Aquac. J., Volume 2, Issue 4 (December 2022) – 6 articles

This study investigated the effect of the inclusion of an enzyme complex-treated rice protein concentrate (RPC) in an extruded diet of juvenile rainbow trout (Oncorhynchus mykiss). A mixture of RPC, corn gluten and soybean meal was pretreated with an enzyme complex before extrusion processing of the diets. An enzyme complex-pretreated RPC (5, 10 and 15%) was formulated with 20% fish meal. A diet without RPC was used as a control. A total of 240 rainbow trouts with an average body weight of 6.04 g were placed in 12 rectangular glass aquaria and fed one of the experimental diets at apparent satiation for 12 weeks. There were no significant differences in growth, feed intake and survival among the groups. Slightly inferior weight gain and specific growth rate were observed in the RPC15E group compared to those in the other groups. The protein and ash contents of the whole body of the final fish were not significantly different among all groups, but the lipid content was significantly lower than that of the control when the dietary RPC level was >10%. Protein digestibility was negatively affected by increased RPC levels in diet. Significantly lower phosphorus digestibility was observed in fish fed the diet containing 5% enzyme-treated RPC than the control. Although there was no significant difference in lipid digestibility in all groups, significantly lower lipid retention was observed in fish fed a diet formulated with more than 10% RPC. These results suggest that the inclusion of RPC in the diet affects lipid retention and the content of rainbow trout. It also decreased protein digestibility. In conclusion, rainbow trout can be fed an extruded diet formulated with 20% fishmeal and 10% enzyme-treated RPC without negative effects on fish growth. Full article

Ensuring the humane harvest of farmed fish without compromising the quality of the fresh product is paramount to the welfare of fish and in meeting consumer demands. Electrical stunning is a quick and effective way to render fish unconscious and it has emerged as the suggested harvest method by EFSA and OIE. The present study evaluated the effects of electrical stunning on the biochemical processes that lead to fillet degradation postmortem, in the red seabream (Pagrus major). Two distinct electrical stunning conditions (low and high) were compared along with the conventional harvest method (an ice slurry). The activity patterns of calpain, collagenase, and cathepsin B and L were assessed and compared to stereological changes in white muscles at different time points up to 13 days post-harvest. Histological examinations, independent of the harvest technique, revealed a progressively declining trend in fiber volume density and increasing interfibrillar spaces over time, indicative of degradation activity within and between the muscle fibers. Strong correlations between the stereological measures and the individual protease activities were recorded. The higher current condition (electric field 1.8 V/cm and velocity 1.6 m/s) consistently exhibited the lowest protease activity levels and the slowest pace of stereological changes, making it the suggested method of all harvest methods explored. Full article

Ulva ohnoi has a big potential in IMTA-RAS fish–seaweed systems. In order to design the best production strategy in these systems, the effect of the main environmental factors, such as pH, nutrient concentration (N, P, and N: P ratios) and dissolved inorganic carbon (DIC), on the productivity, bio filtration capacity, and quality of the biomass obtained was studied. It is concluded that in closed systems, strong pH variations (7.9–10.1) do not influence the growth of U. ohnoi and growth is slowed down due to the depletion of DIC. This fact would not be a problem in IMTA-RAS fish–macroalgae systems, due to the physiological activity of the fish contributing CO₂ to the medium and replenishing it. The results obtained in the wide range of N: P ratios tested (2–410), allow us to conclude that this ratio should not be a limiting factor for the cultivation of Ulva ohnoi in IMTA-RAS systems. Based on those results, the best strategy to follow in an IMTA-RAS sole–sea lettuce would be to maintain the algae with highest level of nitrogen. This procedure implies a high rate of water renewal, which would also guarantee the maintenance of an adequate DIC and the best commercial quality of seaweed. Full article

The continuous expansion of freshwater fish culture is confronted with environmental problems owing to their high antibiotic inputs and antibiotic-resistant bacteria. This study explored the antibiotic resistance gene (ARG) profiles of water and fish gut contents from three fishponds in Tianjin using a metagenomics approach. Proteobacteria, Bacteroidetes, and Actinobacteria were the most abundant phyla in all water samples. However, the microbial composition of the fish guts was distinctly different among the three aquaculture farms. Cetobacterium, Bacillus, Weissella, and Fusobacterium were the dominant genera in the gut contents of all fish. More than 20 unique ARGs with relatively high abundances were detected in both water and fish gut content samples. The dominant genes and pathways of antibiotic resistance mechanisms detected in all samples were antibiotic efflux, antibiotic inactivation, antibiotic target alteration, antibiotic target protection, antibiotic target replacement, and reduced permeability to antibiotics. In addition, our results indicate that antibiotics, such as florfenicol, and heavy metals, such as Zn and Cu, could have a significant correlation with some common ARGs, indicating that antibiotic-resistant bacteria could co-occur with heavy metals. Our study provides a research basis for the development of a strategy for practical antibiotic application and heavy metal monitoring in aquaculture. Full article

This study investigated the effects of oregano (Origanum vulgare) essential oil (OEO) anaesthesia and stress-related physiological parameters in tilapia mossambicus (Oreochromis mossambicus) post-juveniles. Fish were subjected to different concentrations of OEO (25, 50, 100, and 150 μL L⁻¹) to assess the effects of the anaesthesia and recovery time. A second experiment subjected fish to other handling treatments, including a control (no OEO exposure), ethanol solution, 25 μL L⁻¹ (low effective anaesthesia concentration of OEO), and 100 μL L⁻¹ high effective anaesthesia concentration of OEO) for 10 min to assess the haemato-biochemical indices and survival rate at 0 h and after 24 h. Moreover, all the tested OEO concentrations induced anaesthesia in the studied fish, where the anaesthesia induction time decreased with increased OEO concentrations. Meanwhile, the inverse was reported for recovery time. Based on the ideal anaesthetic criteria, 50 μL L⁻¹ and 100 μL L⁻¹ were the suitable concentrations that could be recommended for quick anaesthesia. However, according to the haemato-biochemical parameters and survival results, 25 μL L⁻¹ was safe to anaesthetise tilapia mossambicus for 10 min and could be recommended for time-consuming fish-handling procedures. Future studies should investigate multiple factors that influence anaesthesia in fish for better optimisation of OEO in tilapia mossambicus. Full article

The black soldier fly (BSF, Hermetia illucens) is a prominent insect species and a novel alternative ingredient for aquafeeds. This study aimed to evaluate the replacement of commercial feed with fresh whole black soldier fly larvae (BSFL) for Colossoma macropomum. Juvenile tambaqui (115.2 ± 0.9 g/fish) were stocked in 800 L tanks (11 fish/tank) operating as a recirculating system. The dietary treatments consisted of whole larvae only (BSFL), 50:50 BSFL: commercial feed (BSFL: CF), or only commercial feed (CF), and the fish were fed to apparent satiation for 120 days. At the end of the feeding trial, fish were group weighed, and blood and muscle were collected for hematological and sensorial evaluations. Fish fed BSFL:CF presented with similar growth performance and carcass yield to tambaqui that were fed only CF. The high fat content of the larvae contributed to the highest values for the viscerosomatic index (7.01%; 6.56%), plasmatic triglycerides (312.1 mg/dL; 295.1 mg/dL), and cholesterol (120.6 mg/dL; 119.1 mg/dL) in the fish provided with BSFL or BSFL:CF, respectively. However, a better acceptance in the fillet sensory analysis was observed for these fillets than for the fillets from the CF group. Feeding tambaqui with BSFL:CF did not compromise the production performance and may have improved the acceptance of their fillets. Full article