The aim of this study was to compare survival and growth of juvenile narrow-clawed crayfish (Astacus leptodactylus) and signal crayfish (Pacifastacus leniusculus) fed only with experimental pellets (45% of proteins, 6% of fat, 20% crude fibre) under controlled conditions. The animals were reared in 12 (50 l) circular plastic tanks, the narrow-clawed crayfish set-ups being designated A1 and A2, the signal crayfish set-ups P1 and P2. The initial stocking densities for each species were 600 (A1, P1) and 1,200 (A2, P2) juveniles stage 2 per square metre for each set-up in three replicates. The experiment lasted 92 days under continuous photoperiod (L:D 24:0; 100 LUX) conditions at a temperature of 22.0 ± 0.1°C and an oxygen saturation > 90%. One shelter (plastic tube) was provided for 2 crayfish in each set-up.The highest survival rate was obtained for signal crayfish from set-up P1 (47.5%), the lowest for narrow-clawed crayfish from set-up A2 (22.8%). Crayfish survival evidently depends on the stocking density: in set-ups A1 and P1 it was about 16% higher than in set-ups A2 and P2 (P < 0.05). Mortality was significantly higher (P < 0.05) in both species during the first 30 days of the experiment (48-77% of the total mortality) than during the subsequent two months of the experiment (< 20% of the total mortality).The final body size was the largest in narrow-clawed crayfish from set-up A1 (799 mg, 29.2 mm) and the smallest in signal crayfish from set-up P2 (534 mg, 26.5 mm). Specimens of narrow-clawed crayfish were larger than signal crayfish, and the specimens of both species in set-ups A1 and P1 were larger than those in set-ups A2 and P2. During the first 30 days of the experiment the specific growth rate (SGR) of both species in all set-ups was twice the value reported during the subsequent two months (P < 0.05). La survie des écrevisses dépend évidemment de la densité de départ: dans les lots A1 et P1, la survie est supérieure de 16 % par rapport à celle des lots A2 et P2 (P < 0,05). La mortalité a été significativement plus importante (P < 0,05) pendant les 30 premiers jours de l'expérience (48-77 % de la mortalité totale) que pendant les deux derniers mois (< 20 % de la mortalité totale). Key-words:Les écrevisses à pattes grêles du lot A1 ont présenté les tailles corporelles les plus grandes (799 mg ; 29,2 mm) alors que les plus petites ont été enregistrées dans le lot P2 (534 mg ; 26,5 mm). D'une façon générale, les écrevisses à pattes grêles étaient plus grandes que les écrevisses signal et les individus des lots A1 et P1 étaient aussi plus grands que ceux des lots A2 et P2. Pendant les 30 premiers jours de l'expérience, le taux de croissance spécifique (SGR) de chaque espèce (A1, A2, P1, P2) était plus important que celui mesuré dans les deux mois suivants (P < 0,05).

Summary 1. Most European crayfish species are strongly threatened, mainly as a result of the introduced pathogen, Aphanomyces astaci, transmitted by invasive North American crayfish. Long‐term coexistence of American and European crayfish species is therefore regarded as almost impossible, even though some coexisting populations have been observed. 2. In this study, crayfish were collected from presently coexisting populations of the introduced spiny‐cheek crayfish (Orconectes limosus) and the native noble crayfish (Astacus astacus) from nine standing waters in central Europe. Our aim was to resolve whether the coexistence resulted from reduced virulence in local strains of A. astaci, increased immunity in the native crayfish or an absence of the pathogen in these populations. We used highly sensitive A. astaci‐specific real‐time PCR to evaluate the crayfish latent carrier status, combined with transmission experiments to further validate the molecular results. 3. From the total of 523 crayfish tested (490 spiny‐cheek crayfish, 33 noble crayfish), none positive for A. astaci was detected. Transmission experiments confirmed these results: No abnormal mortality or behavioural changes were seen in noble crayfish kept together with American crayfish from the coexisting populations. If we assume a very low prevalence of A. astaci of 10% in a carrier population, there is a 98% probability of disease being absent in five of the nine coexisting populations tested. Hence, a consistent absence, or an extremely low prevalence, of A. astaci seems to allow the coexistence of European and American crayfish in these central European populations. 4. The results are important for native crayfish conservation and management and demonstrate that disease transmission risk may vary substantially between the different populations of spiny‐cheek crayfish in central Europe.

Research on the ecology of water mites in flowing water has focused mainly on analysis of factors directly affecting these organisms in the aquatic environment. The hypothesis of this study was that apart from factors acting within the aquatic environment, the formation of Hydrachnidia communities in lotic ecosystems may also be affected by factors acting in the terrestrial environment. The analysis was made at three different levels of organization of the environment: (1) landscape level (subcatchments, terrestrial environment), (2) macrohabitat level (sampling sites, aquatic environment) and (3) mesohabitat level (sampling sub-sites, aquatic environment). Some correlation was noted between certain species and some sub-catchment parameters. This may indicate a link between some landscape features (terrestrial environment) and the formation of water mite assemblages in the river. The low percentage for physicochemical parameters together in explaining the variance in occurrence of species, very low correlations between species and physicochemical parameters and the discrepancy in the grouping of sites in the case of faunal data and data on the physicochemical indicates that physicochemical factors had little influence on water mites. Taking into account all three levels of organization of the environment analyzed, we can say that at the landscape level we can find only indirect relationships between environmental factors and the fauna inhabiting the aquatic environment; at the macrohabitat level the description of Hydrachnidia is more precise but still of a general nature. Only analysis at the mesohabitat level fully explains the specific character of Hydrachnidia assemblages.

Using the example of a regulated stretch of the River Krąpiel (NW Poland), an analysis of the impact of dredging on the aquatic beetle (Coleoptera) fauna was made. After dredging the beetle fauna became markedly poorer quantitatively and had lower species diversity. Moreover, the qualitative composition and dominance structure were highly transformed. However, species number and diversity increased rapidly and were restored within just half a year. The structure of the restored fauna was most strongly influenced by vegetation, flow velocity and bottom sediment composition. The first to colonize the river were eurytopic beetles and species typical of small water bodies, which had significant refuges in the form of fish ponds bordering on the river; these species dominated the assemblage throughout the study period. Rheophiles with less dispersal power appeared later, and finally rheobionts, in low numbers but occupying an increasing number of sampling sites.

Conservation of the endangered noble crayfish (Astacus astacus) requires preservation of existing populations and reintroduction into suitable habitats, as currently underway in several regional projects in Germany and Poland. Therefore, knowledge about the genetic variation between populations is vital but, unfortunately, still restricted. By means of ISSR-PCR (Inter-Simple Sequence Repeats -Polymerase Chain Reaction) eight stocks in Germany and Poland were analysed. These semi-arbitrary multiloci markers had not previously been used in population studies of the noble crayfish. A total of 22 unambiguous and polymorphic markers were detected to use in the subsequent statistical analysis. The number of polymorphic loci in one population ranged from 4 to 19. Therefore the ISSR markers proved suitable for assessing within-and between-population DNA variations and establishing significant separation of most of the stocks. Shannon´s Index, a relative estimate of genetic diversity within populations, ranged from 0.06 to 0.51. UPGMA cluster analysis based on pairwise ROGERS' (1972) genetic distance revealed two main clusters corresponding to populations sampled in the western and eastern part of the study area. Results are relevant on the local, regional and supra-regional (landscape) level. The implications for conservational management and restocking programs are presented. Key-words:Astacus astacus, conservation genetics, ISSR, landscape ecology. ESTIMATION DES VARIATIONS D'ADN DE L'ÉCREVISSE À PATTES ROUGES (ASTACUS ASTACUS L.) EN ALLEMAGNE ET EN POLOGNE PAR L´UTILISATION DES RÉPÉTITIONS SÉQUENTIELLES INTERNES SIMPLES (ISSRS) RÉSUMÉLa conservation de l'écrevisse à pattes rouges (Astacus astacus) menacée nécessite la préservation des populations existantes et la réintroduction de spécimens dans des habitats favorables. Plusieurs projets régionaux tenant compte de ces nécessités sont actuellement mis en place en Allemagne et en Pologne. Pour une gestion pérenne de l'espèce, la connaissance de la variabilité génétique des populations est nécessaire.