The city's environment is fighting a losing battle against development and its side effects, including pollution, pleasure boats and abnormal threats like alien seaweeds that have escaped from home aquariums. And with Australia's largest city at 5 million people and still growing, and developers keen to cash in on its glamorous image, beautiful beaches and world-famous harbour, freshwater supplies are barely enough to keep pace.

Even Sydney's harbour, an unusual marine environment that contains hundreds of species of fish, is under threat. An introduced variety of seaweed used in home aquariums called Caulerpa has spread around much of the harbour, from people flushing it down drains.

Whale experts are still unsure what happened to Colin's mother.

But Jean Meaney, a member of the Organisation for the Rescue and Research of Cetaceans in Australia, which was involved in the bid to save the whale, said she may have been hit by a boat.

Rubbish is another problem for whales. Plastic bags, for example, can be mistaken for food.

'Plastic bags look, when they are floating, if they have not been cut, like jellyfish. A lot of whales depend on jellyfish,' she said.

The plastic bags wreak havoc on whales' digestive systems and can kill them. The problem has become so serious that a major campaign has been launched to discourage people from using plastic bags and instead take reusable bags to supermarkets, with some success.

Judy Reizes, founder and manager of the Manly Environment Centre in the northern part of Sydney harbour, said many problems were linked to the activities of pleasure boats. Seagrass nurseries for young fish appeared to be diminishing in places where boats were moored.

'Just imagine grass that is about 1cm wide, and there are long tendrils, say about 30cm long. That covers the bottom,' she said. 'The small fish hide in that. That way the big fish don't eat them. If there's nowhere for the small fish to hide, they never grow up to be big fish.'

Boat owners have even threatened legal challenges as a group if they felt environmental laws threatened their interests. 'These people like to do what they want to do,' Ms Reizes said.

There are some positive signs. Some aquatic reserves have been created, and with the banning of spear fishing, many endangered species have found refuge there. One reserve, set up in 2002, is already showing results. The fish are more numerous, do not fear people and can be enjoyed by scuba divers.

Previous generations were unable to know what was going on far below the water's surface - but now that excuse no longer exists.