Aquatic macrophytes are among those factors which a fishery manager will try to understand and include in his strategies for optimizing capture fisheries in inland waters. Plant species composition, distribution and percentage cover of aquatic plants may determine the fish species composition, individual fish species production, access to fish stocks by fishermen, fishing gear and sometimes also boat access and transport possibilities for getting the fishery product to the markets. Aquatic macrophytes can also be efficient indicators of water quality, and their presence may enhance water quality due to their ability to absorb excessive loads of nutrients. These properties have been used in wastewater treatment as well as in biomanipulation of water bodies for enhancing fish production.
Aquatic macrophytes are present in virtually all freshwater bodies. In lakes they occupy predominantly the littoral shallows, and their importance in a lake ecosystem generally declines with increasing lake size. The portion of the lake occupied by the littoral zone is inversely related to basin slope and to the degree of shoreline regularity, being smallest in large deep lakes with low shoreline development index. Emergent and floating-leaved aquatic macrophytes seldom grow in waters exceeding a depth of 3 m (Canfield and Hoyer, 1992). In small lakes, where the littoral region with aquatic macrophytes occupies a relatively large portion of the lake, there is a relatively low intensity of competitive interaction among consumers that move from the pelagic zone to use littoral resources (Gasith and Gafny, 1998). The cross-boundary movers are typically adult or large fish that move between deep and shallow water for feeding, for cover, and seasonally, for breeding. The littoral residents, on the other hand, are mostly young, or small-sized fish species.
Aquatic macrophytes can influence the lake ecosystem in a number of ways, which fall into three main categories: (1) limnological effects related to changes in physical and chemical conditions in the water and sediments; (2) metabolic effects related to production and processing of organic matter and nutrient cycling; and (3) effect on biotic interactions and community structure related to the role of macrophytes in providing a structured habitat (Gasith and Hoyer, 1998). The importance of macrophytes will decline with increasing size and depth of the water body. The effects of submersed macrophytes on ecosystem processes have been reviewed by Carpenter and Lodge (1986).
While presently eutrophication is often the cause of a change in the aquatic macrophyte community in a particular water body, there are many instances where there are other reasons for such changes. They may take place spontaneously, or under the impact of intensive grazing by birds, crayfish, grass carp, muskrat, coypu, (see Section 3.) or even cattle. Grazing by cattle on aquatic macrophytes has been mentioned for example in the Netherlands, where it is causing the destruction of reed (Best, 1982), and also in Germany (Sukopp and Markstein, 1981). Another reason for change maybe the appearance of an exotic or adventive plant species. Human impact of non-eutrophicating character, such as mechanical or chemical removal of aquatic macrophytes (see Section 13), possibly targeting the unwanted ones, may also lead to a change in the aquatic macrophyte community.
Freshwater fish use aquatic macrophytes for shelter and refuge, as a food source either direct or indirect, in form of epiphyton (periphyton) and associated invertebrates, and as spawning, nesting and nursery sites. Fish species of densely vegetated margins of some water bodies, e.g. in the papyrus margins of Lake Victoria, are very tolerant of low dissolved oxygen concentrations and high temperature. Sparse vegetation or isolated macrophyte patches also attract some fish which use them to shelter, feed or spawn. Floating meadows in the Amazon, or wind-blown patches of floating plants, such as water hyacinth or Pistia, harbour associated fish fauna.
This publication does not claim a complete coverage of the subject. It does not cover interrelationships between fish and mangroves, fish in salt marshes and tidal flats, use of aquatic macrophytes as food or fish feed additives in aquaculture, fish in ricefields, use of macrophytes for removal of aquaculture effluents and in recirculating fish culture systems, and the importance of dead timber in reservoirs as a substrate for fish food organisms. Several topics are covered only briefly: water-borne disease control (only those larvivorous and molluscivorous fish, which also serve as food, are discussed); biomanipulation (the section dealing with this subject focuses largely on the function of fish versus macrophytes); floodplains (the reader is directed to major publications on this topic). Control of aquatic macrophytes is limited to the use of fish, and to the impact of chemical and mechanical control measures on fish.