The fish fauna and finfish fishery of the Leschenault Estuary in south-western Australia

This paper collates unpublished and published data on the fish faunas of the large basin and Collie River regions of the Leschenault Estuary in 1982/83 and 1993/94, and provides information on the commercial and recreational fisheries in that estuary. The most abundant of the 42 fish species recorded in eight six-weekly samples collected from the nearshore, shallow waters of the basin in 1994, were the long-finned goby Favonigobius lateralis , the sandy sprat Hyperlophus vittatus and the atherinids Leptatherina presbyteroides and Atherinosoma elongata ; these four species collectively contributing 83.0% to the total number of fish caught. Of the 42 species, 20 were marine species which use the estuary as a nursery area (marine estuarine-opportunists), whil e 13 complete their life cycles in the estuary, of which seven are also represented by marine populations. The contribution made to the total number of individuals by marine estuarine-opportunists and marine stragglers collectively (32.1%) was far lower than that of species which complete their life cycles in the estuary (67.9%). The presence in shallow waters of large numbers of representatives of species that spawn in the estuary, which parallels the situation found in other south-western Australian estuaries, is probably related to the maintenance of stable conditions and high salinities during late spring and summer when these species, which generally have marine affinities, typically spawn. The composition of the fish fauna of the shallows of Leschenault Estuary differs markedly from that of comparable waters in Koombana Bay into which this estuary discharges. Indeed, the most abundant species in the bay, the flathead sandfish Lesueurina platycephala , which contributed ca 25% to the total numbers in those marine waters, was never recorded in the estuary. The fish catches in offshore, deeper waters of the estuary basin and Collie River comprised larger species and, unlike the situation in shallow waters, were dominated by marine estua- rine-opportunists and the semi-anadromous Perth herring Nematalosa vlaminghi . However, the composition of the fish fauna in offshore, deeper waters of the basin differed markedly from that in corresponding waters in the Collie River. This was mainly due to the presence in the basin of far more species and relatively greater numbers of species, such as yellow-eye mullet ( Aldrichetta forsteri ), tailor ( Pomatomus saltatrix ) and Australian herring ( Arripis georgiana ), and to the occurrence in the river of relatively greater numbers of Perth herring and sea mullet ( Mugil cephalus ). Length-frequency data provide strong evidence that, within the estuary, the small species F. lateralis, L. presbyteroides and A. elongata spawn mainly in summer and typically have a one year life cycle. The small juveniles of the marine species A. forsteri and M. cephalus are recruited into the estuary between mid- or late autumn and early spring, which is consistent with the fact that the spawning period of these two species is very protracted, extending through autumn and winter. Both of these mugilids were represented in some months by three or more age classes. Recruitment into the estuary of King George whiting ( Sillaginodes punctata ) occurs in spring, while that of yellow-finned whiting ( Sillago schomburgkii ) and prickly toadfish ( Contusus brevicaudus ) takes place in summer and autumn, respectively. This reflects spawning occurring in winter and early spring in the case of the first species and in summer with the second and third species. These three species were apparently represented mainly by only two age classes. While length-frequency data show that P. saltatrix also uses the estuary as a nursery area, the modal size classes did not follow such consistent trends throughout the year, possibly reflecting a recruitment, in some years, of the 0+ age class of both the spring and autumn-spawning cohorts. The growt h of all of these species essentially ceased during the cold winter months. The feeding mode of each of the most abundant species can be allocated to one of the following categories. (1) herbivores which feed mainly on the algae associated with seagrass, e.g. Hyporhamphus melanochir, Pelates sexlineatus. (2) detritivores, e.g . M. cephalus, N. vlaminghi . (3) omnivores which feed on algae and a range of invertebrates, e.g. A. forsteri, Amniataba caudavittata . (4) lower-order carnivores which feed on small benthic inverte- brates, e.g. A. elongata, F. lateralis, L. presbyteroides, S. punctata, S. schomburgkii, Torquigener pleurogramma . (5) higher-order carnivores whose prey includes larger and more active invertebrates and fish, e.g. Argyrosomus japonicus, P. saltatrix . The former commercial fishers in Leschenault Estuary, who were restricted to fishing in the basin, shifted from gill nets to haul nets in recent years to reduce the amount of bycatch. Aldrichetta forsteri, M. cephalus, S. schomburgkii and S. punctatia were the main commercial fish species, contributing 55.5, 20.2, 11.1 and 6.6%, respectively, to the total wet weight of all fish species caught between 1981 and 1997. The recreational fishery is dominated by three species of whiting ( S. punctata, S. schomburgkii and Sillago burrus) and tailor P. saltatrix.