Impacts of dams
People are tempted to think that low weirs or cross-river sills are not really obstacles but it is not so much the absolute height of the construction that matters but rather the effect these barriers have on the local species. And these effects can even be devastating if the obstacle is rather low (a couple of decimeters to a few meters) depending on the swimming capacities of the fish species concerned. Nowadays, concern is expressed not only with regard to fish species but all aquatic animals (or aquatic live stages of animals, e.g. macrozoobenthos) that depend on free longitudinal movements in the river.
Impairment by cross-river structures is of two main types. At first, they constitute barriers to upstream and downstream migration for most of the species which, during a certain phase of their life cycle, depend on longitudinal movements in the river, and secondly they affect species through the morphological modifications that result from the damming. These physical modifications are of a wide range and usually include a change in slope, river bed profile, structure of the bottom surface, bottom substrate, submersion of gravel zones or riffle sections, as well as the destruction of riparian vegetation and changes in the thermal or trophic regime. Very often the downstream flow regime is drastically changed.
It is important to note that not only the so-called "migratory species" (i.e. the typical long-distance migrators like salmon, sea trout, shad etc) are affected by obstacles but all species that, for feeding or spawning purposes, change position within the river system during their growing period, and this independent of the distance they cover. Longitudinal passage might be completely interrupted or, at least, migration be delayed. Very often, managers only think in terms of interruption of upstream migration but in most circumstances also downstream passage is impaired and mortality can result from downstream passage through hydraulic turbines or over high spillways. In this respect, also increased predation on migrating young fish, e.g. when passing through the reservoir, must not be underestimated. The cumulative effect of several obstacles on the same river has also to be taken into account.
While obstructed passage can, at least to some extent, be mitigated through a variety of well-functioning passage facilities, i.e. fishpasses (nowadays also called "fauna passes") for upstream migration and by-passes for downstream passage, lost habitat (see the second type of impairment mentioned above) can not easily be compensated for.
As regards fishpass construction, there is long and good experience in North America and Europe. The upstream passage for anadromous and potamodromous species past obstacles can be provided for through several types of fishways, i.e. pool-type fish passes, Denil fish passes, nature-like bypass channels, fish lifts or locks, or through collection and transportation facilities. Over the last two decades, tremendous progress, especially in France, has been made to improve the fish passage facilities, first for upstream and now also for downstream passage. Most recently, a vertical slot fishpass has been constructed at the Iffezheim Dam on the River Rhine to allow inter alia salmon to migrate again upstream and recolonize the affluents.
The critical point in upstream fish passage design is the location of the fish pass entrance and the attraction flow, which must take into account river discharge during the migration period and the behaviour of the target species in relation to the flow pattern at the base of the dam. Some sites may require several entrances and fish passes. Attraction to the fishpass can be improved through a series of measures of which auxiliary flows are amongst the most effective, also in terms of cost/benefit ratio because in many cases these flows can be used for additional hydropower generation, e.g. the Iffezheim fishpass on the Rhine.
The downstream migration problems have for long not been as well studied or fully considered as those associated with upstream migration. The accepted downstream passage technologies to exclude fish from turbines are physical screens, angled bar racks and louvers associated with surface bypasses. Behavioural guidance devices (attraction or repulsion by lights, sound, electricity) have not been proven to perform successfully under a wide range of conditions and are still considered as experimental. For catadromous species, namely for eels, only few special designs have been developed in Europe, Japan, New Zealand and Australia.
Some countries, e.g. France, have amended the relevant laws as to include the obligation to restore free passage at obstacles, at least on classified rivers that are important for migrators. More and more often, the "Polluter-Pays Principle" is applied, i.e. the owner of the dams and weirs has to pay to restore free passage.
Effective design and construction
A prerequisite for the construction of effective (qualitative concept) and efficient (quantitative concept) fish passage facilities is the knowledge of the biology and behaviour of the species concerned. This is what makes transposing of fish passage technology to dam projects in other continents difficult as often basic biological information is missing. If then design standards developed for fish of temperate zones are blindly applied to tropical conditions, failure is quite likely to occur. However, limited knowledge of the biology must not lead to the conclusion "do nothing". On the contrary, the "Precautionary Approach" principle should be applied as again recently discussed at a Fish Passage Workshop in South Africa . Thereby, caution is warranted as to choose a design that most probably suits the local target species.
Fishpass design involves a multidisciplinary approach. Engineers, biologists and managers must work closely together. Fish passage facilities must be systematically evaluated, if possible through a comprehensive long-term monitoring programme. Fishpass technique is empirical in the original meaning of the term, i.e. based on feedback from experience, and that the most significant progress in fish passage technology has been made in countries where the effectiveness of the passes was systematically assessed and where it was obligatory to provide monitoring results.
Considering the impacts mentioned above, dams can have important negative implications for fisheries, especially in tropical regions where rivers often sustain important fisheries. Yield models for large rivers, where river basin area and length of the main channel have been related to catches, suggest exponential increases with increasing river length due to the connectivity and cumulative influences of upstream processes within the system and lateral processes associated with riparian, watershed and floodplain dimension of the stream ecosystem.
Where rivers have extended floodplains, the fish production is even higher. Along the stream continuum, dams and their associated upstream reservoirs have downstream effects on riverine environments and, subsequently, diverse influences on downstream fisheries, even beyond the lotic ecosystem. Cumulative effects of dams in catchment basins and tributary streams can significantly block nutrient flow throughout the ecosystem, affecting fisheries production in downstream reservoirs, river channels as well as estuary and marine environments. Compensation for loss in yield from river fisheries can be difficult to achieve through development of reservoir fisheries. The larger the river, and the more downstream the location of the dam, the less the potential there is for a reservoir fishery to compensate in terms of yield for losses sustained by the river fishery. Compensation potentials apparently are higher in shallower reservoirs in tropical regions than they are in deeper reservoirs and in more northern latitudes.
Productive reservoir fisheries
There exist examples of productive reservoir fisheries that have been seen developing with yields of up to 329 kg/ha/yr in small reservoirs in Africa , up to 125 kg/ha/yr in Latin America and the Caribbean and up to 650 kg/ha/yr in Asia. Thriving reservoir fisheries can also develop where river fisheries contribute little to overall national fishery yields or in drier regions where dams are constructed for irrigation, and fisheries are secondary considerations.
Benefits seem more pronounced for smaller, shallower reservoirs. Stocking of exotic species (both in reservoirs and in tailwaters) can enhance yields, as long as the exotic fishes are environmentally sound and culturally acceptable to the surrounding human population. In this regard, caution is warranted in cultures where fishing and fish consumption are non-traditional activities. Building reservoirs in the context of such cultures may not achieve projected fishery benefits even though exploitable fish stocks may exist.
However, even if compensation is achieved from a fishery perspective, specific needs of fish species not included in the fishery but threatened or endangered must be considered to avoid negative impacts to these fishes.
FAO's Major Programme on Fisheries aims at promoting sustainable development of responsible fisheries and contributing to food security by advocating inter alia the restoration of the aquatic environment as a proper tool for management of inland waters for fisheries. Through its Inland Water Resources and Aquaculture Service, the Fisheries Department of FAO is reviewing the inland water restoration activities on a world-wide scale to give advice to Member States through the appropriate instruments and bodies. The overall objective of the programme is to contribute, through improved management, to the sustainability of fisheries in inland waters for food as well as social and economic benefit in the member countries. A good example for the implementation of mitigation measures is the Technical Cooperation Project on the restoration of migration routes of Baltic Sea salmon through dams in Lithuania where advice has been given as to the modification of an existing fishpass at one dam and the design of a new pass at another dam.
Effective environmental assessment and management coupled with improvements in design of civil engineering structures has made some recent dam projects more fish friendly and environmentally acceptable but there is still an urgent need for drafting legal instruments on a wider scale which will facilitate modification of dam structures to incorporate mitigation measures and help altering dam operation rules to be more beneficial to fish biodiversity and fisheries.