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). Le but de cette étude était de comparer la survie et la croissance de juvéniles chez l’écrevisse à pattes grêles (Asatcus leptodactylus) et chez l’écrevisse signal (Pacifastacus leniusculus) maintenues dans un environnement contrôlé et nourris uniquement à l’aide de granulats expérimentaux (45 % de protéines, 6 % de graisses, 20 % de fibres). Les animaux ont été élevés dans 12 bacs circulaires de 50 l. Les bacs contenant les écrevisses à pattes blanches ont été notés de A1 et A2 et ceux contenant les écrevisses signal P1 et P2. Chaque bac a été testé en triplicat. La densité initiale en juvéniles pour les deux espèces était de 600 individus au stade 2 pour A1 et P1 et de 1 200 individus pour A2 et P2 par mètre carré. L’expérience a été conduite pendant 92 jours sous une photopériode continue (jour: nuit 24 : 0; 100 LUX), avec une température de 22,0 ± 0,1 °C et une saturation en oxygène supérieure à 90 %. Dans chaque bac, un abri (tube en plastique) a été fourni pour deux écrevisses. Le taux de survie le plus important a été obtenu pour le lot P1 (47,5 %) d’écrevisses signal et le moins important pour le lot A2 (22,8 %) d’écrevisses à pattes grêles. 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). 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)

Global warming, eutrophication, fisheries overexploitation, species invasions, and habitat loss are the major threats to freshwater biodiversity. The aim of this study was to determine the species richness and diversity of fish in 535 Polish lakes of different morphometry (area of more than 50 ha and a maximum depth ranging from 0.4 to 108.5 m) and trophic status (from oligotrophy to hypereutrophy). A total of 39 fish species were found in the studied lakes, among which eight species were alien invasive. The Shannon diversity index varied between 0 and 2.04.The most common and frequent species were Rutilus rutilus (99.8% frequency) and Perca fluviatilis (99.6% frequency). Ten fish species, including five alien ones, were characterized by a very low frequency (<1%). The number of fish species in single lakes ranged from 1 to 19. In most of the studied lakes, 11 and 12 species (104 and 108 lakes, respectively) were caught. The richest taxonomic composition (19 species) was recorded in the meso-eutrophic lake with an area of 80 ha and a maximum depth of 34.4 m, slightly poorer (18 species) in the hypereutrophic lake with an area of 168 ha and a maximum depth of 2.7 m. The poorest taxonomic composition (one species) was found in a lake with low conductivity and circumneutral pH. Statistical analysis showed that the number of fish species decreased with increasing eutrophication conditions of the studied lakes, while it increased with an increasing area and the maximum depth of these lakes.