Aquatic macrophytes can contribute to an increase in fish abundance when compared with areas or water bodies devoid of macrophytes (see also Section 2.1.1). Borawa et al. (1979) found that in the Currituck Sound (USA) fish density increased from approximately 1 000 to more than 15 000 fish ha-1 after Myriophylum spicatum became established. Killgore et al. (1989) in their study of fish in the Potomac River (Virginia, USA) found densities of 17 000–98 000 fish ha-1 in areas with plants and the CPUE was two to seven times higher in areas with plants than without plants. Killgore et al. (op. cit.) pointed out that estimates of fish density in aquatic plants are inherently variable due to sampling techniques, plant densities and patchy fish distributions. Seasonal changes in density and species composition of aquatic plants cause a transition in the spatial and temporal distribution of fish. In temperate and cool temperate zones, during the spring fish tend to associate for food and cover with aquatic plants that overwinter or emerge early from the substrate. The fish disperse to the more established macrophyte beds in the summer and autumn (Hall and Werner, 1977). Once the plant community reaches its maximum density in the summer, species capable of establishing dense stands, such as hydrilla, can occupy the entire water column, decreasing fish movement and foraging efficiency. Therefore, fish abundance and condition are often higher in areas of intermediate levels of structural complexity, particularly for piscivorous species (for references see Killgore et al., 1989). In aquatic plants in the Potomac River piscivorous fish were commonly observed occupying small “holes” (spaces) devoid of plants within dense vegetation, from where they ambushed their prey. Too dense concentrations of aquatic plants may lead to a reduction in the condition of fish, resulting in a stunted fish population (Cole and Shireman, 1980).
In a study on Great Lakes (Lake Huron and Lake Ontario, Canada) the fish production index, based on fish abundance and fish size, was significantly higher at littoral sites with macrophyte beds than at sites where macrophytes were sparse or absent (Randall et al., 1996). Fish size was consistently smaller at the high density macrophyte sites, which supported the findings of Barnett and Schneider (1974) who observed that as plant density increased, greater numbers of smaller fish were found.
Summarizing information from the literature, Hoyer and Canfield (1996) listed the most important environmental factors affecting the abundance of aquatic macrophytes in lakes as: general water chemistry, lake trophic characteristics, substrate characteristics, light availability, prevailing winds and lake morphology. These factors can work independently and in combination, varying with the scale of analysis. The factors also suggest that lakes can be divided into four general groups, small and large shallow lakes with abundant aquatic macrophytes, and small and large deep lakes with sparse aquatic macrophytes. The littoral zone of lakes is inversely related to basin slope, depth, and to the shoreline development. When lakes are shallow and other factors are favourable, aquatic macrophytes may cover a large portion of the lake. Studies have shown significant relations between aquatic macrophyte abundance and lake water chemistry; phytoplankton population structure and biomass levels; sediment resuspension and wave action; periphyton and aquatic invertebrate populations, and many other limnological processes; fish growth, abundance and population structure; angler utilization of fish populations; aquatic bird abundance and species composition, etc. The magnitudes of these relations are generally in proportion to the abundance of aquatic macrophytes.
In deep lakes, which have less littoral area for aquatic macrophyte growth than shallow lakes, the importance of aquatic macrophytes to the overall functioning of these lakes decreases proportionately as lakes get larger and deeper (e.g. Tilzer and Serruya, 1990). In some cases, small areas of littoral habitats may play a limiting factor for the reproduction or recruitment of some aquatic organisms in large lakes but not the overall production of the organisms.
Based on the investigations of 60 Florida lakes Hoyer and Canfield (1993) came to a conclusion that total and harvestable fish biomass per unit of adjusted chlorophyll a is at a maximum when the percent volume with aquatic macrophytes ranges from 20% to 40%. However, some of their lakes had high fish biomass also at macrophyte covers of less than 15% or over 85% thus showing that only a potential for depressed fish populations exists at both low and high levels of aquatic macrophytes.
Bettoli et al. (1992) found a strong relation between the submersed vegetation and largemouth bass in Lake Conroe (8 100 ha) in Texas and in Guntersville reservoir (28 000 ha) in Alabama. In Lake Conroe, the density of age-1 largemouth bass declined from about 100 fish ha-1 when submersed vegetation covered 30–44% of the reservoir to about 20 fish ha-1 when vegetation was completely removed. In Guntersville reservoir, young-of-the-year largemouth bass densities averaged 350 fish ha-1 in submersed vegetation compared to 24 fish ha-1 in unvegetated habitats during two years when flushing rates were high. Maceina (1996) cautions that the results of research on aquatic plant-largemouth bass population interactions conducted in different size water bodies, including reservoirs, may not be transferable. His suspicion was confirmed in a study by Hoyer and Canfield (1996a). When they examined the relationships between abundance of aquatic macrophytes and largemouth bass (Micropterus salmoides) for 56 lakes in Florida they found that lakes with higher nutrient concentrations tended to have more adult largemouth bass, but there was no apparent relationship between adult largemouth bass stock and abundance of aquatic macrophytes. They pointed out that lakes with grass carp, virtually free of aquatic macrophytes for ten or more years, supported the same numbers of largemouth bass as lakes with macrophytes. The authors stressed that any study attempting to elucidate relationships between the abundance of aquatic macrophytes and the abundance of largemouth bass, an important sport fish of USA waters, must consider the effect of lake trophic status. From the management point of view, the recommended 20–30% coverage of submersed vegetation (as recommended for example in Texas and Florida) is not needed for largemouth bass. One factor that may contribute to largemouth bass surviving in lakes of different trophic status and in lakes with and without aquatic vegetation is the opportunistic feeding behaviour of largemouth bass. As aquatic macrophytes are removed from the system the bass food habits shift from macrophyte-oriented species (e.g. small centrarchids) to open-water oriented species (e.g. clupeids). However, the authors also pointed out that large lakes may need aquatic macrophytes as refuge for young-of-the-year and subadult largemouth bass more so than small lakes due to the reduced shoreline to surface area ratio.
In a temperate climate, new reservoirs are not always hospitable environments for establishment of submersed plants. Because many reservoirs are operated for flood control, water supply, irrigation or hydroelectric power production, their water level may fluctuate considerably. Flooded terrestrial soils also may not be suitable for rooted aquatic plants, and wave action along the shoreline may soon wash out fine soil, leaving only pebbles or rocks. While stocking reservoirs with fish has been practised for some time, planting desirable aquatic plants as an important component of the ecosystem on which fish and other organisms depend, remains still only an idea. Smart et al. (1996) believe that more experimental work using a variety of locations and species would contribute to developing methodologies for establishing aquatic macrophytes in many reservoirs still lacking them. Planting of submersed macrophytes has been successfully applied in some lakes in China, such as Hongze, with the purpose of enhancing the production of mitten crab (Eriocheir sinensis) (Petr, unpublished), and in the oligotrophic lake Paizhong on the Yangtse lowlands to increase fish production (Zhang and Chang, 1994, for details see Section 9.3). In Lake Puckaway, Wisconsin (USA) aquatic macrophytes were replanted after a biomanipulation removal of weed fish (Brege, 1986).
Reservoirs constructed on the Volga, Dnieper and Dniester rivers in Russia and Ukraine are large shallow water bodies, which serve in the first instance for hydropower production, some also as a storage of water for irrigation. Four reservoirs were constructed on the upper Volga between 1937 and 1957, the largest being the Rybinsk. Fish stocks have been dominated by the phytophilic bream (Abramis brama) and roach (Rutilus rutilus) (long-term average in landings 41% and 19%, respectively), followed by pike (Esox lucius) (14%), and pikeperch (Stizostedion volgensis) (13%). Poddubny and Galat (1994, unpublished manuscript) commented on the decline of aquatic macrophytes in the upper Volga reservoirs, which has resulted in declining catches. While approximately 28% of the Rybinsk are shallows, only 2% of them have aquatic macrophytes, and a constant wave action is attacking the remaining submersed and emergent plants.
Vdovenko and Kravchenko (1980) attempted to determine the optimal density of aquatic macrophytes for bream (Abramis brama)-dominated small (200 to 400 ha) shallow reservoirs situated in the lowlands of the lower Don River. With an increase in density of emergent aquatic macrophytes (Typha angustifolia, Scirpus lacustris and S. maritimus) there was also an increase in the density of zooplankon - until a certain level: in dense stands of macrophytes zooplankton density was low, probably because of the low concentration of phytoplankton and low dissolved oxygen concentration. Bream was found to breed in areas with a very low density of macrophytes, and this notwithstanding that in denser stands there was a higher zooplankton density. The reason for this could be the deterioration in water quality towards the beginning of June, when concentration of dissolved oxygen starts declining, and there is an increase in concentrations of carbon dioxide and humic acids. Vdovenko and Kravchenko (1980) suggested that thin stands of emergent aquatic macrophytes should cover 30–40% of the productive zone to improve bream production in the investigated reservoirs.
In large lowland Ukrainian reservoirs, where because of their size, water level fluctuation is minimal and very gradual, aquatic macrophytes are well developed. In these reservoirs phytophilic fish represented 85 to 90% of the total fish biomass (Taran, 1990), much of it produced in shallows down to 2.5 m depth. These shallows account for 8–10% of the total area of reservoirs, with the reservoirs subjected only to a small summer drawdown of up to 1.5 m. The shallows are characteristic for a well-developed belt of macrophytes, which harbour rich zooplankton and bottom fauna. The production of organic matter in this zone is estimated to be 2.6 higher than that in the open waters.
The littoral of Dnieper reservoirs is responsible for 85–90% of the total reservoir fish production. The littoral is gradually overgrowing with emergent vegetation, which causes a decline in fish production and waterfowl, and decline in zooplankton production on which many fish feed. Zimbalevskaya et al. (1984) believe that aquatic macrophytes should not exceed 50% of the total area of the littoral zone. A gradual degradation through accumulation of organic detritus in the littoral, leading to peat formation and gradual conversion of the littoral plant zone into a swamp, has been taking place in a number of the Dnieper lowland reservoirs.
Land improvement to enhance spawning opportunities for phytophilic fish has been tried on the lower Don River. Tsimlyansk reservoir belongs to the most productive reservoirs in the Ukraine, with a 20-year annual average catch of 10 700 t of fish (Nefedov and Fesenko, 1987), and a yield ranging between 35 and 58 kg y-1. To maintain the high yield, deterioration of floodplain lakes had to be halted. The lakes represent an important spawning habitat, but they have been gradually overgrowing with wetland plants. Part of the vegetation was therefore removed reducing the cover from 20–30% to 5–10%. This resulted in a five-fold increase in spawning rate as compared with the unimproved floodplain lakes. Access to floodplain lakes was also improved by deepening the channels connecting them with the reservoir. As with other hydropower producing reservoirs, Tsimlyansk is subject to a drawdown, which may expose up to 40 000 ha of land. When fully reflooded in late spring, this area provides the best spawning grounds on flooded terrestrial grasses. The authors make a point that maintaining passable channels for broodstock to reach floodplain lakes, and prevention of floodplain lakes from getting overgrown with emergent macrophytes, is not cheap.
Exact data on fish yields from vegetated areas of tropical reservoirs are difficult to come by. In agreement with the observations on temperate and subtropical water bodies, the presence of aquatic macrophytes in the littoral of tropical reservoirs also increase fish production. In the Tanzanian reservoir Nyumba ya Mungu, where a significant portion of the littoral zone was occupied by aquatic macrophytes, the presence of the more open-community emergents Paspalidium germinatum and Cyperus articulatus determined the presence of major shoals of the commercial fish Sarotherodon pangani and S. jipe, which feed largely on periphytic algae growing on aquatic plants, branches of inundated trees, and stone surfaces (Bailey et al., 1978). In an experimental fishery, these two species together constituted 72% of the total (Bowker and Denny, 1978), while Tilapia rendalli, species introduced into the upper catchment of the reservoir, and the only fish specialized in grazing aquatic macrophytes, constituted 7%. The rapid growth of Paspalidium and Cyperus takes place in the nutrient enriched waters, and the same nutrient enrichment is also responsible for a massive growth of periphyton on which Sarotherodon feed.
Floodplains represent areas where the rise in water during the floods inundates extensive areas of land flanking the river channel. Frequently, such a rise in water level connects the river with floodplain lakes, isolated from it during the dry season. While flooding dry land results in covering largely terrestrial grasses and soil with water, reconnecting the river with floodplain lakes provides the fish of the river channel with the opportunity to access their areas covered with aquatic macrophytes, and where such lakes were under a high fishing pressure during the low water level, to repopulate such lakes with fish. The increase in living space, together with the release of nutrients associated with the submersion of the soil and from the decaying plants produces an annual surge of plankton and periphyton, encouraging an explosion in the animal productivity. The present publication does not deal with the significance of seasonal changes on floodplains, and the reader is referred to other publications, such as Welcomme (1979, 1985, 1989) and Gaudet (1992), who deal in more detail with the floodplains of Africa, Howard-Williams and Gaudet (1985) and Thompson (1976) who describe the structure and functioning of African swamps, and to Junk and Weber (1996), Junk (1996) and Welcomme (1979, 1985) who discuss the Amazonian floodplains. Welcomme (1985) constructed a diagram of trophic relationships in a river-floodplain community (Fig. 42).
As an indication of the importance of aquatic macrophytes for the productivity of floodplains in the Amazon varzea may serve the following estimate: the floating meadows of Echinochloa polystachya can accumulate up to 80 t ha-1 biomass, much of which is deposited on the exposed drawdown area after the retreat of flood waters. The decomposition of this plant biomass, together with that of the terrestrial plant species Paspalum fasciolatum plus other herbaceous plants, which grow on the exposed soils during the dry phase, plus from leaf litter etc. from the inundated forest, maintains the high productivity of the floodplain and its water when flooded, and this is reflected in the high fish production. The flood pulse of the Amazon and many other rivers, but especially those with substantial floodplains, is recognized as a major factor in maintaining the high productivity of floodplains. How the flood pulse affects fish of large Amazonian river floodplains is shown in Fig. 43.
Fig. 42. Diagram of trophic relationships in a river-floodplain community. Broken line = influence; solid line = feeding interaction. (From Welcomme, 1985).
Submersed aquatic plants that are abundant in some stream reaches have a potential to provide winter concealment cover for fish. Juveniles of several salmonid species were found in winter in rooted aquatic vegetation in a spring-fed Credit River, Ontario, Canada, in portions of the Kenai River, Alaska, and in a Norwegian river (Griffith and Smith, 1995 and references therein). However, in the Henry's Fork of the Snake River in Idaho, during two years of observations, the cover provided by submersed macrophytes was not adequate to hold 0+ rainbow trout throughout the winter. During one winter the loss of macrophytes resulted from sloughing and foraging by swans and other waterfowl, during another winter the loss was due to the presence of anchor ice (Griffith and Smith, 1995).
Fig. 43. The impact of the flood pulse on fish of large Amazonian river floodplains. (From Junk et al., 1997).
Hoyer and Canfield (1992), taking total fish standing crop data from North American streams located in Wyoming, Vermont, Florida, Iowa, Ontario, Washington and Missouri, noted that the regional estimates of average fish standing crop were significantly correlated to the total phosphorus concentration. Total fish standing crop increased with total phosphorus concentrations throughout the range of reported values, but increased much more rapidly in streams having phosphorus concentrations ≥ 15 μ L-1. However, the results of a survey of 17 Florida streams have shown that macrophyte standing crops were not correlated with instream total phosphorus or total nitrogen concentrations. This suggests that regional differences will be masked when included in a larger geographical coverage.
Mean fish density within the stands of aquatic plants of the Potomac River (USA) ranged from 5 000 to 204 000 fish ha-1 (Killgore et al., 1991). The establishment of aquatic plants had a positive influence on the fish abundance, particularly juvenile sport fish (largemouth bass, yellow perch, pumpkinseed): over 300 largemouth bass were collected in areas with aquatic plants, while only one individual of this species was collected in areas without plants. Considering all fish species in total, up to seven times more fish were collected in areas with plants.
Referring to literature, Killgore et al. (1991) provided comparative figures of fish densities in aquatic plants for some other water bodies: in Currituck Sound fish density increased from 1 000 to more than 15 000 ha -1 after Eurasian watermilfoil (Myriophyllum spicatum) became established. In Orange Lake, Florida, fish density ranged from 13 000–205 000 fish ha -1 in areas with submersed aquatic plants, and up to 86 000–2 500 000 fish ha -1 in several lentic locations in central Florida.
Managing in-stream and riparian vegetation for the enhancement of fish stocks or to provide better conditions for fish species preferred for fishermen, has been undertaken for centuries, mainly in the temperate zone of the northern hemisphere. Because of the vital links between river vegetation and water quality, physical habitats and animals, the manner in which vegetation management is addressed is critical for fishery (Cowx and Welcomme, 1998). The authors have pointed out that without management and the great importance attached to some highly-priced lowland fisheries, many rivers would have been virtually destroyed in the headlong drive for floodplain drainage. See Cowx and Welcomme (1998) for more details on the control of vegetation in water courses, on establishing aquatic and riparian vegetation to benefit fisheries, and on utilizing vegetation to enhance fisheries.
In the 1980s there were over 200 million ha of land under irrigation in the world, and the increase was estimated to be 20% over 10 years (Van Zon et al., 1982). Many irrigation and drainage canals experience a persistent problem of overgrowing with aquatic macrophytes. Spring-fed canals in Europe and Asia, especially in their upper reaches, are characterised by considerable growth of emergent (Phragmites, Typha) and other macrophytes including Bulboschoenus, Schoenoplectus, Potamogeton, Myriophyllum. Filamentous algae Cladophora, Spirogyra and Oedogonium are also common.
In most irrigation systems some form of fishery is caried out, but always extensively. The yield of small-scale fishery is estimated to be 10 to 60 kg ha-1 y-1 (Van Zon et al., 1982). These authors suggested converting aquatic plants, infesting irrigation and drainage canals, to fish protein. This has been tested in Egypt by using grass carp. Sustained harvesting of aquatic plants could produce 200 kg of grass carp per ha, and probably also approximately 100 kg of indigenous fish species. Under a well managed polyculture system, perhaps 500 kg ha -1y-1 could be achieved.
In the late 1950s the Karakum Canal in Central Asia developed considerable coverage of submersed Myriophyllum, which in the early 1960s was successfully controlled by the introduction of grass carp. Myriophyllum was succeeded by Ranunculus, which is avoided by herbivorous fish such as grass carp. Charyev (1984) summarized the experience with the introduction of grass, silver (Hypophthalmichthys molitrix) and bighead (Aristichthys nobilis) carps into the Karakum Canal and emphasized the need to protect the higher aquatic vegetation, particularly in cases where macrophytes represent spawning substrate for other fish, such as the common carp.
Common carp was efficiently used for suppressing aquatic macrophytes in drainage canals in Argentina (Sidorkewicj et al., 1998). In one instance 1 000 and 2 000 fish ha-1 (average weight 20 g) were applied following the mechanical removal of vegetation; another time 500 and 1 000 fish ha-1 (average weight 260 g) were stocked in the existing vegetation. Three months after the introduction of the small fish the regrowth of the dominant submersed plants Chara contraria and Ruppia maritima was well under control. The larger fish completely destroyed all submersed plants within four months. The activity of the common carp resulted in an increase in turbidity of up to ten times that on control plots. The authors noticed that the common carp also fed on aquatic macrophytes.