Trent D'Antignana

Effects of immunostimulants on ranched southern bluefin tuna Thunnus maccoyii : immune response, health and performance

Ranched southern bluefin tuna Thunnus maccoyii were fed baitfishes supplemented with vitamins (predominantly E and C) or vitamins and immunostimulants, nucleotides and β-glucans, over 12 weeks after transfer and monitored for enhancement in immune response, health and performance through their 19 week grow-out period. Fish from two different tows were sampled separately at three different sampling points: at transfer to grow-out pontoons, at 8 weeks post-transfer and at harvest, 19 weeks post-transfer. Lysozyme activity was enhanced during vitamin supplementation compared to control fish. Performance (i.e. survival, condition index and crude fat), health (i.e. blood plasma variables including pH, osmolality, cortisol, lactate and glucose) and alternative complement activity were not commonly improved through diet supplementation. There were some tow-specific improvements in performance through vitamin supplementation including survival, selected parasite prevalence and intensity, and alternative complement activity. Immunostimulant supplementation also showed a tow-specific improvement in plasma cortisol level. Tow-specific responses may suggest that life history, previous health condition and husbandry can affect the success of vitamin and immunostimulant enhancement of immune response, health and performance of ranched T. maccoyii.

Discovery of antimicrobial activities of a marine diatom Thalassiosira rotula

This study investigates the antimicrobial activities of a marine diatom, Thalassiosira rotula against microorganisms including three Gram-stain positive and six Gram-stain negative bacteria and one species of yeast. Well-bioassays were used to evaluate the ability to inhibit the growth of bacteria with extracts derived from algae using different extraction techniques of cell wall rupture and organic solvents. The extracts of T. rotula inhibited the growth of Vibrio harveyi, Staphylococcus aureus, Micrococcus luteus and Bacillus pumilus. The freeze/thaw algal paste extract without solvent extraction showed the highest inhibition against S. aureus and B. pumilus. The hexane:tert-butyl-methyl-ether (H:tBME) extract was the only one inhibiting M. luteus and the chloroform:methanol (C:M) extract was the only one inhibiting Vibrio harveyi. Sonication with beads for 3 min was the most efficient method of releasing the antibiotic substances. In sequential solvent extraction, the vortexed H:tBME extract had a higher ability against S. aureus, B. pumilus, and M. luteus than the sonicated H:tBME extract or the C:M extract. The 1:1 combination of sonicated H:tBME and vortexed H:tBME extracts did not exceed that of the separate extract against S. aureus or B. pumilus. However, the combined sequential C:M and vortexed H:tBME extracts had lower ability against S. aureus than the vortexed H:tBME extract alone, indicating the presence of antagonistic compounds in the C:M extract. This study indicates that T. rotula possess antimicrobial activities but the release of antibiotics depends on physical or chemical rupture of algal cells and extractive solvents.

Random and systematic sampling error when hooking fish to monitor skin fluke (Benedenia seriolae) and gill fluke (Zeuxapta seriolae) burden in Australian farmed yellowtail kingfish (Seriola lalandi)

The Australian farmed yellowtail kingfish (Seriola lalandi, YTK) industry monitor skin fluke (Benedenia seriolae) and gill fluke (Zeuxapta seriolae) burden by pooling the fluke count of 10 hooked YTK. The random and systematic error of this sampling strategy was evaluated to assess potential impact on treatment decisions. Fluke abundance (fluke count per fish) in a study cage (estimated 30,502 fish) was assessed five times using the current sampling protocol and its repeatability was estimated the repeatability coefficient (CR) and the coefficient of variation (CV). Individual body weight, fork length, fluke abundance, prevalence, intensity (fluke count per infested fish) and density (fluke count per Kg of fish) were compared between 100 hooked and 100 seined YTK (assumed representative of the entire population) to estimate potential selection bias. Depending on the fluke species and age category, CR (expected difference in parasite count between 2 sampling iterations) ranged from 0.78 to 114 flukes per fish. Capturing YTK by hooking increased the selection of fish of a weight and length in the lowest 5th percentile of the cage (RR = 5.75, 95% CI: 2.06-16.03, P-value = 0.0001). These lower end YTK had on average an extra 31 juveniles and 6 adults Z. seriolae per Kg of fish and an extra 3 juvenile and 0.4 adult B. seriolae per Kg of fish, compared to the rest of the cage population (P-value < 0.05). Hooking YTK on the edge of the study cage biases sampling towards the smallest and most heavily infested fish in the population, resulting in poor repeatability (more variability amongst sampled fish) and an overestimation of parasite burden in the population. In this particular commercial situation these finding supported that health management program, where the finding of an underestimation of parasite burden could provide a production impact on the study population. In instances where fish populations and parasite burdens are more homogenous, sampling error may be less severe. Sampling error when capturing fish from sea cage is difficult to predict. The amplitude and direction of this error should be investigated for a given cultured fish species across a range of parasite burden and fish profile scenarios.