Introduction
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
seawater.
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.
Planning
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
Gene banks
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
and a
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.
Public opinion
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
treatment.
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.
