Isolation and Identification of major bacteria from three Ethiopian rift valley lakes live and processed fish, and water samples: Implications in sanitary system of fish products

Bacterial pathogens are a great threat to fish production. Gram-negative bacteria are among the major bacterial fish pathogens and zoonotic with the potential to infect humans. This cross-sectional study was conducted to isolate and identify major gram-negative bacteria from live and processed fish, and water samples from Lakes Hawassa, Langanoo and Ziway. A total of 674 different types of samples: 630 tissue samples (210 samples for each intestine, Kidney and liver collected from 210 live fish (Oreochromis niloticus, Cyprinus carpio and Clarias gariepinus), 20 processed fish samples from lake Ziway fish processing center and 24 lake water samples were included in the study from each lake. The mean values of pH, temperature, dissolved oxygen and nitrate in all water samples were within the normal range at which most freshwater fish species become non-stressed. Of a total of 674 samples included in the study, the bacteria were isolated from 154 (22.8%) samples with significant difference (P<0.05) observed in some isolates with respect to sample origin. Of these 154 isolates, 103(66.8%) isolates were gram-negative bacteria consisting of 15 species based on morphology and a range of biochemical tests. From live fish samples, Escherichia coli was the dominant species with 15 isolates followed by Edwardsiella tarda (12), Salmonella Paratyphi (10), Salmonella Typhi (9), Shigella dysenteriae(7), Shigella flexneri (7), Klebsiella pneumonia (7), Enterobacter aerogenes (6), Enterobacter cloacae (5), Pseudomonas aeruginosa (5), Vibrio parahemolyticus (5), Aeromonas sobria (4), Citrobacter freundii (4), Citrobacter koseri (4) and Plesiomonas shigelloides(3). Detection of common fecal coliforms (E. coli, K. pneumoniae and E. aerogenes) and Salmonella spp. in processed fish indicates the potential danger of passage of pathogenic bacteria and/or their poisons to humans via infected and/or contaminated fish products. Human infection by pathogenic fish bacteria and food poisoning is possible through contamination of fish product in fish production chain due to inadequate handling, poor hygiene and contact with contaminated water. Therefore, producers, consumers and all other stakeholders need to be cautious during handling, processing and consumption of fish harvested from the study lakes.

Flow, force, behaviour: assessment of a prototype hydraulic barrier for invasive fish

Migration barriers being selective for invasive species could protect pristine upstream areas. We designed and tested a prototype protective barrier in a vertical slot fish pass. Based on the individuals’ swimming responses to the barrier flow field, we assumed this barrier would block the ascension of the invasive round goby, but allow comparable native species (gudgeon and bullhead) to ascend. The barrier was tested in three steps: flow description, quantification of forces experienced by preserved fish in the flow field, and tracking the swimming trajectories of ca. 43 live fish per trial and species. The flow and the forces were homogenous over the barrier, though gudgeon experienced significantly smaller forces than round goby or bullhead. The swimming trajectories were distinct enough to predict the fish species with a random forest machine learning approach (92.16% accuracy for gudgeon and 85.24% for round goby). The trajectories revealed round goby and gudgeon exhibited increased, but varied, swimming speeds and straighter paths at higher water discharge. These results suggest that passage of round goby was prevented at 130 L/s water discharge, whereas gudgeon and bullhead could pass the barrier. Our findings open a new avenue of research on hydraulic constructions for species conservation.

Performance and Feasibility Study of a Novel Automated Catch-Hauling Device Using a Flexible Hose Net Structure in Set-Net

The labor-intensive catch-hauling method in set-net fisheries faces problems of lower productivity, lower efficiency, and higher operational risk due to aging problem and labor insufficiency. To solve such problems, a novel catch-hauling device using the flexible fire hose and net (hose net), which is placed in the box chamber, was proposed in this study. The hoses were inflated with air injected into one edge of the hose net, and the buoyancy force increased: the net gradually floated up, cornering the fish in the opposite edge. To corner and harvest the fish efficiently and safely, the changing formation and motion parameters of the hose net are significant. A series of floating up, sinking experiments, and catch-hauling tests were conducted to evaluate the performance of this device. The results showed that the hose net could gradually float in an ideal form and sink smoothly through natural exhaust and stretched on the bottom of the water tank. The time spent and average speeds in floating and sinking processes varied with air pressure and airflow rate, allowing the hose net motion to be controlled in practice by adjusting the airflow. Through the catch-hauling test using live fish, most of the fish were directed into the fish bag. Two main capture failure phenomena were also observed. Overall, this newly developed automated catch-hauling device is expected to be successful for use in modern fisheries.

Non-invasive ploidy determination in live fish by measuring erythrocyte size in capillaries

Information about ploidy is important in both commercial and conservation aquaculture and fish research. Unfortunately, methods for its determination, such as karyology, determination of the amount of DNA in a cell using microdensitometry or flow cytometry and/or measuring erythrocytes in a blood smear can be stressful or even destructive. Some of these methods are also limited by the relatively large minimum size of the individual being measured. The aim of this study was to test a new low-stress method of determining ploidy by measuring the size of erythrocytes in the capillaries of a fish, including small individuals. First, we examined diploid and triploid loach (Cobitis sp.) and gibel carp, Carassius gibelio (Bloch, 1782), using flow cytometry and blood smears, with these results being used as a control. Subsequently, we measured the size of erythrocytes in the caudal fin capillaries of anesthetized fishes of known ploidy under a light microscope. For both the loaches and gibel carp, direct observation of the mean erythrocyte size in epithelial fin capillaries provided a consistent and reliable determination of ploidy when compared with the controls based on flow cytometry and blood smears. This new method allows for rapid determination of ploidy in living small fish, where collection of tissue using other methods may cause excessive stress or damage. The method outlined here simply requires the measurement of erythrocytes directly in the bloodstream of a live fish, thereby making it possible to determine ploidy without the need for blood sampling. The method described is sufficiently efficient, less demanding on equipment than many other procedures, can be used by relatively inexperienced personnel and has benefits as regards animal welfare, which is especially important for fish production facilities or when dealing with rare or endangered species.

Non-invasive ploidy determination in live fish by measuring erythrocyte size in capillaries

Information about ploidy is important in both commercial and conservation aquaculture and fish research. Unfortunately, methods for its determination, such as karyology, determination of the amount of DNA in a cell using microdensitometry or flow cytometry and/or measuring erythrocytes in a blood smear can be stressful or even destructive. Some of these methods are also limited by the relatively large minimum size of the individual being measured. The aim of this study was to test a new low-stress method of determining ploidy by measuring the size of erythrocytes in the capillaries of a fish, including small individuals. First, we examined diploid and triploid loach (Cobitis sp.) and gibel carp, Carassius gibelio (Bloch, 1782), using flow cytometry and blood smears, with these results being used as a control. Subsequently, we measured the size of erythrocytes in the caudal fin capillaries of anesthetized fishes of known ploidy under a light microscope. For both the loaches and gibel carp, direct observation of the mean erythrocyte size in epithelial fin capillaries provided a consistent and reliable determination of ploidy when compared with the controls based on flow cytometry and blood smears. This new method allows for rapid determination of ploidy in living small fish, where collection of tissue using other methods may cause excessive stress or damage. The method outlined here simply requires the measurement of erythrocytes directly in the bloodstream of a live fish, thereby making it possible to determine ploidy without the need for blood sampling. The method described is sufficiently efficient, less demanding on equipment than many other procedures, can be used by relatively inexperienced personnel and has benefits as regards animal welfare, which is especially important for fish production facilities or when dealing with rare or endangered species.

Creation of a prototype biomimetic fish to better understand impact trauma caused by hydropower turbine blade strikes

Biomimetic model organisms could be useful surrogates for live animals in many applications if the models have sufficient biofidelity. One such application is for use in field and laboratory tests of fish mortality associated with passage through hydropower turbines. Laboratory trials suggest that blade strikes are especially injurious and often causes mortality when fish are struck by thinner blades moving at higher velocities. Dose-response relationships have been created from these data, but the exact relationship between fish mortality and the actual forces enacted on fish during simulated blade strike testing remains unknown. Here, we describe the methods used to create a prototype biomimetic model fish composed of ballistic gelatin and covered with a surrogate skin to better approximate the biomechanical properties of a fish body. Frozen fish were scanned with high-fidelity laser scanners, and a 3D-printed, reusable mold was created from which to cast our gelatin model. Computed tomography scan data, imaged directly or taken from online data repositories, were also successfully used to create CAD models for use in additive manufacturing of molds. One 3-axis accelerometer was embedded into the gelatin to compare accelerometer data to dose-response data from previous laboratory research on live fish. The resulting model (i.e., Gelfish) had a statistically indistinguishable tissue durometer to that of real fish tissue and preliminary blade strike impact testing suggested its overall flexibility was similar to that of live fish. Gelfish was designed with biofidelity as its guiding principle and our results suggest initial experimentation was successful. Future research will include replication of initial Gelfish test results, quantitative measurement of model flexibility relative to real fish, and inclusion of surrogate skeletal structures to enhance biofidelity. Use of more sophisticated sensors would also better quantify the physical forces of blade strike impact and help determine how said forces correlate with rates of mortality observed during tests on live fish.

The determination of high retention flag marking methods with yellowfin bream (Acanthopagrus latus) fry

With the decline in coastal fishery resources, hatchery-reared fry of yellowfin bream (Acanthopagrus latus) have been commonly mass released to the surrounding waters in China in recent years. Although the release plan has been underway in China for many years, the releasing effects were not often assessed. Therefore, it is necessary to study several suitable mass marking methods that have a high mark retention rate for fry. From October to November 2020, tetracycline was immersed to mark calcified hard tissue (MI), with tagging cut tail fin (TC) and the control, and the species of yellowfin bream by live fish were examined. Moreover, a double marking method, i.e., mark calcified hard tissue (MI) after tag fish for two weeks, was examined to determine any increase in the mark retention rate for yellowfin bream in November 2020. The dual marking and tagging method combining MI and TC is effective and is suitable for yellowfin bream. Both MI of 0.25% treatment and MI of 0.5% treatment also have lower costs, and more individuals can be marked.