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Round goby Neogobius melanostomus (Pallas 1814) has become a significant component in the diet of piscivorous fish from the Pomeranian Bay (Bornholm Basin, Baltic Sea). Proper identification of fish species in the diet of predators is significant in biological studies of fish and other aquatic animal species, and, with regard to N. melanostomus, it is important to the knowledge of trophic web structures in areas this species has invaded. A total of 142 individuals of N. melanostomus, measuring 16-174 mm standard length, were examined. Seventy-two fishes were caught during monitoring surveys in fishing grounds, whereas 70 were found in the stomachs of European perch Perca fluviatilis, pike-perch Sander lucioperca and Baltic cod Gadus morhua. The objective of the present study was to analyse the sagittal otoliths to identify variations in outer shape with increases in fish length; expand and correct descriptions of the sagitta, lapillus and asteriscus otoliths; and evaluate the relationships among otolith dimensions and fish standard length. The otoliths were described morphologically. The analysis of the outer shape of sagittal otoliths using Fourier analysis and multivariate statistics exhibited great phenotypic variability that was associated with fish length, including within pairs in individuals and/or among individuals in length classes. In addition, the asterisci and lapilli of N. melanostomus from selected specimens, which were described for the first time with regard to fish length, were found to be less variable compared to sagittal otoliths. This study presents the first analysis of intrapopulation phenotypic plasticity of N. melanostomus sagittal otolith morphology as it is linked to fish size.

Background Conservation of noble crayfish (Astacus astacus) populations is becoming particularly important since the number of individuals is rapidly declining across the distribution range of the species in Europe. Five crayfish populations in northwestern Poland have been constantly monitored for two decades. However, the genetic structure of these populations has not been analysed, although this information is important to devise effective conservation strategies. Methods Noble crayfish were collected in the autumn of 2014 by scuba diving in Lakes Graniczne, Babinki, Biwakowe, Sęki and Kwisno, all of which are situated in the Bytów Lakeland of northwestern Poland. Genetic diversity of the five populations was assessed based on allele variability in nine SSR regions and six AFLP primer combinations. Results Microsatellite results analysed with AMOVA showed that the diversity between populations corresponds to 18% of total variability, which was confirmed by similar results obtained using AFLP. Additionally, significant genetic diversity was revealed by high average FST values. All of the studied crayfish populations significantly deviated from the expected Hardy–Weinberg genetic equilibrium and were characterised by negative values of inbreeding coefficient (FIS). Discussion The invariably negative inbreeding coefficients (FIS) suggest a low number of mating individuals, a possible consequence of the phenomenon known as genetic bottleneck. However, additional comprehensive analyses are needed to assess the genetic structure, origin and vulnerability of the remaining populations of noble crayfish in the Bytów Lakeland of northwestern Poland, which have high conservation value and are particularly important as a live genetic bank for breeding and restitution purposes.

The taxonomic classification of European crayfish is neither fully examined nor generally accepted as a unifying scheme. At present, one of the most obvious differences exists between astacologists from Western and Eastern Europe. In the present paper, an analysis of the information published on the systematics of European crayfish over the last decades was conducted against the background of own observations on features with importance for the systematics of narrow-clawed crayfish (Astacus (Pontastacus) leptodactylus) from Poland. The objective of this analysis was to standardize the nomenclature of European crayfish at the level of genera. The presented proposal for a taxonomy recognizes three genera within the family Astacidae in Europe: Austropotamobius, Astacus, and Pontastacus. The main (morphological, zoogeographical and ecological) criteria for considering the genus Pontastacus a separate genus are of a discriminatory strength similar to the widely accepted criteria for the separation of the genus Austropotamobius. Further studies on the taxonomy and ecology of crayfish from eastern Europe using modern techniques such as molecular genetics are strongly recommended.Key-words: Astacidae, Astacus, Austropotamobius, morphology, Pontastacus, taxonomy. PROPOSITION POUR ACCEPTER PONTASTACUS EN TANT QUE GENRE D'ÉCREVISSE EUROPÉENNE DE LA FAMILLE DES ASTACIDÉS, SUR LA BASE D'UNE REVUE BIBLIOGRAPHIQUE DE LA TAXONOMIE EST ET OUEST EUROPÉENNE RÉSUMÉA ce jour, la taxonomie des écrevisses européennes n'a jamais été réellement examinée en détail et aucun consensus existe. Les avis sur leur classification divergent dans la communauté scientifique, notamment entre les astacologistes de l'Ouest de l'Europe et ceux de l'Est. De nouvelles recherches sont nécessaires sur la taxinomie et l'écologie des écrevisses dans l'Est de l'Europe, et l'utilisation de techniques modernes telles que la génétique moléculaire est vivement recommandée.

The occurrence of non-indigenous sturgeon species in the Baltic Sea is currently discussed in relation to plans for the re-establishing of the native A. sturio L. Introduced fishes must be considered competitors with regard to the utilisation of the habitat and food resources as well as to the potential risk of interference with the gene pool of the native species (Gessner et al. 1999). Therefore, any information on the biology of sturgeons acquired from open waters can be valuable. The first sturgeon was caught 800 m offshore in the Baltic Sea (Pomeranian Bay) near the village of UnieÊcie, on 17 December 2000. The fish weighed 549 g (before gutting) and its standard length was 38.2 cm (Fig.1). The second fish was caught 3 km offshore near the village of Gàski (Pomeranian coast between Ko∏obrzeg and Dar∏owo), on 20 October 2001. After gutting the fish weighed 1092 g and its standard length was 49.8 cm (Fig. 2).