Impact of municipal effluents and hydrological regime on myxozoan parasite communities of fish

Increased productivity from sewage effluents can enhance species richness locally. Results from a study of spottail shiners (Notropis hudsonius) in 1999 showed that prevalence and the mean number of myxozoan parasite species per host were higher downstream of the wastewater outflow from the Island of Montreal than upstream in the St. Lawrence River, Quebec, Canada. This was attributed to organic enrichment of the sediments which presumably lead to increased densities of oligochaetes, the alternate hosts, downstream of Montreal. Spottail shiners subsequently were collected every August/early September in 2001–2004 to examine the stability and repeatability of these patterns. Prevalence and mean number of myxozoan species per fish typically were again higher downstream of the sewage source each year compared to upstream, although there was no significant difference in these measurements pooled across years between localities immediately upstream and downstream of the effluent outflow. Density of the oligochaete Limnodrilus hoffmeistereri, a common alternate host of myxozoans, was much higher at two downstream localities than at an upstream one. At a larger spatial scale, mean myxozoan infracommunity richness across sites in the St. Lawrence River was negatively correlated with mean water levels measured in the 3 months prior to fish sampling. Results suggest that on a local scale, variations in prevalence and diversity among localities are influenced by municipal effluents, but that at a landscape scale annual variations across sites are affected by the hydrological regime and climate. In effect, water level fluctuation had a landscape-wide impact that was superimposed over pollution-induced local variations.     

Changes in numbers and growth of Ligula intestinalis in the spottail shiner (Notropis hudsonius), and their roles in transmission

Eighteen species of fishes were collected during the open water (1985-1987) and winter seasons (1985-1986) from Dauphin Lake, Manitoba, Canada (51 degrees 17'N, 99 degrees 48'W) and examined for plerocercoids of Ligula intestinalis (L.). Plerocercoids were most prevalent (5.3%) in spottail shiners (Notropis hudsonius), the major fish host for Ligula in Dauphin Lake. Detailed analysis of the spottail shiner-Ligula host-parasite system revealed that the number of plerocercoids differed between years and among habitats but there was no statistically significant seasonal pattern; recruitment of new worms was highest in young fish and decreased with age, and infected spottails had reduced gonad development. Analysis of host and parasite growth revealed that the soma of male spottails infected with Ligula weighed more but had a reduced growth rate. Growth rate of females was unaffected by Ligula but somatic weight was slightly increased. This appears to be related to the greater metabolic stress on infected males. Based on increased mean somatic weight and skewness of the weight distribution for infected fish, we propose that infected spottails are subjected to size-selective mortality. Some of the contradictions in the literature may be attributed to underestimated prevalence due to increased numbers of spawning fish in the spring, mixed age-classes of Ligula in separate age-classes of fish, and differential effects on growth due to infection in male versus female hosts.     

Ultrastructural changes in fish gills as biomarker to assess small stream pollution

In order to verify the principal suitability of gill ultrastructure as a biomarker, semi-field studies with two endigoneous fish, trout (Salmo trutta f. fario) and loach (Barbatula barbatula), were performed. The fish were exposed in flow-through systems to one heavily polluted (K?rsch) and one lightly polluted small stream (Kr?henbach) in South-West Germany. Ultrastructural responses in gills were correlated with limnological and chemical data recorded over a 2 year period in each stream. After 8 weeks of exposure to the heavily polluted stream, fish showed ultrastructural changes in the gills, such as cell proliferation, dilation of the endoplasmic reticulum, hyperplasia, hypersecretion, and epithelial lifting in chloride, epithelial, and mucus cells. The results of the study demonstrate that ultrastructural reactions in the gills of fish kept under semi-field conditions are potentially useful biomarkers indicating small stream pollution.     

Spatial variation in relative abundance of a widespread, numerically dominant fish species and its effect on fish assemblage structure

Collections of fish assemblages from streams in the midwestern United States were used to examine assemblage-level effects of spatial variation in relative abundance of the red shiner, Cyprinella lutrensis, a widespread and highly abundant minnow species. This species has been widely introduced outside its native range and is suspected to have impacted local assemblages where it has become established. Given its overall dominance of midwest fish assemblages, and its suspected impact on assemblage structure, we asked if structure of the residual fish assemblages (red shiners excluded) was a function of the relative abundance of red shiners throughout the native range of C. lutrensis in the USA. Although red shiner ranked first in abundance in half of the assemblages and numerically dominated 28% of the assemblages, red shiner relative abundance in an assemblage had no detectable effect on richness, diversity, evenness, or complexity of other (residual) species in the assemblage. Relative abundance of red shiners did have a positive effect on the abundance of benthic minnows in the residual assemblage, but not on water column minnows that are ecologically most like red shiners. Environmental factors did not explain a significant amount of the variation in relative abundance of red shiners, but did explain some variation in residual assemblage structure. Although widespread and numerically dominant at many localities, red shiners do not appear to have a strong impact on local fish assemblage structure within their native range. This is in sharp contrast to the reported negative effects of red shiners on fish assemblages where they have been introduced outside their native range.     

Diel variation in diet composition of a riverine fish community

Although diel food habit studies have been undertaken on a number of individual species, few studies have examined diel variation in the diets of fish communities. We examined the diel diet variation and feeding periodicity of a fish community in the Juniata River, Pennsylvania. Nine species, totalling 1,098 fish, were collected at 4-h intervals over a 24-h period in October 1989, in numbers sufficient to describe their diel variation in diet composition. Diel variation in diet composition was evident in all species, as no single prey taxon was dominant in the diet of any species during any 4-h interval. Ephemeropterans were the most important prey taxa for four species of centrarchids, whereas chironomids were the main prey of banded killifish, mimic shiners, and spotfin shiners. Algae was the major component in the diet of spottail shiners, whereas bluntnose minnows contained mostly detritus. Feeding activities of rock bass, redbreast sunfish, and pumpkinseed occurred at low levels throughout the day; peak feeding occurred from 2000 to 0400 hours. Food consumption of smallmouth bass increased throughout the day with peak consumption occurring at 2000 hours. Non-centrarchids fed little during daylight hours and showed peak activity at 2000–2400 h. Construction of a 24-hour diet from six 4-h interval estimates and feeding periodicity data provided a comprehensive representation of the diel feeding ecology of all species collected.     

The use of fish metabolic, pathological and parasitological indices in pollution monitoring

  An integrated biological effect monitoring concept has been tested in flounder (Platichthys flesus L.) from four locations with different anthropogenic impact in the German Bight. During 3 years of sampling, biomarkers at all levels of biological organisation from the molecular to the ecosystem level were applied and tested on 742 individual fish of body lengths between 18 and 25 cm. At the ecosystem level, the fish were taken as a habitat for the parasite assemblage. The hypothesis was that changes in the environment might lead to changes in the species diversity of parasites and in the infection intensity of single species, as well as between heteroxenic and monoxenic parasite species (H/M ratio). At the molecular level, activity of the CYP1A-dependent monooxygenase ethoxyresorufin O-deethylase (EROD) was used as a biomarker of exposure. At the subcellular level, the integrity of lysosomal membranes in hepatocytes was taken as an indicator of non-specific acute and chronic toxic effects. Both biomarkers are recommended by the ICES Advisory Committee on the Marine Environment for the application in biological effects monitoring programmes. In addition, neutral lipid content in the liver was used as a marker for pathologically induced fat accumulation. In the same individual fish, a new method for the measurement of macrophage aggregate activity in the liver was tested for its application and reliability in reflecting immunosuppression. Tests were accompanied by chemical analysis of standard organochlorine and heavy metal residues in flounder tissue. A total of 33 parasite species were found. As an indicator species, the mean abundance of Trichodina sp. reflected best the pollution gradient observed with highest infection intensity at the most polluted location. Species diversity was significantly higher in fish caught near the reference site and significantly lower in fish from the polluted Elbe estuary. The use of the heteroxenous/monoxenous species ratio as a marker was not useful at the locations investigated because of the dominance of heteroxenous species at all habitats. Since EROD activity and macrophage aggregate activity were dependent on sex and maturity of female flounder, only male fish were taken into consideration for the integrated evaluation of data. All biochemical and histochemical tests were able to reflect accurately the site-specific differences, as well as an observed pollution event at the end of 1995 as determined by chemical analyses. The correlation analysis revealed a connection not only between the single parasitological and biochemical parameters but also within these groups. The non-specific immune response and Trichodina infection intensity were correlated with all other parameters, leading to the assumption that these may serve as links between the lowest and the highest levels of biological organisation. The simultaneous use of metabolic and parasitological results facilitated the interpretation of the observed variations of the data and the distinction between natural variation and pollution-mediated effects. Received: 1 March 1999 / Received in revised form: 25 May 1999 / Accepted: 7 June 1999     

Histopathological biomarkers and genotoxicity in gill and liver tissues of Nile tilapia Oreochromis niloticus from a polluted part of the Nile River,Egypt

Fish health is affected by water pollution. Oreochromis niloticus collected during summer 2014 from El-Serw, a polluted site on the Nile River, were compared with fish from a reference site, El-Zamalek. Histopathological changes were detected in gill and liver tissue samples using light and electron microscopy. In addition, the degree of DNA damage was measured using the comet assay. To indicate the severity of water pollution at the two sites, physico-chemical properties and heavy metal concentrations were investigated. Gill damage, including lamellar cell hyperplasia and aneurysm, was observed in the fish samples from the polluted site. The livers of fish from the polluted area showed necrosis and an increase in melanomacrophage centres. Histochemical results confirmed a marked rise of gill mucopolysaccharides and a reduction of carbohydrate stored in hepatocytes. Electron microscopy revealed clear alterations in gill and liver tissue of fish from the polluted site. The comet assay showed highly significant DNA damage in tilapia collected from the polluted site, compared to those from the reference site. Histopathological biomarkers and the comet assay may therefore be sensitive indicators of exposure to mixtures of aquatic pollutants in Nile tilapia.     

Feeding,growth, and particle-size-conversion efficiency in white sucker larvae and young common shiners

Synopsis Instantaneous feeding, assimilation, and growth rates for white sucker larvae and young common shiners and guppies (1–2 mg initial dry weight) were measured. Food consisted of natural zooplankton. Linear relationships were obtained between growth and feeding rates for instantaneous feeding rates up to 0.5 mg^-1 day^-1. Maximum gross conversion efficiencies (K₁) were 0.26, 0.31, and 0.24 for suckers, shiners, and guppies respectively. Particle-size-conversion efficiency (-log K₁ per log predator-prey size ratio) was 0.15 for suckers, 0.14 for shiners, and 0.18 for guppies. These values are lower than most available estimates of particle-size-conversion efficiency for carnivorous zooplankton and piscivorous fish, indicating that young planktivorous fish are among the most efficient predators at transferring biomass up the biomass size spectrum in aquatic ecosystems.     

The myxozoan fauna of spottail shiner in the Great Lakes basin: membership, richness, and geographical distribution

Spottail shiner (Notropis hudsonius) from localities in each of the Great Lakes plus some nearby waterbodies, i.e., the St. Lawrence River, and the Chester River, Maryland, were examined for myxozoan parasites. A total of 10 species was found, including 7 histozoic (Myxobolus sp.; M. algonquinensis Xiao and Desser, 1997; M. bartai Salim and Desser, 2000; M. xiaoi Salim and Desser, 2000; M. fanthami Landsberg and Lom, 1991; M. hendricksoni Mitchell, Seymour, and Gamble, 1985; Thelohanellus notatus Mavor, 1916) and 3 coelozoic (Chloromyxum sp., Zschokkella sp., Sphaerospora sp.) representatives. Infracommunity richness varied from 0 to 5 species per fish; mean infracommunity richness varied from 0 to 2.5 species. Component community richness varied from 0 to 7. Significant positive correlations were observed between mean and maximum infracommunity richness and component community richness. Similarly, maximum prevalence of each species at any 1 site was positively correlated with geographic range as measured by number of localities where a parasite species was encountered. Individual species occurred independently of each other. Representative histozoic and coelozoic species displayed similarly widespread distributions from Wisconsin to Maryland, but overall, histozoic species were dominant members within component communities. The study concludes that, under the present taxonomic paradigm, species parasitizing spottail shiner appear to be part of a larger network that cycles, in varying degrees, through certain other cyprinid and catostomid fish. The challenge of future research is to determine whether each parasite species constitutes single or multiple genetically isolated populations.     

Parasites as fish population tags and pseudoreplication problems: the case of striped red mullet Mullus surmuletus in the Spanish Mediterranean

Studies of parasites as fish population tags often apply a single round of sampling to identify potential stocks or predict harvest localities. However, the lack of replication generates pseudoreplication, implicitly assuming that infection levels are more similar between samples from the same locality than between samples from different localities. We evaluated this assumption in the case of the striped red mullet Mullus surmuletus in three localities of the Spanish Mediterranean separated by c. 300 km. Samples of 25 fish of similar size were collected in each locality in the summer and autumn of two consecutive years. Prevalence and abundance of three long-lived parasite taxa differed significantly among localities, indicating their potential as stock indicators. However, a cluster analysis (for prevalence) and a MANOVA (for abundance) indicated strong inter-sample variability, even within the same locality, with poor spatial segregation among samples. A linear discriminant analysis (LDA) based on the abundance of 17 parasite taxa correctly assigned over 80% of fish to their locality, and 95% bootstrap confidence intervals of percent classified fish per locality were narrow, indicating good and stable predictive power. However, when a LDA based on data from the first year was used to predict the locality of fish from the second year, predictive power dropped drastically (46% of correct allocation). Overall, we interpret that parasite communities of mullets change at a much lower spatial scale than that adopted in this study. This finding strongly suggests the need for proper replication to make reliable inferences about stock structure in fish populations based on parasitological data.     

Persistence of Dactylogyrus eucalius (Monogenea: Dactylogyridae) on the short-lived host Culaea inconstans (Pisces: Gasterosteiformes)

The monogene Dactylogyrus eucalius Mizelle and Regensberger, 1945 and its ability to maintain a population from year to year on the annual fish Culaea inconstans Kirkland was examined in a small lake in central Ontario. Fish were sampled toward the end of their annual breeding season, at a time when the host population consisted of 2 cohorts, i.e., young-of-the-year (0+) and mature adults (1+). Prevalence of infection was 94%, with a mean intensity of 8.8 +/- 9.6; neither measure varied significantly with host length or between cohorts (P > 0.05). At necropsy, parasites were characterized as juveniles that included postoncomiracidia (immature, with a ventrally directed haptor) as well as developing protandrous males (body with a near-complete haptor and with little or no pigmented vitellaria), or as adults (with testis, ovarium, darkened vitellaria, and occasionally bearing a tanned egg). The proportion of juvenile to adult parasites differed significantly between cohorts (P < 0.05), with 0+ fish infected with a mixture of juveniles and adults, whereas 1+ fish had almost exclusively adult parasites. Since adult (1+) brook stickleback typically die after spawning, the increased frequency of juvenile parasites exploiting juvenile hosts may represent an evolutionary adaptation, maximizing the chances of parasites infecting hosts that will enter winter. It is suspected that 0+ fish can be infected in the nest within 2 wk of hatching and persist by effectively infecting new host recruits when they are sympatric with their parents.     

Inspection and evaluation of host plant by the butterfly Mechanitis lysimnia (Nymph., Ithomiinae) before laying eggs: a mechanism to reduce intraspecific competition

Parasites of all kinds affect the behaviour of their hosts, often making them more susceptible to predators. The associated loss in expected future reproductive success of infected hosts will vary among individuals, with younger ones having more lose than older ones. For this reason, young hosts would benefit more by opposing the effects of parasites than old ones. In a laboratory study, the effects of the trematode Telogaster opisthorchis on the anti-predator responses of the upland bully (Gobiomorphus breviceps) and of the common river galaxias (Galaxias vulgaris) were examined in relation to fish age. In a bully population where parasites were very abundant, the magnitude of the fish's anti-predator responses decreased as the number of parasites per fish increased, and this effect was significantly more pronounced in age 2 + and, to a lesser extent, age 3 + fish than in age 1 + fish. In another bully population where parasites were 10 times less abundant, similar effects were noticeable but not significant, whereas no effects of parasites on the responses of galaxiids to predators were apparent. Differences in the abundance of parasites and in their sites of infection in fish may explain the variability among host populations or species. However, in the bully population with high parasite abundance, parasitism has age-dependent effects on responses to predators, providing some support for the prediction that young fish with high expected future reproductive success invest more energy into opposing the effects of parasites than do older fish.     

Heavy metal pollution affects dawn singing behaviour in a small passerine bird

Although several studies have suggested that behavioural measures may be more comprehensive than other biomarkers for indicating an organism’s health or welfare, this has rarely been investigated in free-living terrestrial vertebrates. Here we examine the expression of dawn singing behaviour in a free-living small songbird in relation to environmental pollution. We compared the singing behaviour of male great tits Parus major inhabiting an area extremely polluted with heavy metals with that of males inhabiting areas of low(er) pollution (at 4 and 20 km distance from the pollution source). Males at the most polluted site had a significantly smaller repertoire size than males at the two other sites. They also produced a significantly lower total amount of song during the dawn chorus than the males at a distance of 4 km from the pollution source. Our results, although non-experimental and obtained in field conditions, strongly suggest that heavy metal pollution might affect the expression of singing behaviour. Taking into account that previous studies were not able to detect clear, straightforward differences between the health of great tits at the most polluted site and at 4 km distance from the pollution source, our results suggest that the singing behaviour of great tits may be a useful indicator of environmental stress at the population level.     

Complex physiological traits as biomarkers of the sub-lethal toxicological effects of pollutant exposure in fishes

Complex physiological traits, such as routine aerobic metabolic rate or exercise performance, are indicators of the functional integrity of fish that can reveal sub-lethal toxicological effects of aquatic pollutants. These traits have proved valuable in laboratory investigations of the sub-lethal effects of heavy metals, ammonia and various xenobiotics. It is not known, however, whether they can also function as biomarkers of the complex potential range of effects upon overall functional integrity caused by exposure to mixtures of chemicals in polluted natural environments. The current study used portable swimming respirometers to compare exercise performance and respiratory metabolism of fish exposed in cages for three weeks to either clean or polluted sites on three urban European river systems: the river Lambro, Milan, Italy; the rivers Blythe, Cole and Tame, Birmingham, UK; and the river Amstel, Amsterdam, The Netherlands. The UK and Italian rivers were variously polluted with high levels of both bioavailable heavy metals and organics, and the Amstel by mixtures of bioavailable organics at high concentrations. In both the UK and Italy, indigenous chub (Leuciscus cephalus) exposed to clean or polluted sites swam equally well in an initial performance test, but the chub from polluted sites could not repeat this performance after a brief recovery interval. These animals were unable to raise the metabolic rate and allocate oxygen towards exercise in the second trial, an effect confirmed in successive campaigns in Italy. Swimming performance was therefore a biomarker indicator of pollutant exposure in chub exposed at these sites. Exposure to polluted sites on the river Amstel did not affect the repeat swimming performance of cultured cloned carp (Cyprinus carpio), indicating either a species-specific tolerance or relative absence of heavy metals. However, measurements of oxygen uptake during swimming revealed increased rates of routine aerobic metabolism in both chub and carp at polluted sites in all of the rivers studied, indicating a sub-lethal metabolic loading effect. Therefore, the physiological traits of exercise performance and metabolic rate have potential as biomarkers of the overall sub-lethal toxic effects of exposure to complex mixtures of pollutants in rivers, and may also provide insight into why fish do not colonize some polluted environments.     

Toxic pollution and parasitism in freshwater fish

As aquatic habitats throughout the world are increasingly contaminated with toxic chemicals, toxicologist strive to determine what impact these substances will have an biological systems. So far, the effects of this type of pollution on fish parasites has received little attention, despite their important status as determinants of fish health. Robert Poulin discusses the many ways in which toxic pollution may affect infections of metazoan parasites in freshwater fish. Effects on fish immunity, parasites survival and intermediate host populations are not independent, and currently the outcome of toxic pollution and parasitism in fish is impossible to predict. Future research will have to answer several questions if we are to understand and forecast pollution-mediated changes in parasitism, an urgent challenge for ecotoxicologists and fish parasitologists alike.     

Facultative schooling behavior in the spottail shiner (Notropis hudsonius): possible costs and benefits

Synopsis If an individual is less susceptible to predation in a group it should be able to reduce its vigilance for predators and perhaps also its tendency to flee from predators, thereby gaining time and energy for feeding and other activities. These predictions were examined in the field (Veronica Lake, northwestern Ontario) using a facultative schooling species, the spottail shiner (Notropis hudsonius). Schools of various sizes attacked with a fish predator model had a significantly lower reaction distance to the predator than solitary shiners. Solitary shiners must therefore invest more time and energy in defense but may gain a feeding advantage because their visual field does not overlap with conspecific competitors.     

Parasitic anomalies observed in snow trout due to anthropogenic stress in water bodies

There is interrelationship of the environmental conditions and fish health. Decrease or increase of pollution in aquatic ecosystem have direct impact on presence or absence of parasites. Fish living under optimum environmental, well-nourished conditions are more resistant to diseases than fish weakened by malnutrition caused by parasite infestation or due to deterioration of environmental conditions because ofpollution. Fish encounters common parasites in wild and in culture systems. Parasites attach to the host through suckers and hooks and make their way inside the host through different means, which include skin, through mouth along with food, by means of gills. The hosts were collected during Jan 2019 to Jan 2020 from river Veshaw. During this study it was observed that presence of parasites causes some changes in fish which can serve as indicators of deterioration in aquatic habitat. Clinical signs were noticed in fish hosts collected from sites which received waste due to anthropogenic activities. Parasitic anomalies in the host collected from polluted site was observed to include body deformaties, gastric distention, lesions in gut, increased mucus production, damage in gill filaments etc.     

Using indicators of fish health at multiple levels of biological organization to assess effects of stream pollution in southwest Germany

The health of fish populations and assemblagesin two degraded streams in south-westernGermany were investigated in order to validatethe use of biomarkers for the assessment ofsmall streams as part of the Valimar project.In the more severely polluted stream,recruitment was impaired and populationabundances were low. Extinction ofsite-specific species and appearance of nonsite-specific species has resulted in a shiftin the species composition from thatcharacteristic of unpolluted reference streams.In the less severely degraded stream, abundantrecruitment and relatively high densities ofadults indicated healthy, self sustaining fishpopulations. Missing species, however,indicated some effects of human influenceswithin the reference stream. The reproductivestatus of adult stone loach, Barbatulabarbatula, was not affected in any stream asreflected by normal production of eggs andreproductive behavior. Indices of energystorage differed between the investigated sitesbut could not be related to pollution. Theevaluation of biological integrity based onfish assemblages revealed that both sites inthe more severely impacted stream were in poorcondition. The two reference sites wereclassified in moderate and good condition,respectively. By excluding effects of migrationbarriers on the fish assemblage there was astronger correlation between chemical waterquality and biomarker responses. The resultsindicate the importance of assessing theeffects of pollution within the content oflongitudinal gradients in streams and theusefulness of measuring health effects overseveral levels of biological organization.     

The parasitic communities of the minnow (Phoxinus phoxinus L>)

Phoxinus phoxinus being a widely distributed through Palaearctic and ecologically specialised species is a good model to study parasite communities. 72 specimens of P. phoxinus from 5 localities of the Siberia and Mongolia has been studied by the author and 651 specimens--from 5 localities in Kola Peninsula by the other researches. 595 specimens were studied by B. S. Shulman (1983) year around. To describe infra- and component parasite communities the Brillouin, Shannon, Berger-Parker indexes were used. Communities within individual fish were compared within, and between localities with quantitative percentage similarity index (Esch e. a., 1988). The Brillouin, Shannon and Berger-Parker indexes can be calculated based on abundance or total number of parasites of each species for component or compound communities. Intensity cannot be used. Significant negative correlation was observed between the Brillouin and Berger-Parker dominance indexes (r = -0.984, p < 0.01, n = 66) in infracommunities. The Brillouin index was significant positive correlated with the number of parasites in infracommunity (r = 0.407, p < 0.01, n = 66) and with the number of species in infracommunity (r = 0.92, p < 0.01, n = 66). Berger-Parker dominance index was significant negative correlated with the number of parasite in infacommunity (r = -0.815, p < 0.01, n = 66) and with the number of species in infracommunity (r = -0.268, p < 0.05). Total number of parasites and the number of species in infracommunity was positive correlated significantly (r = 0.565, p < 0.01, n = 66). These correlations remain the same at the component community level for the Shannon/Berger-Parker indexes (r = -0.965, p < 0.01, n = 10) and total number of parasites/number of species (r = 0.789, p < 0.01, n = 10) only. Similarity index was not correlate with any other indexes and parameters. No significant correlation was found between the Brillouin, Berger-Parker indexes and geographical characters of localities (longitude, latitude, altitude above sea-level). Weak correlation (r = 0.551, p < 0.098, n = 10) was observed only between the Berger-Parker dominance index and the latitude. 37 metazoan parasites have been found in all localities (specialists--40%, allogenic species--22%, rare species--40%). Allogenic generalist species of the genus Diplostomum (D. phoxini in most cases) were dominant in 60% of the component communities and 36% of infracommunities on average. 20% of the component communities were dominant by autogenic specialists Gyrodactylus magnificus and only 12% of the infracommunities were dominated by this species. Trematode and nematode larvae were core species in most localities. Frequency distribution of infracommunity richness varied from locality to locality being overdispersed in combine sample. Parasite communities at least on the infracommunity level demonstrate high variability of all parameters. Some tendency to decrease the diversity seems to be present depending on remoteness from prospective centre of origin of the genus Phoxinus.     

Accumulation of plerocercoids of Triaenophorus crassus in the second intermediate host Coregonus lavaretus and their effect on growth of the host

Patterns of accumulation of Triaenophorus crassus in its second intermediate host whitefish Coregonus lavaretus s.l. were studied between 1991 and 1996 from two host populations in two separate areas of Lake Saimaa, Finland. Whitefish were infected commonly with several T. crassus plerocercoids and the parasites were aggregated into the oldest hosts. In one host population the annual parasite accumulation was 0·9 parasites in all host age groups between 3 and 8 years. In the other host population the annual accumulation was 1·6 parasites in 3–5-year-old fish, but increased up to 3 to 4 parasites per year in fish over 5 years old. The increase did not coincide with the period of maturation or any increase in whitefish growth, both of which could alter the food intake of the fish. The sharp increase in the annual accumulation suggests a threshold intensity above which the probability of acquiring further parasites increases. In spite of a heavy aggregation of parasites there was no evidence of parasite-induced host mortality. The annual increase in mean abundance was not correlated with the mean annual weight increase in 2–4-year-old fish within cohorts. However, evidence of a negative effect of parasites on whitefish growth was revealed by back-calculation of lengths of uninfected and infected whitefish and correlations between length or weight of fish and intensity of infection with fish age. Both analyses suggested that larger young fish harboured more parasites than the smaller ones while in older fish the reverse was true, a pattern that has not been shown earlier for parasitized fish.