Department of Agriculture, Fisheries and Parks said yesterday.

The slick, which prompted numerous calls to the department, was caused by the spawning of corals, the department said.

Corals produce large numbers of free-swimming larvae in order to populate new areas, and they invariably spawn on hot, calm August days, a news release explained.

Tranquil weather promoted formation of the slicks, and a strong, rancid odour might be produced as the byproducts of the spawn decompose, it added.

The annual phenomenon was not a hazard, but promoted the continued existence of corals in local waters.