By Fiona Hodge

MAF's current collaboration with local iwi to remove invasive sea squirts brings much needed attention to marine pests. It illustrates that marine pests can threaten not just our economy, but our way of life too. The invasive sea squirt in question, Pyura stolonifera praeputialis, threatens to smother and displace other species, including kaimoana.

So what exactly are sea squirts? Sea squirts are filter feeding animals that start out life as tadpole-like larvae, with a head and tail. These tadpoles travel for less than a day looking for a suitable site to settle: time is in short supply when you lack a mouth. Settling tadpoles do a headstand and then attach themselves to the rock or mussel with the suckers on their head. They then digest their own muscle and brain (a nerve cord), and metamorphose into a simple filter feeding sack! It’s almost the opposite of a caterpillar’s conversion into a butterfly. The adult's main trick is to reverse the direction the heart pumps the blood, which it does several times an hour for as yet unknown reasons.

One might think such an odd animal would be no challenge for our biosecurity team. However, MAF has already accepted that P. stolonifera praeputialis is here to stay. The current eradication is from just two locations, places with high local significance and low sea squirt numbers. MAF and locals will be out scrapping the adults off rocks and mussels in these two sites for the next six months. Eradicating the tadpole larvae is unfeasible and a real problem given the likelihood of neighbouring adult mums and dads.

So, as usual, the best cure is prevention. How do marine pests get to New Zealand? Main routes include through ballast discharge and hull fouling. Each year almost three million metric tonnes of foreign seawater is discharged from ship ballasts into New Zealand ports. The problem is it's not just seawater. Ballast seawater contains eggs, sperm, larvae, plankton and life in general. Biologist James Carlton has even found fish in ballast tanks before. “A typical ballast tank could be the size of an auditorium that seats 700 people” he says, giving an idea of the huge volume transported by each cargo ship.

Since 2000 MAF has had a policy requiring ships to swap their ballast water offshore before arriving in New Zealand. The idea being that life collected from the open ocean is less likely to survive and become a pest problem in our coastal port waters. Swapping ballast water off shore is not feasible in all boats and in all weather conditions, and exemptions are granted in these cases. Ideally portside or shipboard facilities would be able available to filter or UV process this seawater. MAF is currently researching into further ballast legislation, and planning a ballast levy to cover inspection costs.

Hull fouling of boats is another way marine pests are spread. Sessile creatures like sea squirts can become mobile by living on the bottom of a boat, their offspring settling on the shores and coasts visited. Currently boat owners are only encouraged to keep their hulls clean and the anti-foul paint job current, but MAF is in the process of developing specific requirements for hulls.

Prevention requires having a well staffed biosecurity office, always on the lookout for new threats. It requires having the baseline knowledge of our ecosystems to spot the unusual before it’s obviously invasive. It requires more legislation around ballast discharge and hull cleaning. All of this costs. Seeing local iwi pick sea squirts off their kaimoana hopefully reminds us that marine pests cost a lot more. Worse, as MAF Marine Biosecurity Coordinator Alan Bauckham notes, “once established here, these [marine] pests are extremely difficult and costly to control, and their impacts are usually irreversible.”