The production and trophic ecology of shallow-water fish assemblages in southern Australia I. Species richness, size-structure and production of fishes in Western Port, Victoria

Abstract

Fishes were sampled by seine and gillnet in three different habitat types (seagrass, unvegetated sediment and channel) within Western Port, Victoria, between 1989 and 1990. Approximately twice as many fish species were collected per seine haul from seagrass habitat ( x ̄ = 8.7 ) compared with intertidal unvegetated habitat ( x ̄ = 4.7 ), while relatively few fish species were collected per haul from deeper unvegetated-channel habitat ( x ̄ = 2.3 ). Most fishes associated with seagrass beds were widely distributed within the embayment, with the notable exception of a number of coastal species that occurred in low numbers at a single site (Cowes Bank seagrass) near the entrance to the bay and had presumably drifted as pelagic larvae from reefs 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 Western Port were not found to provide a significantly more important nursery habitat for commercial species than unvegetated habitat. The production of small fishes at different sites was highly correlated with the production of epifauna, particularly crustaceans. We postulate that these factors are causally linked. This hypothesis is supported by seasonal data, which indicate that when the production of crustaceans was at its lowest during May, populations of small fishes were rapidly declining in Western Port, either by dying or emigrating. Seagrass beds supported over twice the production of small fishes as unvegetated habitat (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 6̃30 tonnes AFDW per year. However, falling catches of only two commercial fishes, six-spined leatherjackets ( Meuschenia freycineti) and blue rock whiting ( Haletta semifasciata), were clearly associated with seagrass loss, with most of the loss of fish production occurring amongst small non-commercial species, particularly syngnathids, clinids and scorpaenids. Differences in production of small fishes between habitats were not reflected higher in the food chain. Although greater numbers of fishes were collected using small-mesh (64 mm) gillnets from seagrass than unvegetated habitats, these differences were due to variation in the catch of the omnivorous mullet Aldrichetta forsteri, the most abundant species, rather than variation in the abundances of piscivorous fish species. No differences were found between catches of fish using large-mesh (108 mm) gillnets placed in different habitats.