Composition, seasonality and distribution of Ichthyoplankton in Port Phillip Bay, Victoria

Abstract

Plankton samples were collected on monthly cruises over 1 year from May 1983 to April 1984, to investigate composition, seasonality and distribution of ichthyoplankton in Port Phillip Bay. Fish egg numbers were dominated by the southern anchovy, Engraulis australis; clupeoids and gobiids were co-dominant larvae. Other common fish larvae were the cobbler, Gymnapistes marmoratus, greenback flounder, Rhombosolea tapirina, and callionymids. Egg abundances were highest in summer, resulting mainly from high abundances of E. australis eggs. Abundances of fish larvae were highest in summer and winter-spring. The summer period was dominated by clupeoid and gobiid larvae, together with callionymids, monacanthids, platycephalids, the blenny, Pictiblennius tasmanianus, the snapper, Chrysophrys auratus, and a number of rarer species. The winter-spring period was almost entirely dominated by four taxa: gobiids, G. marmoratus, R. taplrina and the long-snouted flounder, Ammotretis rostratus. Eggs of E. australis, Sardinops neopilchardus and A. rostratus, and larvae of all common taxa except monacanthids and C. auratus, were distributed widely throughout the bay. Abundance of eggs of E. australis at the mouth of the bay was significantly lower than within the bay proper. Larvae of P. tasrnanianus were significantly more abundant in the shallow, northern region of the bay than in the deeper, central region. Most monacanthid larvae were collected towards the mouth of the bay, whereas C. auratus larvae were only collected at stations well inside the bay. Analysis of winter samples revealed multispecies patchiness of fish larvae, and a positive correlation between larval abundance and volume of net zooplankton. Although juveniles of King George whiting, Sillaginodes punctatus, and yellow-eyed mullet, Aldrichetra forsteri, are abundant in Port Phillip Bay, larvae were virtually absent. It is proposed that these species spawn offshore, and that immigration into the bay occurs at a late larval-early juvenile stage not detectable by plankton sampling.