Welcomme (1985) estimated that yield potentials from African river and floodplain fisheries ranged from 5 to 143 kg/ha/year. Actual yields in large reservoirs subject to moderate to heavy fishing varied from 27 to 65 kg/ha/year (Kapetsky, 1986). For medium-sized reservoirs, mean yield estimated from actual yield values provided by van der Knapp (1994) was approximately 80 kg/ha/year. Mean yield from a variety of Sub-Saharan small water bodies was 329 kg/ha/year (Marshall and Maes, 1994).
Petr (1975) addressed factors associated with initial high fish catches in African reservoirs. High fish catches were related mainly to annual formation of floodplains during the gradual rising of waters over several years, and the formation of aufwuchs (periphyton) on submerged terrestrial vegetation. Peak landings occurred in 5-6 years (Kariba; Volta), and coincided with the year when water levels reached maximum pool level. By this time, the main lake commercial fish stocks were completely different from that of the original river (catches dominated by tilapia). In faster filling lakes with rapid drawdowns (e.g. Kainji) commercial fish landings peaked the first year. Rapid drawdowns restricted spawning by tilapia and reduced their expansion throughout the system. In Lake Kariba, Limnothrissa miodon, an introduced species from Lake Tanganyika, is now an important component of fish landings. Without this fish, catches would be only around 3 000 t (T. Petr, pers.comm.).
In the Kafue River reservoir above Kafue Gorge Dam (Zambia), the large surface area and shallow water of the reservoir benefitted tilapia species that form a major part of the commercial catch (Dudley, 1974). Water level fluctuations can be advantageous to impoundment fisheries (particularly those with expansive shallow-water zones), as well as river floodplain fisheries in the region, because alternating wet and dry conditions reverse nutrient losses through oxidation and leaching. Drought-induced loss of fish stocks in small impoundments can be addressed through restocking once the systems refill (Mheen, 1994), assuming that restocking is economically feasible and that fish for restocking are available.
Dams have reduced overall fishery yield from some African systems. Such yield reductions can be temporary or long term. For example, Lelek and El-Zarka (1973) reported that two years after filling Lake Kainji, fish catches from the system were reduced by 30%. Post impoundment studies of the system suggested that the commercially important Mormyridae were reduced from about 20% of the catch to around 5% (Lelek and El-Zarka 1973; Lewis 1974). Catches tended to increase to pre-impoundment levels as the system stabilized.
Sagua (1997) addressed Lake de Guiers, a natural lake in Senegal. The lake in its natural condition was fed by the Senegal River during floods and dried during drought. A dam was constructed to divert more water into the lake. Fish production in the lake increased from 2 500 t/year to 3 000 t/year. Prior to the drought, fish production in the river and its floodplain was 23 500 t/year. Although the dam controlled flooding, it reduced the river fishery by 50%, and shifted stocks toward marine fishes due to intrusion of saline waters. There was an annual net loss of 11 250 t of fish from the system.
Crul and Roest (1995) compiled fisheries assessments for Africa's four largest reservoirs: Kainji, Kariba, Nasser/Nubia, and Volta. Considered collectively using minimum and maximum yield estimates from each system, average yield for these four reservoirs is 28-38 kg/ha/year. System-specific assessment summaries are provided below.
Balogun and Ibeun (1995) addressed fisheries of Lake Kainji. Catches fluctuated between 4 500 and 6 000 t/year which translates to production values of 3.5-4.7 kg/ha/year. The Niger River pre-impoundment sustained fisheries composed primarily of mormyrids, citharinids and distichodontids. During the first two years post-impoundment, the "false flood" in the newly-formed Lake Kainji stimulated production of these floodplain river fishery resources, particularly for citharinids and distichodontids, but thereafter production of these fishes decreased. Mormyrids never did well post-impoundment. In contrast, catches of cichlids, cyprinids and bagrids were low pre-impoundment but after impoundment significantly increased. Littoral zones of the lake were most productive.
Machena (1995) addressed the fisheries of Lake Kariba (Zambia/Zimbabwe). Yields were 60kg/ha/year for pelagic fishes (e.g. the sardine Limnothrissa miodon). When the inshore fisheries are included, yields were 30-57 kg/ha/year.
Lake Kariba is considered an oligotrophic system with low fish production potential (limited by nitrogen and phosphorus). Most fish production is in shallow littoral areas but most of the reservoir has steeply sloping shoreline. Within the inshore areas, crocodiles consume the equivalent of 10% of the catch. Catch composition is shifting toward benthic fishes, especially catfishes (e.g. Synodontis zambezensis). In 1989 there were nearly 2 000 artisanal fishers in Zimbabwe and nearly 1 000 in Zambia. Most fishers (91%) in Zimbabwe were full time and fished with 2-3 gillnets. The fishery was plagued by high catch spoilage and low productivity. Comparable fishery characterization data for Zambia were unavailable.
Rashid (1995) addressed these fisheries and estimated yield at 36-39 kg/ha/year. Tilapia (T. nilotica and T. galilaea) contribute 89% to total landings. In Egypt, the impoundment of the Nile River by the Aswan High Dam (1964) to create Lake Nasser/Nubia led to increased fish yields in the impounded section of the river, but due to trapping of nutrients in the impoundment that led to this productivity, there were declines in the pelagic fisheries in the entire eastern Mediterranean Sea (Ryder 1978). After construction of the Aswan High Dam, Bernacsek (1984) estimated fish yield in the lower Nile River at 72.75 kg/ha/year.
Braimah (1995) addressed fisheries of Lake Volta. Estimated yield was 42-52 kg/ha/year based on catch statistics, and 12 kg/ha/year based on the morphedaphic index, MEI, (Ryder et al., 1974). Tilapia are a major component of the harvest, with catches influenced by water level (higher catches when water level is low).
During reservoir drawdowns, standing timber is harvested for firewood and to facilitate beach seining. However, standing timber in the reservoir basin is important for periphyton production. Braimah (1995) estimated that 52% of the fish caught were dependant on invertebrates exploiting this periphyton. Removal of standing timber, in conjunction with overfishing, has negatively impacted the fish stocks.
Fish migration is a primary concern throughout this region. Kvernevik (1997) concluded that fishes in Malaysian rivers utilized migration as an important adaptive tactic, and that migratory species were more common in the Kelantan River system, which has no large hydroelectric dams acting as barriers, than in the Perak River where there are four large hydroelectric dams acting as mainstream barriers. Roberts (1995) discussed impacts from 12 hydropower projects on the mainstream of the Mekong River and stressed that the combined impact on fisheries from these dams is greater than the sum of the individual impacts. Each of the Mekong River dams addressed by Roberts (1995) will block fish migrations.
Dams, however, are not the only concerns with respect to riverine fisheries in the region. For example, Roberts (1993a) attributed the 80-90% declines in fisheries below the great waterfalls of the Mekong River (southern Laos) primarily to overfishing and to fishing with explosives. Roberts (1993b) emphasized that tropical rivers in regions subject to deforestation and dams become increasingly simplified ecologically and unable to withstand additional impacts. Following construction of the Pak Mun Dam (Thailand) Roberts (1993b) emphasized the need to consider industrial development associated with the dam, and its impacts to river fisheries. He also expressed concern that 200 fish species occurring naturally in the river would be replaced with only 25 species stocked from hatcheries into the reservoir above the dam.
In Malaysia, there are 51 impoundments (46 in Peninsular Malaysia, 3 in Sabah, 2 in Sarawak) ranging in size from 10 ha (Mahang Dam) to 37 000 ha (Kenyir Dam) (Ho 1995) and 94 major river systems (49 in Peninsular Malaysia, 24 in Sabah, 21 in Sarawak)(Yap 1992). Yap (1992) reported yields for four principal rivers: Rajang (Sarawak, 100 kg/ha/year); Baram (Sarawak, 142-169 kg/ha/year; Gombak (Selangor, 180 kg/ha/year), Perak (Perak, 11.64 kg/ha/year). Khoo et al. (1987) reported that inland capture fisheries in Malaysia are dominated by cyprinids and silurids in the country's larger river systems, and that there have been sharp declines in catches during recent decades. These declines are attributable to a combination of factors, including river regulation (particularly dewatering of stream reaches below dams), (D. C. Jackson, Mississippi State University, pers. observ. 1978-1980, and 1997, 1998, 1999) and pollution, siltation, damming, illegal gear/methods, and overfishing (Khoo et al 1987). In the Selangor River, flows have been reduced from 5 482 000 m3/day to 300 000 m3/day and in Sabah, the release from the Babagon Reservoir dam has reduced streamflow to 5.5-21.0% of the natural river flow (Yap 1992).
Photo 9: The Temenggor Reservoir (Malaysia), which shows extreme drawdowns and exposure of steep littoral zones, doesn't have a good fisheries production. (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Photo 10: Sport fishing in an upper reach of a Pahang River tributary (Malaysia). (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Reservoir systems along the Perak River in Malaysia have received considerable attention with respect to fisheries research. Along the Perak River, evaporation from the reservoirs may exceed streamflow. The result can be dewatering of the tailwaters and loss of riverine fish habitat (D.C. Jackson, Mississippi State University, U.S.A., pers. observ. 1978-1980, and 1997, 1998, 1999). Khoo et al. (1987) reported that Chenderoh Dam blocked movements of Probarbus julieni and reduced breeding and spawning grounds. Ali (1996) reported higher biodiversity in the Chenderoh Reservoir than in the river downstream from the dam but diversity in the reservoir has declined over time and fish standing stock is low. Yield from the reservoir fishery was estimated at 12.2 kg/ha/year which, while low, exceeded that of Bukit Merah Reservoir, a blackwater reservoir in the same region (3.7 kg/ha/year) (Ali and Lee, 1995). Limnological studies of impoundments along Malaysia's Perak River indicate that standing crop of phytoplankton and zooplankton is minimal, species are characteristic of oligotropohic fauna and production generally is low except for a brief period during the dry season (A.B. Ali, Universiti Sains Malaysia, Penang, pers. comm., 1999). Deep water reservoirs in Malaysia that have low retention times, and that are designed primarily for hydroelectric purposes, may have limited fishery potentials.
In Malaysian streams that have not been dammed (e.g. Tembling River, Pahang River system), demands for river fish have resulted in overfishing (Tan and Hamza, Universiti Sains, Penang, Malaysia: undated publication). Semi-sanctuary status provided to the upper Tembling River and its principal tributaries as a result of being in the National Park (Taman Negara) has protected fish stocks somewhat, primarily for recreational fisheries and to support demands for fish by tourist restaurants near the national park headquarters (D.C. Jackson, Mississippi State University, USA, pers. observ., 1997; 1999). Elsewhere in interior Malaysia, pollution and sedimentation impact riverine fisheries, especially during the rainy season when runoff is increased (Ho, 1995).
Lake Kenyir, the largest reservoir in Malaysia, has a surface area of approximately 36 000 ha, a maximum depth of 145 m, and a mean depth of 37 m (Yusoff et al., 1995). The lake sustains a small-scale commercial fishery as well as popular recreational fisheries, with yields estimated at approximately 20 kg/ha/year (Yusoff et al., 1995). These overall low yields are the result of an anoxic hypolimnion, lack of forage in the pelagic zone and few lacustrine fish species (Yusoff et al., 1995). Historical fish migrations in the river were blocked by the dam which does not have a fish ladder (Yusoff et al., 1995).
Sukadi and Kartamihardja (1995) described fisheries associated with reservoirs in Indonesia. There are 23 major reservoirs in the country. Average yield from 13 principal reservoirs was 174.5 kg/ha/year (range 5.3 kg/ha/year - 692.9 kg/ha/year). Yield estimates for river fisheries were not provided but those of floodplain environments ranged from 100 kg/ha/year to 800 kg/ha/year.
Sugunan (1997) reported that fish yields in Indian rivers range from 0.64 to 1.64 t/km (average 1.0t/km) with 3.2-7.8 fishers/km. In the Ganga River, yields declined from 50.3 kg/ha/year (1960s), to 20.0 kg/ha/year (1972) down to 6.5 kg/ha/year (mid-1980s). Specific reasons for this decline were not documented. Based on these figures, Sugunan (1997) determined that rivers in India do not contribute significantly to the country's total inland fish production in terms of volume, although a large number of traditional, artisanal fishers exploit middle and lower stretches of the rivers.
Sugunan (1995) addressed impoundment fisheries throughout India, and estimated fish yields for 291 small (49.9 kg/ha/year), 100 medium (12.3 kg/ha/year) and 21 large (11.43 kg/ha/year) reservoirs. The overall national production rate for Indian Reservoirs was 20.1 kg/ha/year. He reported that supplemental stocking of small impoundments in India can yield on average 146kg/ha/year (range 63-316 kg/ha/year).
Dams have had negative impact on river fisheries in various systems throughout the region. Dam construction on the Sefid River (Iran) resulted in reduced streamflow, increased water temperature and declines in food items for sturgeon (Acipenseridae) (Vladykov, 1964). Reservoirs constructed on rivers emptying into terminal lakes of Central Asia and Kazakhstan severely reduced stocks of migratory fishes in the rivers, encouraged development of stocks more lacustrine in character, exacerbated precipitation/evaporation deficit ratios and have led to accelerated salination of groundwater as well surface waters (Petr and Mitrofanov, 1998). Sandhu and Toor (1984) noted sharp declines in catches of Hilsa ilisha as a result of dams, barrages, weirs and anicuts on the Hoogly, Godavari, Krishna and Cauvery rivers (India), and that mahseer Tor putitora and T. tor no longer are found above Nangal and Talwara dams. Fishways constructed in conjunction with dams are used as fish traps by local fishers.
In addition to impacts on hilsa and mahseer stocks and their associated fisheries, formation of reservoirs in India has had negative impact on snow trout (Schizothorax), and rohu (Labeo) in Himalayan streams, and catadromous eels and freshwater prawns in all major river systems. One of the earliest known impacts to river fisheries in India occurred as a result of construction of Mettur Dam (1935) on the Cauvery River, which formed Stanley Reservoir and completely stopped runs of the Indian shad Tenualosa ilisha. Within the reservoir itself, water level changes, recruitment failures and predation resulted in reduced stocks of Indian major carps.
Sugunan (1995) noted that some fishes (e.g. the exotic tilapia, Oreochromis mossambicus) do well in Indian reservoirs, but that tilapia are more or less restricted to tropical regions of the country. Introduction of tilapia in many Indian reservoirs has resulted in declines in native fishes and, in systems dominated by tilapia, low overall fish yields. Stunting is a problem with tilapia (especially O.mossambicus), and particularly so in small impoundments. In larger reservoirs, tilapia can achieve weights of 2.5 kg (average 0.5-0.7 kg) (Sugunan, 1997). For tilapia fisheries, O. niloticus is considered a better fish and apparently does not have the same stunting problem as O. mossambicus. Tilapia often enjoy high consumer preference, even when the price is the same as Indian carps (Sugunan, 1995).
Gill (1984) addressed the effect of dams on the fish fauna of India's Punjab region. Reservoirs in this region have resulted in good fisheries, with more than 1 800 t/year landed annually at Bhakra Dam and Pong Dam, collectively. However, river fisheries have been negatively impacted, particularly with regard to migratory fishes. Construction of barrages at Ropar, Harike, and Ferozepur has restricted migration of Indian major carps, in spite of fishways. During most of the year, little water is released into the river below the dams from the reservoirs, and fish are concentrated in pools where they are more easily captured by fishers. Fishways designed to promote fish passage past dams are used by fishers to capture fish.
Dams in India's Punjab region have reduced flooding, but in so doing they have also negatively impacted production of Indian major carps, resulting in reduced total fish production for the region (Sandhu and Toor 1984). However, in the reservoirs formed by these dams high yield fisheries have evolved, primarily through development of stocks of exotic fishes (e.g. the fishery for the exotic silver carp Hypophthalmichthys melitrix contributes more than 30% to the catch from Gobindsagar Lake) (Sandhu and Toor, 1984).
George (1995) described the fisheries of Pakistan's six Water and Power Development Authority reservoirs. The combined area of these reservoirs is 99836 ha. During the period 1979-1994, fish yields ranged from 3.8 kg/ha/year (1985-1986) to 25.6 kg/ha/year (1993-1994) with an overall average yield for the entire period of 15.2 kg/ha/year.
In Sri Lanka there are 100 rivers, of which 28 are large (basins greater than 500 km2). However, and with the exception of floodplains, which have yields of 18-284 kg/ha/year, Sugunan (1997) reported that river fisheries in Sri Lanka were insignificant. Inland fisheries contributed 20% to the country's total fish production and most of this came from exotic fishes (e.g. tilapia) in reservoirs. Average yield was 244 kg/ha/year (range 40-650 kg/ha/year) (Sugunan, 1997). The higher production in Sri Lanka compared to that of India can be related to generally shallower water and greater conductivity in Sri Lanka reservoirs, and to the many competing species as well as large predators in Indian reservoirs (Sugunan, 1997).
Lu (1986) reviewed reservoir fisheries in China. At the time of his report, total surface area of reservoirs was two million hectares. Yield from reservoirs historically has been high (127-152kg/ha/year) but this likely reflects the results of intensive stocking programmes, primarily of carps.
There are five major rivers in China: the Heilongjiang (3 101 km); Huanghe (also known as Yellow River, 5 464 km); Huai (1 000 km); Changjiang (also known as Yangtze River, 6 300 km) and Zhujiang (also known as Pearl River, 2 210 km). Dudgeon (1995) conducted an assessment of river regulation influences on fisheries in southern China. On the Zhujiang River system (the largest south of the Changjiang River), there are 40 large reservoirs and 200 smaller reservoirs. Fish catches peaked in the 1950s (10 367 t/year).
By the early 1980s, catches had fallen to 6 463 t/year, likely as a result of a combination of factors, including the influences of dam construction and operation. For example, there have been significant declines in carp recruitment, primarily as a result of pollution and overfishing. Additionally, most of the dams were built without fishways. Construction of the Sijin Hydroelectric Station in 1958 caused a reduction in carp populations. This reduction was attributed to a combination of reduced river flow, blockage of migration routes and lowered water temperatures. Clupeids (e.g. Macrura reevesii and Clupanodon thrissa) once migrated into the upper reaches of the Dongjiang River to spawn. These spawning runs were eliminated by construction of five dams in the lower reaches of the river. Additionally, these dams reduced the abundance of carp, especially Cirrhinus molitorella, to levels where there is no longer an economically viable fishery in the river.
Luo et al. (1992) have projected that the Three Gorges Dam project will cause the fishery of the Changjiang1 River estuary and adjacent waters to shift to the northwest of the estuary. Additionally, and with respect to the Three Gorges Dam, there is some concern regarding the Yangtze sturgeon (Acipenser dabryanus) (Dudgeon 1995). No facilities are planned to facilitate upstream migration of fishes past the dam. This may, however, be a mute point because fish movements (and particularly those of migratory species) in the river were blocked in 1981 by the Changjiang Low Dam at Gezhouba (Zhong and Power, 1996). This blockage reduced populations of the Chinese sturgeon (Acipencer sinensis), partly through alteration of downstream flows and changes in sediment characteristics that reduced spawning success. Xin et al. (1991) reported that the proportion of mature Chinese sturgeon in the system after construction of Gezhouba Dam has fluctuated between 13.5 and 78.0%, and that the population has maintained a 1:1 sex ratio since interception of flows began in 1981. Subsequently, Xin et al. (1991) did not consider the Gezhouba Dam a threat to this species.
Below two high head dams, Xinanjiang Dam (Qiantang River) and Danjiangkou Dam (Han River), fish spawning has been delayed 20-60 days by lower water temperatures (Zhong and Power, 1996). Reduced water velocities and less variable discharges caused spawning grounds below the dams to be abandoned. Changes in the hydrological regime caused extinction of Macrura reevesii, a highly valued fish, in the Qiantang River. In the Qiantang River estuary, the number of freshwater fish species declined from 96 to 85, whereas marine species increased from 15 to 80. Loss of habitat eliminated torrential riverine habitat species from areas inundated by Xinanjiang and Danjiangkou reservoirs. Now lentic species dominate the reservoir fish assemblages. The expanded aquatic habitat has however, been beneficial to overall fishery production. Catches from the two reservoirs have continued to increase 20 years after impoundment, but this may be the result of supplemental stocking from fish hatcheries.
Grass carp Ctenopharyngodon idella, black carp Mylopharyngodon piceus, silver carp Hypophthal-michthys molitrix, and bighead carp Aristichthys nobilis in China are highly adaptable species (Dudgeon, 1995), and are reported to be successfully spawning in Chinese reservoirs (Liu et al 1986). Their spawning grounds are widely distributed, and fishways for bypassing dams are not considered necessary to sustain the fish stocks (Yi et al., 1991).
The Danjiangkou Dam on the Hanjiang River prevents movements of eels (Anguilla japonica) (Liu and Yu, 1992). The dam also has blocked migration of commercially-important carps. Movements now are primarily seasonal downstream from the dam. Overall, the sizes of young-of-the-year fishes have decreased, growth of herbivorous fishes has slowed, populations of planktivorous fishes are small and slow growing, and dietary shifts have been noted (e.g. grass carp now feed on attached algae rather than on vascular plants; mollusk-feeding fishes now forage primarily on mussels rather than snails). Although riverine fisheries have been negatively impacted, fisheries in the reservoirs have greatly expanded (estimated yield: 1 000-1 500 t/year).
Walker (1985) conducted assessments of principal river fisheries in Australia. In northeastern Australia, dams, weirs and tidal barrages have resulted in declining catches of barramundi (Lates calcarifer), a catadromous species in coastal rivers, estuaries and mud flats. In southeastern Australia, a weir on the Lerderderg River (Victoria) disadvantaged the native short-finned eel (Anguilla australis) and river blackfish (Gadopsis marmoratus) and to some extent the exotic brown trout (Salmo trutta), but favoured the introduced perch (Perca fluviatilis), carp (Cyprinus carpio) and tench (Tinca tinca). Dewatering below Tallowa Dam (Shoalhaven River, New South Wales) was destructive to downstream fish populations of Australian bass (Macquaria novemaculeata), Australian greyling (Prototroctes maraena) and Macquarie perch (Macquaria australasica). In Tasmania, dams and gauging weirs have been constructed in a way to assist fish movements, particularly for commercially important migrating eels. Fish ladders also have been installed to assist galaxiids and lampreys. In the Murray-Darling system, a diversion weir 200 km downstream from Hume Dam is a barrier to fish. Weirs and dams have resulted in replacement of the Murray crayfish (Euastacus armatus), a river adapted species, by its floodplain counterpart, the yabbie (Cherax destructor). Most native fish species have declined, including the Murray cod (Maccullochella peeli) and river blackfish. Disruption of seasonal flood cycles has had a negative influence on spawning and recruitment processes of some native fishes.
More recently, Walker and Thoms (1993) reviewed environmental effects of flow regulation on the lower Murray River. Historically, Murray cod and callop or golden perch (Macquaria ambigua) have been the two primary commercial fishery resources in the river. Since 1950, Murray cod have declined in conjunction with expansion of water storages, diversions and irrigated agriculture. The Murray River now has the lowest commercial fish yield per km2 of floodplain of any of the world's major rivers, although historical catches were comparable. Additionally, there is a clear correlation between callop catch and river levels. Proliferation of weir-pool environments, increases in annual proportional flow deviation and general increases in regulation in the Murray-Darling River system, have resulted in reduced species diversity, and conditions more favourable to exotic species like common carp (Gehrke et al., 1995; Walker, 1985). T. Petr (pers.comm., 2000) emphasized that if common carp were included in the commercial catch of the Murray River, yield per km2 would be increased considerably.
In Queensland, reservoir stocking is conducted to enhance recreational fisheries (Petr, 1998). Ongoing release of fingerlings is necessary in Queensland reservoirs because the fisheries are primarily put-grow-and-take fisheries using species that do not reproduce in the impoundments (e.g. barramundi).
Sugunan (1997) reported that there were 68 800
impoundments in Brazil of which 50 are greater than
10 000 ha and 520 are greater than 1 000 ha. These are primarily for hydroelectric and irrigation projects. Most small reservoirs are in the northeast region of the country which is prone to drought. Agencies constructing dams must guarantee that river fauna are not affected by obstructing the river. Fish passes are required if necessary to accomplish this, or affected fishes must be propagated and stocked in the upstream areas. Additional concerns should consider loss of spawning and recruitment grounds resulting from changes in hydrology and from exacerbated sedimentation resulting from dam construction and operation. Introduced species dominate the catches (mostly Oreochromis niloticus and Pescada caucunda). More than half of the catch from northeast Brazil is tilapia. With the exception of northeast Brazil, there are strict regulations regarding stocking of exotic fishes.
Dos Santos and de Oliveira (1999) conducted reservoir fisheries assessments for a 2 360-km2 reservoir located on the Uatum River 170 km upstream from Manaus (Brazil). Fish diversity has been reduced and there have been other environmental and social changes resulting from the project. However some fish species proliferated in the reservoir and have resulted in development of a reservoir fishery. Yields were very low (2.1 kg/ha/year), with the fishery primarily exploiting tucunare (Cichla sp.). Total annual production is estimated at 500 t/year.
Gomes and Miranda (in press) studied reasons for low fishery production in Brazilian reservoirs and concluded that climate and hydrology precluded synchronization of phytoplankton production, thereby undermining the foundation for fish production in these systems. Retention time for water in the reservoirs was considered a critical factor. Unless there was sufficient time, phytoplankton blooms conducive for supporting fisheries could not be developed and sustained.
Itaipu Dam (Parana River, Brazil) encompasses 135 000 ha and has a total catch of approximately 1560 t/year (Sugunan, 1997). This corresponds to a yield of 11.5 kg/ha/year. Sugunan (1997) reported that prior to impoundment there were 113 fish species in the affected river reaches and that approximately 20 species have been negatively impacted by the dam. Borghetti et al. (1994) collected fishes from the Itaipu Dam fish ladder (Parana River, Brazil). The high ratio (72%) of fish in maturing gonadal development stages indicated that the fish were migrating for reproductive purposes. Fish ladder efficiency in Brazil was studied by Godinho et al. (1991) in the Tiguco River (upper Parana basin, southeastern Brazil). The ladder tested had 25 steps, was 78.3 m long and 10.8 m high. Of the 41 fish species captured in the region of the dam, 34 were present in the fish ladder but only 2% reached the upper section of the ladder. The most affected fishes were pimelodid catfishes.
Some of the more successful fisheries programs associated with dams are found in the Caribbean region. Sugunan (1997) conducted assessments of reservoir fisheries in Cuba. Fish is a major source of protein in Cuba but rivers contribute little in the way of fish yield to the country, primarily due to altered flow regimes resulting from dams, sedimentation, and altered river bed configurations. Reservoirs however contributed 19 000 t of the total 99 000 t/year of fish produced for the country, with tilapia and carp dominating the stocks. Average yield of 28 reservoirs was 125 kg/ha/year. Juarez-Palacios and Olmos-Tomassini (1992) calculated yields of cichlids in the 15 largest Cuban reservoirs for the period 1984-1988. They reported yields ranging from 11.5 kg/ha/year to 297.2 kg/ha/year (average 134.7 kg/ha/year).
Photo 11: Artisanal fishers exploit tilapia in a reservoir in the Dominican Republic. (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Jackson (1985b) conducted fishery assessments of rivers and reservoirs in the Dominican Republic. Fishery yield estimates for reservoirs ranged from 29 kg/ha/year to 75 kg/ha/year. Prior to the construction of dams, river fisheries focused on crabs and marine fishes that ascended the country's rivers. Small markets utilized the crab catch but there was little fisheries or market development for riverine finfish. Construction of hydroelectric dams throughout the country created reservoirs and tailwaters that were stocked with largemouth bass (Micropterus salmoides) and tilapia. These fishes expanded rapidly and were quickly recognized by local people as resource bases that could support recreational, artisanal and subsistence fisheries. Local markets accepted these new fish products and a tourist industry evolved around angling for largemouth bass. In tailwaters where flow was maintained, local fishermen harvested both species for subsistence purposes. In the reservoirs, most of the commercial and subsistence catch was tilapia. Primary challenges were access to ice, transportation of the catch, and safety concerns from fishermen encountering standing dead timber while fishing in small craft. Features limiting reservoir fisheries in the Dominican Republic were steep slopes, fluctuating water levels, and limited littoral zones for fish spawning. Additionally, dewatering of tailwaters in some of the systems precluded their contributions as fishery resources for surrounding communities.
In Central America, rivers tend to be short and have high gradients except for reaches near the narrow coastal zones. As a result, river fisheries have been small historically, albeit, utilized for traditional fisheries by indigenous peoples. Reservoirs have had an important role in expanding the base for inland fisheries in the region.
A good example in this regard is the Republic of Panama. As has been the case for Caribbean nations, inland fisheries development in the Republic of Panama has been founded on introductions of exotic species. Peacock bass (Cichla ocellaris) was introduced into the Chagres River, and quickly moved from this river and throughout Gatun Lake, the principal reservoir for the Panama Canal (Zaret and Paine, 1973). The peacock bass is a top predator, and highly piscivorous (Zaret, 1979). It quickly decimated the native fish fauna but founded stocks that were conducive to fishery exploitation. Fishing cooperatives evolved. Yield was estimated at 4.8-5.3 kg/ha/year (Bayley, 1986; Maturell and Bravo, 1994). These cooperatives invested in fishing gear, ground transportation and marketing. Additionally, fishers not affiliated with the cooperatives established roadside stands where fish were sold. Recreational fisheries also evolved as the sporting qualities of peacock bass were discovered both by local Panamanians and by US military personnel stationed in nearby bases. Guide services soon followed and a small tourist industry for anglers was established (D.C. Jackson, Mississippi State University, USA, pers. observ., 1995; 1999).
Photo 12: This highland reservoir in Panama accommodates a good carp, tilapia and peacock bass fishery. (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Photo 13: The Fortuna Reservoir in Panama, situated quite far upstream, is deep and has cold water; it only supports a limited fishery. The water is probably too cold to grow tilapia which are stocked in other Panamanian reservoirs. (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Beyond Gatun Lake, peacock bass, common carp and tilapia were stocked in the interior regions of the country. These combinations successfully established fishery resources for local communities beyond the Canal Zone. Although nearly all of the reservoirs are situated in interior highlands, those at lower elevations have generally been more productive than those at higher elevations, primarily due to temperature influences (higher altitude reservoirs such as Fortuna Reservoir, maintained water too cool for the tropical cichlids) (D.C. Jackson, Mississippi State University, USA, pers. observ., 1984).
A special case in Panama is Bayano Lake. This lake was impounded in the 1970s and is the shallowest major reservoir in Panama (Maturell, 1984). The fishery of Bayano Lake has been fairly productive (63.2 kg/ha/year, Candanedo and D'Croz, 1983), and is apparently exploited primarily by indigenous peoples for subsistence purposes (D.C. Jackson, Mississippi State University, USA, pers. observ., 1984). Prior to construction of the dam, numerous marine species ascended the river, and some of these fish became landlocked when the dam was closed. Over time, populations of these euryhaline fishes have declined in the reservoir. Tilapia are now in the system (R. Gonzales, University of Panama, pers.comm., 1999). Water quality has prevailing influences on fishery production potentials of this low-elevation reservoir (Candanedo and D'Croz, 1983), primarily as a result of decomposition of organic material.
In North America, dams have been responsible for development of highly productive reservoir (Hall and Van Den Avyle, 1986; Miranda and DeVries, 1996) and tailwater (Walburg et al., 1981) fisheries, but have been problematic for migratory species such as anadromous salmonids. In the Columbia River, Eble et al. (1989) reported that fishing and alteration and degradation of river habitat from hydroelectric power dams, irrigation and exploitation of regional resources other than water have greatly altered the system, and have reduced annual returns of anadromous fish from about 10-16 million adults originally to about 2.5 million. Adult fishes must negotiate a variety of passages as well as the still waters of reservoirs in their attempt to reach upstream spawning areas. Young fish must travel downstream and negotiate the reservoirs and pass through or over the dams. High mortalities occur. Valuable fisheries have been severely impacted.
Photo 14: The Bull Shoals Dam, which releases cold water, has changed the White River fishery from a natural world-class smallmouth bass and catfish fishery into a highly productive but artificial "put-and-take" tailwater fishery for rainbow trout. (Photo: D.B. Flynn, fishing consultant, Hot Springs, Arkansas, USA)
Of particular concern are salmon and steelhead stocks (Oncorhynchus spp.) in the Snake River, a principal tributary of the Columbia River. Four federal dams were constructed between 1962 and 1975. Wild salmon stocks that averaged more than 100 000 adults in the 1950s fell to 1 500 in 1995 (American Rivers and Trout Unlimited, 1999). All four Snake River salmon are listed as threatened or endangered and the Snake River steelhead (Oncorhynchus mykiss) is listed as threatened. Comprehensive investigations into the plight of spring and summer Chinook salmon (O. tshawytscha) in the system concluded that restoring some level of pre-dam ecosystem function has a high probability of achieving recovery for these fish (Nemeth and Kiefer, 1999). It is not only a question of restoring spawners to the system but in addition, providing nutrient transport from ocean environments into riverine environments via decomposing carcasses of adult fish post-spawning (Piorkowski, 1995; Cederholm et al., 1999).
In these and other systems of the northwestern USA, exotic species (e.g. smallmouth bass Micropterus dolomieu and channel catfish) have been introduced (Fletcher, 1991; Bennett et al., 1991). Although these exotics are as yet not well accepted by the angling public, and generally are not considered commercial species, their popularity likely will increase as (or if) salmonid stocks in these impacted rivers continue to decline.
In the interior USA, dams have been constructed primarily for hydroelectric, navigation and flood control purposes. One of the first integrated systems in this regard was the Tennessee River, developed by the Tennessee Valley Authority (Voigtlander and Poppe, 1989). Impoundments were developed on the main channel as well as on tributary streams. Prior to impoundment, many of these streams had recreational fisheries for various centrarchid fishes, and in the higher elevation headwater reaches there were brook trout (Salvelinus fontinalis). The dams created slack water environments conducive for largemouth bass, crappies (Pomoxis spp.) and other sunfishes (Lepomis spp.), as well as for landlocked striped bass (Morone saxatilis). Large socio-economic enterprises developed around these fisheries. In tailwaters below dams, highly productive salmonid, sauger (Stizostedion canadense) and smallmouth bass fisheries evolved.
In the White River (Arkansas and Missouri, USA), a similar system was developed. Where once world class stream fishing for smallmouth bass existed, reservoir fisheries focused primarily on largemouth bass now prevail. Highly oxygenated hypolimnetic discharge from the hydroelectric dams proved too cool for native fishes and invertebrate fauna (Hoffman and Kilambi, 1971). To compensate for losses of these warmwater stream fisheries, trout (primarily rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta) were introduced (Fry and Hanson, 1968). Hydropeaking discharges precluded substantial natural spawning of trout in these tailwater, so the fisheries are maintained by stocking from federal and state hatcheries (Baker, 1959). The trout grow exceptionally fast and attain very large size in the tailwaters (commonly > 10 kg). The tailwater trout fisheries have contributed substantially to supporting a multimillion dollar tourist industry (Baker, 1959) that makes significant economic contributions throughout the entire region (which continues, however, to be one of the poorer regions in the country). For example, along the White River below Bull Shoals Dam there are numerous privately-owned fishing resorts catering to sportfishing anglers. These resorts are assisted by local fishing guides and supported logistically by local businesses supplying goods and services (D.C. Jackson, Mississippi State University, pers. observ., 1965-1999). Similar tailwater fisheries for trout have been developed in Tennessee (Parsons, 1955).
In addition to tailwater fisheries, trout are stocked in the reservoirs, and live in the cold hypolimnion zones. These "multi-story" fisheries provide alternative trout fishing alongside warmwater fisheries in the reservoirs.
Photo 15: Jordan Dam on Coosa River supports a good tailwater fishery. (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
Photo 16: A fishing pier has been constructed by the U.S. Army Corps of Engineers to help anglers exploiting the tailwater fisheries of the Columbus Dam on Tombigbee Waterway (Tennessee, USA). (Photo: D.C. Jackson, Dept. of Wildlife and Fisheries, Mississippi State University, USA)
In the southeastern USA, the Tombigbee River historically was considered as one of the more biologically rich and diverse riverine ecosystems in the entire country (Boschung, 1987). The river was comprehensively modified by construction of the Tennessee-Tombigbee Waterway to become a series of slack-water pools and artificial canals, linked by dams and locking systems (Jackson, 1995). Prior to construction of the waterway, the river supported limited local fisheries, primarily for catfishes and centrarchids. With the advent of the waterway, several large reservoirs and numerous tailwaters were established. The reservoirs are relatively shallow with high degrees of shoreline development conducive to production of centrarchid fishes (primarily largemouth bass, crappies, sunfishes). The tailwaters are highly productive for catfishes, crappies and centrarchid basses (Jackson and Dillard, 1993). Fisheries for white bass (Morone chrysops) are developing. The cutoff bendways (old river channels) are excellent crappie fisheries (Sarnita, 1991). Ultimately, these bendways may develop into oxbow lakes as they become isolated from the main channel by sedimentation processes (Jackson, 1995).
Moffett (1949) reported that flows from Shasta Dam on the Sacramento River (California, USA) benefitted salmon and trout in the river below the dam by stabilizing physical environmental features. Flow reductions resulting from operation of dams, however, can negatively impact fish stocks (Bain and Boltz, 1989). Whitley and Campbell (1974) studied the effects of flow reduction resulting from dams on the Missouri River (USA) and demonstrated significant reductions in the amount of inundated floodplain. Reduction of inundated floodplains can result in reduced fish stocks in river-floodplain ecosystems (Welcomme, 1985; Junk et al., 1989; Jackson and Ye, 2000). In a 145 km long reach of the Missouri River, flow regulation resulted in a 67% loss of inundated floodplain area and a greater than 80% decrease in fish catch (Whitley and Campbell, 1974). Declining harvest downstream from dams on the Missouri River also may be the result of physical degradation of channels coupled with fluctuating water levels (Hess et al., 1982). Reservoirs on the Missouri River act as sediment traps. Discharge of clear water accelerates erosion downstream from dams and ultimately can lead to dewatering of backwater areas. This can result in reduced fish production from backwaters as well as stranding of fish.
The Pascagoula River (Mississippi) is the largest physically unmodified river ecosystem in the lower 48 states of the continental USA (Dynesius and Nilsson, 1994). It is located in the state of Mississippi and drains directly into the Gulf of Mexico. One of its principal tributaries, the Bouie River, flows near the city of Hattiesburg, Mississippi. There is, currently, a proposal to construct one or two dams on the Bouie River, primarily to enhance the water supply for the city and surrounding region. First-order estimates of fish yields from the proposed reservoirs (using models advanced earlier in this report) indicated that reservoir fisheries from the reservoirs would compensate for losses sustained from the river. However, there are two fishes found in the Bouie River that are listed as threatened species: the gulf sturgeon Acipenser oxrhynchus desotoi (Zehfuss et al., 1999), and the pearl darter Percina aurora (Ross, in press). On 14 October 1999, a meeting was convened by D.Jackson (co-author of this chapter) at Mississippi State University (USA) to share information and discuss the issues regarding impoundment of this river. Participants at the meeting were representatives from state and federal natural resources agencies, academic institutions, engineering and environmental consulting firms, conservation organizations, and a local citizens group. It was generally determined that fisheries per se were not the critical issues, but that the welfare of the sturgeon and the darter, as well as the physical and biological integrity of the river ecosystem, were of national and international significance. Support from participants representing biological, ecological and natural resources professions was stated as critical to advancing the impoundment project on the Bouie River. This support was not forthcoming.
Lelek (1989) reported maximum fish yields in the High Rhine stretch of the River Rhine (Germany) at 37.6 kg/ha/year and for the lower Rhine at 45 kg/ha/year. Impounded reservoirs in the High Rhine stretch yielded 21.1 to 61.8 kg/ha/year but the highest value (67.7 kg/ha/year) was associated with a flowing stretch of the river between two dams. In the Upper Rhine stretch, reservoir yield was reported at 42.4 kg/ha/year, while the regularly flooded backwaters historically provided a yield of 115kg/ha/year.
Backiel and Penczak (1989) reported yields from the Vistula River (Poland) at 26 kg/ha/year. Bacalbasa-Dobrovici (1989) reported yields for the Danube River in Germany at 65-76 kg/ha/year and in Austria at 32 kg/ha/year. Total fish yield from the lower Danube River is approximately half the yield obtained from the river prior to changes by hydraulic works.
Karpova et al. (1996) documented fisheries characteristics for 49 reservoirs on rivers in the Commonwealth of Independent States (CIS; former USSR). Fish yields over a 12-year period (1980-1991) ranged from 0.14 kg/ha/year in the coolest, most northerly reservoirs, to 48.11 kg/ha/year in a Ukrainian reservoir. They noted also that CIS reservoirs in Central Asia and Kazakhstan have fairly low yields (average 11.48 kg/ha/year) but that this may be more related to cultural and local factors (e.g. preference for red meat rather than fish; distance to markets) than to productive potentials of the reservoirs.
Dams alter river ecosystems and subsequently require development of new relationships between humankind and natural resources associated with these ecosystems. We build dams because we perceive that benefits will accrue to us from them in the form of energy, water supply, transportation, flood control, fishing, recreation, aesthetics, and so on. We must, however, be on guard against developing arrogance with respect to ecosystems and the resources they afford (Catton and Dunlap, 1980), and also with respect to the persons who interact consumptively or otherwise with these resources and who may have little if any voice in decision-making processes. From the human dimension perspective, this is particularly the situation when the resources are non-portable natural resources (e.g. a river and its fish stocks). Persons linked to river fisheries through culture, tradition and economics incorporate these fisheries as dominant components of their human identities (Brown et al., 1996; Jackson, 1991). Reorientation of their values and activities after impacts to or loss of the foundation for their identities can generate considerable socio-economic stress to these people, their communities and their cultures (Baird, 1994; Brown et al., 1996).
From a fishery perspective, dams and their resulting reservoirs can benefit human societies. Dams, however, usually alter traditional riverine fisheries, sometimes positively (i.e. from tailwater fisheries), but more commonly negatively. There typically are faunal shifts from river-adapted species to those more adapted to lentic environments. Species diversity in impoundments usually declines over time as river-adapted species fade from the system. Benefits from impoundment of rivers seem to be more pronounced for smaller, shallower reservoirs that have reasonably high concentrations of dissolved solids and that are located in the upper reaches of their respective river ecosystem. However, several such impoundments within the same river catchment can result in synergistic negative impacts to the downstream fisheries. Introduction 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. Although management of dams can result in acceptable fisheries, when such management is, however, focused solely on fisheries, specific needs of fish species not included in the fishery, and/or that may be threatened or endangered, are at risk of being overlooked.
Compensation for loss in yield from river fisheries can be difficult to achieve through development of reservoir fisheries. Loss in fish yield is assumed also to be loss with respect to human nutrition, primarily protein. The larger the river, and the more downstream the location of the dam, the less potential there is for a reservoir fishery to compensate (in terms of yield) for losses sustained by the river fishery. In this regard, mitigation emphasis should concentrate on surface to volume ratios, and temperature regimes for the reservoirs. Compensation potentials apparently are higher in shallower reservoirs and in tropical regions than they are in deeper reservoirs and in more northern latitudes. If fisheries are directed to stocks of migratory fishes, dams can be disastrous, both to riverine fisheries and, in some cases, to ocean fisheries (e.g. anadromous salmonids). There are, however, exceptions where a dam apparently didn't harm the river fishery. Petr (pers.comm., 2000) emphasizes that, for example, Lake Volta dam at Akosombo (Ghana), is located not very far from the sea and that Lake Volta which extends over several hundred kilometers also floods extensive shallows, thus positively influencing fishery production in this system.
If seasonal flood pulses are lost as a result of dams, there can be substantial losses to the fisheries of floodplain river ecosystems (sensu Welcomme, 1985; Junk et al., 1989). Additionally, and because dams tend to be constructed to enhance socio-economic development activities, they tend to attract people and industry. Subsequently, river ecosystems containing dams must contend with pollution and increased exploitation and extraction of their resources, pressures independent from, but adding to, the direct influences of dams and reservoirs on the physical and biological dimensions of the system. Given the above concerns, regions lacking naturally substantial riverine fishery resources and/or river fisheries tend to derive the greatest degree of net benefit from development of reservoir fisheries (e.g. Dominican Republic, Sri Lanka, Republic of Panama). Caution, however, is warranted when fisheries development is conducted in the context of cultures where fishing and fish consumption is not a tradition. Otherwise, the potentials for altering riverine fisheries as a result of damming rivers must be acknowledged.
We can, however, look at the issue of dams on rivers from an entirely different perspective. If our spatial scale is large enough (planetary, continental, perhaps regional and biome), and our temporal scale is long enough (decades, centuries, millennia), placing a dam on a river does little more than increase atmospheric water vapour (through evaporation from the impoundment, which can be extreme in more arid environments; Petr and Mitrofanov, 1998), reduce long-term streamflows downstream (Jackson and Davies, 1988a and 1988b; Jackson et al., 1991), desiccate terrestrial environments, salinate surrounding areas, and shift bio-energetic processes (some of which can lead to floral and faunal extinction at various scales of resolution). We cannot assign the terms "good" or "bad" to any of these phenomena. They simply reflect anthropogenic activity on the planet. However, if we look at smaller spatial and shorter temporal scales, which we obviously cannot neglect as we have to make decisions that have bearings on the present and future human generations but also on present and future living aquatic resources, we have to keep in mind that dams and their reservoirs, which can under certain circumstances help to better nourish people and make their livelihoods more sustainable, can - if wrongly placed - also lead to significant declines of fisheries and to extinction of aquatic species.
Given sufficient time, geophysical and climatic forces will override and erode the physical influences of dams, and evolutionary forces will alter how life forms interact with the resulting environments. Whether or not humans as a species will be included in these processes is not known. Our perspective is too short. But short-term ( typically ca. 100 years) perspective is fundamental to construction of dams. Eternity, as a concept, is too big for us. We do not usually project caring, or our sense of responsibility, beyond three or perhaps four human generations. Beyond that, people (if there are any) must be able to take care of themselves. Nature does not care. It is we who (hopefully) care_and who must assume responsibility.
The integrity of rivers is challenged by human demands for their products and buffering from their processes. As co-evolutionary components of this planet, humans utilize and modify rivers and their associated resources to meet real or perceived needs. We have used rivers as highways for exploration and conquest and to address basic human needs. We use rivers to transport goods, services and wastes. We have harnessed the waters and the energy of rivers to protect and power ourselves and our civilizations. Our use of rivers has led to our encroachment on them. These anthropogenic activities alter how rivers function and thus how we function with them.
Prevailing intentions are almost always good; designed to advance human civilization and/or alleviate human suffering. At some point, however, we must ask ourselves if we really know enough to make decisions about the future of rivers, decisions that if wrong, can degrade human civilization and increase human suffering. These questions rightly temper our hearts as we ponder the legacy we leave for future generations.
Though trained and disciplined in technology, engineering and the sciences, we who practice these professions realize that ultimately decisions cannot be made solely through mental processes. There are indeed dimensions of the human experience that transcend reason and logic. We discover that if we rely on our minds in decision-making processes, we will be correct most of the time. However, if we add the dimension of the human heart to our decisions, we can substantially increase the probability of being right. This does not discount professional objectivity but rather expands the data bases from which we operate. It underscores the reality that we are humans and should act like humans; and that we must remember that there are other humans who are depending on us, the scientists, the resource managers, the decision-makers, to be right.
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We acknowledge with thanks the valuable assistance of Ms Kelly McCoy (Mississippi State University) and Dr Qifeng Ye (South Australia Fisheries Research Station) in locating and assimilating literature.\
1 also known as Yangtze River