Use of ichthyoplankton ecology to evaluate ecosystem changes: a case study in a large, semi enclosed Australian bay

Intensive night sampling was conducted fortnightly in 1995/96 to investigate the ichthyoplankton assemblage of Port Phillip Bay. Results are compared with those from a similar survey in 1983/84, and are used to ascertain whether major changes have occurred in the composition and abundance of fish eggs and larvae, and whether these are related to ecosystem changes in the bay between 1969 and 1995. The 17 157 larvae caught during this study belonged to 60 taxa from 32 teleost fish families, with Gobiidae (54.2%), Engraulidae (16.7%), Clinidae (10.0%) and Odacidae (5.9%) dominating the catches. Larval concentrations peaked only in summer, in contrast to summer and winter in 1983/84. Larvae from 13 families recorded in 1995/96 did not occur in 1983/84, including Gobiesocidae (9 spp.) and Odacidae (3 spp.). Larvae absent in 1995/96 but present in 1983/84 included Acanthopegasus lancifer (Pegasidae) and taxa from another seven families. Neoodax balteatus larvae ranked fourth in 1995/96 but were absent in 1983/84, while Engraulis australis eggs and Gymnapistes marmoratus larvae were comparatively fewer in 1995/96. We suggest that some of the main differences between the two surveys may be attributable to major ecosystem changes in the bay, particularly the introduction and establishment of exotic marine species.