The production of European perch (Perca fluviatilis) has become notably refined, with numerous physiological and nutritional studies conducted in recent years. However, it is still an expensive undertaking due to the high amount of animal protein required in the feed. Therefore, we investigated the possibility of using hemp meal (HM) as an alternative source of protein in extruded feed for perch reared in recirculating aquaculture systems (RASs). Perch fry (initial body weight of 68.1 g) was divided into four groups (HM0, HM10, HM20, HM30; 100 fish in each) and fed with diets containing different levels of HM (0, 10, 20 or 30%, respectively) for 10 weeks. Overall, dietary inclusion of HM did not affect body parameters, muscle composition, or blood parameters, nor did it cause any serious histopathological lesions. Nonetheless, basic production indices (SGR, FCR, PER) all peaked in the HM20 group, and predominantly positive changes in intestinal mucosa were found in all three HM-inclusion groups. Furthermore, the expression patterns of several genes in the intestine and liver were different in groups HM20-30 than in HM0-10. Lastly, hepatic activities of alkaline phosphatase (ALP) and glutathione peroxidase (GPX) diminished with increasing dietary HM inclusion levels. In summary, there were no negative effects of HM on the homeostasis of studied fish or, more specifically, the physiology of their digestive organs. When accounting for minor tendencies in the results, the dietary inclusion of hemp meal at 20% turned out to be the most promising fish meal alternative for the European perch.
The purpose of this study is to enhance the performance of the mesohabitat model MesoHABSIM by lowering the necessary hydraulic modelling effort. This proof-of-concept study tests an application of the MesoHydraulics model to simulate the hydraulic characteristics of hydromorphological units (HMUs) occurring in a regulated river at different low discharges. In this quantitative approach, hydraulic patterns are transferred from a source site, where depth and velocity distributions were derived from field measurements and a 2D hydrodynamic model, to a target site, where only a single field hydrometric survey was conducted. Instead of modelling changes in individual hydraulic measurement values to estimate hydraulic responses to discharge, the model relies on statistical distributions of these values within HMUs. We were testing whether changes in the distribution of HMU’s and their hydraulics can be transferred between morphologically comparable river sections to serve as a sufficient hydraulic input for mesoscale habitat modelling. The hydrodynamic component of the River2D software (V.0.95a), routinely used in MesoHABSIM, served as a baseline for testing the MesoHydraulic model’s performance and for producing source data for deriving distribution functions. The test was conducted using data from two one-kilometre sites on the upper Oder River (Poland). The model transfers the HMU area distributions, along with corresponding depth and velocity frequency distributions, for a number of flows from one site (the source) to another (the target). The hydraulics at both sites were surveyed under single-discharge conditions. For the source site, the hydrodynamic model was applied to classify the HMU mosaic at three additional discharge stages. At the target reach, the HMU mapping was conducted based on survey data, and statistical frequency functions were used to model distributions of hydraulic patterns at discharges modelled for the source. The hydraulic model’s performance was evaluated at the target reach by comparing simulated hydraulics and HMU patterns with those modelled using River2D. Finally, both models were used to calculate habitat availability for the fish communities, and dissimilarities were observed. The resulting hydraulic distributions were similar, with an average affinity index of 90%. Higher affinity indices were reached at flows close to the measured value, with increasing model disagreement toward flow extremes, most notably for Run and Backwater units. Regardless, habitat models for the fish community were also highly correlated with R2 = 0.98 for amounts of suitable habitat and almost identical habitat distribution among the species. Yet, the MesoHydraulics-based model slightly, but consistently, overestimated habitat availability. While the model was tested in a large and regulated river system, its accuracy may vary depending on the natural river morphology. Further research should evaluate modelling uncertainties and their applicability in less-modified water bodies.
The effects of a four-week dietary transition from pelleted feed to live prey fish were assessed for two morphs of brown trout (Salmo trutta) - river trout and sea trout, which were simultaneously divided into four groups. Group C was given the same diet as before; group E was switched onto an experimental pellet; group EF was given both the experimental pellet and prey fish for two weeks, then only prey fish for another 2 weeks; while group F was only given prey fish throughout. Histological measurements were performed using visceral samples obtained after 2 and 4 weeks from ~ 85-110 g trouts (stains: HE, AB-PAS, Perl's Prussian blue). Fish from groups EF and F had shorter intestinal folds, smaller hepatocytes and lesser vacuolization of hepatic parenchyma than fish from C and E. Hepatocyte nuclei and splenic melanomacrophage centers were larger in fish from group F, and the sea trout had larger centers than the river trout. In summary, dietary transition from pellet to prey should last at least 4 weeks prior to fish release, but the (initially) mixed feeding strategy was surprisingly less effective than an abrupt switch onto live prey, especially for the river trout which adjusted quicker to piscivorous diet.
Fish marking is an essential tool for fisheries management, especially for evaluating the stocking of endangered fish species to support conservation and sustainable use of fish stocks. Batch marking of young European eels Anguilla anguilla (L.) prior to stocking is recommended as the benefits of stocking for the spawning stock can be evaluated by recapturing marked fish over time, therefore mass marking of young eels with substances such as alizarin red S (ARS) is becoming increasingly important. To improve the marking method and reduce marking costs when immersing glass eels in an ARS solution, eight laboratory experiments under varying conditions (e.g., temperature, ARS concentration, immersion time, osmotic induction, fish density) and with ARS from different suppliers were carried out. The results show that optimal marking of glass eels can be carried out in the field or during transport by putting approximately 50 g of glass eels per liter in 150 mg L-1 ARS solution for 3 h at 10-15°C. Lower concentrations did not result in reliable marking. Water temperatures of 5°C and below can have a stunning effect on the eels and increase mortality significantly, regardless of the concentration of ARS. Glass eel densities below 50 g L-1 in the marking bath increase marking costs unnecessarily, while a higher density of 100 g L-1 resulted in significantly higher mortality and lower marking success. A somewhat more difficult but less expensive alternative is to bathe the fish in a saline solution of 1% (10 PSU) of 80 mg L-1 ARS for 3 h at 10°C. Costs can also be significantly reduced by choice of supplier for ARS, but care should be taken as the quality of the powder appears to vary (mean percentage of sufficiently marked eels ranged from 59% to 91% among suppliers in the present study) and can lead to marking failure. The optimal marking conditions can help ensure that stocked glass eels can be reliably identified in future studies to assess stocking benefits while reducing costs.
Fish stocking is common practice in fisheries management, but its effectiveness to support or increase stock size or catches is debated. In Lower Saxony, Germany, a controlled, replicated stocking experiment of five native fish species in eight gravel pit lakes <20 ha was conducted to examine the potential additive effects of fish releases on abundance, indexed by catch per unit effort (CPUE) in electrofishing and gillnet samples. Fish were stocked twice in four study lakes, while the remaining four control lakes were not subjected to any stocking over a 6-year study period. Recapture rates of individually marked fishes were extremely low in all five species. Consequently, stocking did not increase species-specific and total fish community abundance. In additional control experiments in separate ponds all stocked fish species, except bream (Abramis brama), showed high survival in at least one study year, indicating general survival of the stocking material. Fish marking with passive integrated transponders did not affect fish survival, except for roach (Rutilus rutilus), and can thus be ruled out as an explanation for the high post-stocking mortality. We conclude that stocking failed to enhance stocks of five native fish species in small gravel pit lakes.
