More from the book (website):
When returning pink salmon appear the the mouths of streams, they are usually seen swimming near the surface and often exhibit a characteristic leaping behaviour. Individuals leave the water, after a rapid swimming burst, in a forward leaping motion with their body initially oriented dorsoventrally, and then quickly rotated laterally, so the fish "falls" on its side or back.
Large numbers of pink salmon may suddenly migrate into a stream at once. Berg (1948) cited an observation of a mass migration made by I.F. Pravadin in 1926: "Although the weather was calm and sunny, an extraordinary noise could be heard coming from the middle of the river ... similar to the noise of boiling water splashing in a gigantic cauldron. .... the fishermen feasted their eyes upon a tremendous school of fish, wich went up the river, making a very loud noise, as if a new river had burst into the Bolshaya; the fish jumped out of the water continuously. The noisy stretch of fish was at least one verst long (1067m) and not less than 100m wide so that size of the school could be estimated at several millions specimens."