After a pleasant holiday trout fishing on the North Island of New Zealand, Canberra angler Pat Brennan was surprised to find his wading boots impounded at Sydney airport for a month’s decontamination following his return.

The experience shows just how seriously Australia’s Quarantine and Inspection Service (AQIS) is taking the threat of a possible slime invasion of Australia’s fresh waters, like that currently causing havoc across the Tasman. It also highlights the growing importance of biosecurity for anglers - as well as professional fishers, fish farmers, aquarists, kayakers and other water users - as unwanted organisms stage fresh invasions of both fresh and marine waters round the world.

The villain in this story is didymo (Didymosphenia geminata), a particularly grungy-looking freshwater alga - known to frustrated anglers as “rock snot” - that has invaded some of New Zealand’s finest trout rivers in the South Island. Since the first sighting in October 2004, didymo has been reported in 44 rivers and 5 lakes. Experts at Biosecurity NZ estimate the original invasion took place in 2001, so it has taken didymo barely six years to slime its way the length and breadth of the South Island.

When it blooms didymo forms dense fibrous mats with the texture and appearance of dirty yellow-brown cotton wool. These degrade river and lake ecosystems, smothering local aquatic plants and insects in the food chain and so affect fish and bird populations. Didymo fouls fishing and boating gear and blocks industrial water intakes. Apart from falls on slimy rocks, it does not pose a serious human health risk – fish and eels caught in affected rivers and lakes are safe to eat and extracted water is safe to drink – but it can cause eye irritation to swimmers, and it ruins the aesthetic appeal of the affected river.

New Zealand is the latest victim in a worldwide invasion by didymo. Countries where it has now been found include the UK and Ireland, Canada and the United States, Scandinavia, France, Spain, Russia, Turkey, Ukraine, Poland, Romania, Hungary and Iceland. It only takes a single, invisible, algal cell to start an invasion, and biosecurity experts believe it may be travelling the world on damp fishing gear, like nets or the felt soles of wading boots. It is thought the original invasion, on the Lower Waiau River was the result of angling gear contaminated from use in North America.

An economic study puts the didymo damage bill to the NZ economy at between $NZ60-285 million by 2012 – depending on whether or not it invades the North Island. Besides its economic impact on the recreational fishing, tourism, energy and water industries it has unquantifiable effects on Maori cultural values including customary fishing, alluvial gold mining, exports of used fishing and boating gear, and human health from eye irritation in swimmers and injuries from slipping on rocks.

As a result Biosecurity NZ has mobilized a major public awareness campaign, “Check, Clean, Dry” to enlist anglers’ help in preventing its spread and set up cleaning stations on many popular rivers. These measures have been stiffened by the threat of five years' gaol and $NZ100,000 in fines for anyone crazy enough to spread the plague deliberately.

While didymo has so far been confined to the South island, and a recent survey found 67 key fishing rivers to be free of it, the economic study takes seriously the possibility it will eventually hurdle the Cook Strait to the North Island on somebody’s gear.

Australia’s AQIS considers there is a significant risk that Didymo could enter, establish and spread in Australia on used watercraft or sporting and fishing equipment.

“We now know these invasions can occur, sometimes with terrifying speed – as in the case of didymo in NZ,” says AQIS communication manager and ichthyologist Carson Creagh. “Every day something new comes along in quarantine globally, but it is only comparatively recently that people have come to fully appreciate how effective water is for spreading unwanted pathogens.”

This awareness began with marine ballast water and spread to the hulls of ships and yachts as they moved from port to port with their invasive cargoes of hitch-hiking wildlife. An invasion of the notorious striped mussels in Darwin a few years ago led to a whole boat harbour having to be poisoned in order to successfully exterminate them.

New Zealand river managers, at their wits’ end over the didymo invasion, are trying equally radical tactics using chelated copper to try to kill off the worst blooms – not, they hasten to add, in the hope they can exterminate it, but merely to see if it can be managed.

“Fishos are a bit like yachstmen,” says Carson Creagh. “They’re out there not just to catch a fish but because they love where they are, and their involvement in the natural world. Most are appalled at the thought of even accidentally doing anything to endanger what they love.”

But AQIS surveys suggest that awareness among anglers and other water sports enthusiasts of what could come into Australia in the way of waterborne weeds, pests and diseases on their gear is fairly low and patchy. “Until relatively recently there weren’t very tight controls over fishing and other water equipment moving in and out of the country. That has changed. Recreational water equipment all has to be inspected now,” he says.

Today, AQIS advises, if you’re planning to bring fishing rods and nets, waders, kayaks, paddles, life jackets or any other used recreational freshwater equipment into Australia, you should:

• make sure all equipment is thoroughly cleaned and dry before you bring it to Australia — not wet or damp, and
• present all equipment (clean and unclean) to a Quarantine officer for inspection when you arrive. AQIS officers will inspect them and may send them for treatment to make sure they’re safe.

It’s useful to check with AQIS as conditions can change. For example, some of the current treatments for used freshwater articles and equipment are:

For porous materials like felt-sole waders/boots, leather top boots and neoprene boots that have been used in an affected country, decontamination is mandatory. Currently, the approved treatments are:

1. the entire felt sole, porous uppers and other porous surfaces need to be completely immersed for 30 minutes in hot water (45°C or higher) containing 2% dishwashing detergent or nappy cleaner; or

2. if water alone is used, the entire felt sole, porous uppers and other porous surfaces need to be completely immersed and the temperature of the water should be maintained above 45°C for 40 minutes to make sure the interior of the felt reaches 45°C; or

3. store at -18°C for 24 hours : the time should start when the core temperature of the consignment reaches -18°C; or

4. gamma irradiation at 25 kGray.

Doing this effectively can mean the angler — who has to pay for treatment — won’t get his or her equipment back for up to a month.

Patrick Brennan recounts that his boots were taken away on re-entry to Australia and he was told they would be returned in 28 days. “We were wet-wading – but we would have lost our waders too, if we’d been using them. The experience cost us $30 to cover the decontamination and we had to sign a form waiving liability should anything be damaged in the cleaning or freezing process.

“My travel companion and I were both aware of the didymo problem and the risk it poses to unaffected waters and were happy to comply – but the inspecting officers said they had been getting plenty of agro from incoming and returning anglers.”

He adds that visiting anglers from overseas or even passengers with stopovers in Australia risk losing their tackle for a whole 28 days – for many, the duration of their stay in Australia. In today’s more vigilant biosecurity environment, his advice to any recreational angler planning to travel abroad with their gear is to carefully check the quarantine requirements at both ends before they leave.

Carson Creagh acknowledges that the process is inconvenient, but adds, “The threat is so very great that we have to do all we can to minimise it.

“The cost to Australia of an outbreak of a pathogen like didymo or some of the fungal fish diseases found in other countries would be horrendous. That’s why we’re so tough on border security.

“The more we can all do – including recreational anglers – to reduce the risk, the better Australia’s chances of remaining free of these invasions.”

Details of quarantine on fresh water equipment can be found at:

http://www2.aqis.gov.au/ICON32/ and type “fresh water” in the search box.

Julian Cribb is adjunct professor of science communication at the University of Technology, Sydney and edits R&D Review and ScienceAlert.