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Fish Passes: Types, Principles and Geographical Distribution an Overview

Larinier M.1 Marmulla G.2

1 Institut de Mécanique des Fluides, Allée du Professeur Camille Soula, F - 31400 Toulouse, France

2 FAO, Viale delle Terme di Caracacalla, 00100 Roma, Italy

INTRODUCTION

Barriers across rivers often have negative impacts on natural fish populations and, along with other factors, may contribute to the diminished abundance, disappearance or even extinction of species. An example of this is the extinction of the salmon (Salmo salar) in the River Rhine, a stock that supported a thriving fishery in the first half of the twentieth century. Dams are threatening many aquatic species in Europe and North America, as well as in other continents where even far less is known about the biology, behaviour, fishery and population dynamics of the fish species concerned. There is increasing concern today that fisheries and the associated livelihoods are being threatened as a consequence of dam construction.

Fish migrations take place in all three directions, upstream, downstream and laterally but only the first two are dealt with in this paper. The general principle of upstream fish passage facilities (called fish passes, fishways or sometimes even “fish ladders”) is to attract fish that move up the river to a specific location in the river downstream of the obstruction so as to induce them (actively), or even make them (passively), pass upstream by opening a waterway (fish pass in the strict sense) or by trapping them in a tank and transferring them upstream (fish lift or transport systems such as trucking).

Upstream passage technologies are well developed for certain anadromous species, mainly salmonids (e.g. salmon, trout) and clupeids (e.g. shad, alewives, blueback herring) in North America and Europe. Upstream passage can be provided through several types of fish pass, including pool-type fish passes, Denil type (or baffle-type) fish pass, naturelike bypass channels, fish lifts and fish locks or collection and transportation facilities. Special designs for catadromous species (mainly eel) have been developed in Europe, Japan, New Zealand and Australia.

The design of a fish pass, the effectiveness of which is closely linked to the water velocities and flow patterns, should take into account the behaviour of the target species. Thus the water velocities in the pass must be compatible with their swimming capacity and behaviour. A large water level difference between pools, excessive aeration or turbulence, large eddies or low flow velocities can act as a barrier for fish. In addition to hydraulic factors, fish are sensitive to other environmental parameters (level of dissolved oxygen, temperature, noise, light, odour, etc.), which can have a deterrent effect.

Downstream fish passage technologies are much less advanced than those for upstream passage and are the areas most in need of research. Obviously, this is partly due to the fact that efforts towards reestablishing free movement for migrating fish began with the construction of upstream fish passage facilities and that downstream migration problems have only more recently been taken into consideration. Second, the development of effective facilities for downstream migration is much more difficult and complex. Research continues to improve downstream passage, especially at large obstacles where satisfactory solutions were scarce (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 for other species.

A large number of systems exist to prevent fish from being entrained into water intakes but they are by no means as effective as bypasses. They may take the form of physical barriers, which physically exclude fish from turbine intakes, or behavioural barriers that attract or repel fish by means of applying stimuli to elicit behavioural responses. Bypasses for downstream passage can be complemented with such systems. The design of effective facilities for assisting the downstream passage of fish must, of course, take into account the swimming ability and behaviour of the target species and the physical and hydraulic conditions at the water intake.

UPSTREAM FISH PASSAGE FACILITIES

POOL-TYPE FISH PASSES

The concept of pool-type fish passes, which are widely used, is very old. An official survey carried out in France in the nineteenth century by Philippe (1897) revealed that there were more than one hundred. The principle behind pool passes is the division of the height to be passed into several small drops by forming a series of pools. The passage of water from one pool to another is either by surface overflow, through one or more submerged orifices situated in the dividing wall separating two pools, or through one or more notches or slots. Often, hybrid pool passes exist, for example with part of the flow through a notch, slot or over the dividing wall in combination with submerged flow through an orifice.

The main parameters of a pool pass are the dimensions of the pools and the geometric characteris- tics of the cross-walls separating the pools (dimensions and heights of the weirs, notches, slots and orifices). The upstream and downstream water levels also influence the functioning of the pass.

The pools have a twofold objective, i.e. to ensure adequate dissipation of the energy of water, with no carryover of energy from one pool to another and to offer resting areas for fish. There is a large diversity of pool-type fish passes throughout the world, which differ in the dimensions of the pools, the type of interconnection between pools, the differential heads between pools, the flow discharge and the slope. The discharge can vary from a few dozen litres to several cubic metres per second (Larinier 1992a, 1998; Bates 1992; Clay 1995). Design criteria are based on the swimming capacities and behaviour of the species involved as well as hydraulic models and field experience. Ideally, the drop between pools should not be more than 0.30 m. The pool volume is determined by a maximum energy dissipation in the pools that limits turbulence and aeration. This criterion seems to be commonly accepted nowadays but must be adapted for different species, i.e. between 200 watts m-3 for salmonids and less than 100 watts m-3 for small species and juveniles (Larinier 1990; 1992a; Bates 1992; DVWK 1996; Beitz pers. comm. 1999; FAO/DVWK 2002).

Pool passes with deep and narrow interconnections, like vertical slot type fish passes, can accommodate significant variations in upstream and downstream water levels without the need for regulatory devices.

Experience shows that when pool-type fish passes are well designed with respect to the different hydraulic criteria they can allow passage of most species (Travade et al. 1998).

DENIL FISH PASSES

Mr. Denil, a civil engineer, developed the first baffle fish passes in Belgium in the 1910s, mainly for Atlantic salmon. The principle is to place baffles on the floor and/or the walls of a rectangular flume with a relatively steep slope (10 to 25 percent) to reduce the mean velocities of the flow. These baffles, in shapes of varying complexity, cause secondary helical currents that ensure an extremely efficient dissipation of energy of the flow. The shape of the original baffles was later simplified with good results (Larinier 1983, 1992b; Lonnebjerg 1980; Rajaratnam and Katopodis 1984).

A disadvantage is that no resting zones for fish exist in a Denil pass and fish must swim through without stopping. If the total drop is very high and the pass, consequently, becomes very long, the fish must make an excessive effort for a period which may exceed the limits of its endurance and thus result in failure. Therefore, one or several resting pools should be provided at intervals that depend on the swimming performance of the target species (Larinier 1992b).

This type of pass is relatively selective and is really only suitable for individuals > 30 cm of salmon, sea-run trout, marine lamprey and large rheophilic potamodromous species such as barbel. Significant adaptations are needed if smaller fish are to pass.

Three designs of Denil fish passes are now in common use which are mainly distinguished by the shape and the material of the baffles, the slope and the width of the pass (OTA1985; Larinier 1990; Amstrong 1996; Nakamura pers. comm.1999). The herringbone patterned baffles (super active-type baffles) are placed only on the bottom, while the two sides of the channel are kept smooth. The width of such a design is not limited, i.e. several unit-patterns can be juxtaposed according to the size of the river and the discharge required.

NATURE-LIKE BYPASS CHANNELS AND FISH RAMPS

The nature-like bypass channel, being very similar to a natural stream, is a waterway designed for fish passage around a particular obstruction. As noted by Parasiewitz et al. (1998), the function of a naturelike bypass channel is, to some degree, restorative in that it replaces a portion of the flowing water habitat which has been lost due to impoundment. These channels are characterised by a very low gradient. The energy is dissipated through a series of riffles or cascades positioned more or less regularly, similar to those in natural water courses, rather than the distinct and systematically distributed drops of pool type passes (Gebler 1998). The main disadvantage of this solution is that it needs considerable space in the vicinity of the obstacle and cannot be adapted to significant variation in upstream level without special devices (gates, sluices). These control devices may cause hydraulic conditions that make fish passage difficult.

As with any other fish pass, it is recommended that the fish entrance to the artificial river be located as close to the obstruction as possible. Given the very low gradient, for reasons of limited space it is sometimes difficult to position the entrance immediately below the obstruction, which means it must be placed further downstream. This may restrict the efficiency of these passes and, consequently, make them less useful for large rivers.

Fish ramps are constructions that are integrated into the weir but cover only a part of the river width; with as gentle a slope as possible to ensure that fish can ascend. Independently of their slope, all these structures are called ramps; in general the incorporation of perturbation boulders or boulder sills is required to reduce flow velocity (DVWK 1996; FAO/DVWK, 2002).

FISH LOCKS

A fish lock consists of a large holding chamber located at downstream level of the dam linked to an upstream chamber at the forebay level by a sloping or vertical shaft. Automated control gates are fitted at the extremities of the upstream and downstream chambers (Travade and Larinier 1992; Clay 1995). The operating principle of a fish lock is very similar to a navigation lock. Fish are attracted into the downstream holding pool, which is closed and filled along with the sloping shaft. Fish exit the upstream chamber through the opened gate. A downstream flow is established within the shaft through a bypass located in the downstream chamber to encourage the fish to leave the lock.

The efficiency of such a fish facility depends mainly on the behaviour of the fish which must remain in the downstream pool during the whole of the attraction phase, follow the rising water level during the filling stage and leave the lock before it empties. The velocity and turbulence in the downstream holding pool must, of course, be acceptable for the fish. On the other hand, the lock chamber should not be filled too quickly during the lifting phase, since thus would cause excess turbulence and aeration, which might encourage the fish to remain in the lower chamber. The fish should have sufficient time to leave the lock in order to prevent any chance of being swept back downstream when the lock empties.

Numerous locks have proved to be either not very efficient, or else totally inefficient. The main drawback of the lock is that it has a limited capacity (in terms of the number of fish that it can handle) compared to that of a traditional fish pass; this is due to the discontinuous nature of its operation and the restricted volume of the lower chamber. The fish attracted into the lock may also leave the downstream chamber before the end of the trapping stage.

The fish locks constructed at the first dams on the Columbia River (Bonneville, The Dalles, McNary) and elsewhere in the United States were abandoned in favour of pool-type fish passes. Similarly, most locks in France are considered to be ineffective (some of them for obvious design reasons) and pool fish passes have replaced some. Difficulties due to fish behaviour have been solved in the United States (Rizzo 1969), and in Russia (Pavlov 1989). More recently Beitz (1997) forced fish to pass upstream in Australia by installing a crowder in the holding pool and a follower to coax fish towards the surface of the lock during the filling phase.

FISH LIFTS

In fish lifts, fish are directly trapped and lifted up in a trap or a trough together with water. At the top of the dam, the trap or trough empties its contents into the forebay. In order to limit the height of the trap in the case of significant downstream water level varia- tion and to ensure easier maintenance, the fish lift can be installed upstream of a short section of conventional fish passes.

Where the number of fish to be passed is very large and can reach hundreds of thousands of individuals, it is no longer possible to hold the fish in the confined volume of the trap. High mortality may occur for some species. Therefore, the design is improved by incorporating a large holding pool into which migratory fish are attracted. A mechanical crowder is used to force fish to enter the area above the tank. The attraction water for the fish lift enters partly at the upstream end of the tank, partly through side or floor diffusers and gratings. Crowder gates at the entrance remain in a V-trap position to prevent fish moving back out through the entrance. Fish collected in the tank are released into an exit channel with low downstream velocities to swim up into the forebay (Travade and Larinier 1992).

The main advantages of fish lifts compared to other types of fish passage facilities lie in their cost, which is practically independent of the height of the dam, in the little space needed and in their low sensitivity to variations in the upstream water level. They are also considered to be more efficient for some species, such as shad, which have difficulties in using more traditional fish passes. The main disadvantages lie in the higher cost of operation and maintenance. Furthermore, the efficiency of lifts for small individuals (e.g. young eel) is generally low because sufficiently fine screens cannot be used for operational reasons.

NAVIGATION LOCKS

The passage of migratory fish through navigation locks is generally fortuitous, given the low attraction of these facilities, which are located in relatively calm zones to enable boats to manoeuvre. Tests carried out in the United States have shown that less than 1.5 percent of migrating fish use the lock at the Bonneville dam on the Columbia River (Monan et al. 1970).

However experiments have shown that navigation locks may constitute a significant back-up facility, or even a useful alternative to the construction of a fish pass at existing sites, providing that their operation is adapted to fish passage. The first condition that must be fulfilled is that sufficient attraction flow is created in the downstream approach channel to the lock. Opening the filling sluice of the lock with the downstream gates open can do this. Once the lock is full, it seems that it is necessary to maintain sufficient surface velocity to encourage fish to proceed upstream. As an example, more than 10 000 shad passed through the Beaucaire navigation lock on the Rhône River in 1992 in 49 lock operation cycles (Travade and Larinier 1992). However, the use of navigation locks as fish passage facilities is limited because the required mode of lock operation is often incompatible with navigation requirements.

COLLECTION AND TRANSPORTATION FACILITIES

The technique of trapping and transporting migrants is often used as a transitory measure before upstream fish facilities are constructed. For example in the case of a series of dams when the building of fish passes occurs in stages, trapping and transportation can be an interim measure. Fish can be released further upstream in the river near the spawning areas or transported to a hatchery, which is often the case for salmonids during the first stage of restoration programmes. Trapping and transportation can be a more long-term measure in the case of dams where the construction of a pass would be difficult, or in the case of a series of dams where one dam is close to the next, thus creating a reach without valuable habitat for breeding.

Pavlov (1989) describes a floating fish trap used in Russia as part of a system of trapping and transporting fish over dams. It consists of a floating barge, anchored in place and equipped with pumps to provide attraction flow. After a period of attraction, a crowder concentrates fish over a lifting device, which then lifts them to the transportation chute of a container vessel. The container vessel is self-propelled and transports fish upstream. This system has the advantage of being able to be placed anywhere in the tailrace and in the path of migrating fish.

FISH PASSES FOR CATADROMOUS SPECIES

Research efforts to adapt fish passes to the needs of catadromous species, which enter fresh water and migrate upstream as juveniles, have been much less intense and are only relatively recent. Specially designed fish passes for young eels are being developed in Europe, Canada and New Zealand (Porcher 1992; Clay 1995; Mitchell 1995). Research programmes have been more recently launched in Australia, Japan and France to design and test fish passes suitable for very small fish.

LOCATION OF FISH PASSES

For a fish pass to be considered efficient, the entrance must be designed in such a way that fish find it with a minimum of delay (Bates 1992). The width of the entrance is usually small in proportion to the overall width of the obstacle and its flow represents only a limited fraction of the total river flow. The only active stimulus used to guide the fish towards the entrance is the flow pattern at the obstruction. The attraction of a fish pass, i.e. the fact that fish find the entrance more or less rapidly, depends in particular on the location of its entrance and the hydraulic conditions (flow discharges, velocities and flow patterns) in the vicinity of the entrance. The flow that comes out of the entrance must neither be masked by the turbulence of discharge to the turbines or the spillway, nor by re-circulating zones or static water.

In the case of a wide river, fish may reach the obstacle on either side and therefore it may be necessary to provide not only several entrances to one fish pass but even more than one fish pass because a single pass cannot be expected to attract certain species that migrate along the opposite bank.

The siting of the pass entrance at an obstruction is not the only factor to be taken into account when choosing the location for fish pass. The exit of the fish pass should neither be situated in a fast flowing zone near a spillway, weir or sluice, where there is a risk of the fish being swept back downstream, nor in a static area, or re-circulating zone in which the fish could become trapped.

Finding the best position for entrances to the fish pass is not easy and rarely obvious, especially at hydroelectric dams. The hydraulic barrier to the fish may be right at the exit of the draft tubes, upstream of a zone of large turbulent eddies resulting from turbine discharges. On the other hand when the residual energy from the turbine water is significantly high, the hydraulic barrier may occur further downstream. Finally the location of the hydraulic barrier can vary at the same site, depending upon which turbines are in use at any one time.

When the barrier zones cannot be clearly identified at a particular site and are likely to vary depending on dam operating conditions, meaning that the correct fish pass entrance locations are not obvious, the effectiveness will be considerably improved by installing several entrances at points which appear, a priori to be the most favourable. The problem is extremely complicated and difficult to solve in the case where the fish passage facility is intended to suit several species whose swimming abilities and migratory behaviour are very different, or sometimes even unknown. This gives rise to the necessity to define the target species clearly at the outset of the project.

The discharge through the fish passage facility must be sufficient to compete with the flow in the river during the migration period. It is difficult to give precise criteria, but generally the flow passing through the fish pass must be of the order of 1-5 percent of the competing flow. It is clear that the higher the percentage flow of the watercourse passing through the fish pass, the greater the attraction of the pass will be. Although it is quite possible to direct a large fraction of the flow of the river through the fish pass in the case of small rivers, this is not the case in large rivers where the mean flow can exceed several hundred cubic metres per second. It then becomes difficult, in terms of cost, to maintain a sufficient flow through the facility, particularly during high water periods. On major rivers an attraction flow of around 10 percent of the minimum flow of the river (for the lower design flow) and between 1 and 1.5 percent of the higher design flow seem to be satisfactory for a well located fish pass to work.

When a large quantity of water is needed to attract fish into a fish pass (several cubic metres per second) only a fraction should be allowed through the fish pass itself in order to limit the size and the cost of the facilities. The auxiliary flow needed to boost up the attraction is then injected at low pressure and velocity through screens in the downstream section of the pass, or at the entrance itself. The simplest option is to add the auxiliary flow (or supplementary attraction flow) by gravity, after dissipation of the energy in a pool. At large dams, the auxiliary flow can also be created either by pumping water from the downstream pool or by sending water from upstream through one or several small special turbines that reduce, to a certain extent, the electric energy production losses and thus, in general, please the energy companies (Bates 1992; Larinier 1992a).

EFFECTIVENESS AND EFFICIENCY OF UPSTREAM FISH FACILITIES

The answer to the question “are fish passes effective mitigation means” is not obvious. The biological objectives of building a fish pass vary according to site and even on the same site depending on the species considered. The concept of effectiveness is therefore very variable and can only be defined with respect to an objective.

The concepts of effectiveness and efficiency may be used to clarify the degree of mitigation provided by a fish pass. Effectiveness is a qualitative concept, which consists in checking that the pass is capable of letting all target species through, within the range of environmental conditions observed during the migration period. Effectiveness may be measured through inspections and checks, i.e. visual inspection, trapping, video checks (Travade et al. 1998).

The efficiency of a fish pass is a more quantitative description of its performance. It may be defined as the proportion of stock present at the dam which then enters and successfully moves through the fish pass in what is considered an acceptable period of time. The methods giving an insight into the efficiency of a pass are more complicated than those for effectiveness. Marking and telemetry are valuable techniques to assess the overall efficiency of fish passes and the cumulative effect of various dams along a migration path.

The targeted effectiveness for a given site must be defined with respect to the biological objectives sought. It is therefore related to the species considered, the number of obstacles on the river and the position of the obstacle on the migration route.

The objective of a pass designed for diadromous species, such as salmon and located downstream of all the spawning grounds, is that the whole migrating population passes through. If numerous obstacles characterize this river, the aim is to minimize the time needed by the fish to enter the pass, so that the migrating fish reach the reproduction areas “on time”. The efficiency of a fish pass is expressed both in terms of the percentage of the population which negotiate the obstacle and the migration delay, i.e. how long the population, or part of the population, takes to pass the obstacle. On the other hand, if the fish pass is located upstream of some spawning grounds, the requirements on percentage and time taken may be less stringent because fish may reproduce downstream. Whatever the case, the fish pass must be sufficiently efficient so as not to constitute a limiting factor in the long-term maintenance of migrating stock.

When dealing with a fish pass for potamodromous species whose biological objective is, above all, to avoid the sectorisation of populations in the various reaches, it is not necessary that all individuals of a population move upstream. The pass will be effective if a “certain number” of individuals, i.e. a significant proportion with respect to the population downstream of the obstacle, gets through the pass.

When the causes of poor performance (in terms of effectiveness and/or efficiency) of fish facilities are analysed, certain factors are frequently revealed (Larinier 1992; Nakamura 1993; OTA 1995):

Lack of attraction of the facility, resulting from a poor position of the fish pass or insufficient flow at the entrance of the facility in relation to the flow discharge into the river.

Poor design of the facility with regard to the variations in water levels upstream and downstream during the migration period, resulting in undersupply or oversupply of flow to the fish pass, or excessive drop at the entrance. This may be due to poor appreciation of the range of the upstream and/or downstream water levels during the project planning phase, or a subsequent change in these levels.

Poor dimensions, i.e. pools with insufficient volume causing excessive turbulence and aeration, excessive drop between pools, insufficient depth for the fish, or the flow pattern in the pools not suitable for the target species.

Frequent clogging up or obstruction of the fish passage facility, resulting from inadequate protection against debris, or too exposed a position, or quite simply inadequate maintenance on the part of the operator.

Malfunctioning of parts which regulate the flow discharge and the drops between pools (automatic sluice gates, etc.), or which ensure the functioning of the facility in the case of fish lifts and fish locks (automatic sluice gates, hoist for the tank, moving screens, etc.).

However, there are limits to the effectiveness of a fish pass. Even if 100 percent effective, a pass may prove insufficient to maintain the balance of a migratory population in the long term. In addition to problems arising from obstructed fish passage there are indirect effects such as a change in hydrological regime, water quality, an increase in predation and the loss or deterioration of the habitat upstream or downstream, which may also be limiting factors. These aspects are however species-specific and/or site-specific. Other mitigation measures, for example specific water flow management for fish at certain times of the year, may prove indispensable.

BARRIERS AND DOWNSTREAM FISH PASSAGE FACILITIES

PHYSICAL BARRIERS

Fish are often entrained and pass through the turbines at generating facilities. One solution to prevent this involves stopping them physically at water intakes using screens that must have a sufficiently small grid size 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 created to effectively guide fish towards the bypass (ASCE 1995; Larinier and Travade 1999).

BEHAVIOURAL BARRIERS

Knowledge about visual, auditory, electrical and hydrodynamic stimuli has led to the development of a large number of experimental barriers, i.e. 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 use of behavioural barriers, which are still experimental, must be considered with caution (OTA 1995).

SURFACE BYPASSES IN CONNECTION WITH SURFACE BAR RACKS OR DEEP INTAKES

Surface bypasses associated with existing conventional trash racks or angled bar racks with relatively narrow spacing have become one of the most frequently prescribed fish protection systems for small hydroelectric power projects, particularly in the Northeast of the United States 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 the ratio of fish length versus spacing and to response of fish to hydraulic conditions at the front of the structure and at the bypass entrance. Tests showed that under optimal conditions, efficiency can reach 60- 85 percent (Larinier and Travade 1999). Flow discharge in the bypass has also been proven to be critical. The design criteria currently applied in the United States and France call for a minimum discharge of 2 percent to more than 5 percent 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 percent to 10 percent range. The design goal of theses bypasses is to guide at least 80 percent of the juvenile fish (Ferguson, Poe and Carlson 1998).

DOWNSTREAM MIGRATION OF 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 that 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 trash racks would be efficient. Experiments on bottom bypasses need to be carried out, 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 maintenance problems. The principle of behavioural light screens appears promising, taking into account the species repulsion to light (Hadderingh, van Der Stoep and Hagraken 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 United States (Euston, Royer and Simons 1998) and Anguilla dieffenbachii 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).

FISH PASSES AROUND THE WORLD

The following review is not 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 of doubtful scientific bases.

NORTH AMERICA

There are about 76 000 dams in the United States, including around 2 350 operating hydroelectric projects. Among these hydroelectric generating facilities, 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 Atlantic coast, which supports the most important anadromous fisheries and in the Rocky Mountains, which have valuable recreational fisheries.

The main advances in upstream passage technology came from the west coast of United States and Canada where fish passage facilities have gradually become more sophisticated over the years since the building of Bonneville Dam, the first dam with large fish pass on the Columbia River about 60 years ago (OTA 1995). Currently, about 40 large-scale hydropower stations are in place on the Columbia River. Upstream passage technologies are considered to be well developed and well 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 etc.). 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 (Washburn and Gillis 1985). The Denil fish pass is not widely used in the West coast, except in Alaska for salmon (Oncorhynchus 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 United States and Canada, 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.

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

Francfort et al. (1994) completed a detailed study of the costs and benefits of measures used to enhance upstream and downstream fish passage at dams, using data of operational monitoring studies from 16 key projects across the United States which represent the measures most commonly used in the United States. 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 and which have to cope with an increasing number of adult shad, i.e. from 4 000 to over 80 000 between 1984 and 1992 (Cada 1998). Although all 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).

EUROPE

A recent inventory suggests that there are approximately 380 fish passes in England and Wales. 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 (Amstrong 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).

In Germany and Austria, design and construction of fish passes has also been actively pesued 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; FAO/DVWK 2002).

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.

Although a law was passed in Portugal in 1962 that required the installation of fish passes to maintain fish migrations, this was hardly enforced until 1990. After 1991, all projects of new weirs and dams were analysed and if migratory fish populations were concerned a fish pass has to be installed. About 50 new fish passes were built. The problem of the old dams without a fish pass, or with an inefficient pass, remains but the philosophy is changing and the question of maintaining fish migration corridors for diadromous or potadromous is now on the agenda. The first inefficient fish pass will be removed and a new one will be installed in Coimbra Dam on river Mondego for shad (J. Bochechas pers. comm.).

A total of 115 fishpasses were catalogued in Spain (Elvira, Nicola and Almodovar 1998), with about one third constructed after 1990. These fish passes are mainly located at weirs and dams of moderate height. The commonest fish pass found is the pool and weir fish pass, including vertical fish slot. In addition, Denil type and other non-standard designs have been used. Distribution is not uniform, since 87 percent of them are located within the two northernmost basins, North and Ebro. Many of the fish pass facilities were built in rivers where Atlantic salmon and brown trout (both resident and sea trout) are present. The effectiveness was estimated with 58 percent being highly suitable, 15 percent adequate, 19 percent low and 8 percent totally ineffective.

In Northern Europe, the main migratory fish species considered are Atlantic salmon and brown trout, which always had a special value for the inhabitants. Several whitefish species (coregonus sp.), grayling (Thymallus thymallus), northern pike (Esox lucius) and some cyprinids make shorter migrations both in freshwater and between the brackish water of the gulf of Bothnia and rivers running into it. In the coastal areas lamprey (Lampetra fluviatilis) is of great commercial value (Laine, Kamula and Hooli 1993). Norway has a very long tradition for building fishways and has been the predecessor in fishway construction in Scandinavian countries. There are now about 420 fishways. Most of these facilities are of the pool and weir type, but some are Denil fishways. The first fishways were excavated in rock and were usually large pools separated by narrow passes. About 25 percent of the fishways are on regulated rivers and 75 percent on rivers with natural obstructions (Grande 1990). Most of the Norwegian fishways are for salmon. There are just a few for brown trout, grayling and coregonids. Most Swedish fishways are pool and weir type fishways. Combinations of pool and Denil fishways have been built for inland fish, the first of them in the 1950s.

ASIA

There are probably about 10 000 fish passes installed on Japanese rivers (Nakamura and Yotsukura 1987). They are mainly designed for anadromous salmonids (Oncorhynchus 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 that 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 to construct 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, Qinhao and Keming (1990) describe the Yangtang fishway on the Mishui River, which passes 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-1) 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.

In Nepal a couple of fish passes exist that were probably derived from the European or North American pool type and vertical slot passes. However, they do not seem to function well due to the chosen design criteria, resulting in the discharge through the pass being too low compared to the river flow and too turbulent. The dam that was under construction in 2001 on the Kali Gandaki River is 44 m high and the Kali Gandaki “A” Hydroelectric Project will generate about 842 gigawatt-hours (GWh) of electric energy per annum. The dam cuts off a river bent of approximately 70 km and the minimum flow rate below the dam will only be of ca. 4 m2 s-1. Together with the dam on the Andhi Khola River, it blocks the migration of important fish species, e.g. Tor sp. and Bagarius. No fishpass for upstream migration has been incorporated into the project. Instead, “trapping and hauling” of fish at the dam and constructing a hatchery to provide stocking material to stock the river upstream of the dam was preferred. A fish pass for downstream migration has been incorporated. Also the fish passes on the rivers Bagmati, Modi and Trijuga do not seem to function properly.

The construction of major and medium dams and barrages has accelerated in India due to the increased need of water for agriculture, industry and community use. It is noteworthy that over the last forty years almost 200 billion cubic meters of storage has been created, thereby intercepting almost 30 percent of the available surface flow. While providing great benefits for agriculture through irrigation, the water resources projects have concurrently blocked the migration routes of fish leading to considerable reduction in fish catch. The dams modify significantly the historical flow patterns of the rivers and in turn have led to a radical change of the river ecology affecting fish in particular. In addition to an overall loss of fish production, many diadromous species are threatened with extinction because of habitat destruction and obstruction of migration routes at the barriers (P.B. Das pers. comm. 2003).

Bad experience in the past, with catches dropping due to the construction of dams and weirs without or with poorly designed fish passes, has led to the construction of some fish passes in India in more recent years. However, some of these passes have not been effective in the absence of detailed studies of the target species and their swimming capabilities. The economic sustainability of the fishing communities along many large rivers has been affected significantly, with colossal annual losses. With barrages on the main arm of Ganges and its tributaries, even the Dolphin population (rare species) has decreased and isolation of subpopulations makes the species even more vulnerable genetically (P.B. Das pers. comm. 2003).

Data on the performance of fish passes, fish landings, spawning and growth patterns on some of the large Indian rivers, such as Ganges (at Farakka barrage), Yamuna (Hathnikund barrage), Mahanadi (Hirakud Dam and barrage at Cuttack) and Cauveri (Mettur Dam) has been collected by research institutes.

Detailed information on the migratory fish species has been collected by the Central Inland Fisheries Research Institute (CIFRI) before a fish pass was designed at the Farakka barrage, which was built in 1975 on the Ganges River. This information included data on species’ biology and behaviour, their spawning habits, characteristics of migration and swimming performance, number and size of fish passing per hour and, last but not least, the economical value of the fisheries they are supporting. As a result, two fish locks have been constructed but the commercially important Hilsa shad (Tenualosa ilisha) has highly suffered from Farakka barrage blocking of almost 1 000 km of its migratory path. Today, the upstream catches do not show a coherent tendency, with only one of four landing sites reporting an increase in catch after the construction of the barrage. No detailed analysis as to the functioning of the fish locks has been available (P.B. Das pers. comm. 2003).

A drastic reduction in fish yield has been noticed due to Salandi Dam, which became operational in 1970 in Orissa State. The annual catch of the main species has fallen from annually 350 tonnes (1950-65) to approximately 25 tonnes (1995-2000) in the same river reach. After a dam was built on the Mahanadi River, fish catch dropped form 800 tonnes to 500 tonnes Two new barrages, built between 1985 and 2000, have been equipped with Denil-type fish passes, which are reported to have increased upstream catches. A study of Beas River has revealed the adverse impact of water abstraction on aquatic life due to the diversion of Beas water to Sutlej by a dam at Pondo. Tor putitora, another important species, has been negatively affected by being cut off from important spawning grounds. Where large losses of fish occur at high water discharges that require the excess water to be spilled over a dam, fish passes would be an appropriate means to help fish migrating back into the reservoir.

The two major 100-year old barrages on the Ganges at Haradwar and on the Yamuna at Tajewala have proved detrimental to the migration of T. putitora in particular as the old fish pass constructed in the early 1890s did not prove effective. The new barrage on Yamuna, constructed in 1999, was equipped with a Denil-type fish pass, which seems to benefit upstream migration of T. putitora. However, in general the new Indian fish passes have only partially mitigated the migration problem. Therefore, a comprehensive solution is to incorporate fish passes at all major barrages for the benefit of the families fishing along the thousands of kilometres of main rivers (P.B. Das pers. comm. 2003).

The idea of the construction of fish passes and “fish friendly structures” (FPFS) has been introduced in Bangladesh in the 1990s and since then four FPFS have been built in the country (Kabir and Sharmin pers. comm. 2003). The primary objectives were to facilitate fish migration and reduce mortality rate of young fish while moving through the FPFS gates. Unfortunately, technical details of the fish passes and FPFS are not available and on this basis it is difficult to assess the structures. The differences between the two are not really clear but appear to lay in the seasonality, the efficiency for different fish sizes and the construction costs. The fish passes seem to be more efficient than the fish friendly regulators in terms of fish migration. The appreciation of efficiency is different in the different stakeholder groups, i.e. whereas fishers do not see any benefit, local officials think that the structures allow free passage. The main problem with both the structures does not seem to be of a technical nature but a management issue. Management committees have been established but do not function. Also, management regulations are missing and the structures have even been misused as fish traps.

AFRICA

Africa has over 2 000 known species of indigenous freshwater fishes. The construction of dams has multiplied since the 1950s for both irrigation and hydroelectric power generation. Shad populations are present in North African rivers, namely in Morocco, but existing and some recent fish passes seem not to be 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 bound to fail.

Apart from shad in North Africa, no anadromous species are known. As noted in Daget, Gaigher and Ssentongo (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.

AUSTRALIA

In temperate southeastern Australia, there are approximately 66 indigenous freshwater species; over 40 percent 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 the mid-1980s 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 were 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 subtropical 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).

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 (Cheimarrichthys fosteri), grey mullet (Mugil cephalus) and black flounder (Rhombosolea retiaria). There is also one shrimp (Paratya curvirostris) which requires passage and numerous marine migrators 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, can also undertake extensive migrations within river systems (Boubée pers. comm. 2000).

The Fish Pass Regulation of the year 1947 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. Indeed, fisheries managers at that time advocated exclusion of elvers as beneficial to upstream population of introduced trout. By the early 1980s, 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 Freshwater Fisheries Regulation in 1983, 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 continue to exist not only at high dams but also at weirs, culverts and floodgates. Upstream passage for climbing native species has been mitigated 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).

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

LATIN AMERICA

As noted by Northcote (1998), with possibly some 5000 species of freshwater fishes in South America and probably more than 1 300 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 km (Welcomme 1985) to more than 2 000 km (Barthem, Lambert de Brito de Ribeiro and Petrere 1991).

Hydroelectric impoundments are seen as potentially the most dangerous human-induced threat to Amazonian fisheries (Bayley and Petrere 1989). In Brazil, Petrere (1989) recorded about 1 100 dams managed by government authorities. 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. The Itaipu Dam on the Paraná River was originally built without facilities for upstream migration. Only an experimental fish entrance unit was installed to obtain more precise information on the biology of the migratory species. However, the attracting flow was only 0.3 m3 s-1 when the average flow rate of the river was 11 800 m3 s-1 at times of the experiment (Borghetti et al. 1994). A fish pass has now been built covering a differrence in elevation of 120 meters. This pass is more than 7 km long and consists of three different sections, i.e. the lower part of the pass uses the river channel of a small tributary to the Paraná River, the middle section is a pooltype pass and the upper part is built as a by-pass channel. There is one big lake at the outlet of the pool-type section (about half way up the pass) and a small lake two-thirds up the by-pass channel; they can be used as resting “pools” for fish using the pass. Monitoring will show the efficiency of the pass in the years to come.

The first fish passes built in Latin America 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 three 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 percent of them reached the upper section of the fish pass. They mentioned another fish pass at Emas Falls on a low dam that seems to be more efficient.

As noted by Clay (1995), experiences from Latin American seem 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.

CONCLUSIONS AND RECOMMENDATIONS

If a new dam or weir is planned and constructed, the project cycle usually consists of six major phases, i.e. the identification phase, the design phase, the project appraisal phase, the construction phase, the operation phase and the decommissioning phase (World Bank 1991a, 1991b, 1991c). In this respect, fisheries interests should be taken into consideration right from the beginning. Bernacsek (2001) has identified specific fisheries management capacity and information base requirements for the six phases of the dam project cycle. For example, during the dam identification phase, basic information in as much detail as possible should be gathered as regards the status of the aquatic environment, fish biodiversity, fish migration, existing fisheries upstream and downstream as well as regarding the likely impacts the dam might have and possible mitigation measures. The key output during the second phase must contain an assessment of the level of impacts on and the risks for fish and fisheries, as well as a statement with regard to the degree of suitability and acceptability - or need for rejection - of the project from a fisheries point of view. Where the construction of a dam cannot be avoided care has to be taken that the needs for fisheries management are addressed, inter alia through the construction of fish passage facilities (Cowx and Welcomme 1998; Bernacsek 2001).

Fish passes have been developed mainly in North America and Europe for a very limited number of target species, mainly salmonids and clupeids, present in these countries. Today, the design of such passes can be considered relatively well developed for these species. Salmonids and clupeids are the only species for which reliable, quantitative data exists on the effectiveness of passes. In general, data is gathered through monitoring (trapping or video surveillance) or marking/recapture and telemetry experiments. By respecting a certain number of design criteria regarding the pass itself, its location, the position of its intakes and the flow, it is possible to design passes that are relatively effective in terms of percentage of the population able to pass without major delay.

While suitable passes can also be designed for other species, much less data is available on their effectiveness, particularly for potamodromous or catadromous species such as eels. It is often difficult to assess the real efficiency of such passes in so far as the migration needs and the part of the population likely to use the pass are often unknown.

Interrupted upstream fish passage is only one of the aspects of dam-induced problems. Very often, also the downstream migration is rendered difficult or made impossible. In addition, there are indirect effects, e.g. changes in water flow rates, water quality, increase in predation and more particularly the loss or deterioration of upstream or downstream habitat. An accumulation of these factors, especially at high dams or for a series of dams, may compromise the survival of migrating fish populations. This remark is in keeping with the trend in both North America and Europe to decommission dams of limited usefulness or those considered having a major impact on the environment. This is a trend of increasing importance and, for example, in the United States dozens of dams have been removed since 1999 after the breaching of Edwards Dam on Maine’s Kennebec River. In the last years, many more dams have been proposed for removal to restore the native salmon fisheries, e.g. Elwha and Glines Canyon dams, as well as four dams on the lower Snake River. In France, three dams have been destroyed on rivers whose migratory population was the subject of a restoration programme.

In countries where fish pass technology is advanced for a very limited number of species, fish passes may be considered an effective means of mitigation for obstacles that do not drastically modify neither the upstream habitat conditions (by their height or their number in the case of series of dams) nor the water flow and quality.

The situation is very different in other countries, e.g. in particular in South America, Asia and Oceania, where the biology and migratory behaviour, i.e. periods and stages of migration, of many species is not well known or even unknown. There, fish passes must often accommodate species of very different sizes, swimming abilities and migratory behaviour and especially small catadromous species with limited swimming abilities. Very often, fish pass design has been based on American or European experience with salmonids and most frequently with less-than-optimal design criteria, which makes the passes generally unsuitable for the species concerned. The passes are often undersized and not particularly well adapted to the pertaining hydraulic conditions. Also, the attraction aspect of the fish pass entrances has rarely been adequately considered. The lesson learnt is that for such countries, many of which are developing countries, maintaining or restoring free fish passage has, if at all, almost never been given appropriate attention. 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.

When discussing passes in South America, Quiros (1989) noted that the lack of knowledge of the swimming ability and migration behaviour 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. Therefore, the priority must be to acquire a better knowledge of fish communities, their biology and their migratory behaviour which should enable stakeholders 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 defined such as to conduct well resourced and directed research to determine migratory requirements, to design programmes involving the appropriate mix of biologists and engineers, to make commitments to monitor all new or modified fishways and to adopt a holistic approach identifying fish passage within a whole river rather than past individual barriers. The results obtained already appear encouraging.

As also outlined in the FAO Code of Conduct for Responsible Fisheries (FAO 1995) and the related Technical Guidelines (FAO 1997), a precautionary approach should be adopted, i.e. the fact that knowledge of the migratory behaviour and the swimming capacities of many species is scarce or non-existing must not be an excuse for doing nothing. Doing nothing is, however, unfortunately all-too-often the option that is adopted, as was recently the case of the Petit Sault Dam on the Sinnamary River in French Guiana.

In the absence of good knowledge of the characteristics of the species concerned, the fish passes must be designed to be as versatile as possible. Some passes, such as vertical slot passes with successive pools, are more suitable than others when targeting a vast variety of species because the drop between pools and thus the energy dissipated in each pool can be adapted to the fish size. Selective or highly specific passes, such as Denil passes or mechanical lifts, should be avoided. Also, provisions must be made to allow for modifications of the construction, if necessary, e.g. if indicated by monitoring results. Thus, a comprehensive monitoring programme must be part of any fish passage rehabilitation project and 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 improving the pass, if necessary, or for designing 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 fish pass technology had been fully mastered for shad (Travade et al. 1998).

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 one looks at the history of fish pass techniques, it is clear that the most significant progress has been made in countries that systematically assessed the effectiveness of the passes and in which it was required to provide monitoring 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 technology in countries such as the United States, France and Germany and, more recently, Australia and Japan.

However, one should never lose sight of the limits of the effectiveness of a fish pass even if its design is optimum because even if the obstructed passage can be mitigated, there may exist indirect effects of dams as mentioned above which may prove of major significance. Complementary mitigation measures, e.g. modified flow management at certain times of the year, could prove indispensable for sustainable long-term preservation of migratory fish populations. The mitigation measures to be adopted to protect species from the negative impacts of a dam must thus consider a much wider context than the mere aspect of obstructed fish passage alone.

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