Research on fish and fish stocks in freshwater and brackish water habitats is often carried out in isolation. Fish are closely dependent on their total environment, the balanced condition in which can be distorted not only by human activities, but by excesive pressure evolved by introduced exotic organisms. Aquatic macrophytes are one of the major links in the interrelated environment, and some authors believe they are the central link which determines its stability. They have a central position in the network of ecological relations between nutrients, plankton and macroinvertebrates, and in the temperate zone of Europe they determine the carrying capacity for the population of some fish such as pike, most cyprinids and eel (De Nie, 1987). Much of the literature focuses on individual species or restricted groups, and there is a shortage of studies of whole communities, containing more than two trophic levels (Jones et al., 1998).
Aquatic macrophyte-eating fish species are common in Europe and Asia, Africa and Latin America, but there are no fish native to North America for which aquatic macrophytes are an important dietary component (Lodge et al., 1998). But aquatic macrophytes are also subject to herbivory by invertebrates, especially insects and snails, by some mammals, turtles and birds.
Literature which deals with aquatic macrophyte-fish interactions often concludes that intermediate plant densities enhance fish diversity, feeding, growth and reproduction of fish. However, “intermediate density” is a subjective term and recent evidence suggests that both spatial distribution and relative abundance of plants must be considered in plant management strategies. Management decisions are usually based on macroscale, which implies either an entire water body or a water body divided into zones (e.g. littoral, cove). Aquatic plants are mostly assessed on the basis of their area cover and expressed as hectares, percent coverage, or biomass per hectare. Fish are expressed as standing crop, density, catch per unit effort, or percent abundance relative to plant coverage. Macroscale information rarely allows to fully understand what is actually happening within the macrophyte habitat, where behavioural characteristics of fish often determine the complex relationships of temporal and spatial character during the ontogenesis of the individual fish species. Microscale studies that quantify fish behavioural responses such as habitat preference, foraging efficiency, predator avoidance, and social attraction in vegetated areas are required to clarify the role of aquatic plants as fish habitats (Dibble et al., 1996).
The relationship between plant and fish abundance is an important management consideration when plant control strategies are developed, because it provides a basis to evaluate potential effects of the control programme on the integrity of the fish community structure (Killgore et al., 1989). Artificial structuring of plant beds, as for example by clearing a series of deep channels (Olson et al., 1998) or boat lanes, suggests that plant beds could be engineered for increased fishery value. Studies of fish-plant interactions will assist in a better understanding of causal relationships between the structure of plant beds and a specific response from the fish community and verify protocols for enhancement of fish habitats coincident with control of plants (Hoover et al., 1993).
Aquatic macrophytes can contribute to an increase in fish abundance, particularly in areas once devoid of any substantial amount of cover. In the USA Borawa et al. (1979) found that fish densities increased from approximately 1 000 to more than 15 000 fish ha-1 after the invasive plant Myriophyllum spicatum became established in Currituck Sound. Killgore et al. (1989), in their study in the Potomac River, estimated fish density from 17 000 to 98 000 fish ha-1 in areas with plants and the CPUE was two to seven times higher in areas with aquatic plants than areas without plants. In Orange Lake, Florida, Haller et al. (1980) and Shireman et al. (1981) found a density from 13 000 to 205 000 fish ha-1 in areas with submersed plants.
An intermediate density of aquatic macrophytes is still considered as preferable, from which both fish and crayfish benefit most. But Hoyer and Canfield (1993), who investigated 60 Florida lakes for the relationships between aquatic macrophytes and fish stocks, cautioned that only potential for depressed fish stocks exists at both high and low levels of aquatic macrophytes.
Persson and Crowder (1998) produced a summary of potential mechanisms by which fish may affect macrophyte abundance, littoral-pelagic coupling, and overall lake dynamics:
Routes for effects on submersed vegetation (habitat structure) by fish feeding activities in macrophyte habitats feeding on macrophytes feeding on epiphytes feeding on macroinvertebrates feeding-induced uprooting of plants
Routes for effects on submersed vegetation (habitat structure) by feeding activities in open
zooplankton predation induced changes in phytoplankton biomass (transparency) sediment feeding induced fluxes of nutrients to open water affecting phytoplankton biomass (transparency)
Other fish induced coupling of littoral and pelagic habitats
transport of nutrients/organic matter to pelagial due to littoral feeding and pelagic excretion/egestion
transport of nutrients/organic matter to littoral due to pelagic feeding at night by daytime refuging juvenile fish
recruitment of juvenile fish to the vegetation habitat from pelagic larval stages over ontogeny
recruitment of adult fish to the pelagic habitat from the vegetation habitat over ontogeny.
Researchers are now focusing more on the whole submersed macrophyte food web. The trophic cascade, which has been demonstrated for relatively simple food chains, sometime fails in more complicated webs. Crowder et al. (1998) highlighted the need for linking the littoral and pelagic habitats both in terms of prey consumption and translocation of nutrients, to include for example the transfer of epiphyte productivity to pelagic habitat. Macrophytes, as a substrate for epiphyton, and fish, are the major factors in such a simplified system, which shows the importance of a vegetated littoral for the whole ecosystem of a water body.
The need for biocontrol of aquatic macrophytes/weeds still remains high on the list of priorities, not only for fishery managers, but also for maintaining good quantity and quality water for drinking and industrial uses, for engineers of hydroelectric dams, for river and lake transport etc. The need for aquatic weed control can be seen from the situation in India where in the mid-1970s about 320 000 ha of water surface had to be cleared of weeds annually for fish culture, and in eastern states up to 70% of water area was infested by weeds (Chaudhuri et al., 1977). Since then, the situation has probably further deteriorated. Submersed and floating aquatic macrophytes are also a habitat of vectors and hosts of parasitic diseases, but the notion that fish may become an efficient control agent has been revised as it is becoming evident that only integrated control appears to be the solution to the problem.
Fisheries represent only one of the users of the aquatic resource. Wiley et al. (1984) stressed that management strategies for aquatic macrophytes should reflect both the ecological realities of aquatic systems and the practical uses for which water is intended. If recreational or commercial fish production is an important goal, fisheries management and aquatic plant management must be integrated in a compatible way. Barko et al. (1986) pointed out that anagement procedures directed toward aquatic macrophyte eradication are generally ill-advised, not only on ecological grounds, but also because many aquatic systems serve multiple purposes. Major environmental changes are generally related to the amount of vegetation controlled rather than to the control method. Control of aquatic weeds should, therefore, be a major consideration in selecting control methods, especially when monetary resources are limited (Sutton, 1985). Wide information dissemination, including good case studies, and training using a holistic and integrated approach to ecosystem management are needed for faster progress in this field. The start has been already made, as seen from publications such as Jeppesen et al. (1998a) and Gulati et al. (1990).
Much of the research on interrelationships is done in developed countries. There exist centres of excellence, such as in the USA (Florida), Denmark, Germany and Brasil, where most interesting work has been done in the last 10–30 years. Transfer of knowledge from industrialized (often temperate) to less industrialized (often tropical) countries is gaining speed, but not all the knowledge gained in and on waters in a temperate climate is relevant or applicable. It is mostly the latter countries where problems with invasive aquatic macrophytes, such as water hyacinth and salvinia, are most urgent, as such plants interfere with water supply and inland fisheries, both of which are vitally important in tropical countries. For some time to come, industrialized countries may remain the vanguard of indepth, sound, and applicable research and of devising the best ways of aquatic plant management for water bodies of the temperate and tropical climate. But it would be on the countries situated in the tropics and subtropics to critically validate the knowledge already available and test it in local conditions, but also to undertake both field and experimental studies on the function of aquatic plants in determining the level of fish productivity in vegetated water bodies.