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Sydney's coastal seas extend from the lowest tide mark on the shore to about 40 km out to sea.

From this point on, the water is more than 200 m deep and is considered open ocean.

Coastal areas are the richest, most biologically diverse regions of the sea, particularly where the seabed is rocky. This is because enough sunlight reaches the seabed to allow plants to grow while the rocks provide a home for the many different animals and plants that form rocky reef communities. These reefs are home to a great variety of marine animals from crabs to sea dragons and sharks. In deeper water, there are large fish such as tuna and marine mammals such as dolphins and whales. Between May and January, it is common to see migrating Humpback and Southern Right Whales.

Coastal sea ecosystems are vulnerable to:

  • pollution from land run-off;
  • overfishing of commercial species and damaging fishing methods such as trawling;
  • pollution from discarded plastics and other non-biodegradables.