A school of striped fish with yellow tails darts through the water. Larger fish linger in bushes of swaying brown kelp on the ocean floor, and above them two researchers swim past in scuba gear.

They've come to Cabbage Tree Bay, an aquatic reserve at Manly, to check on a plantation of crayweed — a type of seaweed that used to be plentiful on Sydney's coast until it disappeared from a 70-kilometre stretch between Palm Beach and Cronulla some time in the 1980s.

Madelaine Langley and Derrick Cruz climb into Cabbage Tree Bay to monitor crayweed. Credit:Kate Geraghty

Researchers can't be certain what made the crayweed disappear, but strongly suspect pollution including sewage played a part. Treated sewage is now released several kilometres offshore.

Derrick Cruz, 39, and Madelaine Langley, 24, are part of the Sydney Institute of Marine Science's "operation crayweed", which aims to restore the once-flourishing habitat for crayfish, abalone and other marine life. Crayweed also stores carbon from the atmosphere.