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Health problems of the Western Australian dhufish

Stephens, F.J., Cleary, J.J., Jenkins, G., Jones, B.ORCID: 0000-0002-0773-2007, Raidal, S.R. and Thomas, J.B. (1998) Health problems of the Western Australian dhufish. Australian Government. Fisheries Research and Development Corporation/Murdoch University

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The purpose of this project was to describe and investigate health problems in captive West Australian dhufish, Glaucosoma hebraicum. The dhufish is a potential aquaculture species due to its popularity as an edible species and fishing pressure on wild fisheries. The two most significant health problems apparent in captive dhufish were exophthalmos in otherwise apparently normal fish and infestation of gills with a monogenean parasite, Haliotrema abaddon. Several other health problems were also described and investigated during the project.

Exophthalmic lesions were described, followed by investigations into the aetiology and pathogenesis of the condition. Epidemiological data were gathered to identify risk factors that may increase the pre-disposition of dhufish to the development of exophthalmos. The anatomical arrangement of vasculature supplying the eye was described and the haemoglobin-oxygen transport properties of dhufish blood that pre-dispose dhufish to exophthalmos were studied. Oxygen partial pressure in the normal retina and oxygen content of gas bubbles in exophthalmic eyes were recorded. Risk factors for the development of exophthalmos were investigated in an experiment using unaffected fish, variable water temperature, fright- induction and exercise regimes.

Gas and haemorrhage was present in the choroid of exophthalmic eyes, with haemorrhage in retrobulbar tissues resulting from perforation of the sclera in some eyes. Oxygen content of gas in eyes with recently developed exophthalmos was high (up to 73%). In some eyes with retrobulbar haemorrhage, oxygen tension approached zero, indicating severe disruption of blood supply to the eye. Oxygen tension at the retinal-vitreal junction of normal dhufish eyes was high (344 ± 26 mm Hg), with oxygenated blood supplied to the choroid body from the gills via the pseudobranch. The finding of a single haemoglobin with pronounced Root and Bohr effects in dhufish was significant and may contribute to the susceptibility of the species to exophthalmos.

Investigations suggest that exophthalmos is physiological in origin and is related to the environmental differences between the natural habitat of the fish and the conditions that are experienced in aquaculture. Dhufish appear to be highly adapted to a relatively inactive life-style with relatively constant environmental conditions at high hydrostatic pressure. Rapid changes of temperature or blood acid-base characteristics may precipitate the development of exophthalmos.

The monogenean parasite, Haliotrema abaddon, was described and stages of its life-cycle identified. As the parasite was troublesome in captive fish, potential treatments were investigated using in vitro and in vivo studies. Praziquantel was identified as the most effective ‘in water’ treatment of fish infested with H. abaddon. Other useful but less effective and safe treatments were low salinity baths (<1.5 ppt for ninety minutes) and 0.5 mg L-1 trichlorphon for 36 hours.

Life in tanks appears stressful for many dhufish, resulting in health problems such as exophthalmos and disease outbreaks, including severe H. abaddon infestation and bacterial and fungal diseases. Multiple risk factors appear to pre-dispose the fish to these conditions. They include environment al factors such as water temperature, depth and physico-chemical composition, diet and stocking density; host factors such as physiological and social adaptation to a relatively solitary, sedentary lifestyle in a deepwater habitat and pathogen factors such as increased fecundity and decreased generation time in warmer water temperatures. Decreasing fish stress and maintaining environmental conditions close to those in the natural habitat, including increasing tank depth and decreasing light intensity are expected to improve the overall health of captive dhufish.

Item Type: Report
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Series Name: FRDC Project 98/328
Publisher: Australian Government. Fisheries Research and Development Corporation/Murdoch University
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