Gyrodactylus salaris (GS) is a monogean ectoparasite that lives on the skin and fins of Atlantic salmon. GS was first reported from Sweden
and is believed to occur naturally in Russia
and the Baltic regions of Finland, Latvia and
Lithuania. The Baltic strain of salmon has a high
level of tolerance to infection. However, Atlantic
salmon are highly susceptible to the parasite.
GS was introduced to Norway in the 1970s
(thought to be via stocking of infected fish
imported from a Swedish hatchery) and has
caused the complete collapse of the Atlantic
salmon populations in over 40 major rivers.
GS causes high mortality in salmon parr and
fry in many cases up to 95% - but has little
pathogenic effect on adult salmon. It lives and
reproduces in freshwater, but cannot tolerate
GS is not currently found in the UK but if introduced, could rapidly spread across the country and dramatically reduce wild salmon stocks. The UK imposes controls on live fish movements from infected countries to prevent introduction. However, there are other potential routes for introduction, such as through contaminated angling equipment and illegal live fish imports. The UK fish health authorities remain vigilant in preventing the incursion of GS into our waters, and maintain close contacts with the Norwegian authorities to keep abreast of developments in GS control in Norway for contingency purposes.
In Norway, rotenone is used to eradicate GS from some rivers. This treatment kills all the fish in the river, which is then restocked with uninfected eggs and juveniles from the local gene bank. Rotenone treatment and restocking is one of a number of options that could be considered if GS were to be introduced into the UK. In 2004, the UK fish health authorities were invited to Norway to observe a rotenone treatment of a major river system. This invitation established links which have continued with Norwegian experts providing input to disease contingency plans, should GS be introduced into the UK.
In August 2011, members of both the Cefas Fish Health Inspectorate and DARDNI (Department of Agriculture and Rural Development for Northern Ireland) were invited by the Norwegian Veterinrinstittutet to observe a rotenone treatment of the lower River Vefsna, one of Norways largest and historically most productive salmon rivers. The river in Helegland is 163 km long, drains a watershed of over 4,000 km2 , enters the sea through the Vefsnfjord in the town of Mosjen and is, in places, far wider than most UK rivers. Salmon stocks on the Vefsna have been decimated by GS since it was first detected there in 1978: the annual rod and commercial catch on the river has reduced from 100 tonnes per year to less than 100 fish.
Norway produces a large proportion of its electricity from hydro-electric (HE) schemes, and most of its larger rivers have HE dams with built-in fish passes to assist upstream migration of the anadromous stocks. On other rivers such as the Ranaelva (treated in 2004), long term closure of migration barriers has successfully removed all salmon, sea trout and infection from the upper reaches, which effectively decreases the overall length of river that needs to be treated with rotenone. There was also widespread use of temporary long-term barriers. These are used in complex watercourses particularly where lakes are present, where there are long anadromous sections of river, or where rivers are difficult to treat. These barriers are substantial structures having to survive the Norwegian winter, and remain in place for a minimum of 10 years before treatment. They prevent upstream migration of fish and again their function is to restrict the distribution of salmon in the river and so reduce the distribution of the parasite. The four main migration barriers on the Vefsna had been closed for many years. Preparation for rotenone treatment of the Vefsna began in earnest in 2008, and included:
- exhaustive pre-treatment surveys of the main river and all adjoining waters
- development of an eradication plan, with times for treatment of different sections of the water course
- creation of a salmon gene bank
Prior to any rotenone treatment, fish are
caught from the rivers and kept in purpose
built tanks and holding units until they reach
sexual maturity and can be spawned artificially.
Sperm is frozen and taken to a central bank for
long term storage, whilst fresh sperm and ova
are taken to gene bank centres, where they
are fertilized, disinfected and hatched. The ova
from individual females are kept separate and
each family group is maintained in separate
tanks until they are large enough for adipose
fin-clipping (to indicate their gene bank origin).
These fish, or their progeny, will be restocked
into the Vefsna 3 years after the initial treatment.
Sea trout can also be preserved in the same way, but they are often netted out of the river as they migrate upstream (along with any adult salmon that are caught) and placed in sea cages for the duration of the treatment. This was witnessed in the Fusta, a neighbouring river of the Vefsna. GS cannot survive the full salinity and the clean fish are released to return to their native rivers once the rotenone treatment has been completed.
How rotenone works
When poisoned with rotenone, fish swim erratically and move to shallower water or come
to the surface gasping for air. Their ventilation
rate then slows and they sink to the bottom
where they remain until death.
Rotenone does not remove oxygen from the water, but it inhibits a process that occurs during respiration within cells, called oxidative phosphorylation. Rotenone acts on the electron transport system where it blocks a mitochondrial enzyme called NADH ubiquinone reductase. This means that the blood oxygen will be unavailable for respiration. Rotenone is not specific to fish it also kills aquatic invertebrates.
Fish are highly susceptible because rotenone quickly enters the blood stream via the gills. Efficient uptake is helped by the fact that rotenone is relatively insoluble in water and favours the high lipid content of the gills.
A Finnish company called Inter-Agro provide the rotenone to the Veterinrinstittutet under the brand name CTL-Legumin. This particular preparation uses fatty acids as a solvent. Other commercially available preparations, in particular those used for fisheries management in the USA, use oil based solvents which are thought to have a greater environmental impact.
Treatment of the River Vefsna
Rotenone treatment of the river Vefsna was
complicated by the fact that there are three
very large lakes along its course. Lakes Fustvatnet, Mjvatnet and mmervatnet have
a total volume of 381,008,177 m3
maximum depth of 68 m. Veterinrinstittutet
staff had successfully treated these in 2010
using the natural inversion of the lake water to
mix the rotenone successfully (using rhodamine die as an indicator of the mix).
To treat the lower river in 2011, a main rotenone dosing station was set up at Laksfrsen, just upstream of a large waterfall. Treatment was initially delayed as the area was experiencing the wettest summer in living memory: the river flow was measured at 400 m3 /sec, double the maximum flow of 200 m3 /sec required for the correct dilution of 8 ppm. Once river flows reduced, 3 m3 of rotenone were pumped into the river with passage through the waterfall ensuring thorough mixing.
As the treatment progressed below the waterfall, fish could be seen surfacing as the rotenone began to take effect and were collected. Staff from the Veterinrinstittutet and other government agencies were deployed along the river, and collector numbers were augmented by volunteers from local fishing and wildlife organisations. Many of the volunteers had great experience of the river, or had worked on other rivers.
All dead fish were collected, bagged and tagged with the species and river of origin before being transported back to the temporary laboratory for classification, registration and processing. They were sampled for the presence of GS and other parasites and notifiable disease. The carcasses were then frozen and taken away for incineration.
As well as the main dosing station, teams of people in boats followed the initial cloud of rotenone downstream. To ensure thorough treatment, they sprayed additional rotenone into the banks, vegetation and rocks around the edges of the water using boat-mounted spray equipment delivering high-powered jets. Juvenile salmonids are often found in pools of water under rocks that are left by receding waters. The boat teams also treated all of the small creeks and streams that entered the Vefsna, where potential infection could be hidden. Drip stations and watering cans were used to treat creeks where it was not possible to manoeuvre the boat and equipment into position. Rotenone was also poured into manholes and drains along any adjoining roads, as juvenile salmonids were found in these during pre-treatment surveys.
We were taken along the length of the lower Vefsna, as the rotenone made its way towards the mouth at Mosjen. The efficacy of the chemical was evident all along the river. Fish which collecting staff had not been able to remove, regularly turned up drifting towards the fjord. A large collection team was stationed in the Vefsnfjord before the river enters the sea. Here we witnessed many dead sea fish (including cod and mackerel) being collected, before denaturing and dilution made the rotenone ineffective. The treatment was completed by the following morning, although dead fish were still evident in the fjord.
The total quantity of rotenone used on the Vefsna at 200m3 /sec was 30 m3 over two days. A total of 118 staff were involved in the treatment: 65 people operated the treatment equipment, 50 collectors removed dead fish, and 3 leaders coordinated the operation. The Norwegian officers conducted the whole process in a highly professional manner with military-like precision. The cost of this round of eradication was estimated at 15 million Krone, equivalent to 1.7 million.
The river after treatment
Immediately after the treatment, the Vefsna will be entirely devoid of all life, both fish and invertebrate. Historic data for other rivers subjected to rotenone treatment suggest that key species do re-establish rapidly and successfully. Invertebrates should return to pre-treatment levels within 12 months. Some species will take a little longer to re-colonize, depending on drift downstream from untreated sections of the river. Soon after treatment, the anadromous stocks (including those stocks held in sea cages for the duration of the treatment) will begin to return to the river and make their way to the spawning grounds. Parr, fry and ova from the gene banks will be used to restock in the year following a second treatment of the Vefsna). Once this is completed, the migration barriers will be reopened and natural recruitment encouraged once more.
Little opposition to the process was apparent
with most of the local community understanding the reasons why it was being done. The
Veterinrinstittutet had undertaken excellent
educational work in the years leading up to the
There was one small group campaigning against the treatment. Norges Miljvernforbund (NMF), which roughly translates as Green Warriors of Norway, have a strong environmental presence in Norway, shadowed the treatment for most of the time, and made sure their views were heard during any television or newspaper coverage. The NMF argue that GS infection is simply a natural occurrence and therefore so was the demise of the salmon stocks. They also argue that salt could be used as an alternative to rotenone which would save the fauna of the catchment. There seemed to be a fairly good natured relationship between the Veterinrinstittutet and NMF until a large quantity of rotenone was stolen one evening.
Relevance to England and Wales
The Norwegian Authorities are clearly committed to the eradication of GS from infected river
systems. The work is being undertaken in a
professional manner by dedicated and experienced people who are fully committed to the
use of rotenone as the only practical method of
eradicating the parasite. The Norwegians have
over 20 years experience in the control of GS,
and have undoubtedly refined and optimised
their methods over this time. Most importantly,
there is obvious political will to support this approach, not least due to the unique cultural and
economic position that the wild Atlantic salmon
has in Norway.
Prevention of introduction of GS to the UK by strict movement controls is certainly preferable to having to deal with an infection. However, if GS were to establish in an English or Welsh river, management would probably be the only option for control, with rotenone use being improbable.
Rotenone is a drastic treatment and is only possible in rivers with favourable biological, geographical and hydrological conditions; not all rotenone treatments in Norway have been successful. The high usage of hydro-electric power schemes in Norway has meant that most of the affected rivers and catchments already have impassable fish barriers in place that can be shut for long periods of time (25 years on some rivers). In England and Wales there are no such schemes and for a rotenone treatment to be considered, a large number of temporary barriers would need to be constructed to prevent upstream migration of affected stocks and reduce the overall populations of both salmon and parasite. The cost of this would almost certainly be prohibitively high.
Rivers with salmon runs in England, and to some extent Wales, have a far greater biodiversity than in Norway which presents problems with rotenone use. It is anticipated that such a biologically devastating treatment would meet with a huge amount of resistance from UK environmental interest groups, even if it was conducted to protect an IUCN Red List endangered species. Many of the salmon rivers in the south of England also have large populations of commercially valuable coarse fish such as barbel; these are arguably of greater financial importance than salmon, due to the revenue derived from coarse angling.