Paweł Koperski

Food web manipulation in a small, eutrophic Lake Wirbel, Poland: effect of stocking with juvenile pike on planktivorous fish

In a four year experiment (1988–1991) carried out in a shallow, eutrophic lake, an increase in young-of-the-year pike (Esox lucius) density was used to control planktivorous fish assemblages consisting mainly of small-sized cyprinids: roach (Rutilus rutilus), Leucaspius delineatus, and white bream (Blicca bjoerkna). Stocking with pike fry of ca 30 mm in three successive springs resulted in large-scale mortality among prey of vulnerable sizes. Rotenone treatment at the end of the fourth year showed that roach and white bream populations contained few 0+ to 2+ fish, but very high numbers of 3+ to 6+ fish. By this time, short-lived, slow-growing L. delineatus, with no size refuge from predators, had been driven almost to extinction.

Food-web manipulation in a small, eutrophic Lake Wirbel, Poland: long-term changes in fish biomass and basic measures of water quality. A case study

A whole-lake experiment was carried out for seven years(1988–1994) in a shallow, eutrophic lake. In the first phase(1989–1991), repeated introductions of young-of-the-year pike(Esox lucius) were used to control the density ofjuvenilestages of the dominant planktivores (roach Rutilusrutilus,white bream Blicca bjoerkna and Leucaspiusdelineatus).The successive introductions of juvenile pike were accompaniedbyselective removal of large pike, roach, bream Abramisbramaand white bream. No visible improvement in water quality wasregistered in the first three years. Assessments made afterthelake was treated with rotenone, revealed the high efficiencyofjuvenile pike in controlling prey of vulnerable size. Thoughthefirst three age-classes were nearly exterminated, older fish(3+ to6+) remained abundant enough to keep their planktonic prey incheck. Indeed, the non-altered density of dominant cladoceransindicated that they were still heavily preyed upon. Algalbiomassremained high.In October 1991, rotenone was applied to remove all fish.Immigrating fish were gradually recolonizing the lake fromspring1992 onwards. However, the process was kept under control bycontinued stocking with juvenile pike and intensive fishing.Theyears following the rotenone treatment witnessed aconsiderableimprovement in water quality, with water transparency 30%higher,a significant 2.2-fold decrease in seston dry weight and2.8-folddecrease in algal biomass, averaged for the summer months.Theseevents can largely be attributed to substantial changes in theherbivorous zooplankton, above all increased density of LakeWirbels largest cladoceran, Daphniacucullata.

Food-web manipulation in a small, eutrophic Lake Wirbel, Poland: the effect of replacement of key predators on epiphytic fauna

The effect of fish removal on the invertebrate fauna associatedwith Stratiotes aloides was studied in a shallow, eutrophiclake. The biomass of invertebrate predators was approximately 2.5times higher in the invertebrate-dominated year (1992) than in thefish-dominated year (1991), while the density of non-predatoryinvertebrates in 1991 was ca half that in theinvertebrate-dominated year. The decrease was due to a sharp fallin the density of epiphytic chironomids, with the density ofplant-mining chironomids being far less affected. Marked declinesin the density of non-predatory invertebrates in theinvertebrate-dominated year were most probably caused byinvertebrate predators. Once freed from suppression induced byfish, invertebrate predators were able to control the density ofepiphytic prey more effectively than fish.

Differences between two physid species (Gastropoda: Physidae) in antipredator behaviour induced by leeches

The occurrence and the intensity of specific behavioural defensive reactions were investigated in freshwater snails of the family Physidae, which vigorously shake their shells when attacked by invertebrate predators. Physella acuta and Physa fontinalis were exposed to two species of leeches with different hunting strategies— the molluscivorous Glossiphonia complanata and the opportunistic predator Erpobdella octoculata. Parameters of reaction were measured and compared between snails and leeches. Significant differences were found in the reactions of both snail species in 10 out of 11 measured parameters. Physa fontinalis reacted more intensively, with a higher number of shell movements triggered by both leech species. The study showed possible recognition by the prey of the predator species, followed by a specific defensive reaction. In general, G. complanata elicited more intensive reactions, especially in treatment with P. fontinalis.

NoMBSI: a new, Non-lethal Method for Benthos Sampling and Identification for use in biological monitoring of flowing waters: preliminary results

Abstract The new method of biological assessment in flowing waters—NoMBSI (Non-lethal Method for Benthos Sampling and Identification) is presented. To determine the amount of information (informativity), which is possible to obtain during a sample analysis, the results of three procedures were compared (NM-the new one based on 3D digital image analysis, NC-control procedure based on microscopic identification and G, which is the traditional method based on samples collected with Günther sampler). Procedures were applied for the samples collected at 27 sites in North-eastern and Eastern Poland. The NM was found to provide for the correct recognition of an average of 45% of taxa present in samples. Taxa of small size and lower abundance were frequently failed to be identified. However, no significant differences between NM and NC in terms of raw data on taxonomic composition were found, while the results of G procedure were significantly different when compared with both other procedures. Values of commonly used biotic and diversity indices calculated on the basis of those procedures were compared using correlation. The level of identification achieved sufficed for the proper determination of water quality classes at all of the sites sampled.