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Fish are dying at Bushells Lagoon - ABC

Hundreds of dead fish have been found in Bushells Lagoon in north-western Sydney, prompting calls from environmental groups for better water management.

Richie Benson from the Hawkesbury Environment Network (HEN) has been monitoring the lagoon at Wilberforce, near Windsor, and was shocked to find hundreds of dead carp and eels there earlier this week.

His concern began in late December with reports of dead pelicans at Deep Lagoon, about half an hour north of Windsor, and later at Bushells Lagoon, which is extremely low.

"On Sunday I went out there to have a look at Bushells Lagoon and it seems that every carp and every eel in the lagoon is dead," Mr Benson said.

"I haven't seen one alive — fish or eel.

"It smells really bad, it's rotten, it's going make the water really poisonous in the next couple of days especially after today, a [40 degrees Celsius] day."

There is no official explanation for why the fish have died, but Mr Benson said he suspected it was a combination of hot weather and the low water level in the lagoon.

Mr Benson said while the drying out of the lagoon was part of the natural cycle, it was being accelerated by silt settling at the bottom.

When he measured the temperature of the water last week, it was 34C.

"It's filling up with sediment quite quick, and because of that it's getting shallower," he said.

"The water's heating up quicker, then we're losing dissolved oxygen in the water.

"At that point there were some eels still alive, some of the smaller ones.

"The eels were actually getting out of the water and laying up on the mud to get air, but they end up drying out and dying."

HEN said while the event was a natural occurrence, they were calling on Hawkesbury City Council to develop a water management plan to help preserve the natural value of the lagoon.

"There's a whole range of different types of water birds and wetland birds there, and of course lots of water bugs and fish," Mr Benson said.

"It's really important, it's something we need to protect for the future."

Hawkesbury City Council has been contacted for comment.