Saprolegnia
Saprolegina
is a fungus which can affect salmon and sea trout in freshwater, including their eggs. At the present time it is perhaps the most serious pathogen of wild adult salmon in the River Tay and its tributaries. It is a major when looking
after the hatchery,
kelt
reconditioning system and the holding of
brood stock. It is widespead in the natural environment.
As a fungus it can attack damaged areas of live adult fish but it also lives on dead fish, dead eggs
or dead juveniles fry.
Adult salmon are particularly susceptible for several reasons. On entry to freshwater their skin is less thick and scales are looser then after they have been in freshwater for some time. Fresh run fish can be very susceptible. Also the fact that adult fish are not feeding and that body reserves are transferred for gonadal development mean their immune systems are weakened compared to say brown trout or salmon parr. Stress exacerbates the disease as does the number of affected fish around influencing the amount of spores available to infect fish.
In the wild saprolegnia is often association with spawning or spent salmon which are in poor condition and may be damaged. A small amount of infection may not prevent such fish from spawning successfully.
However, saprolegnia can sometimes infect adult salmon ealier in their migration.
Incidence of the disease tends to be highest when large numbers of salmon are present in a small area, particularly if they are prevented from moving upstream by some barrier and the fish are presumably stressed. Thus, congregations of fish in low water conditions downstream of a waterfall, a weir or even just too low water in the river are very susceptible.
In many years it is common for such outbreaks to occur in parts of the Tay district. Parts of the River Ericht for example are often affected especially in the late spring and summer. In July 2000, for example, a particularly large run of grilse became bottled up in very low water and warm conditions below Cargill's Leap. The Board's staff removed something of the order of 700 dead fish in a 500m stretch in that episode. While such episodes can occur in hot dry weather, saprolegnia outbreaks have also occured in the autumn and winter, so it is by no means only a warm weather phenomenon.
Saprolegnia is particularly a problem for the hatchery. Adult salmon taken from the wild and placed in captivity prior to being stripped of their eggs are particularly susceptible, presumably because of the increased level of stress they experience. We have found silver fish are much more susceptible than coloured fish. In the past such infections were treated with a substance called malachite
green, but this chemical is no longer permitted. Therefore, if adult fish have now to be taken for broodstock purposes it must be done close to spawning so that the time they are held in the hatchery is kept to a minimum.
Concerns are also sometimes expressed that the way that salmon are handled by anglers when being released may make them more susceptible to saprolegnia. We have seen instances where saprolegnia has taken hold in the tail "wrist" area both after confirmed handling and where handling has been presumed. Of course this is not to say that all handled salmon might develop infections. That will depend on other factors, but as a precaution anglers should not handle live salmon by gripping the wrist of the tail.
Ulcerative Dermal Necrosis (UDN)
Saprolegnia is often popularly confused with a condition known as Ulcerative Dermal Necrosis (UDN).
UDN was a major issue in the late 1960s particularly and is considered responsible for the deaths of at least hundreds of thouands of salmon in the British Isles at that time. The condition appears to be rarely reported now.
The symptoms of UDN are damage to / ulcers on the skin on the head of adult salmon and sea trout. The exact cause of this condition has never been fully established. No viral or bacterial infection has ever been proven. Some have even speculated that it might have resulted from damage caused by sea lice.
In the 1960s UDN was apparently most prevalent in the early part of the year, often when the water was cold. UDN did not in itself kill salmon and it was possible for the ulcers to heal up. However, UDN lesions were a perfect in-road for Saprolegnia, which did then result in the deaths of very large numbers of fish. Hence the confusion between the two conditions.
So, what of UDN?
The prevalence of UDN declined in the 1970s and is hardly talked about now. The decline in reports of (true) UDN seem to have followed the general decline in large concentrations of spring salmon early in the year. However, while little reported, head lesions were quite common on salmon caught in spring below Morphie Dyke on the River North Esk at least as recently as the early 1990s. At that time this weir acted as a temperature barrier and significant numbers of salmon accumlated downstream of the weir over a period of months. TDSFB staff witnessed healed lesions on the heads of grilse in the River Shee in the autumn of 2000 following the large fish kill which had occurred further downstream in July of that year (described above), indicating that head lesions had also been an issue in that incident. The photograph opposite of a sea trout with head lesions is of a fish caught in the River Almond by TDSFB staff during very low water in the early summer of 2010. These lesions appear to be typical of those described as UDN.
The common factor between these incidents were that migrating adult salmon and sea trout were held up in a confined area and presumably stressed. It does appear that once migration continues and the stress abates the lesions can heal, assuming Saprolegnia has not intervened.
For more information on the mysterious UDN click here.
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An adult salmon infected with Saprolegnia. This disease can be recognised by patches of wooly looking fungus which can vary in colour from white to a dull brown.
The end result of Saprolegnia infection. This fungus can also grow on dead fish and will release spores which can infect other healthy fish. Thus, under low water conditions where salmon are concentrated this disease can rapidly escalate and kill large numbers of fish.
Saprolegnia is often popularly confused with a condition known as Ulcerative Dermal Necrosis (UDN). However, UDN is a condition recognisable by skin lesions on the heads of adult salmon and sea trout (pictured above). Saprolegnia infections often follow these lesions. While it is often Saprolegnia which does kill fish with UDN, Saprolegnia is not UDN and does not necessarily require UDN to be present to infect and kill fish.
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