G.I. Jenkins

The effect of salinity on growth and survival of juvenile black bream (Acanthopagrus butcheri)

Growth and survival of juvenile black bream (Acanthopagrus butcheri) were determined at salinities from 0 to 60 ppt (in 12-ppt increments) and from 0 to 12 ppt (in 4-ppt increments) in two separate trials of 6 and 4 months duration, respectively. Juvenile black bream were able to survive and grow at salinities ranging from freshwater (0 ppt) to 48 ppt. Osmotic stress was evident at 60 ppt, however, survival was not significantly affected. Fish reared at 24 ppt in trial 1 had a specific growth rate of 2.34 ± 0.03%/day, a rate significantly higher only to those fish reared at 60 ppt (2.16 ± 0.04%/day). Growth was greater at 24 ppt in association with the highest food intake and most efficient FCR. Although both food intake and FCR were not significantly higher than those obtained with fish reared at 12, 36 and 48 ppt, the combination of the two factors being optimised at 24 ppt lead to the greatest growth. Analysis of data from the second trial found no significant difference in the growth rate of black bream reared at salinities ranging from freshwater to 12 ppt, with SGR ranging from 1.92 ± 0.05%/day to 2.05 ± 0.02%/day. Variable results in freshwater between the two trials suggested that total hardness of freshwater may influence survival and/or an ontogenetic change in salinity tolerance may occur.

Treatments to control Haliotrema abaddon in the West Australian dhufish, Glaucosoma hebraicum

Haliotrema abaddon is a significant monogenean parasite of the gills of maricultured Western Australian dhufish, Glaucosoma hebraicum, and can cause severe gill pathology. A series of trials was undertaken using infested fish in 120-l glass aquaria to assess the safety and efficacy of some potential “in water” treatments, including trichlorphon, praziquantel, formalin, toltrazuril and a freshwater bath. Treatment efficacy was established by assessment of the number of parasites found in tank sediment immediately following treatment and by examination of gills for parasites using a gill biopsy technique. Praziquantel bathes, 2 mg l−1 of 30-h duration, were the most safe and effective treatment. Freshwater bathes of below 2‰ salinity for 90 min were effective but highly stressful for the fish. Trichlorphon 0.5 mg l−1 for 30 h was safe and relatively effective. Formalin, 25 ppm, and toltrazuril, 12.5 mg l−1, were safe but resulted in the removal of insufficient parasites to recommend it as a treatment for dhufish infested with H. abaddon.

Comparisons of the Growth and Gonadal Development of Otolith-Stained, Cultured Black Bream, Acanthopagrus butcheri, in an Estuary with Those of Its Wild Stock

Acanthopagrus butcheri was cultured from broodstock from a southwestern Australian estuary in which its abundance had declined. After marking their otoliths with alizarin complexone, the cultured juveniles were released into this estuary at 6 months of age. Regular sampling demonstrated that this stain persisted throughout the next 3.5 years and that, during the last two of those years, cultured fish dominated the black bream assemblage. Cultured fish grew slower than wild fish, with females reaching total lengths of about 182, 220, and 243 mm at 2, 3, and 4 years, respectively, compared with about 199, 248, and 286 mm in wild fish. However, cultured A. butcheri grew faster than wild A. butcheri in most other estuaries studied. By 3 years of age, essentially all wild fish, but only some restocked fish, had matured. Cultured females matured at a similar length but at an older age than wild females, and cultured males matured at both a greater length and older age than wild males. We conclude that cultured A. butcheri can be used to increase the abundance of a depleted stock, but the reason for the slightly reduced performance of cultured fish should be elucidated and overcome.

Biological and Genetic Characteristics of Restocked and Wild Acanthopagrus butcheri (Sparidae) in a Southwestern Australian Estuary

Acanthopagrus butcheri was restocked in an estuary in which it had become depleted. The restocked fish were cultured in 2001 and 2002 using broodstock from that estuary. These fish, whose otoliths had been stained with alizarin complexone, were released into the estuary and their biological performance tracked for seven to eight years. The 2002 cohort, introduced at circa four months old in autumn, survived far better than the 2001 cohort, introduced at circa seven months old in winter, when freshwater discharge peaks and temperatures are low. While restocked fish matured and grew nearly as fast as wild fish, the increase in density was accompanied by a reduced growth of wild fish. Genetic comparisons, using seven microsatellite loci, demonstrated that the expected heterozygosity and relatedness of restocked and wild A. butcheri, which is naturally characterized by low levels of genetic polymorphism, were similar. Although culturing did not demonstrably increase the level of inbreeding, it did result in the loss of some rare alleles. The biological and genetic results, together with the contribution of restocked A. butcheri to the commercial catch for this species in the estuary rising to 62–74% by 2007–2010, demonstrates the efficacy of using restocking to replenish depleted A. butcheri stocks.

Factors affecting mark quality of alizarin complexone-stained otoliths in juvenile black bream Acanthopagrus butcheri and a prescription for dosage

The relationships between otolith score, a function of increasing stain visibility, and the ratio of alizarin complexone (mg) to fish biomass (g) for 0.1, 0.6 and 7 g black bream Acanthopagrus butcheri show that acceptable marks are produced in all size classes using a stain-to-biomass ratio of 3.3 mg g(-1) following an immersion period of 24 h.

The effect of CO2-rich ground water on the West Australian dhufish (Glaucosoma hebraicum)

Abstract The effect of high CO 2 saline ground water on the blood oxygenation properties of the West Australian dhufish was evaluated following ill thrift of wild caught dhufish held in the water. The Bohr factor was calculated for whole blood from captively bred dhufish to assess the potential effect of increased p CO 2 on oxygenation of dhufish haemoglobin. Blood pH, p CO 2 , electrolytes and haematological parameters were determined for two groups of captively bred dhufish held long term in seawater and high p CO 2 ground water, respectively. Chemical analysis of the two water sources was performed. Ground water in dhufish tanks had a pH of 7.7 (seawater pH 8.1) and p CO 2 6.45 mm Hg (seawater 1.50 mm Hg). Dhufish whole blood had a large Bohr factor. Dhufish acclimated to ground water had increased plasma p CO 2 , HCO 3 − , haemoglobin content, haematocrit and erythrocyte counts. The increase in haemoglobin content was the result of an increase in erythrocytes.