True blue crab count is on
ABOUT 2000 blue swimmer crabs are being tagged in the first long-term study of the edible crustacean’s stocks in the Swan and Canning rivers.
“We would very much like crabbers to leave the tag untouched, make records, and return the crab to the water so we can find out where it goes next,” Fisheries crab scientist David Harris said.
The three-year, $300,000 study is funded by the Better Initiatives Fishing Fund, and is focusing on crab stocks in the rivers, Leschenault Estuary next to Bunbury and nearby Geographe Bay.
“Stocks in the rivers are very healthy, but there is a high level of fishing pressure, which we are monitoring,” Mr Harris said.
Last month, researchers’ tagging in the Perth rivers started using permanent T-bar tags in their work, which will continue until June.
A four-month pilot survey until February glued other types of tags to 1100 crabs, after which reports indicated the marked crabs comprised about 10 per cent of all captures.
But glued tags may fall off during juvenile crabs’ frequent moulting before they mature at about six months, while mature crabs are thought to shed their shells each summer.
Mr Harris said fishers with tagged crabs should record their captures’ size and sex, tag number, date, location, depth and catch method.
Researchers need to know if the crab had eggs, whether it was returned to the water or kept for eating, and whether it was a legal size.
Volunteers can join the work aboard the research vessel as part of a Department of Fisheries True Blue Swimmer Supporter program.
Main picture: Fisheries technical officer Josh Baker holds a T-bar tag and a blue swimmer crab with the glued identification; inset: a blue swimmer crab with a glued tag across its shell and a T-bar tag inserted in its body.