A Baykeeper shoreline shell survey found a large Cart-rut shell (Thais orbita) at Holloway Bend earlier this year, suggesting the species may still live in the area. This species was central to the ban on use of Tributyltin (TBT) as an anti-foulant on vessels less that 25m in Victoria in 1989.

Cart-rut shell: worse for wear but there!

International studies had shown TBT caused female shellfish to grow penises. A high incidence of the condition (known as imposex) had been recorded at Brighton Harbour in the early 1990s, but had reduced after the ban. We were delighted this week to receive photos of several “fist-sized” Cart-rut Shells on a reef offshore of Brighton.

Cart-rut shells on Brighton reef (pic: Steven Putt)

10 years after the ban was introduced, Marine Pollution Bulletin (Volume 42 # 10) reported that the relative penis size index (RPSI) of female Thais orbita in Port Phillip Bay had generally reduced. But the RPSI had increased at Williamstown, indicating that TBT from large vessels, ship maintenance activities, and release from historically contaminated sediments, continued to pollute the area.

The study concluded that while the ban remained, TBT pollution would only threaten ports areas and be unlikely to impact more widely in the Bay. Unfortunately, their study was restricted to a limited number of sites and they didn’t consider the practice of dumping dredged sediments from the Port of Melbourne into the Bay.