THE PRODUCTION AND TROPHIC ECOLOGY OF SHALLOW-WATER FISH ASSEMBLAGES IN SOUTHERN AUSTRALIA .1. SPECIES RICHNESS, SIZE-STRUCTURE AND PRODUCTION OF FISHES IN WESTERN-PORT, VICTORIA

Fishes were sampled by seine and gillnet in three different habitat ty pes (seagrass, unvegetated sediment and channel) within Western Port,V ictoria, between 1989 and 1990. Approximately twice as many fish speci es were collected per seine haul from seagrass habitat ((x) over bar = 8.7) compared with intertidal unvegetated habitat ((x) over bar = 4.7 ), while relatively few fish species were collected per haul from deep er unvegetated-channel habitat ((x) over bar = 2.3). Most fishes assoc iated with seagrass beds were widely distributed within the embayment, with the notable exception of a number of coastal species that occurr ed in low numbers at a single site (Cowes Bank seagrass) near the entr ance to the bay and had presumably drifted as pelagic larvae from reef s outside. Small fishes (<10 g weight) were more abundant in seagrass than in unvegetated habitats; however, relatively few of these animals were juveniles of commercially fished species. Seagrass beds in Weste rn Port were not found to provide a significantly more important nurse ry habitat for commercial species than unvegetated habitat. The produc tion of small fishes at different sites was highly correlated with the production of epifauna, particularly crustaceans. We postulate that t hese factors are causally linked. This hypothesis is supported by seas onal data, which indicate that when the production of crustaceans was at its lowest during May, populations of small fishes were rapidly dec lining in Western Port, either by dying or emigrating. Seagrass beds s upported over twice the production of small fishes as unvegetated habi tat (3.82 g . m(-2). yr(-1) cf. 1.58 g . m(-2). yr(-1) in unvegetated habitat). Because an estimated 178 km(2) of seagrass habitat has been lost from Western Port since 1973, the reduced production of fishes in unvegetated habitat translates to a decline in small fish production of approximate to 630 tonnes AFDW per year. However, falling catches o f only two commercial fishes, six-spined leatherjackets (Meuschenia fr eycineti) and blue rock whiting (Haletta semifasciata), were clearly a ssociated with seagrass loss, with most of the loss of fish production occurring amongst small non-commercial species, particularly syngnath ids, clinids and scorpaenids. Differences in production of small fishe s between habitats were not reflected higher in the food chain. Althou gh greater numbers of fishes were collected using small-mesh (64 mm) g illnets from seagrass than unvegetated habitats, these differences wer e due to variation in the catch of the omnivorous mullet Aldrichetta f orsteri, the most abundant species, rather than variation in the abund ances of piscivorous fish species. No differences were found between c atches of fish using large-mesh (108 mm) gillnets placed in different habitats.