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Most research on non-consumptive predator effects on amphibian larvae has been conducted in laboratory or mesocosm designs. Here, Pelobates fuscus and Hyla orientalis tadpoles were separately exposed to non-lethal (free-moving, but with tied mouthparts) common carp Cyprinus carpio for one week in enclosures placed in a pond densely stocked with fish. Tadpoles exposed to nonlethal fish did not differ in mortality, body mass, or, except for deeper tail fin depth in P. fuscus, morphological plasticity from controls kept in a fishless pond. Hyla orientalis tadpoles recovered from the fish treatment were subsequently enclosed until metamorphosis in either the pond with fish or the fishless pond. Metamorphs from the pond containing fish were heavier, and did not differ in survivorship or development time from their counterparts initially kept in the fish treatment and then transferred to the fishless pond or from controls kept the entire time under fish-free conditions. The lack of apparent metamorphic costs is consistent with previous experiments on anuran larvae, but the morphological defenses induced (or their absence) are not. In the fish-dominated pond, carp indirectly affected tadpole developmental responses by generating turbidity, through adverse impacts on submerged vegetation and predatory insects, and by increasing food resource (unicellular algae) levels. While the present study does not question the validity of laboratory and mesocosm experiments on the costs of non-consumptive predator effects on amphibian larvae, their outcomes cannot easily be extrapolated to ecologically complex natural habitats. [ABSTRACT FROM AUTHOR]

The Aquatic Warbler is the rarest and only globally threatened passerine bird of mainland Europe and a specialist for sedge fen mires and similarly structured wetland habitats. Its Pomeranian population (NE-Germany/NW-Poland) is critically endangered. In this study, we analyse the effects of a targeted EU LIFE project on habitat suitability in Pomeranian breeding sites of the Aquatic Warbler and present recommendations for their further management, particularly in the largest site, Rozwarowo Marshes. Aquatic Warbler censuses and habitat assessments along transects were performed in 2006–2010. We focused our analysis on parameters that have been identified as crucial for Aquatic Warbler habitat selection in Pomerania in earlier studies: height of the litter layer, vegetation height, and cover of herbs. In the last year of the project (2010), out of 14 transects (with six suitable before the project) five transects can be regarded as suitable, and along five other transects habitat suitability had increased substantially. Winter mowing was not sufficient to maintain habitat suitability in one transect, probably subject to substantial eutrophication. At summer mown transects, vegetation height decreased strongly when mowing was implemented already in June/July. In nutrient-rich sites, vegetation height increased despite of summer mowing. Eutrophication should be immediately addressed as a major threat to Aquatic Warbler breeding sites in Pomerania and relevant measures for habitat improvement should be implemented. Habitat monitoring warrants continuation in all Pomeranian sites.

The Aquatic Warbler is the rarest and only globally threatened passerine bird of mainland Europe and a specialist for sedge fen mires and similarly structured wetland habitats. Its Pomeranian population (NE-Germany/NW-Poland) is critically endangered. In this study, we analyse the effects of a targeted EU LIFE project on habitat suitability in Pomeranian breeding sites of the Aquatic Warbler and present recommendations for their further management, particularly in the largest site, Rozwarowo Marshes. Aquatic Warbler censuses and habitat assessments along transects were performed in 2006–2010. We focused our analysis on parameters that have been identified as crucial for Aquatic Warbler habitat selection in Pomerania in earlier studies: height of the litter layer, vegetation height, and cover of herbs. In the last year of the project (2010), out of 14 transects (with six suitable before the project) five transects can be regarded as suitable, and along five other transects habitat suitability had increased substantially. Winter mowing was not sufficient to maintain habitat suitability in one transect, probably subject to substantial eutrophication. At summer mown transects, vegetation height decreased strongly when mowing was implemented already in June/July. In nutrient-rich sites, vegetation height increased despite of summer mowing. Eutrophication should be immediately addressed as a major threat to Aquatic Warbler breeding sites in Pomerania and relevant measures for habitat improvement should be implemented. Habitat monitoring warrants continuation in all Pomeranian sites.

The freshwater habitats of semi-aquatic carnivores in Europe have undergone substantial alterations due to regulation and construction of artificial watercourses. We compared seasonal estimates of otter Lutra lutrafood composition with concomitantly collected data on fish availability and otter holt location in a strongly human-modified ecosystem comprising the upper stretches of the Wieprz-Krzna Canal, the longest artificial waterway in Poland, and an associated regulated river. Concrete lining of the canal, extreme water level variation and regular vegetation clearance to enhance water flow substantially limited otter food and shelter resources, yet the canal offered attractive habitats for some fish and hibernating amphibians, while otter resting sites were available at the closely adjacent river. The taxonomic composition of fish in the otter diet, reconstructed based on faecal analysis, was largely reflected in electrofishing catches. Fish communities in our study area were numerically dominated by small-sized species and otters fed on very small fish (median seasonal total lengths: 46-54 mm). Either the smaller fish in the population were taken or prey sizes did not differ from those available. The significant contribution of amphibians and waterbirds to otter diet (seasonally 41-75% in terms of biomass) indicated poor fish supplies for otters in the canal-river system. High trophic diversity, compared to other otter studies in temperate climatic conditions, indicates that otters facing unstable food conditions exhibit great flexibility in their diet, which may facilitate colonisation of anthropogenic, depauperate environments. Otter occurrence provides little opportunity for conflict with human economic interests in ecosystems that are highly managed for purposes other than fish production. Since anthropogenic habitats are typically poor in some resources, such as shelter structures, their suitability for otters can be enhanced by ensuring good connectivity with nearby less-disturbed habitats, where the necessary resources can be supplemented.

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.

Abstract: 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.

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.

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.

From 1995 to 1998 seven cases of prolonged incubation of unhatchable eggs by Red-necked Grebes (Podiceps grisegena) were observed in south east Poland, all of relatively constant duration (34–36 days). The regular occurrence of prolonged incubation can be attributed to low hatchability rates. It is argued that the failure in assessment of the current egg viability is a by-product of excessive nest attendance as a primary adaptation to irregularities in hatching spreads.