Parental Background Predisposes Baltic Salmon Fry to M74 Syndrome

M74 syndrome is a thiamine responsive disease of Baltic salmon (the Baltic group of Salmo salar L.) leading to the death of nearly all yolksac fry of certain females (see e.g. Koski et al. (1999)). Börjeson et al. (1995) reported that M74 syndrome in Baltic salmon in Swedish rivers was bound to the females, while milt from wiggling males – fish showing symptoms of a thiamine related-illness (Larsson & Haux 1996, Amcoff et al. 1999) – produced healthy offspring. In Finnish salmon farming, farmed broodfish produce the bulk of the eggs needed in restocking programmes. Both wild (fish which have returned from a feeding migration to the Baltic Sea proper) and farmed salmon eggs and milt were available for this experiment. The experiment was performed to determine whether M74 syndrome is associated only with wild salmon and not with farmed fish, and also whether mortality is associated with the eggs or the sperm or both. A cross-fertilization experiment was performed at Lautiosaari State Fish Hatchery, Keminmaa, Finland in 1993-94. The wild fish were caught from the Bothnian Bay at the mouth of the River Simo in summer 1993 and kept under similar conditions to the broodfish described in Koski et al. (1999). Farmed broodfish of the River Simo strain of the Baltic salmon were held at Taivalkoski Game and Fisheries Research (males) and River Simo State Fish Farm (females) and were fed on standard commercial dry salmon pellets. On 5 October 1993, male and female gametes from these 2 fish farms were brought to the hatchery of Lautiosaari for fertilization together with gametes from the wild salmon. The eggs of 10 farmed and 10 wild females were fertilized with the milt of 10 farmed and 10 wild males. The milt of each individual male was only used to fertilize eggs from one farmed and one wild female. After fertilization there were thus 40 batches of eggs. The water-hardened eggs were disinfected with an iodophor (100 ppm free iodine for 10-15 min) and the eggs from each crossing were incubated separately. The development of yolksac fry was followed as described in Koski et al. (1999). The eggs of three crossings died during incubation; all other groups of eggs originating from the same broodfish were then excluded. Consequently, final experiment comprised 32 groups of yolk-sac fry. The cumulative mortality of the yolk-sac fry was followed from hatching to the exhaustion of the yolk sac. Mortality was statistically analysed by pairwise comparison of outcome for a particular broodfish mating with a wild fish and a farmed counterpart. Acta vet. scand. 2002, 43, 127-130.