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The biology of the Dhufish, Glaucosoma hebraicum, in offshore waters on the lower west coast of Australia

Hesp, Sybrand Alexander (1997) The biology of the Dhufish, Glaucosoma hebraicum, in offshore waters on the lower west coast of Australia. Honours thesis, Murdoch University.

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Samples of Dhufish, Glaucosoma hebraicum, were collected in each month between May 1996 and May 1997 from the catches of commercial and recreational wetline fishers and commercial trawlers. These catches were taken in the offshore waters of Western Australia near Geraldton (ca 28°S) and Perth (ca 33°S). Comparisons were made between the number of circuli on scales and the number of translucent zones on sectioned otoliths, and between the number of translucent zones on sectioned and whole otoliths. The number of circuli on scales was often either greater or less than the number of translucent zones on sectioned otoliths of the same fish. The number of translucent zones observed on whole otoliths were the same as on sectioned otoliths for fish with otoliths that have up to six translucent zones. However, the prevalence of underestimates, when using whole as opposed to sectioned otoliths, subsequently increased progressively as the number of translucent zones increased. The mean monthly marginal increments for sectioned otoliths showed a pronounced decline in spring and then a progressive rise during summer and mid-autumn, before levelling off in winter. These trends provide strong evidence that the translucent zones on the otoliths of G. hebraicum are formed annually and that their numbers in sectioned otoliths can be used to age this species.

The von Bertalanffy growth parameters, L∞, K and t0, derived for the growth curves of G.hebraicum from length-at-age data were 1038, 0.108 and -0.172, respectively, for females and 1087, 0.109 and -0.219, respectively, for males. Males grew slightly faster than females, attaining total lengths of 234, 448, 688, 832 and 921mm after 2, 5, 10, 15 and 20 years, compared with 212,405, 626, 792 and 865mm for females at the same corresponding ages. Females and males reached the legal minimum (total) length (LML) for capture of 500mm at ca seven and six years, respectively. The maximum ages recorded for females and males were 29 and 35 years, respectively, and the maximum lengths for females and males were 976 and 1120mm, respectively. Although the growth curves of both females and males of G. hebraicum caught in waters near Geraldton were shown by a maximum likelihood ratio test to differ from those of fish caught in waters near Perth, the differences in the von Bertalanffy growth parameters for fish in these two regions were not pronounced.

Macroscopic and microscopic examination of gonads showed that female and male G. hebraicum first reach sexual maturity at total lengths of between 250 to 300mm and 350 to 400mm, respectively. Sexual maturity was first attained by females and males at the end of their fifth and eighth years of life, respectively. Histological sections showed that some mature ovaries contained post-ovulatory follicles as well as granular and hydrated oocytes during the spawning period. This provides strong evidence that this species is a multiple spawner. On the basis of the monthly trends exhibited by gonadosomatic indices, gonad maturity stages and different stages in oocyte development, G. hebraicum is considered to breed between December and April, with spawning reaching a peak in late January/early February. For this reason, G. hebraicum was accorded a birth date of the first of February.

The nematode parasite Philometra_sp. was found to infect the gonads of G. hebraicum. Preliminary studies have shown that the prevalence of infection is far higher in females than males, and that it increases with fish size and age. At the completion of spawning, this parasite can occupy 50% of the volume of the ovary. Thus, since philometrid worms are known to feed on blood, it is possible that the parasite has a deleterious effect on the reproduction of G. hebraicum. However, histological sections of ovaries failed to reveal visible signs of egg destruction by the parasite, and nor were there any signs of disruption of normal gonadal development of G. hebraicum. Observations of the life cycle stages of Philometra sp. throughout the year indicate that the life cycle of the parasite is closely synchronised with the pattern of reproductive development of its host. No fish were infected under the size at which sexual maturity is first reached and there was evidence that infection by the parasite occurs when G. hebraicum aggregates during the spawning period, when older, infected fish meet younger, uninfected fish.

Since female and male G. hebraicum reach first sexual maturity at total lengths of between 250 to 300mm and 350 to 400mm, respectively, this means that the majority of both female and male G. hebraicum have had the opportunity to spawn at least once before they reach the LMLof 500mm. However because almost all Dhufish caught in waters >30m die upon release after capture, this species is subject to fishing mortality before they reach the LML. Therefore, in terms of fisheries management, there would be little value in maintaining a legal size limit for this species. Future management strategies for maintaining stocks of this recreationally and commercially important and heavily-fished species could include reducing fishing pressure in heavily-fished areas and closing from fishing certain areas of known high population density of G. hebraicum. However, for conservation purposes, further research is needed to determine locations of high population densities of Dhufish that could be restricted from fishing, and the migratory patterns of this species.

Future studies involve the collection of a greater number of small G. hebraicum (i.e. <300mm), to provide more points for the commencement of the growth curve, more data for determining marginal increment trends in fish with otoliths containing one or two translucent zones, a more precise estimate of the size and age at which females and males reach sexual maturity, and the habitats of these small fish. An histological study of spermatogenesis would improve and consolidate the reproductive data already obtained for males and provide additional information for aiding the aquaculture studies presently being carried out on this species. Further investigations of Philometra sp. infection in G. hebraicum, including a comparison of the fecundity of parasitised fish and unparasitised fish, may provide information on the effects of this parasite on the reproduction of Dhufish.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Supervisor(s): Potter, Ian
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