Summary Survival and growth of wels catfish (Silurus glanis L. 1758) larvae (both non‐feeding and feeding), originating from fertilization with cryopreserved sperm were investigated. Non‐feeding (yolk sac) larvae (n = 100 individuals distributed over five replicates) were tested in the laboratory; feeding larvae (100 individuals in five replicates) were tested in both the laboratory and at a commercial fish farm (1000 individuals in triplicates). Water temperature was maintained at 22–23°C during the 4‐day test period on non‐feeding larvae and 10 days on feeding larvae. Larvae originated from cryopreserved or fresh sperm of different males collected in different years. In the laboratory experiments with feeding larvae, changes in standard length (SL), weight, condition factor, specific growth and survival rates were determined after 10 days, while at the fish farm survival and length growth were determined after 10 days. Final SL, weight and survival rate were analyzed on the non‐feeding larvae. In all cases, larvae from fertilization with fresh sperm served as a control. A significant difference (P = 0.034) was found in the 10‐day standard length between larvae originating from cryopreserved sperm (1.92 ± 0.13 cm SL, N = 250) and the control (1.89 ± 0.14 cm SL, N = 250). Significant differences were also observed in the final SL (0.96 ± 0.05 cm in larvae from cryopreserved sperm vs 0.94 ± 0.05 cm in the control, P < 0.001) and weight (10.09 ± 0.37 g in the cryopreserved group vs. 9.02 ± 0.30 g in the control, P = 0.018) of non‐feeding larvae. No significant differences were found in either the survival rates of fry derived from fresh or from cryopreserved sperm. It is postulated that the observed differences can be attributed to the effect of individual males or broodstocks; however, genetic studies are required to confirm this hypothesis. This study gives evidence that the developed cryopreservation technology has no unfavorable effects on the viability of wels catfish larvae.

In a cultured pikeperch (Sander lucioperca) stock the monopisthocotylean monogenean gill parasite Ancyrocephalus paradoxus caused heavy infection and mortalities. The gills of the affected fish specimens were infected by 50 to 800 monogenean parasites. Severe pathological changes were found in areas where the worms attached to the gills. At the attachment sites the haptoral discs of the worms formed a deep depression in the epithelium of the filaments, and the anchors pierced into and fixed themselves to the connective tissue of the cartilaginous gill rays. At these attachment sites red blood cells released from injured capillaries were found among the damaged epithelial cells. Around the hooks, anchors and body sections coming into contact with the gill filaments a proliferative tissue developed in which only a remnant of the damaged lamellae was found. Due to the damage caused by the worms the tips of the heavily infected gill filaments fused, formed clubs and were composed of epitheloid-type regeneration tissue lacking respiratory lamellae. In the basal parts of the filaments, where most of the worms attached to the gill, only denuded filaments deprived of lamellae were observed among the cross-sectioned worms in histological sections.