Differences in feeding selectivity and efficiency between young-of-the-year European perch (Perca fluviatilis) and roach (Rutilus rutilus) — field observations and laboratory experiments on the importance of prey movement apparency vs. evasiveness

Feeding selectivity and efficiency of young-of-the-year European perch and roach were compared under field and laboratory conditions. In laboratory experiments, the importance of prey evasiveness versus prey movement conspicuousness for fish selectivity was evaluated with respect to changing Cladocera/Copepoda prey ratio. Feeding efficiency was additionally investigated in relation to feeding time (5, 10, 20 min) and prey density (approx. 50, 200, 700 ind. L⁻¹). In Říov Reservoir, the diet of both fish species was nearly exclusively composed of crustacean zooplankton. In roach, diet shifted from rotifers and bosminids in May, towards Daphnia sp. and Leptodora kindtii in June and July. Daphnia contributed almost exclusively to the roach diet since June, composing on average more than 94% of total prey. Cyclopoid copepods, occurred in the roach’s diet only on the first sampling date; later on both cyclopoid and calanoid copepods were completely absent. On the other hand, copepods played an important role in the diet of perch. In early and mid-June when their share in the zooplankton was particularly high, copepods contributed by more than 50% to the diet of perch. Although their contribution dropped with their decline in zooplankton in June/July, by the end of July they again comprised about one third of perch’s diet. In both fish species, the increase in numbers of cladocerans in their diet was related to increase in SL. In roach, the numbers of consumed prey were doubled every twenty days during the investigated period. In perch the increase was not so consistent, but significantly higher efficiency of perch was reported on three out of six sampling dates. In laboratory experiments, roach showed a distinct avoidance for copepods and a preference for cladocerans. Both prey categories were only fed non-selectively when they dominated the prey mixture. Perch selectivity was more diversified. Contrary to roach, perch were fed copepods non-selectively on a balanced prey ratio. Further, with an increasing share of Cladocera, a situation resembling that of roach and Copepoda was avoided. However, when the share of copepods in the prey mixture dropped below ten percent, they were consumed non-selectively and with their ongoing decline in the prey mixture their preference even increased. Feeding efficiency differed significantly between perch and roach when foraging on copepods exclusively or on a prey mixture where copepods predominated. In the short time feeding experiment (5 min) with copepods, perch consumed on average 5.9 times more prey than roach. Although roach increased their success with increasing time it was still 1.7 times greater than for perch in the long time feeding experiment (20 min). Total numbers of prey consumed were positively affected by prey density and feeding time. With increasing feeding time, the consumption rate generally declined. With a fourfold increase in feeding time, the numbers of consumed prey increased on average only two times. Only in roach feeding on copepods did the numbers of prey consumed per minute of feeding increase with increasing feeding time. However, the overall numbers were low. Differences in feeding selectivity and efficiency between perch and roach juveniles were found to be significant both in the field and laboratory experiments. In roach, selectivity was determined solely by prey evasiveness. By contrast, perch’s selectivity was influenced by prey movement conspicuousness; prey escape abilities did not play an important role. Perch were more efficient foragers on evasive prey, but its feeding efficiency for non-evasive prey was not lower than that of roach. According to our observations, we suggest feeding behaviour to be responsible for the roach’s inefficiency in capturing evasive copepods.     

Variation in diet of yellow perch (Perca flavescens) in a Quebec reservoir

We determined the diet of 1 to 1 1/2 a old yellow perch (Perca flavescens) in a Quebec reservoir, relative to occurrences of available prey species and size classes of prey. We used Schindler-Patalas trap samples taken over four 24-h intervals between June and September 1981 to determine size distribution of available prey species. Relative abundances and size distribution of ingested prey species were determined from examination of gut contents of perch trapped by gill nets during the same time intervals. Electivity values of different prey species and of different size classes of prey were determined. Larger zooplankton prey (e.g.Epischura andLeptodora) were generally preferred but there was considerable variability in diet among individual fish that cannot be explained by patterns of prey availability. Within particular size classes of prey, perch have definite preferences, e.g. when perch consume small prey, they preferBosmina to other similar-sized prey. These results are discussed in relation to theories attempting to describe and predict feeding patterns of planktivorous fish species.Deceased, June 6, 1983     

Mensurative approach to examine potential interactions between age-0 yellow perch (Perca flavescens) and bluegill (Lepomis macrochirus)

Bluegill (Lepomis macrochirus) and yellow perch (Perca flavescens) populations are often sympatric in the Great Plains region of the U.S.A. and portions of Canada; however, very little attention has been given to potential interactions between these species for available resources, especially during the early life stages. Relationships between age-0 bluegill and yellow perch growth and relative abundance were explored across multiple lakes and years within the Nebraska Sandhill region, USA. In addition, four habitat patch types (open water, Phragmites spp., Typha spp., Scirpus spp.) were sampled for age-0 bluegill and yellow perch, and food habits were examined for each species during August, September, and October of 2009 in one of these lakes. Age-0 yellow perch growth was negatively related to age-0 bluegill relative abundance across a spatiotemporal scale. Age-0 bluegill and yellow perch exhibited similar habitat use (moderate–high overlap), but generally consumed different important and dominant prey taxa (bluegill consumed both macroinvertebrates—56?% and zooplankton—44?%, while yellow perch consumed more zooplankton—66?%), which resulted in low overall diet overlap between species. Previous research indicates that age-0 yellow perch diet ontogeny often results in feeding predominately on macroinvertebrates and positively selecting them (and avoiding zooplankton prey) at sizes observed in our study. Therefore, yellow perch growth rates may be compromised by the presence of bluegill because of the need to consume less energetically profitable prey items such as zooplankton.     

Resource limitation during early ontogeny: constraints induced by growth capacity in larval and juvenile fish

The presence of and mechanisms behind density-dependent growth and resource limitation in larval and juvenile stages of organisms with high mortality such as fish are much debated. We compare observed consumption and growth rates with maximum consumption and growth rates to study the extent of resource limitation in young-of-the-year (YOY) roach (Rutilus rutilus) and perch (Perca fluviatilis). Diet, habitat use, consumption rate and growth rate were measured under varying YOY fish densities over 2 years in four lakes. In the first year, YOY roach and perch were studied under allopatric conditions. Experimental addition of perch roe in the second year also allowed study of YOY of the two species under sympatric conditions in two of the lakes. The diet of YOY roach was dominated by cladoceran zooplankton and YOY roach habitat use was restricted to the shore region in both years. This restricted habitat use did not involve any cost in foraging gain in the first year as consumption and growth rates were very close to maximum rates. During the second year, when the two species coexisted, resources were limited in late season, more so in the littoral than in the pelagic habitat in one lake while the reverse was the case in the other lake. The diet of YOY perch was also dominated by zooplankton, and with increasing perch size the proportion of macroinvertebrate prey in the diet increased. After hatching, YOY perch first utilized the pelagic habitat restricting their habitat use to the shore after 1 to several weeks in the pelagic zone. During the larval period, perch were not resource limited whereas juvenile perch were resource limited in both years. The fact that YOY perch were more resource limited than YOY roach was related to the higher handling capacity and lower attack rate of perch relative to roach, rendering perch more prone to resource limitation. Estimates of resource limitation based on consumption rates and growth rates yielded similar results. This supports the adequacy of our approach to measure resource limitation and suggests that this method is useful for studying resource limitation in organisms with indeterminate growth. Our results support the view that density-dependent growth is rare in larval stages. We suggest that density-dependent growth was absent because larval perch and roach were feeding at maximum levels over a wide range of larvae densities. Received: 14 June 1999 / Accepted: 29 October 1999     

Influence of benthic and interstitial processes on nutrient changes along a regulated reach of a large river (Rhône River,France)

An enclosure study was conducted in Ranger Lake in south-central Ontario, Canada from 4 July to 5 August 1997 to determine predation effects of the larvae of the phantom midge fly Chaoboruson the zooplankton community. Zooplankton assemblages were established in 12 enclosures (2 m in diameter, 7.5 m deep). Three densities of fourth-instar Chaoborus trivittatus (0 l^–1, 0.1 l^–1 and 0.5 l^–1) were introduced as predator treatments to the enclosures. Temperature, dissolved oxygen and zooplankton community composition were monitored for six weeks. To determine if the zooplankton community composition changed, a repeated measures multivariate analysis was performed on percent biomass of Bosmina and calanoid copepods. There were no significant differences in mean taxon percent biomass among predator treatments. There were significant differences in mean taxon percent biomass between water layers (epilimnion and metalimnion). There were also significant differences in lengths of Bosmina and calanoid copepods among predator treatments at the end of the experiment. Crop content analysis of C. trivittatusshowed that Bosmina constituted 88–98% of the prey items found in the crops. These results demonstrate that the use of deep enclosures, a Chaoborus species which vertically migrates, and lower natural densities of Chaoborus may provide prey with an important natural refuge from predation and so allow a more accurate determination of the predation impact of Chaoborus trivittatusin temperate lakes where fish control Chaoborus densities.     

The short-term effects of perch predation on a zooplankton prey community

An enclosure experiment was performed in 1981 to determine if predation by perch (Perca flavescens) affected the density, depth distribution or size structure of members of the crustacean zooplankton community in Lac Choiniere, Quebec. Perch predation reduces the density of all but the smallest zooplankton in enclosures, and the relative rate of predation is influenced by both size and depth distribution of prey species. More individuals and proportionally more large individuals are found in deep water when perch are present. These results are discussed in relation to theories that size-selective predation alters numerous characteristics of zooplankton prey community structure.     

Assessment of Predation Risk by Juvenile Yellow Perch, Perca flavescens: Responses to Alarm Cues from Conspecifics and Prey Guild Members

In two laboratory experiments we tested juvenile yellow perch, Perca flavescens, for behavioural responses to alarm cues of injured conspecifics and several prey guild members: adult perch, Iowa darters, Etheostoma exile and spottail shiners, Notropis hudsonius. Spottail shiners are phylogenetically distant to yellow perch whereas Iowa darters and perch are both members of the Family Percidae. Groups of juvenile yellow perch increased shoal cohesion and movement towards the substrate after detecting conspecific alarm cues when compared to cues of injured swordtails, Xiphophorus helleri, a species phylogenetically distant from perch. Individual juvenile perch increased shelter use and froze more when exposed to chemical alarm cues from both juvenile and adult perch, shiners and darters compared to exposure to injured swordtail cues or distilled water. The response to cues of darters may indicate that alarm cues are evolutionarily conserved within percid fishes or that perch had learned to recognize darter cues. The response to spot tail shiners likely represents learned recognition of the cues of a prey guild member.     

The effect of a predatory leech,Nephelopsis obscura,on mortality,growth, and production of chironomid larvae in a small pond

Summary The effect of a predatory leech, Nephelopsis obscura, on survivorship, growth, and production of chronomid larvae was studied by enclosure experiments carried out in a small pond. The prey population was composed almost entirely of the tubiculous, microphagous chironomid larvae, Chironomus riparius and Glyptotendipes paripes. Nephelopsis significantly reduced chironomid survivorship within the enclosures, and accounted for most of the measured mortality of fourth instar larvae. The cropping by Nephelopsis was not significantly biased toward either prey species. In long-term experiments (66 d) chironomid biomass in enclosures without leeches reached much higher levels than in enclosures containing Nephelopsis. This increase in biomass was due to growth of surviving larvae, rather than recruitment, since emergence and oviposition were not going on during the course of the experiments. The enhanced survivorship of larvae within leech-free enclosures was eventually accompanied by reduced growth and specific production (daily production/biomass) for C. riparius, which made up about 90% of the larval population. Growth and specific production of G. paripes (10% of larval population) was not affected. Short-term experiments (25 d) involving manipulation of densities and species ratio (9:1 CR:GP and 1:9 CR:GP) of larvae revealed that growth of the majority species was strongly influenced by larval density, whereas growth of the minority species was not. The same pattern was observed both in the presence and in the absence of Nephelopsis and was a result of differences in resource utilization between the two species. In the shortterm experiments, growth rates estimated for larvae exposed to leeches were significantly less than those for larvae in leech-free enclosures. This could be due either to size-biased consumption of larvae by Nephelopsis, or possibly a disturbance factor leading to reduced larval food intake and/or increased metabolic costs.     

Predation by a water mite (Piona exigua) on enclosed populations of zooplankton

Short term, replicated experimental alteration of densities of a predatory water mite Piona exigua Viets, inside 0.45–0.475 m³ litre enclosures, revealed little evidence of the effects of predation on the number and relative abundance of the enclosed zooplankton species. Predation rates more closely approximated those estimated from single prey functional response experiments in the second experimental period (December) than in the first (March). In December Daphnia was the only susceptible taxon present in large numbers, whereas in March, Ceriodaphnia and Chydorus were also present. This result is consistent with laboratory findings that predation rates are lowered in the presence of more than one prey type.The difficulty of obtaining evidence for significant effects of these planktonic predators is in part due to changes in the preferred prey species in the diet of Piona depending on stage and sex of the mite and to aspects of experimental design. The wide variability between replicate enclosures at each predator density reduced the power of the statistical analyses used to test the null hypothesis. Enclosures with no predators are necessary to investigate the effects of enclosure on the zooplankton prey, since these effects may outweigh those due to predator consumption.     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: selective feeding of fish and predation effects on the zooplankton communities

Bag-type enclosures (75 m³) with bottom sheets and tube-type enclosures (105 m³) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1⁻¹). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure, probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed. Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments for examining the effects of fish predation on pelagic zooplankton communities are discussed.     

Impact of Chaoborus predation upon the structure and dynamics of a crustacean zooplankton community

Summary During ice-free seasons of 1975, 1977 and 1978, replicated experimental alteration of spring densities of predatory Chaoborus larvae inside 20–27 m³ enclosures in a fishless oligotrophic lake had relatively small, but significant, short-term effects upon prey species abundances. Enhancement of predator densities generally had greater numerical effects relative to controls than did complete removal of predators. With the exceptions Diaphanosoma and Bosmina under artificially elevated Chaoborus densities, numerical effects on prey species did not persist for more than a few weeks after midsummer in these 3 years. During cooler 1976, however, much larger Chaoborus effects in May and June persisted into September. Low temperatures and small initial population densities slowed population growth and tended to increase the proportion of each species' recruitment lost to Chaoborus predation. As water temperatures increased during 3 of 4 summers, rapid juvenile development and compensating increase in adult fertility generally permitted most prey species to escape regulation by these large, univoltine and semivoltine predators. With growing prey population size, declining food levels suppressed crustacean fecundity in July and August, thereby permitting predatory losses to climb again to substantial fractions of the reduced prey recruitment. Nevertheless, prey densities in predator-free and control or predator-enhanced enclosures differed little from July through September (except in 1976). Thus, summer population growth of most prey species seemed more limited by food shortages than by predators per se. Growth of individual zooplankters was affected by food availability during critical periods in July and August in all years, and Chaoborus predation seemed to influence the timing of this food limitation in at least 2 of the 4 years     

Changes in the diet of Nile perch,Lates niloticus (L), in the Mwanza Gulf,Lake Victoria

Samples of Nile perch (Lates niloticus L.) were collected for stomach analysis from trawl catches conducted in the Mwanza Gulf from September 1986 to September 1988. Initially haplochromine cichlids formed the main food item for the Nile perch. Despite their decline, haplochromines still formed the major part of its diet in 1986. After the virtually complete disappearance of the haplochromines in 1987 and 1988, the benthic shrimp Caridina nilotica, the pelagic cyprinid, Rastrineobola argentea and juvenile Nile perch became the main food of Nile perch. Twenty-four trawl sessions were conducted to determine the relative quantities of respective prey taken. When present, Caridina is the major prey, mainly taken by day. If absent, juvenile Nile perch constitute the main part of the diet. Seasonal differences in diet composition probably reflect seasonal fluctuations in the abundance of the main prey species.     

Bioaccumulation of mercury in yellow perch (Perca flavescens) and common loons (Gavia immer) in relation to lake chemistry in Atlantic Canada

Mercury biomagnifies in aquatic foodwebs in freshwater lakes, and common loons (Gavia immer) breeding in eastern Canada can be exposed to reproductively toxic concentrations of mercury in their fish prey. We assessed the bioaccumulation and biomagnification of mercury in juvenile and adult common loons, and their preferred prey: yellow perch (Perca flavescens) in Kejimkujik National Park (KNP), Nova Scotia by measuring mercury levels and stable isotope ratios in tissues. Total mercury levels and stable-carbon (δ¹³C) and nitrogen isotope ratios (δ¹⁵N) were determined in composite whole-fish samples from lakes in KNP and blood samples from juvenile and adult loons captured on lakes in KNP and southern New Brunswick. Geometric mean mercury concentrations were 0.15 and 0.38 μg/g (wet wt.) in small (9-cm fork length) and large (17-cm fork length) yellow perch, and were 0.43 and 2.7 μg/g (wet wt.) in blood of juvenile and adult common loons, respectively. Mercury concentrations in perch and loons were positively associated with body mass and δ¹⁵N values. Juvenile loons and large yellow perch had similar mercury levels and δ¹⁵N values, indicating similar trophic status despite their 22-fold difference in body mass. Mercury concentrations were higher in yellow perch and common loons in acidic lakes. Our findings highlight the importance of both chemical and ecological factors in understanding mercury biomagnification in lakes and associated risks to fish-eating wildlife. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

The role of trophic bottlenecks in stunting: a field test of an allocation model of growth and reproduction in yellow perch, Perca flavescens

Synopsis Stunting in freshwater fish populations may be due to low availability of one or more prey components within their diet. If the limiting prey constitute a trophic level (i.e. zooplankton, benthos, or fish), we define the phenomenon as a trophic bottleneck. Growth of a non-stunted population of yellow perch, Perca flavescens, was simulated using an allocation model based on reported ontogenetic shifts in diet (planktivory to benthivory to piscivory). The model was then perturbed by limiting the available ration individually for each of the three feeding types. The resulting simulated growth curves all reflected lower growth rates than the unperturbed state and the shape of the curve differed between the limited food types. Only the reduced benthic ration produced a simulated growth that matched that observed for the stunted yellow perch of Lac Hertel (Québec). To test the hypothesis that benthic ration was limiting growth in Lac Hertel, all the fish species in the lake were sampled for diet composition and total length at age. We predicted that species feeding upon benthic invertebrates should exhibit the slow growth characteristic of stunting (based on the results of the allocation model). Of the seven fish species found in Lac Hertel, four were stunted: yellow perch, pumpkinseed, Lepomis gibbosus, rock bass, Ambloplites rupestris, and brown bullhead, Ictalurus nebulosus. Three species were non-stunted: northern pike, Esox lucius, golden shiner, Notemigonus crysoleucas, and white sucker, Catostomus commersoni. All stunted species fed on benthic invertebrates and all non-stunted species, except the white sucker, did not feed upon benthos. The prediction of the allocation model was thus supported.     

A trophic bottleneck?: The ecological role of trout‐perch Percopsis omiscomaycus in Saginaw Bay,Lake Huron

Trout‐perch are abundant in many North American aquatic systems, but the ecological roles of trout‐perch as predators, competitors and prey remain relatively understudied. To elucidate the ecological role of trout‐perch in Saginaw Bay (Lake Huron, North America), the spatial and temporal diet composition was quantified and the frequency of occurrence of trout‐perch in diets of piscivorous walleye and yellow perch was evaluated. From May through November 2009–2010, trout‐perch and their potential predators and prey were collected monthly from five sites in Saginaw Bay using bottom‐trawls. Trout‐perch were abundant components of the Saginaw Bay fish community, and in 2009, represented 13.5% of fish collected in trawls, with only yellow perch (38%) and rainbow smelt (19.1%) being more common. Trout‐perch primarily consumed Chironomidae (84.0% of diet biomass) and exhibited strong, positive selection for Chironomidae and Amphipoda, suggesting that their diet preferences overlap with the economically important yellow perch and juvenile walleye. Energy content of trout‐perch averaged 4795 J g^?1 wet and was similar to yellow perch (4662 J g^?1 wet) and round goby (3740 J g^?1 wet). Thus, they may provide a comparable food source for larger piscivorous fish. However, despite their high energy density, abundance, and spatial overlap with other fish prey species, trout‐perch were very rare in diets of piscivorous walleye and yellow perch in Saginaw Bay, indicating that trout‐perch are a weak conduit of energy transfer to higher trophic levels.     

Feeding behaviour determining differential capture success of evasive prey in underyearling European perch (<Emphasis Type="Italic">Perca fluviatilis</Emphasis> L.) and roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis> (L.))

The effect of feeding behaviour on the prey capture efficiency of young-of-the-year European perch and roach was investigated in laboratory experiments using planktonic crustaceans possessing different escape abilities—Daphnia sp. and Cyclops sp. Two sets of experiments were performed. In the first set, the feeding efficiency and behaviour of 270 fish individuals were determined by stomach content analyses and video record evaluations. In the second set of experiments, analysis of attack-effort, which was evaluated as attack-distance and repeated strikes, was undertaken. Except for situations in which Daphnia was offered at high densities, the feeding efficiency of perch was significantly higher compared to roach in all other combinations of prey types and densities. Roach consumed significantly less prey compared to perch when feeding exclusively on the evasive Cyclops and when it was offered in a 1:1 ratio mixture with Daphnia. The mean swimming speed was similar in both fish species, but behavioural differences were evident during prey search and capture. Perch swam through the aquaria in short and fast movements that were interrupted by many stops. Roach exhibited rather continuous swimming that was punctuated by slowdowns instead of stops. The perch attacks were very intensive and repeated strikes occurred, particularly when feeding on evasive Cyclops. On the other hand, roach revealed strong schooling behaviour restricting the fish during inspection of the experimental aquaria. The distinct differences in feeding efficiency between perch and roach were demonstrated to be closely related to differences in their feeding behaviour. Discontinuous searching for prey, vigorous attacks, occurrence of repeated strikes and the absence of schooling increased perch prey capture efficiency, particularly when foraging on evasive copepods.     

Growth of juvenile sacramento blackfish,Orthodon microlepidotus (Ayres)

Growth rates of juvenile (1–8 g) Sacramento blackfish, an omnivorous California cyprinid which grows to >1.5 kg, were measured at three temperature ranges in laboratory aquaria where several diets were offeredad lib. and at four stocking densities in small ponds where natural food was available. Growth rates generally increased with elevations in environmental temperature. Growth in aquaria was possible with strictly plant-based diets, but was faster with pelleted diets higher in protein, lipids, and total calories. Growth rates in ponds increased with decreased stocking densities and approximated or exceeded those measured in laboratory aquaria for blackfish fed pelleted diets. The data also support the importance of energy-rich food organisms (e.g., zooplankton) in the diet of young-of-the-year blackfish in Clear Lake, California.     

Indirect effects of metal contamination on energetics of yellow perch (Perca flavescens) resulting from food web simplification

1. Benthic invertebrate community composition and yellow perch (Perca flavescens) diet, growth and activity levels from lakes along a metal‐contamination gradient were used to assess the importance of a naturally diverse prey base for maintaining energy transfer to growing fish, and how this transfer is disrupted by metal contamination. 2. Zoobenthic communities had lower diversity in metal‐contaminated lakes, with a notable absence of large bodied invertebrate taxa. 3. The average mass of non‐zooplankton prey items was significantly greater for 2+ and 3+ perch from the reference lake, and increased significantly with age in all except the most contaminated lakes where prey choice was limited. 4. Benthivorous perch from all contaminated lakes exhibited slowed growth. Perch from one of the contaminated lakes exhibited faster growth during piscivory, indicating slowed growth only while benthivorous. 5. Estimates of fish activity, using the activity of the glycolytic enzyme Lactate dehydrogenase in perch white muscle tissue as a proxy, suggested that shifts in diet to larger prey (in reference and intermediately contaminated lakes) lowered activity costs, which may explain how diet shifts maintain growth efficiency as perch grow larger.     

The use of integrated constructed wetlands (ICW) for the treatment of separated swine wastewaters

Haplochromine cichlids used to be the main prey of the introduced Nile perch, Lates niloticus, in Lake Victoria. After depletion of the haplochromine stocks at the end of the 1980s, Nile perch shifted to the shrimp Caridina nilotica and to a lesser degree to its own young and the cyprinid Rastrineobola argentea. In the present study, we investigated the Nile perch diet in the northern Mwanza Gulf after resurgence of some of the haplochromine species and compared it with data collected in the same area in 1988/1989. It became clear that haplochromines are again the major prey of Nile perch. The dietary shift from invertebrate feeding (shrimps) to feeding on fish (haplochromine cichlids) occurs at a smaller size than it did when Nile perch were taking primarily dagaa and juvenile Nile perch as their fish prey. The apparent preference for haplochromines as prey has reduced the degree of cannibalism considerably, which may have a positive impact on Nile perch recruitment.     

The effect of prey type and density on the foraging efficiency of juvenile perch, (Perca fluviatilis)

The foraging efficiency of juvenile perch (Perca fluviatilis), feeding on two types of prey, was studied in laboratory experiments. Waterfleas (Daphnia magna) and phantom midge larvae (Chaoborus flavicans) were offered in a range of densities, either separately or combined. Perch fed more efficiently on each prey type separately than when both were mixed. Foraging efficiency decreased with an increase of mixed prey density with both prey types present in equal numbers, but also when the proportion of Chaoborus increased. This could be caused by the existence of different hunting techniques, each of which is fully efficient in the presence of one prey type only. In the presence of two prey types, the predator constantly has to switch from one hunting technique to another.