to establish broad guidelines regarding the most suitable fish pass designs."

The priority is to acquire a better knowledge of fish communities, their biology and their migratory behaviour. This knowledge should enable us to better define the objectives of a fish pass in a given river and to design more suitable devices.

Suitable technologies should therefore be developed for contexts other than North America or Europe. Countries such as Japan and Australia have become aware of the specific nature of their problems and have undertaken to develop a technology suitable for their own rivers and their own species: two symposia were held in Japan in 1990 and 1995, and two workshops in Australia in 1992 and 1997, which enabled an overview to be drawn up and priorities to be outlined, i.e. "well resourced and directed research to determine migratory requirements, design programmes involving the appropriate mix of biologists and engineers, commitment to monitor all new or modified fishways, holistic approach identifying fish passage within a whole river rather than past individual barriers". The results obtained already appear encouraging.

The fact that we do not know the migratory species, let alone their swimming capacities and migratory behaviour, is not an excuse to do nothing. Unfortunately, however, this is the option all-too-often adopted, such as the recent case of the Petit Sault dam on the Sinnamary river in French Guiana.

In the absence of good knowledge on the species, the fish passes must be designed to be as versatile as possible and open to modifications. Some fish passes are more suitable than others when targeting a variety of migratory species, such as vertical slot passes with successive pools, the drop between pools and energy dissipated in each pool being able to be adapted depending on fish size. Mechanical lifts (for the largest species) are to be avoided, as are Denil fish passes, which tend to be very selective. Furthermore, devices to monitor fish passage must be installed. This monitoring process will enable the fish pass to be assessed and the feedback thus obtained may be useful for other fish pass projects in the same regional context.

For high dams, when there are numerous species of poorly-known variable swimming abilities, migratory behaviour and population size, it is best to initially concentrate mitigation efforts on the lower part of the fish pass, i.e. to construct and optimize the fish collection system including the entrance, the complementary attraction flow and a holding pool which can be used to capture fish to subsequently transport them upstream, at least in an initial stage. This was the policy adopted by France in the 1980s for the first large passes for shad, until the technique had been fully mastered (Travade et al., 1992).

Fish pass design involves a multidisciplinary approach. Engineers, biologists and managers must work closely together. Fish passage facilities must be systematically evaluated. It should be remembered that the fish pass technique is empirical in the original meaning of the term, i.e. based on feedback from experience. If you look at the history of fish pass techniques, it is clear that the most significant progress has been made in countries which systematically assessed the effectiveness of the passes and in which there was a duty to provide results. It is the increase in monitoring and the awareness of the need for checks which is at the origin of progress in fish pass technique in countries such as the USA, more recently France and Germany, and more recently again, Australia and Japan.

However, one must never lose sight of the limits to the effectiveness of fish passes. In addition to problems relating to fish passage, there are indirect effects of dams which may prove of major significance such as changes in flow, water quality, the increase in predation and drastic changes to the habitat up- or downstream. Complementary mitigation measures on flow management at certain times of the year, for example, could prove indispensable to the long-term maintenance of a good balance in migratory fish populations. The protection of migratory species for a given dam must be studied in a much wider context than the strict respect of fish passage alone.


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