South-western Australian Waters Research Articles

Partitioning of food resources by four abundant, co-occurring elasmobranch species: relationships between diet and both body size and season

The present study has tested statistically the hypothesis that the diets of four abundant and co-occurring elasmobranch species differ and change with body size and season and has determined the extent of any differences, to ascertain their potential for reducing competition for food resources. Non-metric multivariate analyses of volumetric contributions of dietary categories to stomach contents demonstrated that the dietary compositions of the rays Myliobatis australis and Aptychotrema vincentiana and the sharks Heterodontus portusjacksoni and Squatina australis in south-western Australian waters differed. M. australis fed predominantly on benthic invertebrates, whereas A. vincentiana consumed large volumes of teleosts. The durophagous H. portusjacksoni ingested a wide variety of prey, including gastropods, cephalopods, bivalves, echinoderms and teleosts, whereas S. australis ingested mainly teleosts and cephalopods. The extent and pattern of change in the diet with increasing body size varied among species. For example, the diet of H. portusjacksoni changed abruptly to larger, harder-bodied prey at ∼400-mm total length, whereas that of S. australis underwent small, gradual changes with increasing body size. The diets of each species changed seasonally. Inter- and intraspecific variations in dietary composition reduce the potential for competition between and within these abundant elasmobranch species in south-western Australian waters.

Life cycle characteristics of the Blue Morwong Nemadactylus valenciennesi, compared with those of other species of Cheilodactylidae

The demonstration by this study that the life cycle characteristics of the reef fish Nemadactylus valenciennesi differed markedly from those of other cheilodactylids led us to compare these differences and hypothesise on their evolutionary implications. The distribution, length and age compositions, growth and the length and age at maturity of N. valenciennesi in south-western Australian waters have been determined. The maximum age (21 years) of N. valenciennesi (maximum total length = 984 mm) is far less than those of the much smaller Cheilodactylus fuscus, Nemadactylus macropterus (both ~40 years) and Cheilodactylus spectabilis (97 years). Yet N. valenciennesi, N. macropterus and C. spectabilis mature at a similar young age (3–7 years). While, on the basis of data for other species, all three species mature at lengths consistent with their asymptotic lengths, the last two species mature at a far earlier age than would be predicted from their long life spans. The early attainment of maturity by N. macropterus and C. spectabilis may reflect a response to high juvenile mortality or constrained adult growth, whereas their long life spans could reflect a response to variable recruitment in the past or compensation for limited annual reproductive output due to very constrained adult growth.

Polydorid species (Polychaeta: Spionidae) in south-western Australian waters with special reference to Polydora uncinata and Boccardia knoxi

Eight species of polydorid polychaetes were found to inhabit mollusc shells from south-western Australian waters. Numerous individuals of Polydora uncinata were extracted for the first time from the shells of both land-based cultured abalone Haliotis laevigata and H. roei, as well as from natural subtidal H. roei and Chlamys australis. Shells of the oyster Saccostrea commercialis cultured in sea-based systems were infested by Boccardia knoxi which was first recorded in these waters. Polydora aura, Dipolydora giardi, D. armata, D. aciculata and Boccardia proboscidea were common among shells of various natural intertidal and subtidal molluscs. A small number of P. haswelli were extracted from their self-excavated burrows in shells of cultured oysters. Boccardia knoxi and D. aciculata were redescribed based on the newly collected materials. Polydora uncinata and B. knoxi exhibited similar larval development patterns (exolecithotrophy and adelphophagy), iteroparity and longer life span, suggesting a high reproductive potential. This study suggests that further monitoring of polydorid species is needed not only from the viewpoint of marine biology but also to survey the risk invasive species pose to commercially important molluscs in this region and worldwide.

Size compositions and reproductive biology of an important bycatch shark species (Heterodontus portusjacksoni) in south-western Australian waters

Heterodontus portusjacksoniwas obtained from the catches of commercial trawl, gillnet and longline fisheries operating in south-western Australian waters, in which this shark is an abundant bycatch species. Ninety per cent of theH. portusjacksonicaught by commercial trawling in a marine embayment were <400 mm in total length, whereas 99 and 100%, respectively, of the individuals taken by commercial gillnet and longline outside that embayment were >400 mm. Although the differences between the size compositions in the catches obtained by trawling vs both gillnetting and longlining, which were similar, are partly attributable to gear selectivity, they also reflect a use by juvenileH. portusjacksoniof protected nearshore waters as nursery areas and a tendency for larger juveniles and adults to occupy reef/rock habitats in a range of water depths. The fact that all but one of the numerousH. portusjacksoni>800 mm were females is reflected in the ratio of females to males differing significantly from parity in the gillnet and longline samples. Yolked oocytes usually take one year to develop to ovulatory size. Ovulation occurs in late winter to early summer and hatching takes place a year later when the embryo is 180–220 mm. The claspers of males commence rapid growth at a total length of ~450 mm, coincident with the onset of clasper calcification and gonadal maturation. The rate of clasper growth relative to total length reached a maximum at ~570 mm and then declined precipitously as the claspers approached full calcification. Based on gonadal criteria, theL50at maturity for females (805 mm) was nearly 40% greater than that for males (593 mm). Values of the deviance information criterion and marked overlap in their 95% confidence intervals demonstrate that theL50of 581 mm derived for males using full clasper calcification as the index of maturity was equally valid as the aboveL50derived for males at maturity using gonadal criteria. The capture of substantial numbers of juveniles by trawling and of females <L50at maturity by gillnetting and longlining may be having localized effects on the population structure ofH. portusjacksoniin south-western Australian waters.

Reproductive biology and diet of the southern fiddler ray, Trygonorrhina fasciata (Batoidea:Rhinobatidae), an important trawl bycatch species

The reproductive cycle of Trygonorrhina fasciata females in south-western Australian waters is shown to include a protracted period of embryonic diapause or delayed development. This characteristic is typically regarded as enabling young to be born when conditions are optimal. The birth of T. fasciata occurs in April/May, when water temperatures are still high and conducive to rapid growth. Furthermore, because gut fullness in this and subsequent months was similar to other times of the year, the prey of neonates, i.e. mysids, carids, pinnotherids, amphipods and isopods, were presumably abundant in that period. Ovulation occurs in April/May and embryos grow rapidly between December and birth in April/May. Females grew to greater lengths than males, i.e. 1460 v. 890 mm, and matured (L50) at larger lengths, i.e. 892 v. 678 mm respectively. As body size increased, teleosts, pilumnid and portunid crabs and molluscs became important dietary components. Overall, the diet consisted predominantly of crustaceans, fish, polychaetes and molluscs, with volumetric contributions of 73.4, 17.2, 5.3 and 2.8% respectively. Dietary composition underwent a cyclical seasonal change and differed among locations, indicating some degree of opportunistic feeding. Interspecific comparisons indicate that benthic food resources are partitioned among and within the main batoid species in south-western Australian waters.

First record of the freshwater sawfish, Pristis microdon, from southwestern Australian waters

Sawfishes (family Pristidae) are large (up to 7m) modified batoids with a blade-like snout edged with pairs of rostra I teeth. They occur worldwide in sub-tropical and tropical shallow coastal sea, estuaries and freshwater systems (Last and Stevens 1994; Compagno and Last 1998). There are between five and seven recognised species worldwide, with five species represented in Australian waters (Last and Stevens 1994). Sawfish populations have been extirpated from many parts of their original global range by gillnetting and trawling and are easily entangled in nets by their toothed rostra (Simpfendorfer 2000). The little that is known about the biology of sawfish suggests they have low rates of reproduction (Tanaka 1991; Compagno and Last 1998; Wilson 1999; Simpfendorfer 2000; Thorburn et al. 2004). This combined with their susceptibility to fishing gear, make sawfish a high risk species and all have subsequently been listed globally as critically endangered under the IUCN Red List Assessment 2006 (Compagno et al. 2006).

Reproductive biology, size and age compositions and growth of the batoid Urolophus paucimaculatus, including comparisons with other species of the Urolophidae

The biology of Urolophus paucimaculatus in south-western Australian waters has been examined and compared with that of the co-occurring Urolophus lobatus, Trygonoptera personata and Trygonoptera mucosa. These four species represent the only two genera of the Urolophidae. Urolophus paucimaculatus conceives in early/mid-summer and gives birth in late spring/early summer, closely paralleling the situation with U. lobatus. Although Trygonoptera species likewise have a 10–12-month gestation period, they conceive and give birth in late autumn to mid-winter. The marked intergeneric differences in birth time help account for pronounced intergeneric differences in the diets of their newly born young. By birth, Urolophus paucimaculatus attains ~50% of its asymptotic size. The relatively large mean disc width of the four urolophids at birth, i.e. 105–128 mm, accounts for the small litters of these species (1–2 young per litter). Size at maturity and maximal size and age of each species are greater for females than males and are greater for Trygonoptera than Urolophus species. The growth of U. paucimaculatus and the other urolophids throughout pre- and postnatal life can be described by a single smooth growth curve. The maintenance of a constant pattern of growth from conception is remarkable because the diet and behaviour of these elasmobranchs changes radically at parturition.

Reproductive biology of the Australian herring Arripis georgiana

Arripis georgiana was collected from along the Western Australian and South Australian coasts at regular intervals between October 1996 and December 1998. The trends exhibited during the year by gonadosomatic indices, gonadal maturity stages and oocyte stages demonstrate that spawning is restricted to south-western Australian waters from at least as far north as ~28˚43′S to as far east as ~119˚24′E, and that it occurs mainly during late May and early June. The simultaneous presence of post-ovulatory follicles and hydrated and yolk-granule oocytes in some ovaries during the spawning period indicates that A. georgiana is a multiple spawner, i.e. females spawn more than once in a breeding season. In south-western Australia, where all life-cycle stages are found, the overall sex ratio in catches collected by netting was close to parity, whereas females predominated in those obtained by anglers. The length at which 50% of fish reached maturity was 197 mm for females and 179 mm for males, and just over 50% of females and ~80% of males attained maturity at the end of their second year of life. The implications of these data for management of the fishery for A. georgiana are discussed.

Mercury content of shark from south-Western Australian waters

Muscle samples from four species of commercially sought shark have been analysed for total mercury and the mercury concentration related to the size of the fish.

Races and Populations of the Australian Pilchard, Sardinops neopilchardus (Steindachner)

The evidence of taxonomic differentiation in the Australian pilchard is reviewed. Three major groups (called races), located respectively in eastern, south-eastern, and south-western Australian waters, are distinguished by differences in growth rate. The boundary zone of the two former is near the New South Wales-Victoria bolder, but it is not certain to which of the two latter races the South Australian fish belong. The two former races are sukdivided into smaller, more or less separate stocks (populations), which are distinguished mainly by differences in mean number of vertebrae and in abundance fluctuations. There are at least two such groups in the eastern race, which meet between Port Jackson and Jervis Bay, and at least two in the south-eastern race. The pilchards of Cook Strait, New Zealand, are probably distinct from those of any Australian locality.