A study by Chick and McIvor (1994) on the abundance and fish species composition reconfirmed that beds of different macrophyte types comprise distinct microhabitats for juvenile and forage fish. From the perspective of these fish, the littoral zone of Lake Okeechobee, the largest lake in Florida, may be better described as a landscape of microhabitats with beds of different macrophyte types forming microhabitat patches comprising a mosaic. The fish abundance was related to macrophyte type, but not to vegetation standing crop or water depth. They suggested a number of mechanisms to be responsible for the fish distribution. For example, juvenile and forage fish feed on a variety of invertebrates, and the distribution of invertebrates are known to differ with macrophyte type (e.g. see Hargeby, 1990). Differences in plant surface area and vegetation decomposition could impact epiphytic communities and detrital food webs associated with these macrophytes (Paterson, 1993). Predation pressure probably also differs among macrophyte types because differences in vegetation density can greatly affect the ability of predators to capture prey (Savino and Stein, 1992). Macrophyte structural complexity can affect both the ability of predators to capture small fish and the ability of small fish to capture invertebrate prey. Vegetated habitats tend to be populated primarily by small forage fish and juveniles of larger species, which use these habitats as a refuge from predators and feeding grounds. Chick and McIvor (1994) suggested that a landscape view of littoral zones will prove useful for applied lake management, such as habitat manipulation and littoral zone restructuring, as well as for ecological investigations on fish population dynamics and lake processes.
In the littoral zone, aquatic macrophytes are thought to enhance the survival probability of prey fish by decreasing encounter rate or predation rate (Hosn and Downing, 1994). Moderately complex aquatic plants are important for young fish to hide and feed. Such plants will produce more stable prey-predator relations between fry and piscivorous fish (Savino and Stein, 1982). In Dutch water bodies, survival of young Esox lucius was higher in vegetated areas than in areas without aquatic macrophytes (Grimm, 1983). In a Florida lake in the presence of a coverage of 75% of Hydrilla verticillata the survival of the young Esox niger and some other American sport fish was higher than outside such areas (Shireman et al., 1983). Small bluegill (Lepomis macrochirus) have been shown to be restricted to dense vegetation in the presence of piscivorous fish (Werner et al., 1983). Submersed aquatic macrophytes provide cover for young largemouth bass, and bass (Micropterus salmoides and M. punctulatus) grow better in intermediate to high plant densities of Hydrilla and edge areas than in areas of uninterrupted high-density plants or areas without plants (Morrow et al., 1991). This is consistent with other studies that have shown aquatic plants to be important to the growth of fish. However, several studies have shown that the growth rate of prey fish is retarded when they are confined to the vegetation due to predation risk (Mittlebach, 1988; Persson, 1993; Persson and Eklov, 1995). The maximum foraging rate decreases when the fish shifts from the pelagic to the vegetation habitat (Persson and Crowder, 1998). The slower growth results from an increase in competition among the prey fish in the refuge.
The presence of vegetation will also affect the foraging mode of specific piscivorous predators. For example, perch and largemouth bass change from an active pursuit foraging mode to an ambush sit-and-wait foraging mode with an increase in vegetation density (Savino and Stein, 1982, Eklov and Diehl, 1994).
In many lakes throughout the north-central United States, the centrarchid bluegill sunfish make up most of total fish biomass (Mittelbach and Ossenberg, 1993). This species hatches in the littoral and moves to open water for a few weeks before moving back to the sheltered vegetation habitat. As an adult, it feeds on zooplankton in open water, and the body size at the shift to this habitat depends on predation risk from largemouth bass. In the vegetation, the young bluegill exerts strong competitive effect on other refuging littoral fish, including the young largemouth bass. As bluegill outnumber the other species, the effect of juvenile bluegill on young of the year bass is larger than the reverse (Olson et al., 1995).
Findings of Hosn and Downing (1994), who studied the influence of cover on the spatial distribution of littoral zone fish in Lac de l'Achigan, Quebec, Canada, which has an average macrophyte biomass, confirmed the hypothesis that prey are less aggregated at sites where predation pressure is low. At low densities, fish in open habitats above the bottom are consistently aggregated. At these low densities, fish in macrophytes above the bottom are nearly randomly distributed. If aggregation is a predator-avoidance response, the authors concluded that fish off the bottom may decrease aggregation when they are found within macrophyte beds. Such flexibility in fish behaviour is important when predation risk is variable. Fish are known to constantly reassess their environment and adjust their behaviour accordingly (Magurran, 1990).
In Myriophyllum beds of Lake Guntersville (Alabama, USA), the fish assemblage was dominated by Lepomis. The difference in the density of plant did not seem to affect the density of the fish. The canopy of Myriophyllum allowed the fish easy access to the underlying water column for feeding while maintaining a relatively high degree of protection from predators (Killgore et al., 1988). Some predators such as pike, are adapted to catching their prey among vegetation, and other predators, such as largemouth bass (Micropterus salmoides), an important sport fish in water bodies of southern USA, may change their predatory tactics from searching to ambushing as plant density increases (Savino and Stein, 1989). Bluegill predation rates decline with increasing plant density (Savino et al., 1992). Young brook trout (Salvelinus fontinalis) was found to stay motionless under the cover of vegetation or rocks on the bottom to lessen the risk of predation. By staying near the bottom, fish can also lessen attacks from wading predators.
Most small impoundments used for fisheries purposes in the southeastern USA are stocked with combination of largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus). Predator-prey relationships between these two species are manipulated by managers to establish a state of balance. Eradication of aquatic macrophytes is a recommended component of fishery management programmes. Where the eradication is either unsuccessful or intentionally neglected and aquatic macrophytes are present, predation on bluegill by largemouth bass declines as cover becomes increasingly complex.
Bluegill is commonly associated with aquatic vegetation in shallow water near shore, but the larger individuals are located in deeper open water. Bluegill is an opportunist, feeding from the surface, off the bottom, or from within the water column. Because bluegills can be easily caught with minimal equipment by inexperienced recreational fishermen, their popularity is widespread and the species has been widely introduced into the western USA, Canada, Europe, South Africa, Japan and Puerto Rico (Martin, 1981). Duffy and Jackson (1994) compared relative abundance, size distribution and condition of small bluegill among stands of three aquatic macrophytes common to lentic environments in the southeastern United States: water lily (Nymphaea odorata), water shield (Braseria schreberi), and pondweed (Potamogeton nodosus). Nymphaea and Braseria are characterised by horizontal, floating leaves, vertical stem, and relatively large interstices between stems, while Potamogeton is characterised by dense underwater leaf/stem complexes and relatively small interstices. Nymphaea and Braseria did not differ with respect to CPUE for bluegill number nor weight. Significantly longer and heavier bluegill were captured from Potamogeton, but fish condition was the same for all the plants. Selective management for plants such as Potamogeton, which have dense, underwater leaf/stem complexes and relatively small interstices, may reduce the predation by largemouth bass and assist in the maintenance of viable bluegill stocks. Apparently, Potamogeton nodosus decreased capture success of larger, more mobile juvenile fish, with the predators switching to alternative invertebrate prey that was less mobile and easier to capture (Dibble and Harrel, 1995). Enjoying such plant protection bluegill may escape predation in greater numbers and at greater sizes. Also, zooplankton abundance and biomass associated with pondweed were greater than those associated with water lily and water shield, thus enhancing the food supply for bluegill.
For predation on zooplankton see also Section 6.4.
For Lake Krankesjon in southern Sweden, Hargeby et al. (1994) summarized the food web structure before and after the shift from phytoplankton to submersed macrophytes (Fig. 25). It shows the favourable effect of macrophytes on the piscivorous pike and large perch, as reported by a commercial fisherman. The favourable effect of macrophytes on the recruitment of piscivorous perch is shown by the increasing size of perch during the plant expansion in the lake. It is probably a result of habitat changes that favour perch before competing cyprinids (Diehl, 1988) and allows a larger portion of perch to shift from feeding on macroinvertebrates to fish. Once a fish assemblage with piscivorous perch and pike is established, this favours the maintenance of abundant submersed macrophytes as the piscivores keep in check the zooplankton-feeding and bottom-feeding cyprinids (Scheffer, 1990).
Fig. 25. Trophic food web before (1975–1984) and after (1988–1991) the shift from phytoplankton to submersed macrophytes in Lake Krankesjon. Boxes: biomass; arrows: energy flow. (From Hargeby et al., 1994).
Jacobsen and Perrow (1998) showed the use of aquatic macrophytes as refuge for roach in a shallow Danish lake: some roach (13%) migrated out from among macrophytes in the absence of predators (pike and large perch) during the day, but during the night roach was out in force (90% of the individuals). When pike was present, roach selected the open water (90–92%) during daylight hours, but kept a 1 m distance from the macrophyte edge. The presence of pike thus reduced the use of macrophytes by roach, which in turn may improve macrophytes and the edge area as a refuge for zooplankton. Adult perch, a generally less effective predator than pike, were concealed within the macrophytes most of the time. Jacobsen et al. (1995) pointed out that the potential indirect effect of piscivores reducing the predation pressure upon grazing zooplankton through behavioural changes of zooplanktivores may be an important mechanism in enhancing the stability of submersed macrophytes in shallow lakes.
For roach, rudd and predation see also Section 3.2: Scardinius erythrophthalmus and Rutilus rutilus.
In a study of the influence of macrophytes on predation in Lake Onalasca, which is a backwater lake of the Mississippi River, age-0 largemouth bass predation on bluegills (Lepomis macrochirus) appears to influence the use of vegetated habitat by bluegills. In summer, when most age-0 bluegills were vulnerable to predation by age-0 largemouth bass, bluegill abundance was strongly correlated with vegetation biomass (Dewey et al., 1997).
Sondergaard et al. (1997), based on the results of their experiments on a eutrophic Lake Lyng, Denmark, suggested that pike stocking can be used as a lake restoration tool to increase lake water transparency by creating a trophic cascade. The effect of stocking, however, seems to last only during the season in which it has been undertaken, the impact being most significant at high stocking densities. The method can be used in shallow, turbid lakes in which the nutrient loading has been sufficiently reduced to allow a substantial and permanent macrophyte coverage if clearwater conditions are established.
Diehl and Kornijow (1998), who studied the influence of submersed macroinvertebrates on trophic interactions among fish and macroinvertebrates, pointed out that many macroinvertebrate-feeding fish are vulnerable to piscivores and use littoral vegetation as a predation refuge. Consequently, the presence of submersed macrophytes in lakes create the opportunity for different classes of macroinvertebrate-feeding fish to segregate by habitat. In the presence of piscivores, small vulnerable classes tend to seek refuge in vegetation. The effects of submersed vegetation on pattern of habitat use and individual growth of fish are likely to feed back on the population dynamics of the fish. For example, vulnerable stages of bluegill sunfish in lakes of north America attain high individual growth rates in the open water when vulnerable stages are restricted to the vegetation by piscivorous largemouth bass. The authors also noted that in temperate European lakes the abundance of piscivores is correlated with the abundance of submersed macrophytes.