Sydney Harbour is one of the richest estuarine areas in the world (an estuary is an area where freshwater and seawater mix). It is home to around 580 different species of fishes and a diversity of underwater habitats. Intertidal rocky shoreline, large rock platforms and kelp covered rocky reef dominate the outer harbour while mangroves forests and seagrass meadows grow around the inner harbour and up the rivers.

There are two main factors which contribute to this richness of fish life and habitat:

Sydney sits on the Australian coast at a point where the warm tropical waters from the north and cooler temperate waters from the south meet and mix. As a consequence both tropical and temperate fish species can be seen in the harbour.

The outer portion of the harbour with its deep, wide mouth, more closely resembles open coastal habitat than a true estuarine habitat.This is also the area where the highest diversity of fish species occur.