Dealing with Didymo

David Freudenberger reviews the impact on the South Island fishery, 15 years on

Didymo is an intriguing yet sobering story of globalisation. The leading character is a humble microscopic diatom (type of algae) that has a remarkable propensity to thrive in cold and nutrient poor water. Through photosynthesis, powered by sunlight streaming through clear water, it can lay down centimetre thick mats of goo (aka river snot). The mats can smother a stony riverbed during a summer bloom. But add a bit of phosphorus from a volcanic catchment or farming land and it essentially disappears. When starved of phosphorus it switches from harmless reproduction (splitting of microscopic cells) to production of smothering long filaments. Fortunately, a scouring flood, laden with sediment, can clear it out. So optimal conditions are cold and sediment-free rivers that flow out of lakes. Just the sort of places us fisher folk are attracted to from around the world. Thanks to cheap overnight flights to just about anywhere, this little alga travelled from some North American river (based on DNA profiling) and first found its niche in the renowned Waiau River of Southland. It made the journey by 2004, likely nestled into the damp fibres of a felt-soled wading boot (now banned). This tenacious slime can survive weeks in darkness as long as it’s damp and cold, just like the baggage hold of a 747. Since then it’s been recorded in over 200 rivers in New Zealand, but only on the South Island. But just like a red-letter day of trout fishing, its abundance is patchy across space and through time. Rivers that regularly flood and are above natural or hydro lakes seldom have smothering blooms. It’s also less prevalent in low elevation rivers in the northern regions of the South Island. Analysis of its global distribution (it has also made it to parts of Chile and Argentina) suggests that average temperatures above 5°C during winter may limit its distribution, or at least its capacity to bloom. When it does bloom it’s a problem to both fish and fisher folk. Blooms fundamentally change the food web. Life in a river, as on land, is dependent on photosynthesis. Most of the large nymphs like mayflies graze on thin algal slime that covers the rocks of any healthy river or stream. As an angler I’m always excited about the potential of a stream or river with a stony, rather than fine sandy or gravel bottom. There is usually a lot more perennial nymph food on stones. A didymo bloom changes that. The goo is too thick for grazing nymphs of mayflies and caddis. But it is habitat. Tiny midge larvae thrive in the stuff. The total mass of drifting insects in a heavily affected river is just as much as a nearby unaffected river, but this mass has shifted to tiny midges to the exclusion of mayflies and other large nymph types that trout thrive on. Large trout certainly feed on midge forms in still waters, but appear unable to thrive on them in fast flowing rivers. The impact on large trout populations may be substantial. Decades of drift dives by NZ Fish and Game have shown, on average, a 50% reduction in large trout per kilometre of river. But percentages don’t tell the full story. Prior to the discovery of didymo on the Waiau River below Lake Manapouri, it had an astonishing density of around 250 catchable trout per km of river. It’s now down to around 100 trout. Certainly still worth fishing at that density. But such a percentage reduction in the nearby Mararoa River below the Mavora Lakes has had a much bigger impact on angling. Pre-didymo trout numbers were just 16/km, now it’s around 6. This makes for tough fishing as I found from a couple of days on various stretches of the river this past March. The impact on juvenile trout may be less. They do feed heavily on midges particularly in shallow and calm backwaters. Research on a small trout stream in South Dakota, USA found that there were more small trout (100–200 mm) in a stream moderately affected by didymo compared to a nearby didymo free stream. The didymo-affected trout had stomachs full of midge larvae and were in better condition. Similar research hasn’t been done in NZ, but I witnessed dense evening rises of fingerlings in Southland river reaches that were regularly affected by thick blooms. Perhaps these didymo reaches are now the nurseries suppling trout that move on to less affected areas as they grow up and need to shift to large prey like mayflies. Didymo blooms certainly diminish angling pleasure. I enjoy a break from up-stream nymphing to swing a team of wet flies across stony riffles. No such fun when a river reach has a didymo bloom. The faster swing of wet flies picks up snot on every cast. Better to move on. That’s one of the delights of NZ fishing. Moving on is often just a 20-minute drive to the next river. I escaped the snot of the Mararoa by driving over the hill to the Oreti. Its braided waters remain clear and its boulder bottom covered with that all-important faintly green film. Its wily 2 kg browns are still in abundance. Densities (~25/km) have even increased since appearance of didymo across Southland. Please move on with clean gear. This means wiping down and drying waders and giving boots a scrub, then 5 minute soak in a 5% solution of detergent. Any kind of detergent will break up the cell contents of didymo or any other algae. Adding a bit of bleach is useful and will kill pathogens that you might have picked up walking across a farm paddock. I advise against fishing the North Island immediately after a South Island visit. Do it the other way around, or have a few days on a beach to make sure all your gear, landing net included, are bone dry. The North Island may be naturally resistant to the establishment of didymo. The North is generally warmer and more fertile. But let’s not test this hypothesis. Don’t take wet gear from the South. Likewise, don’t immediately go fishing in Australia with gear that’s been recently used on the South Island. You should have cleaned it well enough to pass quarantine inspection coming into Australia, but give it all plenty of time to dry. Fishing one day out of Queenstown, NZ and the next day out of Hobart is certainly possible. But don’t do it. Boots in particular take days to dry properly, so take a break. Fifteen years of research and experience shows that didymo has seriously impaired trout fishing, but fortunately only in particular places and times. Trout populations have never been ubiquitous. They vary from season to season depending on flooding, spawning conditions and food supply (e.g. a mouse year). Trout density also varies from river to river and reach to reach depending on water temperatures and structures such as willows, stony riffles and pools. The presence of didymo adds another layer of complexity. I can predict where didymo blooms will make fishing undesirable — during summer in rivers flowing out of lakes in the southern half of the South Island for example — but there are plenty of exceptions. The Waiau coming out of Lake Te Anau has plenty of didymo, but drift dives show no drop in trout numbers, whereas below Lake Manapouri there has been a 50% decline. So to avoid disappointment, hire a guide, or ask around, or drive to find suitable conditions. Most importantly, fish clean, no matter where you are. In this world of globalisation, any of us could be the vector for the next pest introduction. The research papers used to prepare this article can be obtained from

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