Ruffe Research Articles

Assessment and conservation of gwyniad (Coregonus lavaretus (L.)) in Llyn Tegid, U.K.: persistence in the face of eutrophication, water level fluctuations and ruffe (Gymnocephalus cernuus (L.)) introduction

The whitefish (Coregonus lavaretus (L.)) exists as only one native population in Wales, U.K., where it occurs in Llyn Tegid and is known as the gwyniad. This population is of national conservation importance but faces a number of environmental threats. Although eutrophication has not directly impacted on gwyniad habitat availability, associated low oxygen levels in the hypolimnion periodically approach its tolerance limit of 2 mg L-1. Anthropogenic water level fluctuations and a 1980 introduction of ruffe (Gymnocephalus cernuus (L.)), a predator of gwyniad eggs, also pose potential threats. Despite these concerns, gill-netting surveys in the 1960s, 1991, 2003 and 2008 have shown no marked changes in gwyniad population biology in terms of length, weight and condition factor. Furthermore, hydroacoustic surveys from 2003 to 2009 have shown an increasing trend in total fish abundance. On the basis of gill-netting data, this increase is shared by gwyniad which in July 2009 reached a population density of 2,302.2 fish ha-1 (lower and upper 95% confidence limits of 1,795.8 and 2,951.4 fish ha-1, respectively). Nevertheless, a precautionary approach has been adopted to the conservation of this unique population and from 2005 to 2007 a translocation programme transferred 81,300 eggs from 366 male and 50 female gwyniad to the nearby water body of Llyn Arenig Fawr. Monitored hatching rates were generally high and in 2009 a hydroacoustic and gill-netting survey revealed that although the fish community of this site was sparse (1.7 fish ha-1, with lower and upper 95% confidence limits of 0.8 and 3.8 fish ha-1), it now included adult gwyniad. Although it does not yet confirm successful translocation, this observation does demonstrate that the gwyniad is able to reach sexual maturity in the recipient site.

Annual development cycle of gonads of Eurasian ruffe (Gymnocephalus cernuus L.) females from lower Odra River sections differing in the influence of cooling water

The subject of this study was gonadal development of Eurasian ruffe females collected at three sites: the Regalica River carrying natural waters of the Odra River, Warm Canal to which cooling water from a power plant is discharged, and Lake Dąbie in which the waters from East Odra and Warm Canal are mixed. According to histological analysis, under the natural conditions in the lower Odra River, ruffe's spawning season usually begins in April and lasts till the end of May. The fish from Lake Dąbie started their spawning season in a similar period, while the fish from Warm Canal started their spawning season in March, a month earlier, and also finished their spawning season earlier. The size of oocytes from the ruffe collected in the Warm Canal at the beginning stage of vitellogenesis was smaller (mean diameter of 184.3 μm) and at the end of vitellogenesis larger (755.3 μm) than the size of oocytes from the fish collected at the other two sites (p < 0.05), although their sizes were within the typical range. In the gonads of the fish caught from June to September at all the sites, the presence of degenerating oocytes was confirmed. Folliculi atresia at the beginning of summer was observed in the last not-laid portion of oocytes, while the degeneration of a part of newly produced vitellogenic September oocytes was caused by a high temperature of the lower Odra River water, of over 20°C at that time. Both types of oocyte degeneration are the natural adaptations to the conditions of a changing natural environment.

Associations of fish with various types of littoral habitats in reservoirs

AbstractFish associations with different types of littoral habitats were studied in four canyon‐shaped reservoirs in the Czech Republic in years 2010 and 2011 by gillnets. Two to three habitats per reservoir–beaches (former meadows), stump fields (former forest) and rubble slopes–were defined and sampled along the longitudinal axis of reservoirs. Effects of reservoir, habitat and locality (position along longitudinal axis) on fish biomass, abundance and species structure were tested for juvenile and adult fish separately. Hierarchical analysis of variance revealed that habitats differed significantly in fish biomass and abundance. Redundancy analysis showed that analysed environmental variables had significant influence on fish community structure. Most variability in community structure was explained by reservoir and then by combination of habitat and slope steepness. Locality position had the smallest influence on community structure. For both adult and juvenile fish total abundance and biomass, the most inhabited habitat was beaches; rubble slopes were the least inhabited. Habitat associations differed among species. Among adults, bream Abramis brama, white bream Blicca bjoerkna and roach Rutilus rutilus were associated with beaches and stump fields, whereas perch Perca fluviatilis, ruffe Gymnocephalus cernuus, asp Aspius aspius and pike Esox lucius were associated with rubble slopes. Bream, white bream, bleak Alburnus alburnus, roach, ruffe and pikeperch Sander lucioperca were associated with beaches among juveniles, whereas the only juvenile associated with rubble slopes was perch. We showed that most common species are associated with distinct habitats and also that utilisation of various littoral habitats differs in general.

Long-term effects of extreme weather events and eutrophication on the fish community of shallow Lake Peipsi (Estonia/Russia)

The fish kill in lake Peipsi (Estonia/Russia) during the extraordinarily hot summer of 2010 evoked an investigation into the effects of environmental extremes and long-term eutrophication on the fish community of the lake. Current data on lake Peipsi indicate that temperature extremes and synergistic interactions with eutrophication have led to a radical restructuring of the fish community. Commercial landings of lake smelt, Osmerus eperlanus eperlanus m. spirinchus (Pallas), the previous dominant species of the fish community, have decreased dramatically since the 1930s, these declines being coupled with summer heat waves coinciding with low water levels. Gradual decline in smelt stock and catches was significantly related to a decline of near-bottom oxygen conditions and to a decrease in water transparency. The first documented fish kill in 1959 occurred only in the southern, most shallow and eutrophic lake (lake Pihkva). Recently, summer fish kill have become more frequent, involving larger areas of the lake. In addition to the cold-water species, e.g. smelt and vendace Coregonus albula (L.), the abundance of bottom-dwelling fishes such as ruffe Gymnocephalus cernuus (L.) and juvenile fish have significantly decreased after the 2010 heat wave probably due to hypoxia and warm water temperatures. This study showed that fish community structure in large shallow lakes may be very vulnerable to water temperature increases, especially temperature extremes in combination with eutrophication.

The population biology and life history traits of Eurasian ruffe [Gymnocephalus cernuus (L.), Pisces: Percidae] introduced into eutrophic and oligotrophic lakes in Northern Italy

We describe the population biology and life-history traits of two Eurasian ruffe [Gymnocephalus cernuus (L.), Pisces: Percidae] populations which have recently colonised two small lakes in the lake Maggiore catchment of Northern Italy, the eutrophic lake (l.) Ghirla (ruffe first recorded in the early 1990s) and the oligotrophic lake (l.) Mergozzo (ruffe first recorded during the present sampling in 2010). Survey gill net catches revealed that ruffe has become one of the most abundant fish species in both lakes. Relative weight and weight-length relationships indicated better growth of ruffe in l. Mergozzo. Thus, gonad weight, adjusted for body size, was higher in l. Mergozzo, indicating a stronger individual reproductive potential in the more recently colonised lake. The stomach contents of ruffe were dominated by chironomid larvae and other benthic organisms, while pelagic microcrustaceans only occurred in small amounts. The ratio of benthic vs pelagic prey in the diet increased with ruffe size. Ruffe intraspecific food niche overlap (based on prey numbers) between age classes was slightly higher in oligotrophic l. Mergozzo than in eutrophic l. Ghirla and decreased with age in both lakes

Evidence of a recent decline in river lamprey Lampetra fluviatilis parasitism of a nationally rare whitefish Coregonus lavaretus: is there a diamond in the ruffe Gymnocephalus cernuus?

Lamprey-induced scarring of the nationally rare Coregonus lavaretus, a known host of a freshwater-resident population of European river lamprey Lampetra fluviatilis, was found to have declined precipitously since the establishment of several non-native fishes in Loch Lomond. Evidence presented in this study points to the possibility that L. fluviatilis in this lake may have altered its trophic ecology in response to the negative impact that non-native species, in particular ruffe Gymnocephalus cernuus, have had on their favoured host.

Interactions between invading benthivorous fish and native whitefish in subarctic lakes

Summary Many species are expanding their distribution towards higher latitudes and altitudes in response to climate change. These range shifts are expected to change fish community structure and alter food‐web dynamics in subarctic lakes. However, the impacts of invading species on native fish and invertebrate prey communities remain understudied. The trophic ecology of invasive species determines the likelihood of direct resource competition with native taxa. In Northern Europe, perch (Perca fluviatilis), a trophic generalist, and ruffe (Gymnocephalus cernuus), a benthic specialist, are expanding their distribution ranges northwards, colonising lakes inhabited by a native generalist, whitefish (Coregonus lavaretus). We predicted that increased fish diversity and density would deplete the invertebrate community and increase resource competition between native and invasive species. To assess the degree of resource competition between native and invasive species, we compared (i) fish and invertebrate community structure; (ii) diet and stable carbon and nitrogen ratios of whitefish, ruffe and perch and (iii) growth, condition and relative population size of whitefish in two non‐invaded lakes with two lakes containing one and two lakes containing both invasive species. Each lake was sampled on a single occasion between August and September. Benthic macroinvertebrate density and community structure were unaffected by increased consumer diversity, while top‐down control of pelagic zooplankton density and size was evident in lakes with increased fish diversity. Differences in diet and stable isotope ratios were evident between all whitefish populations, although these were not directly related to the presence of invasive species. Specialised adaptations of invasive species may confer a competitive advantage in invaded lakes; ruffe dominated the profundal niche, while perch displayed an ontogenetic shift to piscivory, reducing niche overlap with native whitefish. Growth rate and population density of whitefish were largely independent of fish community structure and were governed by local variations in lake productivity. However, there was a sign of lowered condition of whitefish in invaded systems. Shallow and more productive lakes with higher food availability supported populations of native and invasive species. Our findings indicate that trophic specialisations of invasive species play a key role in determining their impacts on the systems they invade. This study focussed on early stages of invasion, and the outcome of species interactions may change following the establishment of new species. In addition, these impacts will not be uniform across the invaded landscape as lake‐specific variations in morphometry and resource availability will alter the competitive balance between native and invasive species.

Within‐lake distribution patterns of fish assemblages: the relative roles of spatial, temporal and random environmental factors in assessing fish assemblages using gillnets in a large and shallow temperate lake

In this study, the relative role of spatio-temporal factors and associated environmental variables (water transparency and temperature) were quantified in relation to gillnet samples of fishes in a large and shallow lake (Lake Balaton, Hungary). Most of the variance (56·1%) in the relative abundance data (%) was related to the vertical segregation of fishes. This gradient substantially affected the catch per unit effort (CPUE) by number of the dominant species, the surface-oriented bleak Alburnus alburnus and the benthic common bream Abramis brama. It also influenced total CPUE, mean fish mass and species richness and diversity. At the lake level, horizontal habitat heterogeneity (i.e. littoral v. offshore) accounted for only 8·3% of the total variance in relative abundance data, but was important in structuring the CPUE of the ruffe Gymnocephalus cernua and the pikeperch Sander lucioperca. The longitudinal environmental gradient (i.e. lake basin), year and season of sampling, water transparency and temperature had significant effects on relative abundance only at the habitat level, but were also important components of variability of CPUE in some species at the lake level. As sampling schemes need to consider the main gradients in fish assemblage distributions, the use of surface and pelagic gillnets should be more intensively incorporated in the study and monitoring of fish assemblages in shallow lakes and lake habitats.

Stock status and current tendencies in the population dynamics of commercial fishes of Pskov-Chudskoe water body

Dynamics of eight commercial populations of fish from Pskov-Chudskoe Lake forming the main bulk (up to 80%) of annual catch was analyzed using an ADAPT-VPA model making it possible to reconstruct the dynamics of the total stock biomass and fishing mortality. The populations of five species (European smelt Coregonus albula, whitefish C. lavaretus maraenoides, perch Perca fluviatilis, roach Rutilus rutilus, ruffe Gymnocephalus cernuus) in the present period demonstrate a tendency for a decrease in biomass while the stocks of two species (bream Abramis brama and zander Sander lucioperca) beginning from the 1980s considerably increased. A particularly drastic (multifold) increase in biomass is recorded in zander population. The causes of the observed population changes are related mainly to ecosystemic transformations taking place in the water body; only with respect to two species-perch and roach-can one state that the decrease in stocks is determined in part by the fishery impact. The increase in zander population is, in all probability, the result of the combined effects of eutrophication forming favorable conditions for the feeding and survival of its juveniles and fishery that led to the decrease in the biomass of one of its main food competitors-perch. The drastic increase in zander numbers in turn promoted the decrease in the biomass of species being objects of its feeding.

A simple method to correct the results of acoustic surveys for fish hidden in the dead zone

Summary In lentic freshwater systems, vertical acoustics may underestimate fish abundance in the acoustic dead zone where fish detection capability is limited. To estimate this bias, the height of fish above the lead-line of a benthic multi-mesh gillnet (1.5 m high) was used to quantify both the vertical distribution of fish near the bottom and the proportion residing within the acoustic dead zone. The study was carried out at the percid-dominated Biesbosch Reservoirs in the Netherlands. Acoustic dead zones were estimated at 7 cm above flat bottoms, and 12–34 cm above 8° sloped bottoms at depths of 5–27 m, respectively. Depending on the habitat, 36 to 75% of the gillnet catch by number was present in the acoustic dead zone, representing 5–51% of the biomass. Near-bottom depths were highly preferred by ruffe Gymnocephalus cernua, often used by perch Perca fluviatilis and pikeperch Sander lucioperca, plus seemingly devoid of smelt Osmerus eperlanus. The total amount of fish hidden in the acoustic dead zone was estimated to be 13–39% of the whole water column. The proportion of biomass obscured in the dead zone was lower (1–12%). The conclusion is that undetected fish in the acoustic dead zone can seriously bias density assessment, which can be corrected by concurrent sampling with benthic gillnets.

Eel attacks—A new tool for assessing European eel (Anguilla anguilla) abundance and distribution patterns with gillnet sampling

Because of its complex life cycle, cryptic behavior, body shape, ability to excrete mucus and excellent maneuvering, the European eel ( Anguilla anguilla ) is difficult to monitor. During many years of gillnetting in Dutch and Czech freshwaters, we registered characteristic involutions of gillnet netting with a partially eaten fish inside. We concluded that these involutions were the result of eels attacking fish caught in gillnets and called the occurrences eel attacks. When we compared the abundance of eel attacks in gillnets with the abundances of eels recorded using active gears (beach seining and trawling), we found a positive and significant correlation (partial Spearman R = 0.640) that can be used for rough eel-abundance assessment. The most frequently attacked fishes were young individuals of abundant species ( Perca fluviatilis , Sander lucioperca , Rutilus rutilus and Osmerus eperlanus ) and small species ( Gymnocephalus cernua ) up to 100 mm standard length. The eels preferred pikeperch and avoided roach and smelt. The reason for prey species preferences was most likely prey species distribution rather than size selectivity. The eels attacked fishes in gillnets during the night, most likely after midnight. The eel attacks were not distributed homogenously among the studied reservoirs, geographical areas and years. The frequency of eel attacks was higher in benthic than in pelagic habitats. Eel attacks were more frequent in Dutch than in Czech reservoirs, corresponding to a general decline of eel densities with increasing distance from the sea. The number of eel attacks also declined significantly from 1998 to 2008. The present study showed that eel attacks can be successfully used as a new, simple and nonintrusive tool for monitoring eel abundance using gillnets. Gillnets are widely used in fish monitoring, and hence, gillnet sampling performed throughout Europe has a great potential for eel-abundance assessment.

Conservation of the vendace (Coregonus albula), the U.K.’s rarest freshwater fish

Although also formerly present in two lochs in South West Scotland, populations of vendace (Coregonus albula) were until recently believed to persist in only two U.K. lakes, i.e., Bassenthwaite Lake and Derwent Water in North West England. However, although Derwent Water has retained its natural mesotrophic conditions and the status of its vendace population is acceptable, monitoring at Bassenthwaite Lake has failed to record any specimens since 2000 and the species has recently been declared locally extinct. Bassenthwaite Lake has experienced increasing problems from eutrophication, sedimentation and the introduction of fi sh species including roach (Rutilus rutilus) and ruffe (Gymnocephalus cernuus), both of which have more recently also been introduced to Derwent Water and give concern over possible food competition and egg predation. In addition to monitoring, considerable efforts have been made in recent years to conserve vendace through the protection and improvement of their habitats and the establishment of refuge populations. The latter has resulted in the establishment of a population originating from Bassenthwaite Lake in Loch Skeen of South West Scotland, with further attempts still in progress at two other sites. Public awareness of the conservation of this species, which is the U.K.'s rarest freshwater fish, has also been actively promoted.

Evaluating the physical removal of ruffe (Gymnocephalus cernuus) with bottom trawling

The ability to reduce a population of ruffe (Gymnocephalus cernuus) with intense repeated bottom trawling was examined at two locations in Chequamegon Bay of Lake Superior. Trawling significantly depleted the population at the Kakagon River site but not at the Second Landing site. At least 93% of the estimated population of ruffe was removed from the Kakagon River site, but ruffe re-established in the area within 1 year. The relative success of short-term reduction at the Kakagon River site was likely due to habitat and biological characteristics that led ruffe to congregate in large numbers. Physical removal does not appear to be an effective way to effect long-term removal of ruffe.

An invasion record for the swimbladder nematode Anguillicoloides crassus in European eel Anguilla anguilla in a deep cold-monomictic lake, from invasion to steady state

This study is the first account of the establishment and development of the neozoic nematode parasite Anguillicoloides crassus in its host, the European eel Anguilla anguilla, in a deep, warm-monomictic [corrected] lake. A 21 year study of A. crassus took place in Upper Lake Constance (ULC), Europe's second largest pre-alpine lake. The study included two extensive surveys, one in 1991 during the initial parasite invasion phase and the second in 2006 when the infection was well established. The subtropical swimbladder nematode A. crassus was first recorded in A. anguilla in ULC in 1989. Prevalence reached 60% in 1992 and remained at this level until 2007. In 2008, prevalence decreased to 48%. Infection intensity peaked in 1993 at a mean value of 16 adult parasites per host fish. Around 90% of all A. anguilla examined displayed swimbladder lesions, with a significant trend to increasing severity over time. Moreover, heavy swimbladder lesions were seen in c. 10% of A. anguilla ready to migrate to their spawning habitat. Both ruffe Gymnocephalus cernuus and sunfish Lepomis gibbosus serve as paratenic hosts for A. crassus in ULC. Gymnocephalus cernuus seems to be the main vector, and infection is especially frequent in spring possibly caused by reduced immune system efficacy of G. cernuus during winter. In 1991, hypochromic anaemia was prevalent in ULC A. anguilla acutely infected with A. crassus, whereas in 2006 blood values were indicative of chronic infection. The growth and survival rates of A. anguilla during their continental phase were not noticeably altered in infected fish, but damage to the swimbladder probably impairs migration potential and thus the subsequent breeding success of the oceanic phase.

The influence of diel period on fish assemblage in the unstructured littoral of reservoirs

Abstract Diel changes in littoral fish assemblage were studied in four reservoirs in the Czech Republic (Central Europe). The sampling was performed by beach seining in an unstructured littoral zone. Perch, Perca fluviatilis L., roach, Rutilus rutilus (L.), bream, Abramis brama (L.), carp, Cyprinus carpio L., ruffe, Gymnocephalus cernua (L.), pikeperch, Sander lucioperca (L.), eel, Anguilla anguilla (L.) and A. brama × R. rutilus hybrids exhibited higher densities at night. Only bleak, Alburnus alburnus (L.) exhibited higher densities during the day. The number of species was higher in night hauls, and mean body size of roach and bleak was also higher at night. Diel changes in fish densities resulted in the change in species composition between day and night. The study has implication for the sampling design to assess littoral fish assemblages using beach seine netting and recommends night sampling for a representative assessment or sampling during both diel periods for a robust assessment.

Winter diet of great cormorant (Phalacrocorax carbo) on the River Vltava: estimate of size and species composition and potential for fish stock losses

Abstract. The winter diet of the great cormorant (Phalacrocorax carbo) was studied by means of examining regurgitated pellets, individual fish bones and fish remains collected from below the roosting trees in two sites on the River Vltava in Vyssi Brod and at Slapy Reservoir, Czech Republic, and by analysis of stomach contents of birds shot on the River Vltava in Prague. Using diagnostic bones (os pharyngeum, dentale, maxillare, praeoperculare) and own linear regression equations between measured dimension of the diagnostic bone and fish total length (LT), a total of 1152 fish of 22 species and 6 families were identified in the diet of great cormorants and their sizes were reconstructed. At all three localities on the main stream of the River Vltava, roach (Rutilus rutilus), bream (Abramis brama), bleak (Alburnus alburnus), European chub (Squalius cephalus), European perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus) made up at least 74.2 % of the cormorants' diet. A great potential for fish stoc...

Establishment patterns of non-native fishes: Lessons from the Duluth–Superior harbor and lower St. Louis River, an invasion-prone Great Lakes coastal ecosystem

We conducted a 2-year, multi-gear survey in the lower St. Louis River, which includes the Duluth–Superior harbor, an international shipping port and non-native species invasion “hotspot.” Our objectives were to quantify the contribution of non-native species to the overall fish assemblage and assess their spatial distribution and abundance. We captured 10 non-native fishes; none, however, were first detections. Non-native fishes composed roughly one quarter of the total species richness, were found in 84% of samples, and composed 15% of the total abundance. The spatial distribution and abundance of non-native fishes was dependent on gear selection (and thereby habitat selection), sampling location, and abundance measure (individuals or biomass). We used a recently published non-native species establishment framework to integrate catch data from the different gears. Viewed in the context of this framework, we identified only two non-native fishes, Eurasian ruffe ( Gymnocephalus cernuus) and round goby ( Neogobius melanostomus), as both widespread and abundant, whereas we identified three as localized and rare. Moreover, the time since first detection of non-native fishes was not predictive of their frequency of occurrence across the study area, underscoring the importance of environmental and biological factors in controlling fish establishment success. Although non-native fishes constitute a considerable portion of the trawl catch in the river channel, from a multi-gear system-wide perspective, native fishes collectively make up the majority of the fish biomass and abundance in the lower St. Louis River.

An experimental approach to estimating vulnerability of European whitefish (Coregonus lavaretus) ova to predation by invasive ruffe (Gymnocephalus cernuus)

Abstract – The complexity of substrate covering spawning grounds of Coregonus lavaretus is assumed to have a role in the protection of incubating ova from predation. It is believed that the Loch Lomond population of C. lavaretus is adversely affected by invasive ruffe (Gymnocephalus cernuus) predation on eggs. To discover the protective ability of substrate commonly found on whitefish spawning grounds, predation experiments of ruffe on artificial eggs were conducted. These were presented to ruffe over different substrates: sand, gravel, pebbles and cobbles. It was found that the greatest protection was provided by pebbles and gravel. Eggs are exposed on sand, but are protected by small gaps between pebbles and gravel, while in cobbles the gaps between substrate particles are large enough to sometimes allow ruffe to foraging within the substrate. Using these results, a comparison between the potential protective ability of substrates of spawning grounds in four Scottish whitefish sites was attempted.

The dependence on temperature and pH of the effects of zinc and copper on proteolytic activities of the digestive tract mucosa in piscivorous fish and their potential preys

The dependence of the effects of zinc and copper on the activities of proteinases of the stomach and intestinal mucosa on temperature and pH in four species of boreal piscivorous fish (pike Esox lucius, zander Zander lucioperca, perch Perca fluviatilis and burbot Lota lota) as well as in some of their potential preys (kilka Clupeonella cultriventris, ruff Gymnocephalus cernuus, perch and roach Rutilus rutilus) was investigated. Species-specific differences of the effects of these heavy metals upon the activities of proteinases depending on temperature and pH were demonstrated. It was revealed that the stomach mucosa proteinases were more tolerant to the effects of the studied factors than the intestinal mucosa proteinases, especially true for pike. The effects of the heavy metals on the whole body proteinases of the fishes' potential preys were mostly dependent on temperature than on pH. At pH 3.0, the negative action of zinc and copper on the fish digestive tract mucosa proteolytic activity to a considerable degree was compensated by the high activity of the hemoglobinlytic proteinases, probably, cathepsine D.

Eye fluke-induced cataracts in natural fish populations: is there potential for host manipulation?

Manipulation of host phenotype (e.g. behaviour, appearance) is suggested to be a common strategy to enhance transmission in trophically transmitted parasites. However, in many systems, evidence of manipulation comes exclusively from laboratory studies and its occurrence in natural host populations is poorly understood. Here, we examined the potential for host manipulation by Diplostomum eye flukes indirectly by quantifying the physiological effects of parasites on fish. Earlier laboratory studies have shown that Diplostomum infection predisposes fish to predation by birds (definitive hosts of the parasites) by reducing fish vision through cataract formation. However, occurrence of cataracts and the subsequent potential for host manipulation in natural fish populations has remained poorly explored. We studied the occurrence of eye fluke-induced cataracts from 7 common fish species (Gymnocephalus cernuus, Rutilus rutilus, Leuciscus leuciscus, Alburnus alburnus, Osmerus eperlanus, Coregonus lavaretus and Gasterosteus aculeatus) from the Bothnian Bay in the Baltic Sea. We found that the parasite-induced cataracts were common in fish and they also reached high levels which are likely to predispose fish to predation. However, we observed such cataracts only in species with the highest parasite abundances, which suggests that only certain hosts may be strongly affected by the infection.