Summary Survival rates and growth parameters of hatchery‐reared sea trout (Salmo trutta trutta L.) fry were determined after stocking in the wild. The larvae were hatchery‐reared for 12 weeks in two groups: fry were fed either on live zooplankton and live chironomidae larvae (LFG), or fed a pellet diet (PFG). The survival rate and specific growth rates were higher in the LFG than in the PFG group. Most effective for hatchery‐reared fish intended for stocking was the natural, live feed. The mean number of chironomid larvae found in the stomachs of fish that were initially captured in the wild was significantly higher in the LFG than in the PFG group. The live diet supplied in the rearing period had a positive impact on the foraging skills of the sea trout fry and their survival in the wild after their release on 24 April 2010.

The aim of this study was to make a comparative analysis of macrozoobenthos composition at different sites at selected sections of lower the Odra River with relation to different physicochemical factors. The observations were made on the lower section of Odra River at five study sites: two of them were localized in the main channel, one in the left branch of river, another one in the channel carrying post-cooling water from Dolna Odra power plant, and the last one was in the channel connecting both branches of Odra River. At all sites, 26 taxa were found representing by: Bivalvia, Gastropoda, Oligochaeta, Hirudinea, Malacostraca and Insecta. The greatest biodiversity and the highest abundance of zoobenthos organisms were noted in the channel joining the two branches of the river, site 5, characterised by the lowest water flow rates and the densest coverage of the macrophytes at the bottom. Temperature was the sole parameter to be studied that affected the composition of benthic invertebrates. A clearly negative impact of temperature on the diversity of invertebrates was observed only in the channel with post-cooling water discharged from the power plant.

Summary The food base and stomach contents of stocked sea trout (Salmo trutta) fry were determined in the first 4 months (April–August 2010) of life in two small lowland streams after resorption of the yolk sac. In each stream, 600 individual trout were released in a 200 m stocking‐section. The macrozoobenthos given as a food base for the fry were collected once a month from the streams using a bottom scraper. Three subsamples of macrozoobentos were considered as one sample from the stocking‐section. On the same day, 50 stocked fry were captured in each stream using electric fishing gear. Preferred food components were usually the taxa represented by number in a given stream in a particular month. Width size range of the prey in fish stomachs in May was from 0.10 to 1.47 mm, and from 0.11 to 3.78 mm in August. All food items found in both streams during the study months were also represented in intensity in the fish: Cyclopoida, and larvae of Baetidae and Chironomidae. Almost all Helodidae and Simuliidae larvae as well as the majority of Limoniidae and Nemouridae larvae were available as food each month.

-In studies of abiotic and biotic factors influencing macroinvertebrate assemblages, there is always the problem of which factor -fish predation or environmental conditions -has the strongest impact on the invertebrates and whether the impact is positive or negative. The aim of our study was to determine the impact on the structures of macrozoobenthos in a small field watercourse exerted by abiotic conditions, with the concurrent lack of predators and varied intensity of anthropopressure. During the entire study period, the presence of 49 taxa of macroinvertebrates was recorded. The highest number of taxa and value of biodiversity was observed in the upper part of the watercourse, and subsequently decreased down the stream, reaching the lowest value at the sites located near the outlet. The tributaries significantly differed between each other in the number of taxa. In the tributary carrying water from wetland, a much higher number of taxa was noted than in the tributary carrying municipal water where the density achieved a significantly higher value of individuals than the remaining sites. The most limiting factors for the abundance of the investigated taxa were the oxygen concentration, nutrients and ammonia.Keywords: Macroinvertebrates / small streams / environmental conditions / anthropopressure Résumé -Variations des structures des macroinvertébrés dans un petit cours d'eau de plaine sans poissons exposé à la pression anthropique influant les conditions environnementales. Dans les études sur les facteurs abiotiques et biotiques qui influencent les assemblages de macroinvertébrés, il y a toujours le problème de savoir quel facteur À prédation des poissons ou conditions environnementales À a l'impact le plus fort sur les invertébrés et si l'impact est positif ou négatif. Le but de notre étude était de déterminer l'impact, sur les structures du macrozoobenthos dans un petit cours d'eau de plaine, des conditions abiotiques, avec absence concomitante de prédateurs, et l'intensité variable de la pression anthropique. Pendant toute la période d'étude, la présence de 49 taxons de macroinvertébrés a été observée. Le nombre le plus élevé de taxons et la valeur de la biodiversité la plus élevée ont été observés dans la partie supérieure du cours d'eau et ont ensuite diminué le long du cours d'eau, pour atteindre la valeur la plus faible aux sites situés près de l'exutoire. Les affluents différaient considérablement les uns des autres quant au nombre de taxons. Dans l'affluent transportant l'eau d'une zone humide, on a observé un nombre beaucoup plus élevé de taxons que dans l'affluent transportant de l'eau domestique, où la densité a atteint une valeur significativement plus élevée d'individus que dans les autres sites. Les facteurs les plus limitants de l'abondance des taxons étudiés étaient la concentration d'oxygène, les nutriments et l'ammoniac.Mots-clés : Macroinvertébré / petits cours d'eau / conditions environnementales / pression anthropique

In this study we attempted to determine the effect of various feeding methods (bottom and surface feeding) used in the hatchery, on the survival and growth rates of hatchery-reared sea trout ( Salmo trutta trutta L.) in the wild. Rearing was performed in two variants: a bottom-fed group (BFG) and a surface-fed group (SFG). At the end of the rearing time, we observed that BFG fish gathered at the bottom of tank, as opposed to SFG fish, which swam in the whole water column. After 4 weeks of rearing, the fish were released into two similar streams. After about 2 months the fish were captured and the foodbase of the streams were examined. 30 fish from each group have been randomly selected for stomach contents analysis. In the shallow stream the growth rates were better for the BFG fish than the SFG and also a significantly higher number of typical benthic taxa was found in stomachs of the BFG fish than the SFG fish. In the deeper stream more food was found in the stomachs of the SFG fish than in the BFG fish. The analyzed results showed that factors such as stream depth, current velocity, and turbulence can also affect the rearing success of juvenile salmonids in hatchery streams. Bottom feeding fish during rearing has a positive impact only on the fish in shallow watercourses, where there is no turbulence, and the food is not carried by the current drift or washed out from the bottom into the drift.