ABSTRACT Native freshwater fish of Australia have a diverse but largely undescribed endemic pathogen and parasite fauna. However, due to long-shared evolutionary histories and virulence/transmissibility trade-offs, effects of these endemic pathogens and parasites appear to be subtle: significant impacts are rarely observed and epizootics have not been recorded. In contrast, a number of alien pathogens and parasites are now established across southern Australia, causing manifestly harmful effects to native fish species and known or suspected epizootics in native fish populations. Undetected and/or undescribed alien viral pathogens are also suspected of being present. Alien pathogens and parasites were introduced to Australia with imports of live alien fish or their fertilised eggs. A review of the scientific and historical evidence indicates that they have had, and continue to have, greater impacts on native fish species than previously realised—especially for freshwater species. This review also documents a previously unknown, Murray-Darling-Basin-wide epizootic of Murray cod Maccullochella peelii in 1929–30, which may have contributed to strong declines in the related eastern freshwater cod Maccullochella ikei . A serious Chilodonella epizootic of M. peelii in 1982 is also examined. In addition, a possible role for alien viruses of the family Iridoviridae ( Ranavirus and Megalocytivirus ) in the general decline of the critically endangered silver perch Bidyanus bidyanus , and the rapid collapse of two specific native fish populations—upper Murrumbidgee River B. bidyanus and Shoalhaven River Macquarie perch Macquaria australasica —is suggested. It is argued that the severity of the impact of the virulent alien oomycete Saprolegnia parasitica , both historical and present day, has been underestimated. Finally, action is recommended against emerging new pathogen and parasite threats, and the extreme risk current alien fish importations pose in introducing them. These will further threaten already stressed native fish populations in southern Australia, particularly across the Murray-Darling Basin.

A survey of fish species occurring in the marine waters of northern New South Wales was conducted at five sites between October 1988 and February 1996. The principal study site was at Julian Rocks, located approximately five km north of Cape Byron, Australia's most easterly mainland point. The other study sites included: Windarra Bank, located 16 km east of Mooball; Wilsons Reef, located 2 km south-west of Julian Rocks; Lennox Head Moat located at the southern extreme of Seven Mile Beach, Lennox Head and Belongil Creek, located two km west of Byron Bay township. A total of 530 fish species in 293 genera and 115 families was recorded from the study area. The total was composed of 394 (74.3%) tropical species, 103 (19.4%) temperate species and 31 (5.8%) subtropical species. The remaining two (0.4%) species have circum-global or circum-Australian distributions. Eighty-four (15.8%) of the species recorded are restricted to the South-Western or South Pacific Oceans and 96 (18.1%) are endemic to Australia. The endemics comprised: 19 (3.6%) tropical species; 48 (9.1%) temperate species; and 29 (5.5%) subtropical species. Two (0.4%) of the species recorded occur in both tropical and temperate seas and two species are primarily freshwater fish, one of which, Gambusia holbrooki (mosquito fish) is introduced. The 394 tropical species dominated the fish assemblage at the study sites. However, when the relative abundances of tropical, subtropical and temperate fish species are compared, the percentage of “common” tropical species (6.3%) was significantly less than the “common” subtropical (38.7%) or “common” temperate (19.4%) counterparts. The most speciose families were: Labridae (74 species); Pomacentridae (43); Chaetodontidae (26); Serranidae (24); Acanthuridae (19); Monacanthidae (16); Carangidae (16); Scorpaenidae (14); Pomacanthidae (13); Scombridae (13) and Tetraodontidae (13). A total of 114 (21.5%) tropical species in 31 families had not previously been recorded from New South Wales. Similarly, 12 (2.2%) temperate species in 10 families had not previously been recorded as far north as the study area.

Gambusia holbrooki, the “Mosquito Fish” introduced to Australian waterways from North America to control mosquito larvae, is widely distributed and abundant in both disturbed and undisturbed aquatic environments. Because it has become established in areas where Litoria aurea was once abundant, there is some suspicion that G. holbrooki's non-specific predatory behaviour may be instrumental in the decline of L. aurea . Laboratory and field experiments were conducted to assess the impact of G. holbrooki predation on L. aurea tadpole survivorship. Laboratory trials involved placing 30 L. aurea tadpoles per 46-litre aquarium in each of the following four treatments: control treatments (two aquaria with just tadpoles and two aquaria with tadpoles and the pondweed, Egeria densa ) and two predator treatments (two aquaria with fish and tadpoles and two aquaria with tadpoles, fish and pondweed). A daily census of the tadpoles was taken over four days. Based on ANOVA of six aquaria for each treatment, G. holbrooki significantly reduced L. aurea survivorship, but pondweed had no significant effect until the final census. The same experimental procedures were used for L. dentata . Again, tadpole number was significantly lower in aquaria housing G. holbrooki than in those without these fish, and pondweed influenced L. dentata's survival only on the final 72-hour census. Field experiments were conducted by placing 30 L. aurea tadpoles in 1.5 x 0.6 x 0.6 m screened cages which permitted water-flow but excluded predatory fish. A pair of cages (one with six G. holbrooki and one without) were placed in three separate ponds on two occasions. A daily tadpole census taken over four days revealed that tadpoles exposed to G. holbrooki had significantly higher daily mortality than tadpoles which were caged alone. These results show conclusively that G. holbrooki is a predator of L. aurea and L. dentata tadpoles. Future management decisions regarding L. aurea populations should include a consideration of G. holbrooki's impact as well as methods for their removal from breeding sites.

Examples are given of a number of New South Wales freshwater fish species which have declined in abundance since last century. Reasons for the declines include river regulation, changes to riparian vegetation, erosion and siltation. The effects of alien species on the stocks of native fish are also discussed together with possible remedial measures to restore both habitat and fish communities.

Gambusia holbrooki is a small, aggressive fish introduced into Australia in 1925 to control mosquitoes. It has been suggested that this species may also prey on tadpoles and be a contributing factor in the decline of some Australian frog populations. We examined the influence of tadpole body size (10 mm. 15 mm and 20 mm). predator prey ratio (1 : 1, 1 :2 and 1 :4) and nutritional status of the predator on the level of predation by G. holbrooki on tadpoles of Limnodynasfas peronii . Unfed fish attacked all three size classes of tadpoles without any significant preference but did so more vigorously when the predator/prey ratio was densest. Predator/prey ratio had no significant overall effect on attack of tadpoles by fed fish but there was a preference for the smaller tadpoles during the first six hours of the experiment. Unfed fish also consumed more of the tadpoles they attacked than did the fed fish. Field surveys were carried out on 10 permanent water bodies in north-west Sydney to examine any correlation between the abundance of G. holbrooki and frog species richness or abundance. The most abundant species was C. signifera . Regression analysis showed a negative relationship between density of G. holbrooki and the abundance of frogs. A positive relationship between the abundance of tree frog species and the cover of aquatic vegetation was also found. While this study did not investigate other factors contributing to declining frog populations, the results of the laboratory experiments and field surveys are consistent with the hypothesis that G. hoibrooki is contributing to declining frog populations.

A diverse fish fauna (49 species) was recorded in four tidal 0.11 ha prawn farming ponds which appear to act as fish traps. The potential deleterious effects of twelve of these species on prawn growth or survival are discussed in terms of the diet and reproductive biology of these fishes. Tarwhine (Rhabdosargus sarba) may be the most serious competitors with prawns for food while short- and long-finned eels (Anguilla australis and A. reinhardtii) are probably the greatest threat as predators especially as individuals appear to bypass the screens which act as filters during tidal water exchange.

Length - weight relationships for 29 fish species occurring in the Murray-Darling River System in inland NSW were determined using data collected principally between 1957 and 1970. In Golden Perch Macquaria ambigua , this relationship is examined with regard to locality, year and sex. In the Murrumbidgee River large females were more rotund than males. In the Murray River fish caught in November appeared to be more rotund than those caught in September supporting an increase in ovary size to be expected at this time of year, and 1960 fish were more rotund than fish from 1963 and 1964 also reflecting time of year and river heights. Twenty one of the species were examined with regard to all fish and sex differences. The remaining eight were examined for all fish combined only. In most cases a single curve adequately describes the relationship for each species, except when the population includes large numbers of maturing females or when substantial sexual dimorphism occurs, such as in Philypnodon grandiceps and Mogurnda adspersa . The relationships provided support for using shape to sex some species.

The conservation of the Oxleyan Pygmy Perch Nannoperca oxleyana would benefit from the adoption of a standardised, non-destructive sampling protocol that effectively and efficiently detects the presence, and quantifies the relative abundance, of extant populations. The objectives of this study were to: (i) quantify the minimum number of traps required to obtain precise relative abundance estimates, (ii) assess the effectiveness and efficiency of various trapping regimes, and (iii) compare the relative detectability and short-term mortality rates of trapping, seine netting and backpack electrofishing. Previous survey data were utilised and augmented with field experiments. Te n traps provided relatively precise estimates of relative abundance. Unbaited and baited traps set for 30 and 60 minutes detected the species on all occasions, whereas traps set for 15 minutes did not. Positive correlations were found between set time and both overall (fish/trap) and standardised (fish/trap/ minute) catch rates, although only the former relationship was significant. The addition of bait to traps did not significantly affect catch rates. Trapping, seine netting and electrofishing detection rates were 88%, 71% and 83%, respectively. Associated mortality rates were 10%, 55% and 0%, respectively. The deployment of multiple gear types increased the likelihood of detecting N. oxleyana . A sampling protocol is recommended that includes saturating sites with unbaited traps set for at least 30 minutes and sampling with a backpack electrofisher. Seine netting should be reserved for situations where an electrofisher is unavailable or non-deployable.

An intertidal rock pool at Seal Rocks, New South Wales, was repeatedly defaunated during summer or autumn between 1969 and 1987 using the ichthyocide rotenone. The fish assemblage was of moderate diversity, dominated by juveniles of subtidal species. Changes in the composition and numbers of species of this assemblage were assessed on both a long-term and short-term basis. The long-term study covered a period of 19 years; the short-term study was from March to May 1986. While considerable variation was observed in the numbers of the component species over the long-term collections, species composition remained relatively constant. In short-term studies recolonization of the area was initially rapid. Initial recolonization was dominated by intertidal species, such as Torquigener pleurogramrna , while subtidal species were more numerous in the long-term. This study is a preliminary observation of the fauna composition in one selected tide pool subjected to repeated defaunation.

The natural diet of the camaenid land snail Thersites mitchellae (Cox, 1864) was investigated by examination of the faecal contents of specimens collected from a range of substrates. The composition of faecal pellets from 22 snails obtained from three different substrates was determined. The results demonstrate that T. mitchellae has a generalist feeding strategy that varies with substrate. Fungal material contributed a high proportion of the diet, suggesting that coarse woody debris (a common fungal substrate) may be an important requirement for populations of T. mitchellae in rainforestassociated habitats. Thersites mitchellae was the first species for which a critical habitat determination was made under the New South Wales Threatened Species Conservation Act 1995. This study adds to our knowledge of the biology of this poorly known land snail. Future studies would benefit from obtaining data pertaining to the timing and frequency of fungal dispersal and substrate preferences to gain further understanding about the availability of fungi as a food source.

A series of letters written from Australia by the naturalist and collector John Gilbert, to his employer John Gould in London, was recently discovered in Liverpool City Libraries and has since been edited for publication (Fisher 1985). One of these is a copy by the thirteenth Earl of Derby of a letter from Gilbert to Gould dated 8 June 1844, written from the Darling Downs (then part of New South Wales but now in Queensland). The letter mainly consists of the description of a new parrot discovered by Gilbert in the Darling Downs, which he hoped might be named after him; a commentary that was later used verbatim in the type description of Platycercus pulcherrimus Gould, 1845. This species, now named Psephotus pulcherrimus (Gould), is the Paradise or Ant Hi Parrot, which has not been seen alive since 1927 but may still exist in remote parts of Queensland (Forshaw and Cooper 1981).