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The biology of Nannatherina balstoni and Lepidogalaxias salamandroides, including comparisons with the other teleosts found in inland waters in south-western Australia

Morgan, David L. (1999) The biology of Nannatherina balstoni and Lepidogalaxias salamandroides, including comparisons with the other teleosts found in inland waters in south-western Australia. PhD thesis, Murdoch University.

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The first aim of this thesis was to determine the length and age compositions, growth rates, reproductive biology and diets of Nannatherina balstoni and Lepidogalaxias salamandroides. Sampling for these two species of teleost, which are found predominantly in the acidic pools of the southern peat flats in the extreme south-western comer of Australia, was carried out monthly over approximately two years by using seine and dip nets and, in the case of larvae, also light traps. Nannatherina balstoni spawned at the end of their first year of life, when, on average, the males and females had reached 60 and 63 mm, respectively. In contrast, less than 30% of both the males and females of L. salamandroides that reach the end of their first year of life spawn at that time. Nannatherina balstoni almost invariably died in the months after they had spawned, i.e. when they were just over one year old. Several L. salamandroides lived for two years and a few for even three or four years. All of the L. salamandroides that reached the end of their second year of life spawned at that time. In contrast to their males, the females of L. salamandroides continue to grow throughout the whole of their life, which accounts for the fact that female fish reach a greater maximum size than the males, i.e. 7 4 versus 50 mm TL.

Nannatherina balstoni and L. salamandroides both spawn during winter after heavy flooding and when water temperatures and day length are close to or at their annual minima. Spawning at this time enables the larvae and juveniles of these species to capitalise on the rich food supplies that are present during the late winter and spring. The considerable growth that is achieved during this period by L. salamandroides has the advantage of enabling this species to attain a substantial size, prior to the time when it commences aestivation when the pools in which it lives start to become dry. Although both species feed on particularly cladocerans, copepods and ostracods during larval and early juvenile life, and L. salamandroides subsequently continues to feed on these and other arthropods, the diet of N. balstoni shifts markedly towards the terrestrial insects that alight or are deposited on the water surface.

The fecundity of N. balstoni ranged from 550 to 1600, with a mean of 1243, while for L. salamandroides the range was 37 to 166 and the mean was 82. Histological sections of the ovary, allied with observations of females laying eggs in the laboratory, demonstrate that L. salamandroides is a multiple spawner, i.e. produces small numbers of eggs on a number of different occasions. The prominent anal fin of the males of L. salamandroides becomes greatly modified, into a structure that can clasp the female and direct sperm in through their cloaca, when they become sexually mature and is then retained throughout the rest of life.

The distributions and habitats of the small suite of fish found in the inland waters of south-western Australia have been determined from samples collected using nets, traps and an electric fishshocker at 410 sites in the region that is bounded by Capel in the north (33°30'S, 115°34') and Cheyne Beach (34°53', 118°24') in the south-east. The distribution data were then collated with those of Christensen (1982) and Jaensch (1992) and with the Records of the Western Australian Museum.

The most abundant and widespread endemic teleosts are Galaxias occidentalis, Bostockia porosa and Edelia vittata, these species being found throughout all of the major river catchments in the study area. Within the study region Galaxiella nigrostriata, L. salamandroides, N. balstoni and Galaxiella munda, are each almost invariably restricted to water bodies south of Margaret River (33°56', 115°05'). Although the first three of these species are essentially confined to the acidic pools in this region, the first two show a marked tendency to occur in ephemeral pools, i.e. pools that dry up in the summer, while the third species occurs in permanent pools. The fourth species, G. munda, is found in the streams and rivers within and bounding the southern peat flats. However, small localised populations of G. munda, G. nigrostriata and N. balstoni are present well to the north of the study area at or near Gingin (31°21', 115°54'). It is suggested that these populations once formed part of a continuous distribution in inland water bodies along the lower west coast of Australia, which became disrupted through the destruction of the majority of the water bodies in that coastal strip by human activities during this century. The fact that the only other endemic freshwater teleost in south-western Australia, Tandanus bostocki, was caught on only two occasions probably reflects the fact that the sampling methods were not ideal for catching this far larger species. Larvae (ammocoetes) of the pouched lamprey were regularly found in the soft silty deposits of several of the rivers in the study region. Galaxias truttaceus and Galaxias maculatus, which are abundant in south-eastern mainland Australia and Tasmania, were found only in catchments in the extreme eastern part of the study region. These two species are known, however, to be present in rivers to the east of Albany. The atherinid Leptatherina wallacei and the gobiids Pseudogobius olorum and Afurcagobius suppositus, which are often abundant in estuaries, were collected from numerous rivers and lakes. The most widespread exotic teleosts were Gambusia holbrooki and Perea fluviatilis, these species being particularly abundant in water bodies that had been modified, e.g. by the effects of the construction of dams. Oncorhynchus mykiss and Salmo trutta were only caught in those systems in which they had been stocked by the Western Australian Fisheries Department. Finally, a summary is provided of the biology of each of the freshwater fish species that is found in south-western Australia.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science
Supervisor(s): Gill, Howard and Potter, Ian
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