Why were the effects of M74 mortality not visible in the spawning run of Baltic salmon in 1997 in the Gulf of Bothnia?

The Baltic salmon fishery exploits salmon during their sea phase and spawning run by catching the fish in offshore areas with drift nets and long lines, and along the coasts with trap nets, respectively. In these fisheries, salmon of both wild and hatchery origin are mostly caught together. When the fish enter a river mouth or a river, the stocks can be harvested separately. On the basis of catch samples from the offshore and coastal fishery, the strength of wild salmon year classes were compared with the volume of the spawning run in rivers, and with the M74 fry mortality in experimental incubation of salmon eggs. The aim of this communication is to explain the reasons for an unexpectedly strong spawning run in 1997 in spite of the negative impact of M74 syndrome causing high yolk-sac fry mortality.