Although aquaculture has been the fastest growing food sector for decades, there are no standardized parameters for most of the fish species regarding physical meat quality. Therefore, this study provides for the first time an overview of the physical meat characteristics of the most important fish species of the German Baltic Sea coast. Traditional farmed salmonids (rainbow trout (Oncorhynchus mykiss) and maraena whitefish (Coregonus maraena) as well as two percids (European perch, Perca fluviatilis and pikeperch, Sander lucioperca) were utilized for this comparison. The results demonstrate that the meat of the salmonids is very analogous. However, the post mortem degradation process starts faster in trout meat. In contrast, the meat quality characteristics of the percids are relatively different. The meat of pikeperch has comparatively low shear strength with a high water-holding capacity resulting in high meat tenderness. The opposite situation is present in European perch. The results indicate that it is not possible to establish the overall quality characteristics for fish or production form, as there is a high range of variability. Consequently, it is particularly important that meat quality characteristics are developed for important aquaculture species for further improvement through changes in husbandry conditions when necessary.

Pikeperch (Sander lucioperca) is a freshwater fish that has become increasingly popular as a food fish. Within this study, the influence of the spawning process on meat quality was investigated. For this purpose, adult pikeperch was examined directly before and after spawning, and compared regarding physical meat parameters and lipid composition. The results indicate that after spawning, the values of the pH, the electrical conductivity and the yellowness value of fillet were significantly higher than those of the animals sampled before spawning. Analysis of the sum of the total lipid content indicates no differences before and after closed season, but differences in the fatty acid profile were present. Despite significant lower MUFA concentrations, the EPA and DHA showed unaffected high contents. Therefore, the fish muscle indicated an equivalent meat quality. Nevertheless, the significant changes of some physical meat quality parameters after spawning season could have a particular impact on the shelf life and storage of the pikeperch fillet, highlighting the need for further research.

The use of selected Baltic coastal organisms as potential alternative feed material in the formulation of rainbow trout diets was studied. German coastal water organisms (Delesseria sanguinea, seaweed red algae (A); Mytilus spp., Baltic blue mussel (M); Alitta virens, king ragworm (W)) were analyzed for nutrient, amino acid and mineral composition, and tested in comparative feeding trials. Five dietary treatments were supplied to a total of 165 juvenile rainbow trout (778 ± 111 g) for 75 days, allotted in 15 special brackish (3–5 practical salinity units (PSU)) water basins consisting of 11 fish each (3 fish tanks (300 L) at 12 °C per feeding group). The fish were fed as follows: C group, 100% basic diet (control); A group, 10% red algae in C diet; M group, 10% mussel in C diet; W group, 35% ragworm in C diet; AW group, 10% algae + 30% ragworm in C diet. Feed provision was performed manually, once a day, with the feed offer adjusted to 1.8% of fish weight for the respective tank. The fish weight gain (WG) and feed conversion ratio (FCR) were recorded. In the proximate analysis of the different coastal organisms, the lowest crude protein content in dry matter (DM) was found in blue mussels (10.9%), whereas it was almost doubled in algae (21.8%), with the highest being found in the ragworm (63.1%). By contrast, the crude ash content was the highest in the mussel (84.4%, mostly due to CaCO₃ from the shell), much less in the red algae (28.1%) and the lowest in the ragworms (20.1%). The gross energy (GE) concentration was the highest in the ragworm (18.8 MJ × kg⁻¹), 12.1 MJ × kg⁻¹ in the algae and the lowest in the blue mussel (2.93 MJ × kg⁻¹). The final weight of the fish ranged between 1780 and 2310 g at the end of the feeding trial, being the lowest for the fish fed the diet combined with red algae (A diet group) and the highest for the fish fed the control diet. No differences in FCR were found for the fish fed the five dietary treatments (p > 0.05), except for the W diet group (king ragworm has a lower FCR than that of the A group red algae, p < 0.05). The results from this trial suggest that at the tested amounts, both king ragworm and blue mussels are promising alternative feed material for rationing the rainbow trout diet, but not red algae, unless combined with ragworms.

Newly hatched Coregonus maraena (Bloch) larvae reared in aquaria were observed during two trials in 2014 and 2015 for 42 and 34 days after hatching in order to determine the on-feeding effects of different feeding strategies on growth and survival. Feeds of commercial dry feed, live Artemia nauplii, frozen rotifers, and mixtures of these were compared. Larval whitefish were fed brine shrimp for up to 13 days beginning 4 days post hatch (dph). Whitefish larvae fed live Artemia for 8-9 h per day over a period of 10-12 days grew considerably larger (mean value of final total length (TL) 27.3-33.0 mm)) and were heavier (mean value of final body weight (BW) 112-165 mg)) than those fed dry feed, frozen rotifers, or Artemia for 3-6 days (TL= 22.6-30.2 mm; BW= 50-102 mg). The growth of larvae fed for 24 h solely on dry food was comparable (BW= 101 mg, TL= 26.6 mm) at 42 dph. Survival differed among groups (31-63%) and was highest for larvae fed dry food exclusively. Our results can improve the first feeding of Maraena whitefish larvae in small-scale fish farms.

In vertebrates, the carbohydrate polymer polysialic acid (polySia) is especially well known for its essential role during neuronal development, regulating the migration and proliferation of neural precursor cells, for instance. Nevertheless, sialic acid polymers seem to be regulatory elements in other physiological systems, such as the reproductive tract. Interestingly, trout fish eggs have polySia, but we know little of its cellular distribution and role during oogenesis. Therefore, we localized α2,8-linked N-acetylneuraminic acid polymers in the ovaries of Coregonus maraena by immunohistochemistry and found that prevalent clusters of oogonia showed polySia signals on their surfaces. Remarkably, the genome of this salmonid fish contains two st8sia2 genes and one st8sia4 gene, that is, three polysialyltransferases. The expression analysis revealed that for st8sia2-r2, 60 times more mRNA was present than st8sia2-r1 and st8sia4. To compare polysialylation status regarding various polySiaT configurations, we performed a comparable analysis in Sander lucioperca. The genome of this perciform fish contains only one st8sia2 and no st8sia4 gene. Here, too, clusters of oogonia showed polysialylated cell surfaces, and we detected high mRNA values for st8sia2. These results suggest that in teleosts, polySia is involved in the cellular processes of oogonia during oogenesis.