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Summary The study analyses for the first time the diet composition of grey seals inhabiting the Polish Baltic Sea coast. Samples of seal scat were collected in the Mewia Łacha Nature Reserve at the mouth of the Vistula River. Using genetic and osteological methods, the remains of organisms included in the grey seals diet were analysed for their taxonomy (families and species). Based on the analysis of 49 scat samples from grey seals, 761 hard parts that could be identified by taxon were isolated. The predominant species in the samples were perch, Perca fluviatilis (almost 78% of samples); pikeperch, Sander lucioperca (67%); lamprey, Lampetra fluviatilis (almost 35% of samples); Baltic cod, Gadus morhua callarias (almost 31% of samples) and sea trout, Salmo trutta trutta (26.5%). Genetic analysis confirmed the presence of Atlantic cod DNA in 69% and sea trout in 63% of samples. The genetic material of the Atlantic herring, Clupea harengus has not been identified in the analysed scat samples. Information on grey seals feeding on river lampreys seems to be valuable in the context of lack of knowledge on the occurrence of lampreys in the Vistula River. The methodology used showed that seals fed on species that were the most abundant in the area which is directly associated with the migration cycle of fish. The results of our study allowed the conclusion that the grey seal is an opportunistic predator and its diet reflects and exploits the variations in its habitat.

Cyprinid herpesvirus 2 (CyHV-2) is the agent of herpesviral haematopoietic necrosis (HVHN), a serious disease affecting two fish species of the Carassius genus, i.e. goldfish (C. auratus) and Prussian Carp (C. gibelio). In Poland, no asymptomatic vectors of CyHV-2 and no farms in which the virus would cause mortalities have been identified. In 2016 and 2017, mass mortalities of ornamental varieties of C. auratus were observed in a cage culture submerged in a channel receiving post-cooling water from the Dolna Odra power plant (north-western Poland). End-point PCRs and sequencing of the mA marker confirmed the presence of CyHV-2 DNA in the gills and pool (kidney, liver, gut and spleen) samples collected from four ornamental types of goldfish, i.e. Veiltail, Wakin, Red Cap Oranda and Ranchu imported from Thailand and UK. Further analysis revealed that the obtained sequences represent a new type of a 295-bp mA sequence (RSD-PL, GenBank accession no. KX852452.1), which had the greatest similarity to the AMS-8 and FR types of CyHV-2 identified in diseased fish imported from Israel and Asia, respectively. Additionally, our study identified common carp (Cyprinus carpio) individuals reared in neighbouring cages as asymptomatic carriers of CyHV-2. The virus was not detected in fish, i.e. ide (Leuciscus idus), perch (Perca fluviatilis), pumpkinseed (Lepomis gibbosus) and common bleak (Alburnus alburnus), collected from the environment of the power plant discharge channel. Identification of CyHV-2 is the first case reported in Poland and the only one in which blood, gills and pool samples of common carp were CyHV-2-positive. Our study identified the cause of mortalities of ornamental goldfish in cage culture, but also shed a new light on issues concerning the species-specificity of CyHV-2.

Catch and consumption of torpedo scad (Megalaspis cordyla) over the western Indian Ocean, but also the western Pacific from Japan to Australia is constantly increasing. Taking into account the degree of exploitation and missing information on the population structure of torpedo scad stocks it is crucial to provide population data. The analysis included individuals obtained in 2012 and 2013 from local markets in Madagascar, Tanzania, Vietnam and Cambodia and after successful DNA extraction fragment of the nuclear rhodopsin gene (RH1) and 9 microsatellite regions (SSRs) were amplified and analysed. Based on the obtained results it was found that there was no 100% overlap between the compared RH1 sequences and those from GenBank. In the case of the studied SSRs, the results allowed the initial characterisation and assessment of the genetic diversity of populations. Moreover, population assignment test distinguished the studied populations into two geographically distant subpopulations.

Declines across global fishery stocks forced aquaculture feed manufacturers to search for new and sustainable components. Therefore, the aim of study was assessing nutritional value and sensory properties of meat of common carp (Cyprinus carpioL.) fed for 116 days with two blends. The control feed contained 5% of fishmeal and vegetable oils (rapeseed and soybean) as sole fat sources. While in the experimental diet half of the fishmeal was replaced with a blend of microalgae (Spirulinasp.,Chlorellasp.), macroalgae (Laminaria digitata) and vegetable oil was replaced with salmon oil. Proximate composition, energy value, fatty acid profile of meat, nutritional characteristics of fat and protein as well as culinary properties of fillets were assessed. Fillets of carp fed experimental diet had a higher level of protein, lower level of fat and energy value. Intramuscular fat of fish fed with the experimental diet had a better parameters of quality. Protein in the meat of fish from both groups was characterized by a high quality comparing to the protein standard. Our study showed that meat of carp fed with experimental feed enriched with sustainable and natural feed ingredients can be a sensorily attractive source of nutritious ingredients in the human diet.

The tench ( Tinca tinca L., 1758) is a promising freshwater species in which numerous studies have been conducted recently to develop a dedicated feed for optimized growth rate. The aim of this study was to assess the stability of seven candidate reference genes, including 18S ribosomal RNA (18S rRNA), β-actin ( actb ), α-tubulin ( tuba ), ribosomal protein l8 ( rpl8 ), 40S ribosomal protein s11 ( rps11 ), glyceraldehyde-3-phosphate dehydrogenase ( gapdh ) and elongation factor 1 alpha ( ef1a ), to select the best calibrator for nutrigenomic studies that would improve our understanding and illustrate the molecular basis of nutrient-related gene expression in response to feed mixes in the tench. An additional aim was to verify the hypothesis that the functioning of the somatotropic axis in the tench (growth hormone receptor, ghr , and growth hormone, gh ) depends on dietary treatment. The analysis was based on five tissue samples (liver, spleen, gut, heart and muscle) collected from adult tench females fed with 0% and 100% replacement of fish meal (FM) with poultry by-product meal (PBM). The study demonstrated that the most stable genes in most tissues were rpl8 and rps11 , with gapdh showing the greatest stability, along with rpl8 , in the liver. Moreover, a comparison of relative transcript abundance between the tissues revealed that ghr demonstrated the highest expression in the liver, while gh demonstrated the highest expression in the spleen. Based on the selected reference genes, it was determined that the feed did not affect the level of expression of ghr and gh in most tissues except the spleen ( ghr ). The study also involved a phylogenetic analysis based on the amino acid sequences of Rpl8 , Rps11 and Ghr , and it was confirmed that the tench is the most closely related with Asian cyprinids (i.e. Megalobrama amblycephala ). The study identified suitable reference genes for RT-qPCR in the tench, and the effect of two feed variants on the somatotropic axis was determined. The study opens the way for further nutrigenomic studies aimed to develop a feed best matched for the needs of the tench and optimizing its growth. Statement of relevance The effect of feed intake on gene expression in fish species is of particular interest of aquaculture sector which seeks for alternative protein sources, but also new species to adapt for intensive culture. The study demonstrated that the most stable genes in most tissues were rpl8 and rps11 , with gapdh showing the greatest stability, along with rpl8 , in the liver. Comparison of relative transcript abundance between tissues revealed that ghr demonstrated the highest expression in the liver, while gh demonstrated the highest expression in the spleen. Presented work characterizes genes with stable expression across various tench tissue types and identifies suitable reference genes for RTq-PCR for nutrient-related gene expression studies. Presented study was performed for tench for the first time and provides information on influence of poultry by-product meal on the tench organism. Information on tench genetic studies, especially combined with dietary treatments are sparse, therefore presented results address this gap and will bring attention of Aquaculture readers.

Spiny-cheek crayfish (Faxonius limosus, Rafinesque, 1817) females and males were examined as an alternative source of highly valuable material currently sought by the food industry. The aim of the study was to assess the profiles of 14 macro- and microelements (Al, Ca, Cd, Cu, Fe, K, Mg, Na, Ni, P, Pb, Se, Si, Zn) in the meat, exoskeleton and eggs (females) of crayfish collected in the spring and autumn seasons in the Lake Sominko (Poland). The content of heavy metals was analysed, and the degree of their accumulation in crayfish tissues was determined. The profile of the investigated elements in the tested samples was variable depending on the body part, sex of individuals and season of sample collection. A portion of crayfish abdomen meat or eggs (100 g) is particularly rich in P (41.5–73.9 %) of daily requirement), Zn (14.1–38.5 %), Mg (7.9–21.5 %), K (3.3–13.1 %) and Na (4.6–7.7 %) and small quantities of Ca (0.8–2.5 %) and Al (0.9–4.7 %). The study confirmed that consumption of crayfish meat is toxicologically safe and nutritionally complete.