The Baltic is a semi-enclosed sea characterised by decreasing salinity in the eastern and northern direction with only the deeper parts of the southern Baltic suitable as spawning grounds for marine species like cod. Baltic cod exhibits various adaptations to brackish water conditions, yet the inflow of salty North Sea water near the bottom remains an influence on the spawning success of the Baltic cod. The eastern Baltic population has been very weakly studied in comparison with the western population. The aim of this study is to demonstrate for the first time genetic differentiation by the use of a large number of SNPs between eastern and western Baltic populations existing in differentiated salinity conditions. Two cod samples were collected from the Bay of Gdańsk, Poland and one from the Kiel Bight, Germany. Samples were genotyped using a cod derived SNP-array (Illumina) with 10 913 SNPs. A selection of diagnostic SNPs was performed. A set of 7944 validated SNPs were analysed to assess the differentiation of three samples of cod. Results indicated a clear distinctness of the Kiel Bight from the populations of the eastern Baltic. FST comparison between both eastern samples was non-significant. Clustering analysis, principal coordinates analysis and assignment test clearly indicated that the eastern samples should be considered as one subpopulation, well differentiated from the western subpopulation. With the SNP approach, no differentiation between groups containing 'healthy' and 'non-healthy' cod individuals was observed.
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.
A total of 142 cods: 60 from the South−East Ground of Bear Island and 82 from the Pomeranian Bay (Baltic Sea) were examined for their ecto− and endoparasites. Twenty different parasite species, comprising one Myxosporea, three Cestoda, four Digenea, seven Nematoda, three Acanthocephala and two Crustacea were found. The parasite component communities comprised 1446 individuals (17 species, six higher taxa) from the Bear Island and 6588 individuals (nine species, three higher taxa) from Pomeranian Bay. The observed parasite host specificity was low, and the intensity in a single fish ranged from one to 279 specimens. The eudominant parasite species were Echinorhynchus gadi, Hemiurus levin− seni and Contracaecum osculatum. The dominant parasite communities from the Bear Is− land were nematodes, but acanthocephalans dominated in cod from the Baltic Sea. It ap− pears that one group of parasites, better adapted for the specific conditions of the macro− habitat, has replaced another. The most prevalent parasites were E. gadi, Anisakis simplex, C. osculatum and Hysterothylacium aduncum, and the mean values of crowding were the highest for E. gadi and Pomphorhynchus laevis. The nematode Camallanus lacustris was noted in this host species for the first time. Only six species of parasites were common to cod from both fishing grounds.
Analysis of stomach contents of 52 cod (Gadus morhua callarias) individuals (TL 37.0Á58.0 cm) caught in November 2003 in the coastal waters of the Gulf of Gdań sk showed the presence of the prawn Palaemon elegans, a malacostracan species that has been recently reported as abundant in various parts of the Baltic Sea. This is the first observation of P. elegans as a cod food item in the Baltic Sea. The prawns were found in 88.5% of the stomachs analysed, accounting for 17% and almost 7% of food weight of cod B45.0 cm and ]45.0 cm, respectively. The prawns consumed by the cod measured from 20 to 48 mm (average9SD: 31.795.3 mm), the prawn length increasing slightly with the cod length. Although P. elegans seems to be the most important invertebrate food item, diet in the cod was still dominated by fish. Neogobius melanostomus and Zoarces viviparus were the most important fish species found in the cod stomachs examined. In addition, the contents of some of the cod stomachs featured other malacostracan species, mainly Crangon crangon, as well as other invertebrates; however, these were of lesser importance. The significance of P. elegans as food of the cod fished commercially in the coastal waters of the Gulf of Gdań sk in comparison with other species preyed upon by cod there is discussed.
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