The term “aquatic macrophytes” includes largely flowering plants (angiosperms), some ferns and mosses (pteridophytes) and macroscopic algae (stoneworts, e.g. Chara, and the alga Cladophora). De Nie (1987) used the following division for aquatic macrophytes, which, expanded by the major tropical and subtropical plants, is also applied in this publication:
Submersed macrophytes: these include the algae Chara and Nitella, moss Fontinalis, and angiosperms Myriophyllum, Elodea, Potamogeton etc.
Cowx and Welcomme (1998) listed a number of characteristics of aquatic plants which make them important to fish:
Aquatic macrophytes are essential if a healthy fish stocks are to be maintained in natural water body. Opuszynski and Shireman (1995) pointed out that some plants or combination of plant species seem to be better fish habitats than others. Plant density is of importance, and the trophic state of the water body also plays an important role in determining what species and what biomass of fish will be present. The more research results become available, the more complicated the aquatic macrophyte-fish interrelationships appear to be.
The shallow marginal littoral with aquatic macrophytes contains a greater diversity of aquatic animals than the offshore open water areas. For example, in Lake George in Uganda, the vegetated littoral contained 19 fish species compared to around 10 species in the mid-lake (Burgis et al., 1973).
Submersed macrophytes have major effects on productivity and biogeochemical cycles in fresh water because they occupy key interfaces in stream and lake ecosystems (Carpenter and Lodge, 1986). Changes in aquatic macrophytes can have major consequences for commercially important fish species. Most macrophytes are rooted, and constitute a living link between sediments and the overlying water. Carpenter and Lodge (op.cit.) found by 1985 seventy-two papers on the effects of environmental factors on macrophytes, but only 22 papers which emphasized the effects of macrophytes on the environment. Since their publication there have been a number of detailed field studies, and an increasing number of field experimental studies on the function of macrophytes in aquatic environment and their impact on the environment.
In the northern temperate lakes fish community structure and stability have been associated with the presence, abundance, species composition, growth form, and structural heterogeneity of macrophytes in various studies (Weaver et al., 1996). The mechanisms that maintain or alter community structure, and their relative importance, differ among communities and vary with spatial and temporal scale. Aquatic and marginal plants may be a key factor determining whether or not there is any fishery interest in the highly modified urban watercourses which are most accessible to the majority of people.
Aquatic macrophytes have great effect on physical environment, especially light penetration through the water column, water temperature, water flow and substrate. Within stands of aquatic vegetation the light intensity quickly decreases with depth, although great differences exist in the degree of light attenuation betwen species.
The vertical temperature gradient within macrophyte stands is as steep as 10°C m-1, compared to gradients of less than 0.2°C m-1 in neighbouring unvegetated areas (Dale and Gillespie, 1977).
Diel oxygen changes as large as 8 mg L-1 occur in the waters of dense submersed macrophyte stands (Ondok et al., 1984). Oxygen may be released into water by some floating plants, such as water hyacinth from rootlets of which oxygen actively diffuses into water (see also Section 7.3.1). Submersed macrophytes oxygenate the water more effectively than floating-leaved macrophytes, as solid cover of the latter prevents effective reoxygenation from the air. Decaying macrophytes consume large amounts of oxygen and macrophyte leachate stimulates oxygen consumption by suspended bacteria. Large mats of floating aquatic plants, such as Salvinia molesta, Eichhornia crassipes and Cyperus papyrus will produce almost totally deoxygenated conditions underneath these plants, because they reduce the light penetration and prevent oxygenation of water blocking the water-air interface.
Macrophytes have positive effect on water clarity through a number of mechanisms. These include provision of habitat for plant-associated macroinvertebrates and a refuge for cladocerans, both of which graze on phytoplankton and epiphytes. While food and habitat provided by plant beds support diverse fish populations, piscivores among them reduce numbers of small planktivorous fish, thereby reducing predation pressure on zooplankton. Macrophytes may also suppress phytoplankton growth through the release of alleopathic substances, and through luxury nutrient uptake and enhanced denitrification. Phosphorus may be reduced by certain plant species that oxidize the sediment. In addition, reduced water movement in plant beds can lead to increased sedimentation rates and reduced turbidity. Where light conditions are deteriorating, mechanisms such as the shedding of epiphyte-burdened leaves, and a shift from low-growing species to taller ones may stabilize the plant state (Moss, 1990).
It is now recognized, that through various pathways, submersed macrophytes have a strong positive effect on the resource base of epiphytic and benthic macroinvertebrates. While many macroinvertebrates adjust their behaviour in response to fish by seeking refuge and/or decreasing risky actrivities, the feeding and individual growth rates of many fish species are positively related to the abundance of macroinvertebrates (Diehl and Kornijow, 1998). However, the negative effects of vegetation on the encounter rates of fish and the positive effects of vegetation on the standing stock and production of macroinvertebrates make it impossible to derive a general relationship between the growth rates of fish and the density of submersed vegetation. It is becoming obvious that the vegetation-macroinvertebrate-fish interaction cannot be understood in isolation from other components of the lake food web and from processes that link the littoral zone to benthic and pelagic habitats (Diehl and Kornijow, 1998). (See also Sections 1.1.1 and 6.5.)
Extensive macrophyte growth in lakes and reservoirs can alter trophic interactions. Typically, there is a negative relationship between abundance of submersed plants and planktonic algal biomass (Canfield et al., 1983). Factors causing this include: competitive uptake of nutrients by macrophytes; development of extensive algal epiphytes on macrophytes; and a decline in nutrient recycling as macrophytes reduce mixing and nutrient resuspension. Productivity of the epiphyte complex may reach up to 93% of host macrophyte productivity (Jones, 1984). Fish that feed directly on planktonic algae could be negatively affected by dense macrophyte coverage. Aquatic plants can also affect fish populations by altering the abundance and composition of zooplankton and macroinvertebrates, by directly influencing predator-prey interactions, and by increasing the reproductive success of some predators (Maceina et al., 1991).
While aquatic macrophytes represent an important habitat for fish, herbivory by fish, birds, invertebrates and mammals is important in determining macrophyte abundance, diversity and productivity. Herbivory may influence the timing and magnitude of nutrient turnover by macrophytes, water quality and other aquatic biota, including fish (see e.g. Sections 2.1, 3.4).
Nearly all the fish species connected with littoral zone of European lakes consume some aquatic plant material during their lives. Prejs (1984) listed 15 species, of which ide (Leuciscus idus), roach (Rutilus rutilus), and rudd (Scardinius erythrophthalmus) had a high content of plants in their food; bream (Abramis brama), white bream (Blicca bjerkna), chub (Leuciscus cephalus), crucian carp (Carassius carassius), gudgeon (Gobio gobio), stickleback (Gasterosteus aculeatus), tench (Tinca tinca), and carp (Cyprinus carpio) had small amounts, and bitterling (Rhodeus sericeus amarus), bleak (Alburnus alburnus), perch (Perca fluviatilis), and juvenile pike (Esox lucius) had sporadic occurrence. The main plant consumers, roach and rudd, use the majority of the species of submersed macrophytes which occur in sufficiently high densities, but they prefer soft submersed plants. The absence of some plant species in the food, even common species, is due primarily to their physical properties or to their odd taste or toxic properties, e.g. Ranunculaceae.
Several indigenous Euroasian fish species inhabit aquatic macrophytes as adults: tench, rudd and pike. The submersed macrophytes form important feeding habitats for perch, roach, bleak, and eel (Anguilla anguilla), while the ruff (Gymnocephalus cernua) prefers the Chara beds (de Nie, 1987).
In the southern USA, the fish Enneacanthus gloriosus, Lucania goodei, Lepomis macrochirus, Heterandria formosa, among others, are substantially more abundant in vegetation than other fish species (Hoover et al., 1989). In the South African lake Swartvlei the fish species Monodactylus falciformis and Rhabdosargus holubi directly depend on periphyton and associated macroinvertebrates of submersed aquatic plants (Whitfield, 1984). The Australian tropical lungfish Neoceratodus forsteri feeds on Hydrilla verticillata, Vallisneria spiralis and some other aquatic plants (Kemp et al., 1981).
Among the important freshwater culturable fish species of India, Jhingran (1977) named Barbus carnaticus and Etroplus suratensis as partially feeding on aquatic macrophytes.
In plant beds rich food supply in form of periphyton, its associated invertebrates and bottom fauna, is generally more numerous and more diverse than in open waters. Within the diverse aquatic plants, the habitat-specific differences in invertebrate composition directly influence habitat selection by fish and foraging behaviour of individual fish (Werner, et al., 1981).
Many young fish need aquatic macrophytes as shelter and protection from predation or to avoid cannibalism. Aquatic macrophytes also serve some fish as a spawning habitat, for the attachment of eggs, and some fish form nesting sites among the macrophytes.
Recent habitat restoration programmes conducted in the Great Lakes Basin of northern America have focused on the importance of aquatic macrophytes for the production of northern pike. It was found that macrophytes are of a greater significance as nursery rather than spawning environment. Adult fish abundance was found to be related to the extent of macrophyte cover, which was found optimal from 35 to 80% but inversely related to body size (Casselman and Lewis, 1996), this resulting in a lower fish biomass in the stands of heavy density.
A thorough study by Killgore et al. (1991) addressed the habitat value of aquatic plants for fishes in North American water bodies, but more specifically in the Potomac River. Their general conclusions can be widely accepted for numerous water bodies in temperate regions of the northern hemisphere. They followed the seasonal change in density and species composition of aquatic plants in relation to the spatial and temporal distribution of fishes. In their discussion, they included references to findings of other authors, supporting their conclusions.
During the spring when plant density is relatively low in the Potomac River, fish abundance is highest in areas without plants because of the occurrence of anadromous, pelagic-oriented species, or in areas with mature stands of Myriophyllum, rather than emerging short stands of Hydrilla. During the spring, fish tend to associate with aquatic plants that overwinter or emerge early from the substrate for food and cover and disperse to the more established macrophyte beds in the summer and fall.
Once the plant community reached its maximum density in the summer, plants capable of establishing dense stands, such as hydrilla, can occupy the entire water column. This slows down fish movement and foraging efficiency of fish. Therefore, more fish are often found in areas of intermediate levels of structural complexity, particularly for piscivorous species. In late summer through autumn immediate densities of submersed aquatic plants usually contained more species and numbers of fish than areas having dense growths. Some fish species such as pumpkinseed fish (Lepomis gibbosus), however, prefer high plant densities. The piscivorous largemouth bass (Micropterus salmoides) and yellow perch (Perca flavescens) were commonly observed occupying small spaces devoid of plants in dense vegetation, ready to ambush their prey. Banded killifish were found at the water surface immediately above the plants, at the periphery of the dense stands, in spaces (“holes”) formed in the plant beds, or the bottom directly below dense canopy formations to utilise both open and structurally complex areas for foraging and predator avoidance. The presence of largely small fish in dense stands of aquatic plants, and larger, piscivorous fish being more common at the periphery of the plant beds, as reported in the literature, was not confirmed from the Potomac River.
Cailteux et al. (1994), who compared the growth of largemouth bass in vegetated versus non-vegetated lakes in central Florida observed that bass from lakes without aquatic macrophytes became piscivorous at the size of 60 mm which resulted in significantly faster first year growth than bass from vegetated lakes which did not become piscivorous until when larger than 120 mm. Although largemouth bass from unvegetated lakes grew faster than in vegetated lakes, survival to recruitment appeared to be much lower.
Studies of European fish species have shown that differences in stem densities of aquatic macrophytes result in different foraging rate: the foraging rate of rudd (Scardinius erythrophthalmus) on Daphnia decreased at stem densities greater than 200 m-2 and that of juvenile perch (Perca fluviatilis) did not decrease even at the highest stem density of 600 stems m-2 (Winfield, 1986). The foraging efficiency of roach (Rutilus rutilus) decreased substantially even at the lowest stem density. Similar results were obtained by Diehl (1988) in a study of the foraging efficiencies of roach, bream (Abramis brama), and perch feeding on chironomids: the foraging performances of roach and bream decreased strongly in the presence of vegetation whereas the foraging performance of perch was only slightly affected by vegetation. Because perch and roach make up most of the total fish biomass in many European lakes, the effects of vegetation structure on the interactions between these two species will have ramifications for overall community and lake ecosystem dynamics. Roach as a competitor with juvenile perch may severely limit the recruitment of perch to large piscivorous stages in the absence of vegetated habitats (Persson and Crowder, 1998).
In other experiments, when simultaneously offered perch and rudd in field enclosures, pike captured more rudd than perch in environments lacking vegetation, whereas the opposite was the case in environments with vegetation (Eklov and Hamrin, 1989). Pike has been found to be a more efficient predator than perch and pikeperch (Stizostedion lucioperca) in vegetation.
Crowder and Cooper (1982) who, in experimental ponds in the state of Michigan, USA, investigated the relationship between macrophyte density (dominated by Ceratophyllum demersum) and the interaction between bluegills (Lepomis macrochirus) and their invertebrate prey noted that at high macrophyte densities fish captured less prey and grew more slowly despite the presence of a higher biomass of invertebrates. Fish diets at high macrophyte density did not broaden; the fish simply consumed less (but larger prey, than at low macrophyte densities) and grew more poorly as a result. At low macrophyte density fish diets tended to be narrower. The authors concluded that in the long term, intermediate macrophyte density may be the best habitat both in terms of foraging and growth of the predator and stability of the fish-prey interaction. However, they also stressed the importance of considering multiple variables (food, habitat structure, temperature, predators) and assessing resource availability, if we are to better understand resource use in spatially complex environments.
Randall et al. (1996) who studied the density of fish in relation to the density of macrophytes in the Great Lakes of North America, found fish production significantly higher in littoral habitats with abundant submersed macrophytes, as measured by percent bottom cover, than in adjacent areas with low macrophyte abundance. Vegetated sites had higher densities of fish, smaller fish, and greater species richness than unvegetated sites (Fig. 1). The authors recommended that conservation should be directed at protecting not only the productive vegetated littoral habitat but also adjacent areas which benefit from the high productivity as well.
The architecture or spatial arrangement of plant species has an important impact on fish frequenting aquatic macrophyte stands. Structural characteristics unique to aquatic plants create spatial complexity in aquatic habitats which are important to growth and survival of freshwater fishes. The littoral of lakes and ponds with aquatic macrophytes is physically complex and spatially patchy. Dibble and Killgore (1994), using literature information, listed the following salient points showing the significance of plant structural complexity, i.e. habitat at the microlevel, for freshwater fish: (i) it deters predation by altering the outcome of predator-prey interactions; (ii) serves as critical refuge sites for smaller fishes; (iii) is important for successful spawning of fish; (iv) increases survival and recruitment of juvenile fishes; (v) growth rates of young fish increase in these habitats because plant leaves and stems offer a habitat rich in food resources for fish by providing attachment substrate and protection for many microinvertebrates. Caffrey (1993) summarized the significance of aquatic plant complexity as follows: broad and dissected leaves provide abundant cover and concealment for adult fish and nursery habitats for fry. They also provide a wide range of microhabitats for fish food invertebrates and periphyton, and spawning substrates for fish and invertebrates. Coarse fish which deposit their adhesive eggs on most plant species prefer submersed plants with broader or complex forms. Floating-leaved plants offer minimal cover, direct or indirect food supply or spawning substrates for fish or invertebrates. This is reflected in the low mean fish standing crop. Filamentous algae do not provide cover, but harbour a large number of invertebrates. This probably explains the good fish crops in Irish canals with a moderate algal growth.
Plants with a year-round presence, such as submersed Hippuris vulgaris, Myriophyllum spicatum and charophytes, all broad or complex-leaved, provide favourable conditions for fish and have a high standing crop. However, plants which in a temperate climate die down for the winter months and initiate their growth cycle late in the spring, provide poor habitat for fish (Caffrey, 1993).
Habitat that is spatially complex may incorporate a variety of microhabitats and thereby support a more diverse community. But special complexity may also restrict access to preferred habitat and thereby limit species distribution. Thus, patchy distribution of habitat is important in structuring and stabilizing communities. Even macrophyte beds that are evenly distributed around a lake often are locally patchy, consisting of patches or bands of vegetation that are separated by open areas. Weaver et al. (1997) examined how the patchy distribution of submersed macrophytes affects the distribution of littoral fishes within Lake Mendota, Wisconsin (USA). When patchiness in the structure of macrophyte beds was considered along with macrophyte abundance, the distribution response of fish to habitat complexity was better explained. Patchiness within vegetation provides prey fish with a habitat in which shelter is in close proximity to open spaces that harbour zooplankton, the preferred prey of many age 0+ fishes. Reduced density within the macrophyte patches increases the manoeuverability of prey fish, and the visibility of macroinvertebrates, and thereby improves their capture success on their prey (Dionne and Folt, 1991). In Lake Mendota decline in the abundance of native plants and small fish and in the growth rate of prey and piscivorous fish coincided with the invasion of Myriophyllum spicatum in the mid-1960s. During the decline of Myriophyllum in the late 1970s native plants have recolonized, and with them the fish community has been recovering (Weaver et al., 1997).
The plant structure complexity also determines the invertebrate biomass in plant stands. In Lake Krankesjon in Sweden Chara tomentosa, which grows there in dense beds and constitutes a more complex structure than Potamogeton pectinatus, another common plant in this lake, has a higher invertebrate biomass (Hargeby et al., 1994) (Fig.2). The complex structure also offers a more efficient shelter against predation from fish (Diehl, 1988). Chara overwinters with green parts and thus offers a more permanent habitat beneficial for the aquatic invertebrates, and consequently also more beneficial for fish than P. pectinatus, which enters senescence during the long winter period (Hargeby et al., 1994).
Fig. 1. Comparison of species richness (a), number (b), and average weight (c) of fish captured at sites with varying densities of macrophytes. Macrophytes densities were: absent, 0% cover (n=7); sparse, 1–30% (n-30); medium, 31–70% (n=7); heavy, 70% (n=24). Error bars indicate one SE (Standard Error). (From Randall et al., 1996).
Fig. 2. Macroinvertebrate biomass (g dry mass m-2) in submersed vegetation consisting of Chara tomentosa and Potamogeton pectinatus, and in a habitat free from vegetation in Lake Krankesjon 1985–1987. Means of samples taken in February, May, July and September. (From Hargeby et al., 1994).
The predation by fish on in-benthos (i.e. tubificids and chironomid larvae) is also affected by the density of vegetation. In experiments in enclosures with various densities of floating-leaved macrophytes (nymphaeids) the abundance of chironomids in the upper sediment layer was negatively affected by perch at low and medium Nuphar densities (Kornijow and Moss, 1998). Densities of Tubificidae were similar in the surficial sediments in the presence and absence of perch, but in the deeper layers were about twice as high in the presence of fish than in the controls. At high vegetation density perch had no impact on chironomid larvae in the benthos. The authors concluded that the behavioural responses of chironomid larvae and tubificid worms to perch differ, and that a high density of floating-leaved macrophytes protects the chironomid prey from fish.
In addition to the structural benefits provided by aquatic macrophytes for fish, the shade that is created by plants also attracts and may benefit fish. Availability of aquatic plant species that provide appropriate shade may increase both foraging efficiency and predator avoidance by offering a microhabitat that improves detection of food items and predators, which ultimately increases growth and survival (Dibble and Killgore, 1994).
Substrate type and submersed vegetation are important factors in cichlid habitat preferences (Lowe-McConnell, 1987). The American cichlid species occupy a great variety of habitats but prefer muddy bottom areas with abundant aquatic vegetation (Resendez-Medina, 1981). In Lake Bacalar in the Yucatan Peninsula, Mexico, cichlids were found exactly in such a habitat, probably because of the abundance of prey and shelter (Gamboa-Perez and Schmitter-Soto, 1999).
While herbivores may directly feed on aquatic macrophytes, to other fish aquatic vegetation provides a high density invertebrate prey and periphyton (epiphytic algae) associated with the macrophytes. Fish have feedback effects on vegetation by their direct consumption of vegetation and indirectly via other trophic components or abiotic routes such as sediment-feeding induced turbidity. Fish may therefore also affect habitat structure (Persson and Crowder, 1998).
Artificial, macrophyte-like structures were found to impair the foraging of juvenile roach), but were no obstacle to feeding on cladocerans by young rudd and perch (Winfield, 1986). In a study in a hypereutrophic pond in England, Venugopal and Winfield (1993) found that juvenile cyprinids (roach, rudd) and perch were found both among dense, peripheral macrophytes, represented largely by water lilies (Nuphar lutea), and in the open water of the pond, although the cyprinids were apparently more abundant among the macrophytes. The higher abundance of cyprinids was confirmed by a significantly higher CPUE within the macrophytes, indicating that they were more abundant or more active in this highly structured habitat. Winfield (1986) found that a dense stand of artificial Nuphar sp. had no effect on the feeding rate of zooplanktivorous roach and actually increased the performance of rudd and perch relative to those in open water, possibly due to the reduced predation.
Perch feed effectively on cladocerans in weed beds (Winfield, 1986), and the fish also find a refuge among the plants. Some submersed aquatic plants, such as Potamogeton berchtoldii, provide some refuge for zooplankton against predation but this is not the case with the nymphaeid Numphar lutea (Timms and Moss, 1984). Further studies of Moss et al. (1998) have shown that predation of perch on Daphnia increases with less dense stands of the water lilies. Numbers of other small crustaceans and rotifers were mostly unaffected by fish predation. Moss et al. (1998) suggested that the refuge will operate even more effectively with cyprinids such as roach and bream who are not as effective hunters among plants structures as perch. Perch may also find alternative food among the plants, such as benthic/epiphytic invertebrates. If these animals are easier to catch than mobile ones in dense vegetation, then this may give an additional refuge mechanism for forms such as Daphnia.
A prolonged aquatic macrophyte senescence may have a negative impact through the removal of shelter and food base for fish. It can be compared to a management measure using mechanical control of aquatic vegetation. In the warm temperate coastal lake Swartvlei, South Africa, a littoral plant community dominated by Potamogeton pectinatus and two charophyte species underwent a marked seasonal regression in winter months. This resulted in a 60% decline in primary production, 74% slump in littoral invertebrate biomass, and a 54% decline in the abundance of two fish species associated with macrophytes, for which invertebrates and filamentous algae from the littoral zone were of major importance as food source. The disappearance of the macrophytes resulted in a decrease in the condition of Monodactylus falciformis and Rhabdosargus holubi (Whitfield, 1984). The virtual disappearance of aquatic macrophytes also had an impact on the physico-chemical features of the aquatic environment (Figs. 3a and 3b).
It is not the purpose of this publication to give a detailed account of all freshwater fish species which use aquatic macrophytes for attaching their eggs or making nests. Therefore, only some examples are given here. Obligatory and non-obligatory plant spawners (phytophils) stick adhesive egg envelopes to submersed live or dead plants. Among these are many cyprinids such as Labeo and small Barbus, characin and siluroid fish species, Puntius (=Barbodes) spp, Polypterus spp, Clarias batrachus (Welcomme, 1985). Gymnarchus niloticus make floating flask or raft shape masses of aquatic weeds in which they lay their eggs. The siluroids Callichthys callichthys and Hoplosternum littorale both make a raft constructed of bubbles and aquatic plants. Many cyprinids lay their adhesive eggs on floodplains on flooded terrestrial vegetation. In Africa, Heterotis niloticus builds nests in flooded plants at the margins of floodplains. Catfish Pangasius sutchi scatters its eggs among submersed weeds and bushes in the shallow inundated margins of the klongs or canals of Indochina.
Figs. 3a and 3b. The structure of the Potamogeton zone in Swartvlei during January 1975 (a) and 1980 (b), showing the biomass of the various components. Vertical profiles of temperature, dissolved oxygen and photosynthetically active radiation at noon are also shown. (From Whitfield, 1984).
The following European fish release their eggs among submersed macrophytes where they adhere to the plants: Tinca tinca and Scardinius erythrophthalmus spawn for example among Myriophyllum beds, carp Cyprinus carpio utilises a range of plants, including Carex, Glyceria, fresh shoots of Phragmites and Salix roots. Other species include: Blicca bjoerkna, Carassius carassius, C. auratus gibelio, Rutilus rubilio and R. arcasi. Some other fish stick their eggs to submersed plants but other substrata are utilized in their absence: Rutilus rutilus, Leuciscus idus, L. leuciscus, L. souffia, Alburnus alburnus, Rutilus frisii, R. lemmingi (Cowx and Welcomme, 1998). R. rutilus spawns on Fontinalis and Elodea beds, Scirpus, Salix roots, stones and submersed logs. Abramis brama may spawn on stones in lakes, in rivers it utilizes only areas with a weak current and macrophyte substrata, e.g. Rorippa, Botumus, Sagittaria, Glyceria and Nuphar. Pike (Esox lucius) spawns among aquatic plants, such as Equisetum, Phragmites, Carex and littoral Poaceae in Finland (Hakkari and Bagge, 1984).
Many North American fish are obligatory plant spawners: these include members of Amiidae (e.g. Amia), Esocidae (e.g. Esox), Cyprinidae (e.g. Cyprinus, Cyprinella, Notemigonus), Catostomidae (e.g. Ictiobus), Cyprinodontidae (e.g. Fundulus), Atherinidae (e.g. Labidesthes), Umbridae (e.g. Umbra), Centrarchidae (e.g. Elassoma), and Percidae (e.g. Perca, some Etheosoma) (Pflieger, 1975; Robinson and Buchanan, 1988). Most data on spawning success relative to structural complexity and plants come from studies of adult sport fish that construct nests (i.e. largemouth bass and bluegill). Although nest spawners successfully spawn in areas devoid of vegetation, they prefer sites with aquatic plants or some other type of structure nearby for refugia. Sunfish of the USA (Perciformes, e.g. Micropterus salmoides, M. dolomieu, and Lepomis gibbosus) form nesting sites among the macrophytes. Aquatic vegetation is used as nursery habitat for larvae by at least 12 families of the North American fish (Dibble et al., 1996). Some species exhibit ontogenetic shifts in habitat use. For example, prolarvae of yellow perch prefer shallow, dense macrophyte areas, while postlarvae prefer deep, low density macrophyte zones (Gregory and Powles, 1985).
Many phytophils and phytolithophils lay eggs on substrates just below the water substrate where they are vulnerable to sudden falls in water level. Removal of instream vegetation during the spawning period can have a dramatic effect on spawning success. Mechanical control of aquatic plants in streams can also directly remove the eggs of phytophilous fish species and food organisms for the fish (see also Section 11.3).
In Lake Ontario fish which lay eggs in submersed stands include northern pike (Esox lucius), perch (Perca flavescens), and largemouth bass (Micropterus salmoides) (Crowder and Painter, 1991). In five marshes near Toronto, Stephenson (1989) found over 20 fish species using the wetland during the breeding season.
The nesting of the American Perciformes may lead to changes in the aquatic macrophyte species set up. After clearing Myriophyllum for their nests and the nesting period, the nests of Lepomis gibosus, Micropterus salmoides and M. dolomieu have been gradually invaded by diaspore-propagated plant species, including Isoetes, which do not spread vegetatively. Every following spawning season about 20% of the original nest sites are not occupied, and the result is a mix of Myriophyllum and the other species of aquatic plants which occupy the abandoned nests. The nesting contributes to the formation of a mosaic of different aquatic plant species (Carpenter and McCreary, 1985).
In South America, Serrasalmus spilopleura spawn on floating vegetation including Eichhornia crassipes, for example in southeastern Brazil. Serrasalmus is an important food fish for riverine people in several places in central and northeastern Brazil (Sazima and Zamprogno, 1985).
