Large Perch Research Articles

Ecological implications of fish removal: Insights from gut-content analysis of roach (Rutilus rutilus) and European perch (Perca fluviatilis) in a eutrophic shallow lake.

Large reductions in fish biomass are common both as a method of managing lake ecosystems by fish removals (biomanipulation) and as naturally occurring fish kills. To further understand how fish reductions change feeding patterns of fish, we studied the diets of small- to medium-sized roach (Rutilus rutilus) and European perch (Perca fluviatilis) on a monthly basis using gut-content analysis during an 18-month period before and after a whole-lake fish removal in a eutrophic shallow lake. Further, we performed in-depth analyses of zoobenthos communities of the profundal and littoral zones, as well as analysed the zooplankton community in the littoral and pelagic parts of the lake to estimate abundance and biomass of potential diet items. We found that, in general, there was a trend toward increased zoobenthivory in both species and among all-sized fish after fish removal, regardless of prior diet preference. Reduced piscivory among larger perch (>150 mm) and reduced zooplanktivory among smaller perch and roach (<150 mm) were also observed. Moreover, during a short period of high zooplankton biomass after fish removal, both perch and roach (all sizes) shifted their diet toward daphnids, which likely caused a decrease in daphnid population. We suggest that such change toward periodical zooplanktivory across fish species and size groups may lead to unexpectedly high top-down control by fish after lake restoration by fish removal.

Are perch (Perca fluviatilis L.) getting larger or smaller in Swedish lakes?

AbstractSpace‐for‐time approaches are often used to indicate current or future changes over time. A European gillnet standard facilitates spatial‐scale comparisons of fish communities in European lakes. Fish size was generally lower and densities higher in warmer lakes, but less is known about trends over decadal time scales. We analysed the size of European perch (Perca fluviatilis L.) in 2121 Swedish lakes sampled during 1996–2021. The aim was to test whether size changed over time, and whether trends were similar in southern and northern regions. We analysed mean length and occurrence or relative abundance of size classes from the smallest (&lt;100 mm) to the largest individuals (≥350 mm), and length at 1 and 5 years. The large data set was used to find general trends over time within regions, and within‐lake trends were tested for 40 time‐series lakes. The mean length of perch decreased in the southern and increased in the northern region. The proportion of the smallest perch increased in the southern, but not in the northern region. The most general trend was an increasing occurrence and proportion of the largest‐sized perch in both regions in the large dataset. The average trends in the southern region of the large dataset also appeared as predominant trends within time‐series lakes, although opposite trends occurred within some lakes in each region. This study also revealed that perch size at age 1 often increased within lakes over time, while more variable trends were found for size at age 5. The duration of the growth season has generally increased during the time frame of the present study, while other changes may differ between lakes in the same region. We recommend more studies based on time series of fish monitoring, including studies on possible reasons for the increased occurrence of very large perch in Swedish lakes.

Mercury–Selenium Accumulation Patterns in Muscle Tissue of Two Freshwater Fish Species, Eurasian Perch (Perca fluviatilis) and Vendace (Coregonus albula)

Mercury (Hg) in the form of highly toxic methyl mercury (MeHg) accumulates in aquatic food webs to an extent where it may threaten fish health in many freshwaters. Selenium (Se) mitigates the toxicity of accumulated Hg by forming strong bonds with it, a drawback being diversion of Se from biosynthetic pathways of essential selenoenzymes. We measured Se and Hg in muscle tissue of Eurasian perch (Perca fluviatilis) and vendace (Coregonus albula). For the perch, Se and Hg correlated positively. For the vendace, a positive relationship was seen when the effect of fish size was accounted for. All fish displayed surplus Se (mol Se – mol Hg > 0). For both fish species, the Se molar surplus ((nmol Se – nmol Hg)/g ww) decreased with fish weight. It was higher in the perch than in the vendace and showed the least variation among the small perch (4–34 g). For the large perch (79–434 g), the Se molar surplus decreased with increasing Hg below 0.5 µg Hg but then increased with further Hg increment despite Hg being a negative term in the Se molar surplus. In case the Se molar surplus reflected the weight-specific Se requirement, the latter clearly decreased with increasing fish size for the vendace. This was less clear for the perch because of the strong correlation between Hg and weight. Together, these Se–Hg relationships suggest that Se accumulation was at least partly subject to homeostatic control and responded to the Hg body burden and the Se requirement.Graphical abstract

The Contribution of Perch (Perca fluviatilis L. 1758) to Maintaining the Diphyllobothriasis Focus in the Krasnoyarsk Reservoir and the Yenisei River

The study investigates infection rates of perch by plerocercoids of the Dibothriocephalus latus (Linnaeus, 1758) Lühe, 1899 (syn. Diphyllobothrium latum) broad tapeworm in the creeks of the Krasnoyarsk Water Reservoir and in the Yenisei River (within the limits of the city of Krasnoyarsk). Plerocercoids were found in the perch in all study areas. The rates of invasion of the fish in the Yenisei River and in the creeks of the lower part of the Reservoir – Biryusa and Shumikha – were low (9.8, 6.9, and 13.5%, respectively); the infection of perch was higher in the middle part of the Reservoir, in the Sisim Creek (21.4%). The fish purchased at an unauthorized market was the most infected – 68.5%. The sample included large fish, weighing from 70 to 970 g. A positive correlation was noted between invasion rate and fish age (r=0.7). The plerocercoids found in the fish were viable. They were mainly localized in the fish muscle. Diphyllobothriosis is a commonly occurring helminthiasis in the riverine communities transformed because of the damming of rivers. In the Krasnoyarsk Water Reservoir, the broad tapeworms are usually transmitted along the plankton – perch – pike chain. The data obtained in this study show that small perch, consuming infected plankton, plays a leading role in maintaining the abundance of D. latus in the water reservoir. However, the main contribution to the epidemiological situation in the region is made by the large perch, which is heavily infected with broad tapeworm plerocercoids because of its cannibalistic feeding behavior. Both recreational fishermen and the people purchasing fish at unauthorized markets prefer large perch. Commercial and recreational fishing, settlements on the shores of the Reservoir, and the increasing number of tourists have contributed to the invasion of pathogenic organisms in the Reservoir. The perch caught in the Krasnoyarsk Water Reservoir or in the Yenisei River or purchased at unauthorized markets, which have not been inspected for disease, could be a source of diphyllobothriosis infection for people

Predation selects for smaller eye size in a vertebrate: effects of environmental conditions and sex.

Increased eye size in animals results in a larger retinal image and thus improves visual acuity. Thus, larger eyes should aid both in finding food as well as detecting predators. On the other hand, eyes are usually very conspicuous and several studies have suggested that eye size is associated with predation risk. However, experimental evidence is scant. In this study, we address how predation affects variation in eye size by performing two experiments using Eurasian perch juveniles as prey and either larger perch or pike as predators. First, we used large outdoor tanks to compare selection due to predators on relative eye size in open and artificial vegetated habitats. Second, we studied the effects of both predation risk and resource levels on phenotypic plasticity in relative eye size in indoor aquaria experiments. In the first experiment, we found that habitat altered selection due to predators, since predators selected for smaller eye size in a non-vegetated habitat, but not in a vegetated habitat. In the plasticity experiment, we found that fish predators induced smaller eye size in males, but not in females, while resource levels had no effect on eye size plasticity. Our experiments provide evidence that predation risk could be one of the driving factors behind variation in eye size within species.

Long-Term Changes in the Importance of Aboriginal and Invasive Fish Species for Feeding of Predatory Fish of the Rybinsk Reservoir

Influence of biotic and abiotic environmental factors on feeding of the predatory fish in the Rybinsk Reservoir in 1953−2012 has been analyzed. It is revealed that the feeding spectrum of the majority of piscivores (sander Sander lucioperca, burbot Lota lota, pike Esox lucius and adult perch Perca fluviatilis) includes 16−18 fish species, while only seven in Volga sander Sander volgensis. Three aboriginal fish species (perch, ruff Gymnocephalus cernuus and roach Rutilus rutilus) and two invasive species (European smelt Osmerus eperlanus and Black-Azov tyulka Clupeonella cultriventris) are the main food items for piscivores. In some years, these species accounted for 97% of the total number of prey. Introduction and naturalization into the reservoir of smelt (during the period of climatic norm) and Black-Azov tyulka (during regional climate warming) considerably changed proportions of species serving as potential prey for predatory fish. During these periods, the share of invasive species in the ration of pelagic zone piscivores was increasing, but this increase was not proportional to the abundance of these prey species. After decline in abundance of smelt and tyulka (while, at the same time, these species were still dominant) their shares in the feeding of predators decreased, while the role of more available mass aboriginal species increased again. It is revealed that predatory fish are selective to certain prey items: sander, Volga sander, and large perch prefer to feed on perch yearlings; burbot, on yearlings of perch and on ruff; and pike, on roach.

The food web positioning and trophic niche of the non-indigenous round goby: a comparison between two Baltic Sea populations

The food web positioning of the non-native round goby (Neogobius melanostomus Pallas, 1814) was studied in a new invasive population in Mariehamn, Aland Islands (northern Baltic Sea). The trophic position and isotopic niche space was compared to other benthic-feeding fish species in the same habitat. The trophic position (TP) was estimated based on stable isotope analysis of carbon (13C:12C) and nitrogen (15N:14N) ratios and compared to that of an established invasive population in Hel, Gulf of Gdansk (southern Baltic Sea). Ontogenetic changes in isotope signatures were evaluated with a regression analysis and compared between the two populations. The round goby positioned as a second-order consumer on the third trophic level among other benthic-feeding fish. It showed a similar trophic position and significant isotopic niche overlap with large perch. The trophic position of round gobies in Mariehamn is significantly higher than in Hel, likely due to different prey items. Furthermore, the ontogenetic patterns differ between the two invasive populations, also suggesting differing resource and habitat availability and levels of intraspecific competition between areas.

The benthos as the basis of vendace, Coregonus albula, and perch, Perca fluviatilis, diets in an oligotrophic sub-Arctic lake

Many small lakes in northern high latitudes are oligotrophic, with very low planktonic primary production, so that benthos may be the important source of carbon for planktonic and benthic fish. To clarify this question, the stomach content analysis in fish and the stable isotopes of carbon (δ13C) and nitrogen (δ15N) in pelagic and benthic organisms were conducted in a sub-Arctic lake (Lake Krivoe, White Sea coast, northern Karelia, Russia). We expected to recognize the main food items, their contribution to the diet of dominating adult fish: perch, Perca fluviatilis, and vendace, Coregonus albula, and trophic links between members of lake food webs. Samples were collected in June–October 2009 (ice-free period), from littoral, profundal, and pelagial habitats. Stable isotope analysis allocated four trophic levels (TL) in lake food webs. The large perch (> 180 mm) were found as piscivorous (top) predator (TL > 4), while vendace and smaller perch as omnivorous predators (TL 3.7–3.9). Stable isotope mixing model (IsoSource, δ13C and δ15N) estimated low proportion (1–5%) for crustacean zooplankton in the diet of both fish species that allocated definitively to benthic–littoral food webs. Great part of energy sources for fish derived from benthic/littoral compartments, including 67–75% from benthic amphipods. Our study also provides evidences for size-dependent (ontogenetic) dietary shifts in the foraging strategies for perch and for amphipods. A great importance of omnivory by consumers was revealed for lake food webs.

Eurasian perch,Perca fluviatilis, spatial behaviour determines vulnerability independent of angler skill in a whole-lake reality mining experiment

To understand the determinants of angling vulnerability arising from the interplay of fish and angler behaviour, we tracked 33 large Eurasian perch, Perca fluviatilis, with fine-scale acoustic telemetry at a whole-lake scale while simultaneously tracking boats of small groups of experimental anglers (n = 104) who varied by self-reported skill. We report two key findings. First, perch vulnerability was strongly related to a repeatable habitat choice behaviour, but unrelated to swimming activity as a personality trait; importantly, highly vulnerable perch were captured throughout the lake and not only in their preferred habitat, suggesting covariance between spatial habitat choice and a behavioural determinant of vulnerability. Second, catch per unit effort of large perch increased with self-reported angling skill, an effect unrelated to skill-dependent lure use or an angler’s ability to encounter perch. Importantly, high-skill anglers captured more fish but not different spatial behavioural phenotypes. Our study has implications for designing protected areas by showcasing that angling could systematically alter the habitat use of exploited populations at whole-ecosystem scales, without necessarily changing average swimming activity and home range extension.

Does forest harvest increase the mercury concentrations in fish? Evidence from Swedish lakes

A number of studies have evaluated the effects of forest harvest on mercury (Hg) concentrations and exports in surface waters, but few studies have tested the effect from forest harvest on the change in fish Hg concentrations over the course of several years after harvest. To address this question, mercury (Hg) concentrations in perch (Perca fluviatilis) muscle tissue from five lakes were analyzed for two years before (2010−2011) and three years after (2013–2015) forest harvest conducted in 2012. Fish Hg concentrations in the clear-cut catchments (n=1373 fish specimens) were related to temporal changes of fish Hg in reference lakes (n=1099 fish specimen) from 19 lakes in the Swedish National Environmental Monitoring Programme. Small (length<100mm) and large perch (length≥100mm) were analyzed separately, due to changing feeding habitats of fish over growing size. There was considerable year-to-year and lake-to-lake variation in fish Hg concentrations (−14%–121%) after forest harvest in the clearcut lakes, according to our first statistical model that count for fish Hg changes. While the effect ascribed to forest harvest varied between years, after three years (in 2015), a significant increase of 26% (p<0.0001) in Hg concentrations of large fish was identified in our second statistical model that pooled all 5 clearcut lakes. The large fish Hg concentrations in the 19 reference lakes also varied, and in 2015 had decreased by 7% (p=0.03) relative to the concentrations in 2010–2011. The majority of the annual changes in fish Hg concentrations in the clearcut lakes after harvest were in the lower range of earlier predictions for high-latitude lakes extrapolated primarily from the effects of forest harvest operations on Hg concentrations in water. Since the risk of forest harvest impacts on Hg extends to fish and not just surface water concentrations, there is even more reason to consider Hg effects in forestry planning, alongside other ecosystem effects.

Growth, survival and interspecific social learning in the first hatchery generation of Eurasian perch (Perca fluviatilis)

New insights into present-day cultivation practices are needed when new species are taken into aquaculture. Understanding different features of survival selection operating on wild fish in artificial hatchery environment is topical also in the contexts of fish conservation and reintroduction programs. We used wild Eurasian perch (Perca fluviatilis) to produce the first generation hatchery stock, and studied survival and growth of the offspring at the age of two years through the transition from a seminatural pond to the hatchery, and the first growth season in the tank environment. First, we tested whether hatchery-raised brown trout (Salmo trutta) could be used as tutors for tank-naïve perch in training to accept commercial dry feeds, and thereby increase survival and growth of the perch in tank culture. Second, we tested whether supplementing the dry feed with minced prey fish would increase perch survival and enable faster growth compared to dry feed diet only. The brown trout tutors did not increase survival or growth of tank-naïve perch, but both improved markedly if the perch were habituated to tank culture conditions already during cold-water seasons. Survival and consequent biomass gain were the highest when the pre-habituated perch were on the fish-supplemented dry feed diet. Survival on the fish-supplemented diet was significantly higher (73.1%) compared with the survival on the dry feed diet (44.4%). The perch exhibited bimodality in final size distribution in both experiments. Although the mean body size of the fast-growing perch did not depend on the diet, the total biomass gain during the first growth season was substantially higher on the fish-supplemented diet (+104%) than on the dry feed diet (+41%). The difference was mainly due to higher survival of the large perch on the fish-supplemented diet but also because the fish supplement supported significant growth among the small fish. To conclude, commercial feeds may exert a strong survival selection on wild fish in hatcheries, which can be mitigated by adding natural elements to the diet. Although perch ability to copy the feeding behaviour from interspecific tutors was negligible for large-scale aquaculture purposes, our experiment encourages further studies on the potential of inter- and intraspecific social learning in the context of feed training in aquaculture. Statement of relevanceAccording to this study, high mortality in perch aquaculture can be reduced by supplementing commercial dry feed diet with minced, less valuable prey fish.

Pathogens trigger top-down climate forcing on ecosystem dynamics.

Evaluating the effects of climate variation on ecosystems is of paramount importance for our ability to forecast and mitigate the consequences of global change. However, the ways in which complex food webs respond to climate variations remain poorly understood. Here, we use long-term time series to investigate the effects of temperature variation on the intraguild-predation (IGP) system of Windermere (UK), a lake where pike (Esox lucius, top predator) feed on small-sized perch (Perca fluviatilis) but compete with large-sized perch for the same food sources. Spectral analyses of time series reveal that pike recruitment dynamics are temperature controlled. In 1976, expansion of a size-truncating perch pathogen into the lake severely impacted large perch and favoured pike as the IGP-dominant species. This pathogen-induced regime shift to a pike-dominated IGP apparently triggered a temperature-controlled trophic cascade passing through pike down to dissolved nutrients. In simple food chains, warming is predicted to strengthen top-down control by accelerating metabolic rates in ectothermic consumers, while pathogens of top consumers are predicted to dampen this top-down control. In contrast, the local IGP structure in Windermere made warming and pathogens synergistic in their top-down effects on ecosystem functioning. More generally, our results point to top predators as major mediators of community response to global change, and show that size-selective agents (e.g. pathogens, fishers or hunters) may change the topological architecture of food webs and alter whole ecosystem sensitivity to climate variation.

Behavioural strategy of large perch Perca fluviatilis varies between a mesotrophic and a hypereutrophic lake.

Behaviour of large perch Perca fluviatilis was studied in two lakes differing in environmental state i.e. mesotrophic v. hypereutrophic. A total of 20 adult perch P. fluviatilis (29-42 cm total length) in each lake were tagged with radio-transmitters, tracked and located eight times a day during six 24 h tracking periods over a year, enabling detection of differences in diel activity patterns and habitat use during summer and winter under two different environmental regimes. During summer, P. fluviatilis in the mesotrophic lake showed a distinct crepuscular activity pattern and a change from pelagic residency during daytime towards the littoral zone at night. In contrast, P. fluviatilis in the hypereutrophic lake were active during the entire diel cycle and were spread throughout the lake also during dark. During winter, crepuscular patterns of activity were seen in both lakes. Condition factor of large P. fluviatilis did not differ between the two lakes. Thus, it is suggested that P. fluviatilis in the hypereutrophic turbid lake adopted an alternative behaviour for successful foraging, being uniformly active throughout the diel cycle.

Cormorant predation on PIT-tagged lake fish

The present study use data from recovered PIT (Passive Integrated Transponder) tags to explore species- and size-specific annual predation rates by cormorants on three common lacustrine fishes (size range 120-367 mm) in a European lake; roach (Rutilus rutilus), common bream (Abramis brama) and perch (Perca fluviatilis). In addition, we quantify the level of age/size truncation that cormorant predation could introduce in a population of perch, an important fish for recreational angling as well as for trophic interactions and ecosystem function in European lakes. Based on three years of PIT tagging of fish in lake Viborg and subsequent recoveries of PIT tags from nearby cormorant roosting and breeding sites, we show that cormorants are major predators of roach, bream and perch within the size groups we investigated and for all species larger individuals had higher predation rates. Perch appear to be the most vulnerable of the three species and based on a comparison with mortality estimates from lakes without significant avian predation, this study suggest that predation from cormorants can induce age/size truncation in lake Viborg, leaving very few larger perch in the lake. This truncation reduces the likelihood of anglers catching a large perch and may also influence lower trophic levels in the lake and thus turbidity as large piscivorous perch often play an important structuring role in lake ecosystem functioning.

Competition, predation, cannibalism: the development of young-of-the-year perch populations in ponds with bream or roach

Summary Piscivory of perch can occur within a few weeks after perch hatch, leading to the development of two size-cohorts, with the larger perch becoming cannibals. However, the possibility of early piscivory is assumed to depend on the hatch timing of the prey and predator. Two species, bream (2006) and roach (2007), were tested as the prey fish. The bream (hatching 12 days after perch) were preyed upon by the perch, leading to the predicted development of two sizes of cohorts as well as to cannibalism. With roach (hatching simultaneously with perch), however, no piscivory or cannibalism occurred and the perch population was unimodally distributed. The results of this experimental pond study underpin recent theoretical findings that size differences between predator and prey, determined through differences in the timing of hatching as well as differences in juvenile growth rates, foster the occurrence of early piscivory in YOY perch that may lead to bimodality and finally to intra-cohort cannibalism.

Importance of maternal size on the reproductive success of perch,Perca fluviatilis, in small forest lakes: implications for fisheries management

Abstract Abundance, sex ratio and size structure of perch,Perca fluviatilisL., populations were studied in two forest lakes. The influence of maternal size on the amount and quality of eggs and larvae was investigated, and the effect of the spawning stock’s length structure on population egg and fry production was evaluated. The importance of large females was essential for the reproduction of perch. Large perch spawned more and bigger eggs that had higher hatching percentage resulting in considerably higher fry production. Larvae of large females were heavier and survived longer with bare yolk sac reserves. The relation between egg dry weight and female length differed between the study lakes suggesting varying maternal influence in different environments. Decrease in the share of large individuals in a perch population can substantially reduce the amount and quality of larvae produced. The results can be applied to improve the sustainability of perch fisheries.

Composition of the Eurasian perch ( Perca fluviatilis) catches in ice fishing: Does capture order predict body size?

Many ecological and evolutionary effects of recreational fishing depend on the selectivity of fishing with regard to body size, sex, behaviour and maturity. Here, we recorded the body size and condition of the Eurasian perch, Perca fluviatilis, in relation to the order of capture in standardized ice fishing, in ten different populations, to study if larger perch would become captured before smaller individuals. In addition, we studied the composition of the catch with regard to sex, size and maturity within and among different waterbodies. Both the total length and body mass but not the condition factor of the captured fish decreased with the order of capture (on average 1 mm and 1 g per fish) suggesting potential size based dominance in feeding hierarchy. According to additional one-sample t-tests on the population-specific slopes between the total body length and capture order, the effect was statistically significant in two independent populations. However, no relationship was found between the order of capture and the deviation of individual's length-at-age from the population mean length-at-age in these two populations. In addition, fish caught in one location were more homogenous in size than fish within the population in general, suggesting that perch form size assortative shoals in wintertime. Our study shows that the ecological implications of capture order effects depend on the mobility of anglers, but are in general population dependent and likely of minor importance compared to the potential overall size-selectivity of recreational angling. However, more studies are needed to examine if angling is selective for behavioural, growth-related traits in general.

Changes in abundance of Nile shrimp, Caridina nilotica (Roux) following the decline of Nile perch and recovery of native haplochromine fishes, Lake Victoria, Tanzanian waters

The abundance of Nile shrimp, Caridina nilotica (Roux) in Lake Victoria has fluctuated significantly over time, from the periods of 1900–1950s, 1980s–1990s and from 2000s to present day. To elucidate its current abundance, its contribution to the Rastrineobola argentea fishery catches, and its importance to the composition of the diet of Nile perch (Lates niloticus L.), and to the bottom trawl catches, were investigated. From August 2006 to January 2007, the mean C. nilotica CPUE was 1.68 ± 1.20 kg boat−1 day−1 at Kijiweni and Igombe, and its 0.37% contribution to the R. argentea total catches was insignificant. Of the 230 Nile perch stomachs containing prey items, C. nilotica comprised 54% by volume of the diet of fish &amp;lt;50 cm TL; while haplochromines contributed 41% to the diet of fish &amp;gt;50 cm TL. Mean catch rates of C. nilotica in bottom trawls in 8 months between 2005 and 2008 ranged between 0 and 2.45 ± 2.50 kg hr−1. Low catches of the C. nilotica in the R. argentea fishery, and the dietary shift of Nile perch of &amp;gt;50 cm TL to once again include haplochromines, may indicate a decrease in C. nilotica's abundance in the lake, and vice versa. Overfishing and the selectivity of the fishery to take only large Nile perch for fish filleting factories, have resulted in reduced stocks and dominance of juveniles in the perch's populations. The highly reduced Nile perch stocks are currently leading to an apparent reversal of the 1980s regime with a shift to a new cichlid-dominated and C. nilotica low abundance state. Increase in predation on juvenile C. nilotica by the recovering haplochromines, and juvenile perches, as well as environmental degradation, especially eutrophication and pollution, along with the effects of global warming impacts, account for the observed decline in C. nilotica.

Antagonistic selection from predators and pathogens alters food-web structure

Selection can alter predator-prey interactions. However, whether and how complex food-webs respond to selection remain largely unknown. We show in the field that antagonistic selection from predators and pathogens on prey body-size can be a primary driver of food-web functioning. In Windermere, U.K., pike (Esox lucius, the predator) selected against small perch (Perca fluviatilis, the prey), while a perch-specific pathogen selected against large perch. The strongest selective force drove perch trait change and ultimately determined the structure of trophic interactions. Before 1976, the strength of pike-induced selection overrode the strength of pathogen-induced selection and drove a change to larger, faster growing perch. Predation-driven increase in the proportion of large, infection-vulnerable perch presumably favored the pathogen since a peak in the predation pressure in 1976 coincided with pathogen expansion and a massive perch kill. After 1976, the strength of pathogen-induced selection overrode the strength of predator-induced selection and drove a rapid change to smaller, slower growing perch. These changes made perch easier prey for pike and weaker competitors against juvenile pike, ultimately increasing juvenile pike survival and total pike numbers. Therefore, although predators and pathogens exploited the same prey in Windermere, they did not operate competitively but synergistically by driving rapid prey trait change in opposite directions. Our study empirically demonstrates that a consideration of the relative strengths and directions of multiple selective pressures is needed to fully understand community functioning in nature.

The role of the freshwater shrimp Caridina nilotica (Roux) in the diet of the major commercial fish species in Lake Victoria, Tanzania

The major commercial fish species of Lake Victoria at the present time are Lates niloticus, Oreochromis niloticus and Rastrineobola argentea. The contribution of Caridina nilotica in their diet was studied in the Tanzanian waters of Lake Victoria between March, 1999 and January, 2002. Stomach samples were collected during routine bottom trawl surveys in Tanzania. The results show that haplochromines dominate the diet of Nile perch, followed by C. nilotica, R. argentea, juvenile Nile perch, fish remains and other prey. Caridina nilotica predominance in diets decreased as size of the Nile perch increased, as this species switched to haplochromines. Larger perch also feed on their own juveniles. The importance of C. nilotica in the diet of L. niloticus was relatively greater in deeper water than that in shallower stations. The diet of O. niloticus was predominantly algae followed by C. nilotica, dagaa, Chaoborus, Odonata and others. O. niloticus has shifted its diet from strictly herbivory to a more omnivorous diet, feeding opportunistically on the most available food material. The overall diet of R. argentea was predominantly copepods, followed by Chaoborus, Cladocera, C. nilotica, Chironomids and insects. In the present food web, C. nilotica is an important food source for the fish stocks of Lake Victoria. The sustainability of the fisheries of Lake Victoria depends among other things on the abundance and availability of C. nilotica.