Search

Data on the phyllosoma larvae of P. cygnus caught on seven cruises carried out between April 1976 and January 1977 off the west coast of Australia are presented and discussed. The studies were undertaken to determine the length of the plankton cycle, the distribution of the phyllosoma in the south-eastern Indian Ocean and the extent of the offshore dispersal. Analyses showed the average length of the planktonic life to be less than one year and probably between 9 and 11 months. After hatching most early phyllosoma were transported offshore. Few larvae remained on or near the continental shelf. The maximum westward extent of the phyllosoma distribution of P. cygnus was not determined because significant numbers of phyllosoma were still found as far offshore as 99 001E. (1500 km offshore), the furthest station sampled. The phyllosoma were distributed over an extensive area of the south-eastern Indian Ocean, and there were few stations beyond the continental shelf between 24 30'S. and 35 00'S., from 99 00'E. to 115 00'E., during midwinter, at which phyllosoma were not caught. The highest densities of mid-and late-stage phyllosoma occurred between 111 00' E. and 104 00' E. (between 375 and 1030 km offshore), at 29 30'S. The area of greatest abundance was due west of the approximate centre of the adult distribution on the coast. The significance of the circulation of the south-eastern Indian Ocean in the transport of the larvae is discussed.

Investigation of the age and growth of snoek, L. atun, from south-eastern Australian waters indicates differences in growth rates of fish taken in Bass Strait and eastern Tasmania. These findings add support to earlier suggestions, based on other evidence, that there are at least three populations of this species inhabiting, respectively, Bass Strait, eastern Tasmanian, and eastern Victorian and southern New South Wales waters.

Estimates of von Bertalanffy growth parameters were made for mature (>3-year-old) gemfish (Rexea solandri), using otoliths sampled biennially from commercial catches between 1980 and 1986. During this period, there was a significant decline in the mean length of mature gemfish in the catch. Large variations in growth-parameter estimates were found over the period sampled (e.g. L∞ ranged between 87.3 and 130.3 cm for males and between 113.1 and 134.7 cm for females). Likelihood-ratio tests showed many of the growth-parameter estimates to be significantly different between the years sampled. Inclusion in the analyses of data for juvenile (1- to 3-year-old) fish considerably reduced both the standard errors of the parameter estimates and the significance of the variations between them. Comparison of parameters estimated by following the growth of individual cohorts spawned in 1975 and 1981 suggested that the variations in the parameters for the 1980-86 data were caused by differences in the length composition of the samples rather than by changes in the growth rate of the fish. The results suggest the need for care when comparing growth parameters estimated for different populations, especially when there are large differences in length composition. The best estimates of growth parameters for gemfish were considered to be those derived from the aggregate data for the whole of the period sampled, including the data for juveniles. For male gemfish these best estimates (with asymptotic standard errors in parentheses) were L∞ =97.5 (0.8) cm, K=0.212 (0.005) year-1 and to = - 0.54 (0.05) years, and for females the estimates were L∞ = 109.4 (0.6) cm, K= 0.180 (0.003) year-1 and to= -0.63 (0.04) years. Likelihood-ratio tests showed these estimates of L∞ and K to be significantly different between the sexes.

Mesh netting of beaches along the more populous sections of the NSW coast for the protection of swimmers and surfers against shark attack has been carried out since 1937 in Sydney and since 1949 in Newcastle and Wollongong. Data for the catches of sharks by taxonomic groups are presented for the period from 1950 to 1990. Although there were large year-to-year fluctuations, neither catch nor catch per unit effort showed any discernible trend for the period up to 1972. In 1973, the catches of all taxonomic groups increased substantially following changes to the specifications of nets and their deployment. Since 1974, there has been a continuing decline in both the numbers of sharks caught and the catch per unit effort. Although Carcharhinus spp. (whalers) and Carcharodon carcharias (white pointer sharks) have shown an almost unbroken decline since the commencement of meshing, Sphyrna spp. (hammerheads) and Squatina australis (angel sharks) have shown large fluctuations over the entire study period, particularly in the period following the upgrading of netting effectiveness (post-1973). Substantial changes have occurred in the size compositions of a number of the taxonomic groups between the first and second 20-year periods of the meshing programme. High proportions of S. Australis and Carcharias taurus (grey nurse sharks) were females. There were major differences in the species compositions of catches between the three major meshing areas and between the pre- and post- 1973 periods. Data on the catches of nontarget species for the Newcastle region for the period from 1965-66 to 1980-81 indicated significant increases in the catches of rays and jewfish in the post-1973 period. Post-1973 changes in the catches of dolphins, turtles and tunas were not statistically significant.

This study examined spatial and temporal variability in the composition, distribution and relative abundance of by-catch from prawn trawl sampling in the Hawkesbury River, New South Wales. It also determined whether there were differences in the fauna between sections of the river that are open and closed to prawn trawling. By-catch was sampled monthly between March 1986 and February 1988 in three areas along the river that differed in distance from the mouth of the estuary and in salinity. The total incidental catch comprised 75 species of fish, 13 species of crustaceans and 5 species of molluscs: 42 species were commercially and/or recreationally important. Classification and ordination analyses showed that the species composition of the by-catch differed between the area closest to the mouth of the estuary and that furthest upstream, and that this difference was relatively consistent throughout time. The number of species in the by-catch decreased with increasing distance upstream. Annual and seasonal changes in the number of species were similar in all areas along the river: more species were caught in 1987 than in 1986, and in autumn and winter each year. In contrast, the number of individuals caught differed among areas and these differences varied between years. Similarly, seasonal fluctuations in the numbers of individuals caught varied between years, but these variations were similar in all areas. In the area furthest upstream there were no apparent differences in the numbers of species and individuals caught by prawn trawl sampling in sections of the river open or closed to prawn trawling.

Phytoplankton species occurrence was studied during 1978-1981 at a coastal station off Sydney and succession patterns were sought using polythetic agglomerative classification analysis. Three major phytoplankton categories were distinguished: (1) a large group of species (including most nanoplankton), which were present throughout the year; (2) a group of diatom species, which bloomed following episodic nutrient enrichments in spring, early summer, autumn and winter; (3) a group of warm-water species (e.g. Trichodesmium), which were associated with admixture of tropical water masses. Gross features of the phytoplankton cycle in these waters are related to the erratic flow pattern of the East Australian Current. However, clear short-term (4-14 weeks) species successions were evident within the spring and summer diatom blooms. These began with small diatoms (Asterionella, Leptocylindrus, Skeletonema, Thalassiosira) followed first by large diatoms (Detonula, Rhizosolenia, Stephanopyxis) and then by large dinoflagellates (Ceratium, Protoperidinium). This sequence is identical to that observed in coastal waters of other parts of the world. A phytoplankton checklist (280 species) for Sydney coastal waters is included as an appendix.

A double-tagging experiment was carried out on a temporarily landlocked population of barramundi. Shedding rates were determined for Floy FT-2 dart tags on fish with a total length over 350 mm and FD-67 anchor tags on fish shorter than 350 mm. Immediate shedding was negligible for both tag types, and the estimated instantaneous daily shedding rate was 0.00057 for FT-2 tags and 0.0018 for FD-67 tags. Results from a small number of long-term recaptures suggest that the shedding rate may decline with time, so that tag loss over long periods could be overestimated if the above shedding rates are applied.