HOW DOES PREDATION FROM FISH INFLUENCE SPECIES AND PHENOTYPE COMPOSITION AMONG BENTHIC INVERTEBRATE PREY IN THE LITTORAL ZONE OF LAKES. CHAPTER ONE INTRODUCTION Predation is attributed as a major selective force characterizing the structure of the benthic invertebrate communities (Wellborn et, al. 1996). It has also been shown that habitat differences and its gradients may influence such predation and the community structure. The community structures of freshwater shallow lakes include species of fish, numerous invertebrates, amphibians, some reptiles and birds. Some invertebrates are known to be associated with some types of the submerged vegetation and had evolved some phenotypes expressions in terms of their sizes and pigmentation. Within the complex domain of the aquatic plants or macrophytes, the predatory activities of the fish on invertebrates is perceived to be limited (Gilinsky, 1984; Diehl 1992; Beckett et al., 1992); thus providing a suitable niche for the invertebrates. Also outside the area covered by macrophytes (in the open water), the predatory fish will be more effective on the invertebrates’ prey, so a different pattern of community structure may be expected (Tolonen et al., 2003). Fish is considered a top predator in most permanent aquatic systems. They exhibit high preference for large invertebrates ‘prey that swims actively; while invertebrates’ predator may have preference for less active, small size preys (Wellborn et al., 1996). To this ends, environment with high fish predation will in turn has greater abundances of small size macro invertebrates, while on the other hand when invertebrate predation is dominant, the small size invertebrates populations will be reduced, while there will be growth in the size of the invertebrate predator populations (Nilson 1981; Crowder and Cooper 1982; Bechara et al., 1993; Blumenshine et al., 2000). Lampert and Sommer 1997 have indicated that small size macro invertebrates are more vulnerable because of the fact that the predatory macro invertebrates cannot effectively handle large size prey. In coastal wetlands, large size macro invertebrates were shown to be dominant towards the shores, where fish predation is considered less significant (Cardinale et al., 1998). Many studies have been conducted in looking at the combine influences of fish and invertebrates predator on the community structure of benthic macro invertebrates. Many of such investigations usually happen in flowing water systems. Shallow lakes or ponds provide suitable spots to investigate more closely how the two predation pressure can affect the community structure of the aquatic macro invertebrates. A study of this nature is useful in evaluating the impacts of human activities and eutrophication on the biodiversity and community structure of an aquatic system in a basin. Lake Eutrophication has strong impacts on fish status and the alternative stable states of the lake systems. Human activities such as agriculture, aquaculture, fisheries, livestock grazing, deforestation ,among others , have strong implications for the habitat quality .Invertebrates benthic community in a lake system is highly regarded when defining the status of the lake, in trying measure the impact of human activities ( Water Directive 2000/60/EC ; Solimini et al., 2006). Thus a research of this nature will be useful in developing skills and knowledge in environment monitoring and evaluation RESEARCH OBJECTIVES 1. To identify the fish species and the macro invertebrates present in the area studied. 2. To find out the predator-prey relationship between the fish and the invertebrates, by means of stomach analysis. 3. To find out the differences between macro invertebrates species compositions and phenotype frequencies with respect to body size and pigmentation in the area with submerged vegetation and area with emergent vegetation. 4. To relate the findings to the optimal foraging model of a predator- prey. HYPOTHESIS Ho There is no significant differences in macro invertebrates’ species compositions and their frequencies in the two different habitats. Ho There is no differences in the handling time for gammarus and other invertebrates of same size. DESCRIPTION OF STUDY SITE Lake Takern is located in south of Sweden in Ostergotland provincial district constituting Linkoping ,Norrkoping , Mjolby, Odeshog Motala and Vadstena as the major urban settlement, close to the lake. It attracts numerous birds, thus very famous as bird lakes. It spans across area of 12km long and 8km wide, with depth averaging 0.8metres. The lake and its contiguous area are nature reserve since 1975 and it is part of Ramsar Convention list of protected nature. The site remains an enviable resource for nature conservation, tourism and research. The nature reserve is managed by both private ownership and State and about 5400 hectares is the size of the reserve area. 270 birds’ species had been found feeding and nesting in the area by birds’ watchers and other visitors. Agriculture, livestock graze land and forest tree productions are the common land use in the basin. The lake habitat is structured by the type of vegetation present. At the reeds, there are aquatic grasses dominating the landscape and with huge deposit of organic matter. Further, there are clear water areas with sand and mud bottom, also area with submerged vegetation distinctly, Stonewort (Chara spp) and water milfoil (Myriophyllium spicatum). The Chara dominated area has also clear water, as the Chara is seen some few centimeters under the water, while the myriophyllium are seen up to the water surface. This lake as noted earlier has got a strong reputation as a bird’s lake; it provides a breeding ground for numerous visiting and residents birds in the north Europe. Ecological studies of this lake have concentrated mostly on birds’ inventory and lately some interest had been rekindled in the area of understanding the limnoligical past and future of the lake. Not much has been known on the interaction between fish and invertebrates in the two contrasting habitats defined by the presences of Chara and myriophyllium submerged vegetation. It was the interest of this study to understand the predator prey relationship in the lake Google earth 2009 Figure 1. Map of Lake Tarkern CHAPTER TWO REVIEW OF LITERATURE THE SHALLOW LAKE ECOSYSTEM Shallow lakes refer to a body of permanent or semi-permanent water, which is sufficiently shallow , less than 5 meters in depth ,that the light penetration extends to the lake bottom and making primary production possible for the flora community (Wetzel, 2001). Valley et al. 2004 further classify shallow lakes based on the surface area it covers and the alkalinity: small area <200hectares, large area >200 hectares and alkaline >100ppm mg/L CaCO3, Not Alkaline <100ppm mg/L CaCO3. Shallow lakes ecosystem is very productive, there are numerous aquatic plants due to high nutrient availability and sunlight in the shallow lake. Emergent and floating plants are common features, examples include: Cattails Typha spp, Bulrush, water lily Nymphaea spp and reeds. There are also the submerged vegetation like the myriophyllium spp, the Chara spp, and coontail. The vegetation provide habitat for the zoo plankton, macro invertebrates, fish, birds and other wildlife (Conroy, 2005). There is no vertical temperature stratification in shallow lakes. The water is consistently mixing and the current is determined by the prevailing wind velocity. There is a high activity in shallow lakes during summer period and in winter, the activity is low, and most benthic organisms go into inactivity. Fish population in particular is reduced by occasional fish kill due to low dissolved oxygen. The energy relationship in a shallow lake stems from simple periphyton and phytoplankton, with sunlight energy for photosynthesis. There are zooplankton and numerous invertebrates’ at different stages of their life cycle. There are adult flying insects that intermittently visit the water. Fish have different feeding behavior; some are planktivore, some benthivores and piscivores. Fish evolve different feeding adaptation at different stage of development. A juvenile piscivore like perch Perca fluviatilis L might feed on zooplankton and subsequently improve on its range of prey choice , to include macro invertebrates of various kinds and in adult stage, its choice will include cyprinids. Gammarus are intensely preyed upon by fish in most freshwater system; this might be due to its food value and its vulnerable body nature, its size and its activity (Wooster 1998, Mac Neil et al. 1999). Large size Gammarus are often more vulnerable to fish predation as compare to smaller ones, same applicable to other common prey. However, it is important to note that size of the predator determines its capacity to handle its prey and thus a presenting a very strong factor on the choice of prey and its size (Allan 1983, Allan and Malmqvist 1989, Andersen et al. 1993). Gammarus spp is associated with detritus from leaf packs in Lake Bottom and also found closely associated with periphyton in macrophytes rocks and also found to forage directly on dead macrophytes (Newman 1991, Kornijow et al. 1995). Fish predation on invertebrates: theories and applications The interactions among organisms can be intra and inter specific. Intra specific is when the interaction in within the same species either to enhance the species group efficiency in the community or to promote individual dominance with a population towards resource access. Inter specific interaction is one that demonstrates organisms of a population having a feeding interaction with other different population; the later, describes the case of fish predation on macro invertebrates. To understand the phenomenon of predation, some theories had been advanced by researchers to describe the processes that modulate predator and prey relations. One of such theories is the optimal foraging concept (Emlen 1966; MacArthur and Pianka, 1966; Charnov, 1976; Mittelbach 1981). It’s proposed that, in selecting prey size, predators weigh the cost of handling a prey, to the benefits derivable in terms of energy gain from the consumption of the prey; a predator will select prey size that will generate maximal energy uptake per unit of handling time (Smallegange et al. 2008). The adequacy of this theory was demonstrated in many studies involving molluscivores predator for example crabs, selection of prey size that will maximize its energy gain (Elner and Hughes, 1978; Hughes and Seed, 1995; Mistri, 2004). However, these foragers on mollusc also express a flexible approach to prey selection, by feeding on sizes that are less energy giving in order to reduce the cost or risk incurred during the handling, or as a response to hunger, presence/absence of a competitor; this described another theory which is called contingency theory (Hughes 1988; Visser 1995; Johnstone and Norris, 2000; Rutten et al. 2006). Optimal foraging theory assumes an important tool in studying feeding ecology because of its quantitative appeal in the prediction of predator – prey relationship (Persson and Greenberg, 1990; Jackson and Rundle 2008). Essentially, the optimal foraging model is to maximize E/t; where E, is the net energy gain from the consumption of the predator, and t, the time taken for handling and searching its prey; within the consideration, the cost also include the risk incurred in the handling of the prey; so there is the question of the prey profitability, when different preys are available to the predator at the same time.. Thus Jackson et al.2004; Jackson and Rundle 2008, adapted the net energy optimal foraging model to describe the diet shifts of goby (Pomatoschistus microps), when variety of prey were available. The mathematical expressions of the predator- prey relationship were as follows: Where E (J), t (s), λi rate of predation i (items s−1), A is assimilable part of energy, ei is energetic content of prey i (J), Ch is handling cost (J s−1), Hi is handling time for preyi (s) and Cs, the cost of searching (J s−1). A, assumed constant at 0·7, according to Elliott 1976 The results of the study demonstrated among others that an increase in prey size for goby shows an increase in the handling time and also the predator size relate directly with the size of the prey they consume (figure 1&2. Jackson and Rundle 2008). Figure 2 Jackson and Rundle 2008. The leading role of fish, in the structuring of the ecosystem in lakes and other aquatic ecosystem, is very important in most aquatic ecosystem investigations. Much effort is focus on this crucial role of fish as a leading predator as against other predators. Moss (1976) observed an emergence increase in biomass of some kinds of macrophytes and epiphytes, in artificially fertilized pond, which was stocked with bluegill sunfish. The reason was that the fish population may have preyed on the invertebrates, which include the community of amphipods and mayflies. Yellow Perch had been shown to have had a very strong impact on the abundance of amphipods and chironomids and indeed leading to an increase in the concentration of periphyton and particulate phosphorus; in the absence of the fish population, the invertebrate were in abundance and thus decrease the periphyton activities (Mazumdar et al. 1989). The feeding behavior of fish is also modified by the factor of stage of its development, the size of its prey and also its natural feeding niche. Thus there is the tendency for a fish to evolve its feeding from a simple phytoplankton feeder, to zooplankton, larger invertebrates and becoming piscivores (feeding in other fishes) at some point in time during its growth. Schilling et al. 2009, investigated the phenomenon of fish absence in a lake as it affects the macro invertebrates community structure , abundance , taxonomic constitutions and the species richness. Their findings were that fishless and fish containing lakes have many differences in the macro invertebrates’ structures and functions, thus fish absence or presence pose a very strong factor that regulates the community structure and functions. The introductions of fish in some lakes also have the tendency of declining of native species of invertebrates and amphibians, due to fish predation. (Bradford et al.1993; Denoel et, al. 2005). Schilling et al. 2009 identified the abundance of six taxa that can serve as bio indicator of fish absence in a lake, instead of undertaking a more rigorous task of fish sampling to satisfy same purpose. In contrast, Wissinger et al.2006, proposed a modest effect of introduced fish on invertebrates community, due to the presence of submerged vegetation, the fish characteristics and the attributes of the prey. In fact, most invertebrates were considered as habitat generalist as no local extinction was possible due to fish predation. However, the threat of introduction of a top predator to the native biodiversity cannot be disputed, due to evidence of decline in major native prey species and cascading effects on ecosystem interactions and compositions following such practice. (Townsend and Crowl , 1991; Larson et al. 1992; Flecker and Townsend ,1994 ; Liss et al. 1995; Mack et al. 2000 Schindler and Leavitt, 2001; Nyström et al. 2003; Simon and Townsend ,2003; Greig and McIntosh 2006; Dunham et al. 2006) CHAPTER THREE MATERIALS AND METHODS Different approaches had been used by researchers in a bide to understand the complex nature of interaction in an ecoligical community. Some seems to narrow investigation to a particular population ,while others may prefer a mix of population . In shallow lakes , the use of cages or similar barriers to partition experimental block or units is well accepted( Christer Bronmark 1994). However, such block or partitioning may occur in nature as exemplified by the vegetative landscape of the studied area. Also, common is the use of core samplying device to get the invertebrate community in the mud bottom in the studied area. There is also the possibility of making experimental set up with plastic bowl or aquaria and having different substrates bottom and introducing the predator and the prey. I adapted my predator prey experiment in a laboratory set up using plastic bowls and aquaria . Samplings were done in the field . Invertebrates were collected in the field . Fish were collected from the local fisherman . The approach to the study was a combination of both field sampling and laboratory investigation of a predator fish and its prey. At Lake Takern, the benthic invertebrates were sample with a sweep net and a sieve at the reeds and also in the area with submerged vegetation . The samples were taken in plastics bucket, with a lid. In the laboratory the benthic samples were sorted in a flat bowl containing clean water and the invertebrates were picked into glass cylindrical tubes and preserved in ethanol. The invertebrates were then identified into their various families with the aid of a microscope and an invertebrate manual. Measurement were made in (mm) with the aid of square papper mark placed under a petri disc and observed through the microscope. Fish sampling was conducted in Lake Takern using gill net. The gill nets were deployed overnight, both in the reeds and in the area with submerged macrophytes. The fish were removed from the gill net, sorted, and the standard length and weight were measured. Samples of the fish tissues were taken and subsamples of the fish were taken for gut content examination. Pair of surgical blade and scissors were used for this purpose; the gut content were in turn introduced to petri disc and then view with the microscope to identify invertebrates parts remains, in the gut.. Fish sampling were done at both northern and southern landscape of the lake. To investigate the benthic invertebrates structures and compostion is very vital in understanding the food chain and the intractions that is possible within an ecosystem. Thus samples of the benthic invertebrates were taken at two different contrasting habitats in the lake landscape. It was also imperative to know the type of fish composition in the lake and also the frequency of their distribution in the two habitat type. Thus fish sampling was carried out, inorder to know what perhalps is the predator and the herbivores fish species in the lake environment. EXPERIMENTAL DESIGN In the laboratory, perch predations on Gammarus spp and Corixidae (water boatman) were investigated using 100mm- 105mm size of perch. Two size groups each of Gammarus spp and Corixidae were fed to the fish at different feeding occasions in an aquarium and two plastic fish holding bowls fitted with aerators and filters. The aquarium was partioned with vertical slide glass attached to a string which manually released to enhance drop down to partioned fish and prey or pulled to removed the barrier ,thus creating fish and prey access. There was also a paper screen to minimize human interaction on the fish activity in the aquarium. The searching time and handling time were estimated in each feeding time using a stop watch and the number of prey taken by the fish per time were measure (the attack rate). Aselus was equally used as the third prey organism and the handling time were observed . In all, there were three different prey organisms and only one predator fish species replicates each. Ten individual prey were introduced to the aquarium stocked with Perch in a group of three individuals at a time and one individual was observed and the handling time taken. The proceedure was repeated 7 times in a day using different fish individual, and for three days, each day taken as a replicate. 21 individual fish were used the average handling time was taken. Fish that were observed to be less active were replaced by the active individuals that were on a standby. The fish were stocked three in an aquarium because of their known social behaviour which promote activities In the experiment with Corixidae and Gammaru ,handling time was estimate for the prey size category and three different fish individual was represented across the size category of the prey and replicate were made for three days. DATA AND DATA ANALYSIS Primary data were obtained both from the field sampling and exprimental set up. Data from the field include fish lenght and weight relationship, the benthic invertebrates distribution, fish gut content , among others. In the experimental set up, data on Perch handling time for three different invertebrate category were obtained. Some of the data were analysed using Excel. The mean, standard deviation and the covariance of the samples were calculated. 30 25 20 Hta 15 Htb 10 Htc 5 0 1 2 3 Figure 4. Perch predation on Corixidae, ( hta. 6-7mm, htb.8-9mm, htc 10mm) for days 1, 2 and 3, and the y axis shows the mean of the handling time. 30 25 20 hta 15 htb htc 10 5 0 1 2 3 Figure 5. Perch predation on Gammarus pulex, ( hta 6- 7mm, htb. 8-9mm, htc 10-11mm) for days 1, 2 and 3. 9 8 7 6 5 4 3 2 1 0 1 2 3 Figure 6 .Perch predation on Aselus of size 6-7mm. The pattern found in the Perch predation on Aselus shows a tendency towards an increasing learning and experience of the fish; from day one to the third day, see a reduction in the prey handling time of the fish. Day 1 and 2 were above 6 seconds per prey while in day 3 it was less than 6 seconds. CHAPTER FOUR RESULTS AND DISCUSSION 4.1. FISH SAMPLES The samples of the fish taken from the gill net were mainly Roach (Rutilus rutilus), Perch ( Perca fluviatilis), Tench (Tinca tinca), and Rudd ( Scardinius erythrophthalmus) The sampled populations are given in the following summary table and with a chart. Table 1 category of fish caught by gill net. roach perch Tench Rudd chara 83 5 2 6 reeds 273 56 0 0 3 individuals Perch were collected from the reed, the rest of the Perch were got from the Chara habitat and their sizes ranges between 55mm and 180mm. Also in the Chara habitat, 2 Tench of 400mm in lenght and 6 Tudd of between 160mm to 240mm were found (figure 7 below 1 0.9 0.8 0.7 0.6 0.5 chara 0.4 reeds 0.3 0.2 0.1 0 roach perch Tench Rudd Figure 7. The proportion of fish sampled with gill net at Lake Tarkern 4.2. THE BENTHIC INVERTEBRATES SAMPLES AND THE FISH GUT CONTENT Benthic invertebrates’ samples were taken at random sweep using a hand net, in Lake Tarkern. The about 16 families of the invertebrates were identified from the samples, which include. The details are as follows, in figure 8. Liplus sp Physa sp Ixodidae Planorbis Lymnea stagnalis Bithynia Lymnea poregra Limnephilidae Hirudinea Zygoptera larvae Polycentropodidae Chironomidae Corixidae Asellus aquaticus reeds chara 0 0.1 0.2 0.3 0.4 Figure 8. The benthic invertebrate sampled in Lake Tarkern. There was more invertebrates representation in the Reeds than in the Chara . The reason could be as result of increasing complexity of the reeds vegetation of the lake, than the chara dominated area. Also, in the fish gut, four different invertebrates were found to be eaten by the perch, and were at some stage of digestion, thus the body parts such as the head, abdominal segment, wings and limbs of the invertebrate were examined and counted. 120 100 80 60 gut expected 40 20 0 Asellus Corixidae Chironomidae Zygoptera larvae Figure 9. Bar chart showing the invertebrates found in the gut of Perch, the fish caught by the gill net. The expected numbers were estimated using Chi square statistics. A Chi square statistics was used to test the significant of the samples found in the fish gut The critical value of the chi square was 7.815, at degree of freedom 3 that is (n-1), n , number of different category and at 0.05 error level, thus the results indicate a significant difference between these distributions( figure 9 ). 4.3. FISH PREDATION EXPERIMENT The results of the predation experiment using Perch of 100mm in size and invertebrates prey of (6- 8mm, hta ) size were compared for the same size of prey across the three prey category and are shown in the following figure. 9 8 7 6 5 asellus 4 Corixidae 3 Gammarus 2 1 0 1 2 3 Figure 10. The Mean and SD of perch handling time for Asellus, Corixidae and Gammarus. hta Days 1 Mean Asellus 6.38 Corixidae 6.574 Gammarus 6.068 2 3 1 SD 2 3 6.513 5.945 6.25 5.541 7.443 6.9 1.637 1.469 1.579 0.488 0.575 0.154 0.398 0.556 1.35 Perch predation on Asellus aquaticus showed a pattern of reduced handling time from the mean of 6.4, and 6.5 seconds for day 1 and 2 to 5.5 seconds in day 3. For Corixidae it was 6.5 seconds, then 6 and 7.4 seconds the third day demonstrating increasing constraint to predation. Gammarus was from 6 seconds, 6.2 and 7 seconds the third day, also, demonstrating an increasing constraint to predation. Differences in pattern of handling can be attributed to factors such as the willingness of the predator itself to respond to the prey availability, familiarity of the predator with the prey, and also the predator response to the observation environment. Part of the procedure was to get the predator use to the prey items a day prior to the experiment as they were being fed on the prey items. The water quality was equally monitored to minimize water quality stress factor. Prior to the experiment with the Asellus aquaticus, there was water quality problem and the fish were off feed, the water was then replaced and leftover food items in the water were removed, this could have posed a potential biochemical oxygen demand. DISCUSSION SAMPLING TYPES AND WHY? In the investigation of the research questions sampling were carried out in the field and in the experimental set up. Fish sampling was done in order to have an insight of the type of fish present in the lake that could be prominent a benthic invertebrate predator. The gut content of the fish that were caught was also examined and data of the gut gives an enhancing indication of the variety of prey organisms available and eaten by the fish. In the fish sampling the Roach were more in number but their gut content analysis revealed so little of their prey organism, but as for the Perch it was possible to identify different prey organism, both in parts or appendages and in whole. Thus using Perch as the benthic predator in the predator and prey experiment was an adequate choice. Also, hand net samples of benthic invertebrates samples were taken at both reeds and chara macrophytes vegetation area of the lake. However, there were no differences in the benthic invertebrate’s samples distribution; there was the general habitat complexity in the two areas. Evidently, preyed organisms found in the fish gut relates to the hand net samples of the benthic invertebrates. This procedure is of many ways to understand the interactions between organisms in a community. FISH PREDATION ON INVERTEBRATES The invertebrates’ distribution in an aquatic environment is a noted factor that affects the fish carrying capacity (Wetzel 2001). The fish depends on a wide variety of invertebrates’ prey existing at different stages of life cycle. Fish has been described as the dominate habitat regulator, a top predator in an aquatic system. (Vanni 1986, Hansen and Jeppesen, 1992, Scheffer 1998). The presence or absence of some species of invertebrates in a lake is used as an indicator of fish or fishless lakes in some recent work. In some habitat regulation or management, it is not uncommon to have fish introduction to water that was in hitherto fishless or with less fish, either to satisfy habitat need or a sustainable resources. It is vital to study the fish predation and the interactions with other members of the community. In the fish predation experiment Perch predation on Asellus aquaticus, Gammarus pulex, and Corixidae were observed. There was evidence of the Perch catching, chewing and swallowing the prey organism. It was equally important to observe the role of social group behavior of the predator. A single predator in an aquarium is but redundant and uninterested, that is different when they are in threes. Handling time was used as means of estimating further the biomass intake of the prey item. Handling time defines the time taken between one action of attacking the prey and the commencement of another attack. To be effective, some handling time may be longer due to an incorporating search time in between actions. Handling time may also be due to the prey size or familiarity to the predator. The nature of the body of the prey may also affect the handling time. Prey with soft body are likely to be taken easily that those with rough body appendages. These and other factors may simultaneously affect the fish handling time of the invertebrates. In the experiment a progressive time reduction of handling time for Asellus aquaticus was observed, this could have been as a result of learning and experience with the prey item. However, for Gammarus pulex and Corixidae such progress was not prominent. . RESULTS AND THE RESEARCH QUESTION The research questions constructed from the hypotheses’ include: Are there any significant difference in the macro invertebrate species composition and their frequency in the two habitats? Secondly, are there any differences in the handling time for Gammarus pulex and other invertebrates of same size? Statistical evaluation of these two questions from figure 8 and 10 respectively. Using one way ANOVA. SUMMARY AND CONCLUSION LIST OF REFERENCES Allan, J. D and Malmqvist, B. (1989). Diel activities of Gammarus pulex (crustacean ) in a south Swedish stream , a comparison of drift catches and baited traps. Hdrobiologia 179 (1), 73- 80. Allan, J. D 1982. The effects of reduction in trout density on the invertebrate community of a mountain stream. Ecology 63 ,1444-1455 Andersen , T.H, Friberg, N., Hansen, H, O., Iversen , T , M., Jacobsen , D. and Krojgaard, L. (1993) The effects of introduced brown trout ( salmo trutta L ) on Gammarus pulex L. different density in two fishless Danish streams. Archiv .fur Hydrobiologie 126(3), 361- 371. Bechara , J. A., Moreau,G and Hare ,L. (1993). The impact of brook trout (Salvelinus fontinalis) on an experimental stream benthic community, the role of spatial and size refugia.Journal of Animal Ecology 62, 451- 464. Beckett, D. C., T. P. Aartila and A. C. Miller. 1992. Invertebrate abundance on Potamogeton nodosus: effects of plant surface area and condition. Can. J. Zool. 70:300-306. Blumenshine, S. C., D. M. Lodge, and J. R. Hodgson. 2000. Gradient of fish predation alters body size distributions of lake benthos. Ecology 81:374–386. Bradford D. F., Tabatabai F. and Graber D.M. (1993). Isolation of remaining populations of the native frog, Rana muscosa, by introduced fishes in Sequoia and Kings Canyon National Parks, California. Conservation Biol., 7, 882-888. BRONMARK, C. 1994. Effects of tench and perch on interactions in a freshwater, benthic food chain. Ecology 75:1818-1828. Cardinale B. J, V.J. Brady and T.M. Burton (1998) Changes in the abundance and diversity of coastal wetland fauna from the open water/macrophyte edge towards shore. Wetlands Ecology and Management Volume 6, Number 1, 59-68. Charnov, E. L., 1976. Optimal foraging, attack strategy of mantid. Am . Nat. 110, 141151. Convey. P.1988. Competition for perches between larval damselflies, the influence of perch use on feeding efficiency, growth rate and predator avoidance. Freshwater Biology 19, 15-28 Crowder , L. B. and W. E . Cooper. 1982. Habitat structural complexity and the interaction between blue gills and their prey. Ecology 65. 894- 908. Denoel, M., Dzukic, G. & Kalezic, M.L. (2005). Effects of widespread fish introductions on paedomorphic newts in Europe. Conserv. Biol., 19, 162–170. Diehl, S. 1992. Fish predation and benthic community structure, the role of omnivory and habitat complexity. Ecology, 5, 1646-1661. Elliott, J.M., 1976. The energetics of feeding, metabolism and growth of brown trout (Salmo trutta L) in relation to body weight, water temperature and ration size. J .Anim. Ecol. 45, 923-948 Elner, R. W., Hughes, R. N., 1978. Energy maximization in the diet of the shore crab, carcinus maenas . Journal of Animal Ecology. 47, 103-116. Emlen, L. M., 1966. The role of time and energy in food preference. Am. Nat. 100, 611617. Flecker, A. S and Townsend , C.R( 1994) Community with consequences of trout introduction in New Zealand Streams. Ecol . Appl. 4, 798-807. Gilinsky, E. 1984. The role of fish predation and spatial heterogeneity in determining benthic community structure. Ecology, 65, 455-468. Greig, H.S. and Mclntosh, A. R ( 2006) Indirect effects of predatory trout on organic matter processing in detritus based streams food webs. Oikos , 112, 31-40. Hansen, A. M. & E. Jeppesen, 1992. Changes in the abundance and composition of cyclopoid copepods following fish manipulation in eutrophic lake Væng, Denmark. Freshwat. Biol. 28: 183–193. Hughes, R.N., 1988. Optimal foraging in the intertidal environment, evidence and constraints . In Chelazzi, G., Vannini, M. (Eds). Behavioral adaptation to the intertidal life. NATO ASI Ser. A . Vol 151. Plenum press , New York, pp 265-282. Hughes, R. N., Seed, R., 1995. Behavioral mechanism of prey selection in crabs. J . Exp. Mar. Biol. Ecol. 193, 225- 238. Jackson A.C. & Rundle S.D. (2008). Diet shifts by an estuarine goby (Potamoschistus microps) in the face of variable prey availability.Journal of Experimental Marine Biology and Ecology 361: 1-7. Jackson A.C., Rundle S.D.,Attrill, M.J., Cotton, P. A., 2004. Ontogenetic changes in metabolism may determine shift for a sit and wait predator. J anim. Ecol.73,536-545. Johnstone, I., Norris, K., 2000. Not all oystercatchers. Haematopus ostralegus, select the most profitable common cockles cerastoderma edule> A difference between feeding methods. Ardea 88. 137-153. Kornij6w, R., R. D. Gulati, and T. Ozimek. 1995. Food preference of freshwater invertebrates: comparing fresh and decomposed angiosperm and a filamentous alga. Freshwater Biology 33:205-212. Lampert, W., and U. Sommer. 1997. Limnoecology: The ecology of lakes and streams. Oxford Univ. Press. Larson, G. L., Wones A., Mclntire, D.D and Samora (1992). Limnology of subalpine lakes and high mountain forest lakes in mountain Rainier National Park. National park service tech. report. OSU/ NRTR , Seattle, W. A . USA. Liss W. J., Larson G.L, Deimling E et al. (1995) Ecological effects of stocked trout in natural fishless mountain lakes, North cascades National park USA. PNROSU /NRTRR- 95-03. National park service Northwest region Seattle, W.A. MacArthur, R. H ., Pianka , E. R., 1966. Optimal use of patchy environment. Am . Nat. 100, 603- 609. Mack R. N., Simberloff, D. Lousdale, W. M., Evans, H. Clout M., and Bazzaz, F (2000) Biotic invasions, causes, epidemiology, global consequences and control. Ecol. Appl. B, 10, 689-710. MacNEIL, C., J. T. A. DICK, AND R. W. ELWOOD.( 1999 ) The dynamics of predation on Gammarus spp. (Crustacea: Amphipoda). Biol. Rev. 74: 375–395 Mazumder, A. , W. D. Taylor, D.J. McQueen , and D .R. S. Lean. 1989. Effects of nutrients and grazers on periphyton phosphorus in lake enclosures. Freshwater Biology 22, 405-415 Moss, B., 1998. Ecology of fresh waters. Man and medium. Past to future. Blackwell Science, London. 560 pp Mistri , M., 2004. Predatory behavior and preference of a successful invader, the mud crab, Dyspanopeus sayi cpanopeidae, on its bivalve prey. J. Exp. Mar. Biol. Ecol. 312, 385-398. Mittelbach G.G. 1981. Foraging efficiency and body size, a study of optimal diet and habitat use by bluegills. Ecology, 62, 1370- 1386. Newman, R. M 1991. Herbivory and detritivory on freshwater macrophytes by invertebrates, a review. Journal of the North American Benthological Society 10, 89114. Nilsson, B. I. 1981. Susceptibility of some odonata larvae to fish predation. Limnologie 21. 1612-1615. PER NYSTROM, CHRISTER BRO¨NMARK AND WILHELM GRANE´ L I Patterns in benthic food webs: a role for omnivorous crayfish? Freshwater Biology (1996) 36, 631–646 Persson, L. and Greenberg , L. A. 1990. Optimal foraging and habitat shift in perch (perca flaviatilus) in a resource gradient. Ecology 71. 1699- 1713. Rutten, A. L , Oosterbeck , K Ens, B. J. Verhulst, S., 2006. Optimal foraging on perilous prey < risk of bill damage reduce optimal prey size in oystercatchers. Behav . Ecol. 17. 297-302 Scheffer, M., 1998. Ecology of Shallow Lakes. Chapman and Hall. London. 357 pp. Schilling E. G., Loftin C.S., DeGoosh K. E, Huryn A.D (2009). Macroinvertebrates as indicators of fish absence in naturally fishless lakes. Freshwater Biology, 54, 181-202 Schindler, D .E, Knapp, R. A and Leavitt, P. R ( 2001) Alteration of nutrient cycle and algal production resulting from fish introduction into mountain lakes. Ecol. 4, 301-321. Simon , K.S and Townsend ,C.R (2003) Impacts of water invaders at different levels of ecological colonization with emphasis on salmnids and ecosystem consequences . Freshwater Biol. 48, 982- 994. Smallegange, M. I., Bert Hidding, Janneke ,M.A. Eppenga, Jaap vander Meer. (2008).Optimal foraging and risk of claw damage> How flexible are shore crabs in their prey size selectivity? J. Exp. Mar. Biol. And Ecol.367, 157-163. Tolonen, K. T., Hämäläinen, H., Holopainen, I. J., Mikkonen, K. & Karjalainen, J. (2003) Body size and substrate association of littoral insects in relation to vegetation structure. Hydrobiologia, 499, 179-190. Townsend, C. R. and Crowl , T. A (1991) Fragmented population structure in a New Zealand fish, an effect of introduced brown trout. Oikos, 61, 347-354. Vanni, M. J., 1986. Fish predation and zooplankton demography. Ecology 67: 337–354. Visser, M. E,. 1995. The effects of competition on oviposition decision of leptopilina heterotonia (Hymenoptera. Eucoilidae). Anim. Behav. 49, 1677- 1687. Wellborn, G.A, Skelly, D.D and Werner, E.E. 1996. Mechanism creating community structure across a freshwater habitat gradient. Annual Review of Ecology and Systematics, 27. 337-363. Wetzel R.G. (2001). Limnology (Lake and River Ecosystems). Academic Press, London. Wissinger, S.A, Whissel, J.C, Eldermire, C., and Brown, W. (2006) Predator defence along a permanent gradient, roles of case structure, behavior and phenology of caddis flies. Oecologia 147, 667-678. Wooster, D. E. 1998. Amphipod (Gammarus minus) responses to predators and predator impact on amphipod density. Oecologia 115:253–259 Appendix Corixidae Day 1 2 mean sd Covariance. 6.574 0.488 7.417 9.9 0.381 3.849 22.08 2.04 9.241 3 5.945 0.575 9.677 9.847 0.375 3.813 15.02 2.665 17.75 7.443 0.154 2.07 7.353 0.853 11.6 11.43 0.634 5.546 Gammarus Day mean sd Covariane Aselus 1 6.068 0.39 6.56 6.613 1.52 23.0 17.6 3.666 20.8 2 6.25 0.55 8.89 5.493 0.625 11.3 16.8 2.078 12.3 3 16.8 2.078 12.37 6.995 1.351 19.31 Day 1 2 3 Mean 6.38 6.513 5.541 Sd 1.637 1.469 1.579 Cov(%) 25 22.56 28.5 n 21 6.767 1.383 20.44 22.02 2.659 12.07