Observed people throwing rocks at seals during a local chum run, because the seals had gotten pretty efficient at making off with the fish on the end of your line...fish would beach themselves rather than get picked off by the seals (of course, they were then picked off...er...up...by less ethical Slavic drones)...when the few winter-runs showed and I hooked a fish, I noted the seal head bobbing in the water and, while running after the fish, pretended to pick up a rock and pretended to throw it...the seal backed off. I wouldn't recommend this method, however, as it made me feel quite goofy (a sentiment confirmed by the loss of the fish right at my feet). Worth to try in an emergency, though...