Janusz Kloskowski

Impact of common carp Cyprinus carpio on aquatic communities: direct trophic effects versus habitat deterioration

The common carp Cyprinus carpio, a worldwide introduced benthivorous fish, has been implicated in the degradation of native environments through initiation of a shift to a phytoplankton-dominated turbid state, which is associated with dramatic biodiversity loss. This study combined surveys of ponds containing either low total biomass of small-sized carp or high densities of large-sized carp with an enclosure/exclosure experiment, in order to quantify the direct (trophic) and indirect (via habitat deterioration) impacts of carp on pond communities. High-density ponds supported substantially lower biodiversity and were more turbid than low-density ponds. The subsequent field experiment examined the effects of carp presence/absence and of clear-water versus moderately turbid conditions mediated by carp on the survival to metamorphosis of larval anurans Pelobates fuscus and Hyla arborea, on Zygoptera and Anisoptera densities, and on the biomass of submerged macrophytes. The presence of enclosed one-year old carp resulted in the complete elimination of larval anurans and the absence of Odonata. The effects of the habitat conditions were not significant, apart from better survival of P. fuscus in the moderately turbid carp exclosures than in clear water. Submerged plants were more abundant in clear-water than in turbid treatments, with a negligible effect of enclosures/exclosures. These results suggest that carp predation and related effects may be primarily responsible for animal diversity loss in invaded communities, as they may act prior to, or independent of, the ecosystem switch to a turbid phase.

Human-wildlife conflicts at pond fisheries in eastern Poland: perceptions and management of wildlife damage

Wildlife occurring at aquacultural sites can ignite conflicts over resources managed by humans. A telephone survey concerning nuisance wildlife occurrence, perceptions of inflicted damage, and use of preventive measures at pond fisheries was conducted in 2003–2004 in eastern Poland. Significant economic losses to wildlife were reported by 80% of the respondents, with 41% claiming severe losses to more than one species. Serious damage was attributed primarily to otters Lutra lutra (56% of farms), cormorants Phalacrocorax carbo (26%), grey herons Ardea cinerea (23%), and beavers Castor fiber (21%). Two alien species, mink Neovison vison and muskrat Ondatra zibethicus, were widespread, but rarely blamed for causing substantial harm. Lethal controls were the most popular means of damage reduction and were more likely to be deployed at fish farms reporting significant losses. Cormorants, shot at half of the farms visited, were the most widely persecuted. Both otters and beavers were culled, notwithstanding their protection status and compensation payments available for beaver damage to ponds. The survey indicated poor cooperation between conservation authorities and fish farmers in managing wildlife concerns. Monitoring conflict interactions, i.e., wildlife occurrence and alleged damage at fish farms vs damage management processes, is an essential step toward conflict alleviation. Considering the wide range of species interacting with fisheries, adoption of more flexible policies to address the conflicts than a single-species conservation approach is recommended.

Differential effects of age-structured common carp (Cyprinus carpio) stocks on pond invertebrate communities: implications for recreational and wildlife use of farm ponds

Sustainable development of common carp Cyprinus carpio pond fisheries in Europe postulates their multifunctional use, integrating exploitation of aquaculture resources with recreational services and maintenance of high levels of local biodiversity. Age classes of farmed carp are grown separately and pond ecosystems may be differently affected by different ontogenetic stages of fish. To examine these relationships, a study was conducted on spring and summer diet of carp, invertebrate abundance and community structure, and water quality characteristics in ponds stocked with three carp age classes in SE Poland. With the exception of young-of-the-year fish in spring, benthic dipterans prevailed in the diets of all carp age classes and their consumption increased from spring to summer. Zooplankton featured in the diet of carp only in spring. Medium- and large-sized cladocerans predominated among microcrustaceans found in the guts of one- and two-year-old carp. Consequently, in summer, total biomass of medium- and large-sized cladoceran grazers was substantially lower in ponds stocked with older-age fish than in ponds used for production of 1-summer-old fingerlings. The relatively sparse submerged vegetation cover and low water transparency in ponds with older fish stocks compared to ponds with young-of-the-year carp indicate a transition to a turbid water state mediated by a trophic cascade mechanism in the presence of older-age fish. Densities of water-column macroinvertebrates decreased with the age of carp in the ponds. These results suggest that non-aquaculture use of carp ponds should be diversified according to their environmental quality. Fingerling rearing ponds are more suitable for environmentally friendly multifunctional use than ponds with older stocks.

Otter Lutra lutra damage at farmed fisheries in southeastern Poland, I: an interview survey

Abstract Fish farmers were interviewed on the occupancy and impact of otters Lutra lutra at common carp Cyprinus carpio fisheries in southeastern Poland during a preliminary survey in 1994–1995 and in 2003. Otters occurred at 104 (91%) of 114 surveyed fisheries, and in 71 (62%) fish farms the perceived otter predation pressure increased over the last decade. Of the interviewees, 65 (57%) reported serious losses to otters, and of these 51 (78%) claimed to have no otters or to have had the otter problem under control in the mid-1990s. The most frequently listed type of damage was killing or serious injuring of the commercially most valuable brood fish and surplus killing of cultured carps. Nondestructive attempts to protect stocks from otters were reported only exceptionally. Of the farmers with depredation problems, 17% admitted that illegal otter killing occurred at their farms. Field surveys revealed that fish farmers correctly identified otter presence at their ponds. However, farms differing in the perceived otter impact on cultured stocks did not differ in their annual overall fish losses. Private pond owners perceived losses to otters as higher than managers of state-owned fisheries. Spraint analysis at two intensively monitored adjacent fish farms, one with perceived serious economic losses and one which reported a minor otter problem, showed similar proportion of carp by weight (44 vs 41%, respectively) in otter diet.

Consequences of the size structure of fish populations for their effects on a generalist avian predator

Size-structured interspecific interactions can shift between predation and competition, depending on ontogenetic changes in size relationships. I examined the effects of common carp (Cyprinus carpio), an omnivorous fish, on the reproductive success of the red-necked grebe (Podiceps grisegena), an avian gape-limited predator, along a fish size gradient created by stocking distinct age-cohorts in seminatural ponds. Young-of-the-year (0+) carp were an essential food source for young grebes. Only adult birds were able to consume 1-year-old (1+) fish, while 2-year-old (2+) fish attained a size refuge from grebes. Amphibian larvae were the principal alternative prey to fish, followed by macroinvertebrates, but the abundance of both dramatically decreased along the carp size gradient. Fledging success was 2.8 times greater in ponds with 0+ versus 1+ carp; in ponds with 1+ carp, chicks received on average 2.6–3 times less prey biomass from their parents, and over 1/3 of broods suffered total failure. Breeding birds avoided settling on 2+ ponds. These results show that changes in prey fish size structure can account for shifts from positive trophic effects on the avian predator to a negative impact on the predator’s alternative resources. However, competition did not fully explain the decrease in grebe food resources in the presence of large fish, as carp and grebes overlapped little in diet. In experimental cages, 1+ carp totally eliminated young larvae of amphibians palatable to fish. In field conditions, breeding adults of palatable taxa avoided ponds with 1+ and older carp. Non-trophic interactions such as habitat selection by amphibians or macroinvertebrates to avoid large fish may provide an indirect mechanism strengthening the adverse bottom-up effects of fish on birds.

Habitat selection by breeding waterbirds at ponds with size-structured fish populations

Fish may significantly affect habitat use by birds, either as their prey or as competitors. Fish communities are often distinctly size-structured, but the consequences for waterbird assemblages remain poorly understood. We examined the effects of size structure of common carp (Cyprinus carpio) cohorts together with other biotic and abiotic pond characteristics on the distribution of breeding waterbirds in a seminatural system of monocultured ponds, where three fish age classes were separately stocked. Fish age corresponded to a distinct fish size gradient. Fish age and total biomass, macroinvertebrate and amphibian abundance, and emergent vegetation best explained the differences in bird density between ponds. Abundance of animal prey other than fish (aquatic macroinvertebrates and larval amphibians) decreased with increasing carp age in the ponds. Densities of ducks and smaller grebes were strongly negatively associated with fish age/size gradient. The largest of the grebes, the piscivorous great crested grebe (Podiceps cristatus), was the only species that preferred ponds with medium-sized fish and was positively associated with total fish biomass. Habitat selection by bitterns and most rallids was instead strongly influenced by the relative amount of emergent vegetation cover in the ponds. Our results show that fish size structure may be an important cue for breeding habitat choice and a factor affording an opportunity for niche diversification in avian communities.

Size-structured effects of common carp on reproduction of pond-breeding amphibians

The role of fish in driving amphibian communities has been widely recognized. However, little is known about size-structured interactions between amphibian and fish populations. This study compared the taxonomic occurrence and densities of larval amphibians between unstocked ponds and ponds stocked with different age cohorts of common carp Cyprinus carpio differing in average body size. The average total densities of early and late breeding anurans known to be vulnerable to fish were by 1–2 orders of magnitude greater in the presence of young-of-the-year carp than that of older cohorts. The probabilities of occurrence of the most common taxa did not differ between ponds stocked with young-of-the-year fish and ponds free of carp, but were significantly larger in those ponds than in ponds stocked with large-size cohorts. No significant differences between pond categories were found for densities of unpalatable Bufo bufo larvae. In aquatic systems harbouring size-structured fish populations, a fish age/size gradient may explain differential habitat suitability for breeding amphibians better than the fish presence/absence dichotomy. When dominated by young cohorts incapable of predation or of adverse habitat alteration, fish-abundant waters are suitable for amphibian reproduction. Conversely, even a ‘non-predatory’ fish, after attaining large body size, may exert a detrimental impact on amphibian breeding success. These findings may be particularly important for amphibian conservation at pond fisheries characterized by spatial separation of age/size distributed stocks.

Fish farms as amphibian habitats: factors affecting amphibian species richness and community structure at carp ponds in Poland

SUMMARY Open fish ponds are a potential consideration in the conservation of biodiversity, yet information on their suitability for breeding amphibians remains lacking. This study examined amphibian responses to attributes of pond habitats at common carp Cyprinus carpio farms in eastern Poland, focusing on features typical of traditional European cyprinid culture. Canonical correspondence analysis identified age of fish stocks, density of predatory Anisoptera and pond hydroperiod as the factors most strongly influencing amphibian community composition expressed in terms of larval abundance. Separation of fish agecohorts into different growing ponds was decisive for differential occurrence patterns of individual amphibian taxa. The amphibian assemblage was significantly nested: ponds stocked with young-ofthe-year fish supported relatively high numbers of amphibian taxa, including endangered species known to be vulnerable to fish, while ponds containing one- and two-year-old carp harboured only more common taxa. These results suggest that traditional carp pond culture characterized by a spatially discrete distribution of inter-annual cohorts includes habitats compatible with amphibian conservation. However, a flexible conservational approach is needed: ponds stocked with young fish produce secondary breeding habitats for amphibians, but presence of large-sized carp is associated with severe species loss.

Otter Lutra lutra damage at farmed fisheries in southeastern Poland, II: exploitation of common carp Cyprinus carpio

Abstract I carried out a 2-year study on otter Lutra lutra dietary composition at a fish farm in southeastern Poland, where otters were perceived to inflict serious damage to common carp Cyprinus carpio stocks. The aim of my study was to determine the role of commercial fish in the diet of otters living in habitats comprising carp fisheries. Otter diet was examined by scat analysis. On an annual basis, carp dominated otter diet with 43% by weight. However, in terms of numbers, smaller wild species such as Prussian carp Carassius auratus gibelio (27%) and roach Rutilus rutilus (12%) were more abundant in the diet, whereas carp accounted for only 10%. The availability of farm cohorts of different age compared with frequencies of carp age classes recorded in spraints indicated otter preference of 1+ over 2+ carp. The exploitation of farmed stocks was most intense from autumn to early spring, unless the access to stocks was prevented by the presence of ice cover. In areas with carp farms, otters may rely on cultured stocks, especially during periods of food stress.

Sexing Red-Necked Grebes Podiceps grisegena by Molecular Techniques and Morphology

Abstract. Sexual size dimorphism was analysed in the Red- necked Grebe in southeast Poland. A DNA-based procedure was utilised to sex individuals and to assess the accuracy of morphological criteria for the sex identification of adult breeding birds: discriminant analysis on the sample level and within-pair comparisons. Males were significantly larger than females in all body measurements used in the discriminant function selection process. Owing to considerable overlap in measurements, however, the sexes cannot be accurately separated by biometrics at the population scale. Sexual dimorphism was most pronounced in bill length measured from the corner of the gape to the tip, but only 79% of individuals were correctly identified on the basis of this parameter alone. When two variables, bill length and wing length, were combined, the discriminant function was of similar efficiency (80%) in determining the sex. The accuracy level of sexing may be improved by comparing mates within pairs: combined comparisons of bill length and body mass were as accurate as the genetic technique, but sex assignment was restricted to 76% of the measured pairs.