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Health problems of the West Australian dhufish, Glaucosoma hebraicum

Stephens, Frances (2001) Health problems of the West Australian dhufish, Glaucosoma hebraicum. PhD thesis, Murdoch University.

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The purpose of the studies reported in this thesis was to describe and investigate the 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 were exophthalmos in otherwise apparently normal fish and infestation of gills with a monogenean parasite, Haliotrema abaddon. Other health problems were described and investigated as they occurred throughout the tenure of the project.

Exophthalmic lesions were described following gross and histological examination, 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, followed by an investigation of the haemoglobin-oxygen transport properties of dhufish blood that were considered to be relevant to pathogenesis of the condition. Oxygen concentration and tension in the normal retina and in exophthalmic eyes was recorded. Risk factors for the development of exophthalmos were investigated in an experimental design using unaffected fish and variable water temperature, fright-induction and exercise regimes.

Gas and haemorrhage were 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 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.

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. Multiple risk factors appear to pre-dispose the fish to these conditions. They include environmental 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 deep-water habitat and pathogen factors such as increased fecundity and decreased generation time in wanner 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: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Raidal, Shane, Jones, Brian and Thomas, Jan
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