Today’s catch lays in front of me. As I marvel at how used to the stink of dead fish I’ve become, the fisher who brought them in points out a couple of larger Arctic charr as the researchers standing by him gush over the presence of a couple of tiny sticklebacks. He is fascinated by their appreciation of the smaller fish, which I had almost mistaken for baby trout. He offers to bring in more tomorrow if he can find them. It might not seem like the most significant of interactions, but it’s one more example of the willing assistance that the researchers here have enjoyed over the last week from local fishers.

I’m sitting fifty metres from the northernmost of a series of three lakes in Fustavassdraget, northern Norway. Seventy years ago there was a flourishing salmon river fishing community in the area, and the lakes were populated by healthy Arctic charr and brown trout, in the heart of their native geographic range. But in the early 70s the locals started to notice Atlantic salmon juveniles washing up dead in the river Fusta. A few years later the adult population collapsed.

The culprit was Gyrodactylus salaris, a tiny parasite native to Sweden which causes mortality in juvenile Atlantic salmon in coastal Norwegian rivers. Swedish salmon have evolved defences to the parasite, which is obligate to fish, and attaches to the host with two generations ready to be released inside it. But when an aquaculture station in southern Norway imported Swedish salmon in the 1970s, it didn’t take long for the parasite to spread throughout the unprepared Norwegian populations. It helped by the then fairly commonplace practice of fishers supplementing local salmon populations by releasing bought salmon into their waterways, and the station was happy to provide fish for this purpose.

Soon after the collapse of the adult salmon population occurred, scientists identified Gyro as the cause. Salmon traps at a downstream waterfall were closed to prevent further spread of the parasite throughout Fustavassdraget and heavy penalties were imposed on anyone releasing foreign salmon, but the parasite had already established into three largest nearby lakes (Ømmervatnet, Mjåvatnet and Fustvatnet). Worse still, in the mid-80s they discovered the parasite residing on Arctic charr. Was there any point getting rid of the parasite in the salmon population if it could simply jump back to the salmon through the charr?

University of Tromsø researcher Dr. Rune Knudsen and his PhD student Anja Winger were brought in in 2005 to test whether or not the charr could maintain a Gyrodactylus population.

“We found a controlled stretch of river and introduced around one hundred Arctic charr which were infected with the parasite.” recounts Rune. “We wanted to see whether it could survive the long cold winter period, effectively to see whether the charr could sustain a population of the parasite without salmon present.”

The next spring, Rune and Anja found two infected Arctic charr. It may seem like a small number, but it meant that even if the parasite was eradicated from the salmon population, it could sustain on charr and be reintroduced to salmon immediately. Worse still, whilst the charr populations stayed in their own lakes, the parasite could also most likely be spread through the system by the local sea-trout, which move freely between the lakes.

In the early 2000s the idea was floated to treat all three lakes with the piscicide rotenone. Rotenone effectively wipes out fish populations, so that the lake can be restocked – something of an ecosystem reboot. In Norway and Sweden it’s often used to get rid of invasive species that have been brought in from other areas. Normally however, the lakes are much smaller. An undertaking like this was new for Norway, and a competing region in the south (also infected with Gyro) wanted the treatment as well. Eventually the Fusta region won out, and treatment began in 2012.

This is where the team I’m currently spending time with comes in. Headed by Rune and parasite ecologist Dr. Rachel Paterson of Cardiff University, the team also consists of Estonian Bachelor’s student Jelena Nefjodova, New Zealander Karolyn O’Connor and Associate Professor at Linköping University Dr. Hanne Løvlie, plus many local fishermen. They’re trying to ascertain whether the fish and local parasite populations have recovered after the treatment, working out of a local community hall, now filled with dissecting instruments, microscopes, and a very distinctive stench of fish organs. Whilst rotenone is generally effective in eradicating Gyro by removing all fish it infects, rotenone may also impact other aquatic organisms, including beneficial local parasites.

“The rotenone was put in before the winter, to ensure it stayed under the ice and wiped out Gyro-infected charr all the way to the bottom of the lake. But there’s still a chance that a few native parasites survived the treatment.” says Rachel.

The local fishers are obviously concerned with the project, as survival of the parasite would mean another round of treatment and a few more years of empty lakes, or worse, a permanently crippled local salmon population. But even where scientific projects directly benefit locals, communication between the two parties isn’t always present.

The difference here is Eilert Hatten, head of the local fishers’ association. Rune and Eilert have been in contact for some years now, with Rune often stopping at Eilert’s house on his journeys between Tromsø and Trondheim. It’s Eilert that coordinates the local fishers in bringing in the fish that Rachel and Rune dissect and inspect every day. Hanne and I spent an hour out on Ømmervatnet bringing in nets with Eilert and Rune’s brother Bjørn, followed by picking fish out of the nets, a pretty tiring and time-consuming task for any researcher. Having local fishers dump some tens of fish on their doorstep every morning means Rachel and Jelena spend more time examining fish and less time extracting hopelessly tangled fish from nets.

Rachel Paterson and Hanne Løvlie are two members of the research team who spent last week dissecting the fish brought in by locals in the nearby community centre (Image Credit: Sam Perrin, CC BY 2.0)

Facilitators like Eilert are hard to find though. Last year I wrote about the lack of communication between fishers and fish scientists in Australia, and similar problems exist all over the world, with Norway being no exception.

“Sometimes fishing clubs just won’t talk to you. They’re worried because they think you’ll reduce their catch quotas, or that you’re trying to hunt down fishers who have illegally introduced other fish.” says fish ecologist and avid fisher Bastian Poppe.

But the researchers aren’t just here to allay concerns about the rebounding of the fish population. Rachel wants to make sure that the local parasites that existed before Gyro arrived have established their previous populations. Starting an ecosystem from scratch after rotenone treatments means that the invertebrate communities might rebound at different rates, leading to a very different parasite community composition than before. So the parasites tells a story of changes in the whole lake ecosystem.

“It’s easy to think of parasites as a negative, but they tie an entire ecosystem together” insists Rachel. “Often fishers look forward to a golden age, free of parasites. But it’s one of the worst things that could happen. You’d upset the balance of the entire community.”

Many of the native parasites inhabit two or more different host species (inverts, fish, birds) over their lifetimes, with the trout and Arctic charr fulfilling many of these roles. Trematodes like the eye flukes Jelena is currently staring at move from water snails to the charr and trout, where they impair vision. It may sound nasty, but it’s the impaired vision that allow local bird populations to prey on the fish. Without them, bird populations would plummet, as fish become harder to catch and fewer chicks are raised. It’s just one example of the links between species that local parasites facilitate.

Thankfully, it looks like many of the parasites that existed before the rotenone treatment are now back, with the exception of Gyrodactylus. It’s a relief for both the fishers and for Rachel’s team. Either a disturbance of the community or the persistence of Gyro would be a dent for Scandinavian freshwater managers looking to restore their local lakes through rotenone treatment.

For the moment, it looks like Gyro has been successfully exterminated from Fustavassdraget. But if Rachel and Rune’s team return again to continue their monitoring of the catchment’s recovery, the local fishers will be waiting for them, with the day’s catch ready to go.

Cover image credit: Rachel Paterson, Cardiff University, CC BY 2.0