Stopping Didymo
Essential biosecurity for Patagonia anglers
Didymo (Didymosphenia geminata) is one of the most persistent ecological threats to the rivers and lakes of southern Chile and Argentina since its discovery here in 2009/10.
The alga forms dense, fibrous mats that smother riverbeds, disrupt insect life, and alter the character of streams that many anglers consider some of the finest on earth. Once established, it is almost impossible to remove — which makes prevention the only meaningful defense.
For traveling anglers, the responsibility is simple: treat every river crossing as a biosecurity checkpoint.
Why didymo spreads so easily
Didymo cells cling to anything that stays damp — waders, felt soles, boots, nets, leaders, anchor ropes, boat bilges, even gravel stuck in tyre treads. A single drop of contaminated water can start a new outbreak. The southern Andes are vast, but the pathways between river systems are short. One wet pair of boots can connect entire watersheds.
Angler protocol: what you must do
Chile’s Sernapesca and Subpesca, along with Argentina’s Ministerio de Ambiente and regional authorities — as well as in New Zealand where didymo has been present since 2004 — all recommend the same core practices. Build these into your routine.
1. Check
Before leaving any river, inspect boots, waders, reel, net, socks, packs, boat and trailer. Remove any algae, mud, and plant material and dispose of it in the rubbish — never down drains or back into waterways.
2. Clean
Use one effective method every time you change catchments:
Hot water — Drench or soak gear in water 60°C or hotter for at least one minute.
Bleach solution — Use a 2% sodium hypochlorite solution; ~200 mL household bleach per 10 L water. (Household bleach broadly translates to ‘lavandina’ in Argentina, or ‘cloro’ in Chile.)
Non-porous items: 1 minute
Porous items (waders, lifejackets, cork rod handles): 30 minutes
Detergent — When hot water or bleach is unavailable, some agencies accept a 1–5% detergent solution for at least a minute.
3. Dry
If you cannot clean immediately, then drying is mandatory. Gear must be completely dry — no cool, damp spots — for at least 48 hours before entering another waterway.
Patagonia’s weather often demands 72 hours or more, leaving bleach as the most practical method when traveling between river systems.
4. Boats & Trailers
Drain any bilge water, empty buckets or containers holding river water, open hatches, and let everything dry fully. Check trailer frames and tyre treads for trapped vegetation or debris.
5. Dedicated Gear
If you fish a river known to have didymo, consider dedicating boots, waders, and net to that system alone if possible. Some guides now operate with ‘river-specific’ kits for this reason, so ask your lodge or outfitter ahead of your visit.
Felt soles on wading boots have been broadly banned in Argentine Patagonia and whilst not yet universally banned in Chile, they are strongly discouraged. Some outfitters are still foolishly advising that bringing clean felt soles into Chile is acceptable, despite high risk of cross-contamination of streams whilst moving between waterways. There’s a clear reason why both New Zealand and Argentina have banned felt soles, so play the long game and leave them behind.
6. Report Suspicious Mats
If you are a return visitor to a stream and encounter unfamiliar algal growth:
Chile: Report through Sernapesca channels.
Argentina: Report to provincial environmental or fisheries authorities.
Contact details are listed at the end of this article. Early reporting helps protect neighboring watersheds.
A small routine that protects a huge place
Patagonia’s rivers are rugged in character, yet complex, delicate fisheries. A minute to check, apply hot water, a bleach soak, and a drying cycle is insignificant compared to the damage didymo can inflict. New Zealander’s know the damage this alga can inflict on southern waterways. International visitors to Chile and Argentina must take the same care as is demanded in New Zealand’s South Island.
These fisheries rely on the choices anglers make at the tailgate, the put-in, and the take-out. Treat your gear with the same care you give the fish, and you help keep Chilean and Argentine waters healthy for decades to come.
Authorities to report didymo sightings
Argentine Agencies
Ministerio de Ambiente y Desarrollo Sostenible (MAyDS) – National Environment Ministry
Email: inspecciones@ambiente.gob.ar
Website: argentina.gob.ar/ambiente
Autoridad Interjurisdiccional de Cuencas (AIC) – Watershed Authority
Phone: +54 299 443 1943 (Neuquén office)
Website: aic.gob.ar
Chilean Agencies
Servicio Nacional de Pesca y Acuicultura (SERNAPESCA) – National Fisheries and Aquaculture Service
Phone (National): +56 32 281 9176
Email: oficinadepartes@sernapesca.cl
Website: sernapesca.cl
Superintendencia del Medio Ambiente (SMA) – Environmental Superintendency
Phone: +56 22 617 1860
Email: oficinadepartes@sma.gob.cl
Website: portal.sma.gob.cl