Higher growth variability and stronger responses to temperature changes in wild than hatchery-reared sea trout (Salmo trutta L.).

Each year, millions of hatchery-reared sea-run brown trout Salmo trutta L. (the sea trout) juveniles are released into the natural environment in the Atlantic region. The aim of this work was to investigate the growth responses of sea trout to changing temperature conditions and to compare the growth plasticity between wild and hatchery-reared fish. Scales were collected from sea trout in a selected river flowing into the southern Baltic Sea. We analyzed the scale increment widths as a proxy of somatic growth and investigated the interannual variabilities and differences in growth between fish groups (wild and hatchery-reared). We used mixed-effects Bayesian modeling and ascribed the variances in growth to different sources. Furthermore, we developed indices of interannual (2003-2015) growth variation in the marine and freshwater phases of the life cycle of the fish and analyzed the relationships between trout growth and temperature. Temperature positively affects fish growth, regardless of the origin of the fish. We observed stronger relationships between fish growth and temperature conditions in the marine phase than in the freshwater phase. Additionally, wild sea trout are characterized by stronger responses to temperature variability and higher phenotypic plasticity of growth than those of the hatchery-reared individuals. Therefore, wild sea trout might be better suited to changing environmental conditions than hatchery-reared sea trout. This knowledge identifies possible threats in management actions for sea trout with an emphasis on ongoing climate change.

Gyrodactylus proterorhini Ergens, 1967 (Monogenoidea, Gyrodactylidae) in gobiids from the Vistula River--the first record of the parasite in Poland.

During a parasitological survey of two non-native fish species-racer goby Neogobius gymnotrachelus and monkey goby Neogobius fluviatilis-in the Wloclawek Reservoir on the lower Vistula River, the monogenean Gyrodactylus proterorhini was recorded for the first time in Poland, and for the first time, the racer goby was listed as the parasite host. Specimens of G. proterorhini were detected on body surface, fins, and gills of the fish studied. In 2006, the monkey goby was infected with the prevalence 41%, racer goby with the prevalence of 47%, at maximal intensity of 6 and 9 parasites per fish, (respectively). The infection level in relation to the fish size and sampling season was also discussed. The presented study supports the hypothesis of progressive introduction of the parasite with gobiids to the colonized areas.

First record of Nippotaenia mogurndae Yamaguti and Miyata, 1940 (Cestoda, Nippotaeniidae), a parasite introduced with Chinese sleeper to Poland.

A parasitological survey of an Asian fish, Chinese sleeper Perccottus glenii, inhabiting the Wloclawek Reservoir on the lower Vistula River (central Poland) revealed the occurrence of an exotic cestode Nippotaenia mogurndae Yamaguti and Miyata, 1940, which spreads spontaneously with a host to European waters. This paper documented the first record of the parasite in Poland. Specimens were described, and the effect of fish size and season on the infection level was analyzed. Within the 3 years, 112 fish were examined. Samples were collected by electrofishing in the upper part of the Wloclawek Reservoir three times (seasonally) in 2006 and 2007 and once in autumn 2008. Parasitological indices (prevalence, intensity of infection, and abundance) in particular samples as well as in two size groups of fish were calculated. Statistical tests were applied to analyze the results (Spearman's correlation coefficient and Mann-Whitney's U test). The infection level of Chinese sleeper with N. mogurndae in the Wloclawek Reservoir (prevalence 54.7%, mean intensity of infection 7.2, maximum 29) was comparable to infection noted in the area of the natural range of the host. The infection of smaller fish (less than 80 mm, TL) increased gradually with increasing body size. The infection of bigger specimens was in general higher, but the parasite recruitment was of a more incidental character. The spreading of N. mogurndae with Chinese sleeper is still in progress. The Wloclawek Reservoir provides suitable conditions for parasite development.

Oogenesis in Vimba vimba (L. 1758) from Drawienski National Park (NW Poland).

The objective of the studies was to analyse the process of oogenesis in vimba from a non-migratory population living in the waters of Drawienski National Park in north-west Poland. The character of spawning of this species is an obstacle in determining the right moment to catch spawners or developing artificial spawning biotechniques. Previtellogenesis of vimba begins about six months after hatching and lasts three years. The trophoplasmatic growth of oocytes (October-March/April) begins when carbohydrate vesicles appear near the nuclei oocytes of sexually mature females (aged 4+). Just before spawning, granulated, lipoprotein-like substances are cumulated. The resorption of pre-ovulation corpora lutea (non-ejected oocytes) and post-ovulation corpora lutea (ruptured theca folds and follicles) begins in the ovary of vimba in the middle of June. These were observed in histological cross sections for about two to three months. Describing the process of oogenesis can provide a foundation for developing practical applications in aquaculture aimed at preserving the biodiversity of the park's waters and this critically endangered species of the Polish ichthyofauna.