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Studies conducted in Ireland and elsewhere demonstrate that water bodies rich in vegetation generally support productive recreational fisheries. This reflects the cover, direct and indirect food supply and spawning substrates that aquatic macrophytes provide for fish and invertebrates. The composition and structure of the vegetation has also been shown to influence fish stock levels, through its impact on macroinvertebrates and periphyton supply, and by interfering with freedom of movement of the fish.

For recreational canals in Ireland with 20% to 70% plant cover, Caffrey (1993) reported the mean fish standing crop of approximately 2.5 times higher than in canals with vegetation exceeding 70%, and 4 times higher than in those with less than 20% cover. Sparsely vegetated sections supported 30–82 kg ha-1 of fish. In densely vegetated sections, dominated by Vaucheria sp. (filamentous alga), macroalga Chara spp, and Potamogeton pectinatus, 10 out of 17 sections had less than 100 kg ha-1 and represented poor recreational fishery. Most supported mixed fish communities, dominated by bream, although pike and perch, both piscivorous species, were well presented. In moderately weeded canal sections, the lowest fish standing crop was 116 kg ha-1, with 16 out of 24 sections having over 200 kg, and six of these in excess of 400 kg ha-1. The highest mean standing crop of 451 kg ha-1 was recorded from the sections where submersed plants with broad leaves, such as Potamogeton, or with complex leave types, e.g. Myriophyllum spp, charophytes and Ceratophyllum demersum dominated. Where strap-shaped or streamlined plants (Sparganium emersum, Scirpus lacustris, Sagittaria sagittifolia) dominated, the mean standing crop was 120.4 kg ha-1. In sections with a mixture of growth forms the mean standing crop was 201.5 kg ha-1, and where algae dominated, the mean standing crop was 148 kg ha-1 (Caffrey, 1993).

Good angling may be expected in moderately vegetated canals, which support more than 200 kg ha-1 of coarse fish. Denser vegetation stands restrict free movement of open water fish, such as roach, rudd, pike and perch (Canfield and Hoyer, 1992).

In Lake Conroe, Texas, elimination of aquatic vegetation from an area of over 3 600 ha using grass carp led to a change in the structure of the sport fish community (Bettoli et al., 1993) The largemouth bass-crappie-hybrid striped bass fishery was replaced by a channel catfish-white bass-hybrid striped bass-largemouth bass-black crappie community. Whether sport fishing on this lake got better or worse the authors were unable to say. Creel survey results indicated a decline in catch rates and an increase in average size of largemouth bass after vegetation removal. Lower catch rates were probably due, in part, to the lower density of largemouth bass after vegetation removal.

The Lake Conroe experience has several management implications, summarized by Maceina et al. (1991). In lakes and reservoirs where crappies (Pomoxis spp) depend on threadfin shad (Dorosoma petenense) for food, managers may consider control of excessive aquatic macrophytes to enhance crappie fisheries when open-water limnological changes have occurred. Macrophyte reduction should influence the trophic structure of the limnetic fish population, and crappie growth rates should improve. However, Maceina et al. (1991) caution that an optimal level of plant coverage for maximizing sport-fish standing crop may not be applicable, because of a number of variables, such as regional geology, water level fluctuation, plant community type, hydraulic flushing rate, basin morphometry, and external and internal nutrient loading rates. There are also differences in the type of fishery to be managed.

In Red Haw Lake, Iowa, reduction in vegetation biomass by 91% in four years using grass carp increased the opportunity for anglers to fish from shore (Mitzner, 1978). During the investigation popularity of shoreline fishing increased by 241% with catch success remaining greater than 0.70 fish per hour. Boat anglers also became more numerous and usually more successful than shore fishermen, and they caught nearly twice as many fish. Bailey (1978), who assessed the impact of grass carp control of aquatic macrophytes in over 100 lakes in Arkansas, expressed the opinion that submersed aquatic weed control with grass carp is far superior to chemical control, mechanical control, or no control. As lakes in the USA are managed for the benefit of fishery, with the ultimate goal to satisfy the greatest number of fishermen, Bailey concluded that the use of grass carp for submersed weed control is an effective and desirable tool.

For the impact of grass carp on other fish, see also Section 3.1.2.

In New Zealand, temporary removal of aquatic plants in Lake Parkinson, a small, eutrophic lake, resulted in a number of changes to the population of stocked rainbow trout (Oncorhynchus mykiss). In the first summer following weed removal the number of cormorants (Phalacrocorax carbo) counted at the lake increased and their predation resulted in a decline in trout density. However, the growth rate and condition of the trout population then exceeded that of trout present before weed removal. During the second summer after weed removal a cladoceran bloom was followed by low phytoplankton levels and high ammonia concentrations. These low oxygen levels eliminated the trout population, but other fish species present survived. The experiments demonstrated the importance of weed beds in maintaining a stable fish community in lakes such as Lake Parkinson (Rowe, 1984).

In the lower Potomac River, Virginia (USA), where anglers target especially the piscivorous largemouth bass (Micropterus salmoides) and yellow perch (Perca flavescens), spaces (‘holes’) within the dense stands of hydrilla were found to provide a favourable habitat for the piscivorous fish. Expansion of such holes, or creation of new ones in dense aquatic plants would also result in better fishing success for such sport fish. Another management method is creation of open lanes (channels) through the aquatic plant beds. Crowder and Cooper (1979) recommended creation of rectangular strips of intermediate vegetation density to create sufficient “edge” for larger fish to forage on small fish. This type of control strategy can allow access by boats, while maintaining a productive interaction between the aquatic plants and fish. Such management approach is more suitable for lakes than rivers.

In the USA, where largemouth bass (Micropterus salmoides) is an important sport fish, Trebitz et al. (1997) developed a model for heavily vegetated temperate lakes that simulates the interactions of bluegill (Lepomis macrochirus), largemouth bass and their invertebrate prey in the context of vegetation structure. Bass growth rates and numbers increased after most simulated vegetation removals because their access to prey increased. Bluegill grew fastest when about 30% of the vegetation was cut, but responded negatively to mowing more than about half the plants. Bluegill responded to the manipulation more by changing growth rates, while bass responded more by increasing numbers. Moderate plant removals accomplished by mowing many narrow channels are most likely to simultaneously benefit both species.

Recreation along river banks and shores may cause a major damage to the littoral aquatic vegetation and through this impact the spawning and nursery habitat of some fish. Along some lakes and reservoirs in Germany and Hungary especially, the summer invasions of tourists seeking water may cause considerable damage to the water plants through trampling, as well as by using boats. Further damage may be caused to terrestrial bank or shore vegetation, the shade of which is known to attract some fish species to the littoral. Pleasure boats may increase water turbidity, which is the result of stirring bottom sediments. This may lead to recycling of nutrients and cause increase in phytoplankton. Strong correlation has been found in the English canals between the intensity of boat traffic and the decrease in submersed and floating-leaved macrophytes, especially Potamogeton natans and Nuphar lutea (e.g. Murphy and Eaton, 1981). On the other hand boat traffic performs the useful function of maintaining a satisfactory amount of vegetation against the natural tendency for plant growth to become excessive. The study of 13 canals in England has shown that canals with little or no boat traffic had mostly good fish stocks.

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