it is a slaughter, they went to the dam on their own free will... the seals do not need that much food. the fish stack up there and the seals come in for an easy meal, killing thousands of fish, barely eating a whole one each time. its like going to a park and throwing a loaf of bread in the pond at the same place each day, after awhile all the carp, bluegill and ducks will be stacked up waiting for a free meal. they have plenty of food to eat elsewhere, but its a free, easy meal. look at trawlers, all the bycatch gets thrown behind the boat, you can find thousands of Cobia, jack, sharks, barracuda, snapper ect lined up for a free meal. these fish would not be there otherwise. sure, behind a trawler they clean up all the excess and its not as bad, but say we had bullsharks in california, and they found the bonneville dam, in no time you would have hundreds stacked up each year awaiting the migration. hopefully eating a few seals while there, but its just the same.
same with farmers. they have chickens and sheep and such. the animals are in mass numbers in a confined area, wolves, coyotes, eagles, and hawks all see this as a free easy meal. but farmers are allowed to protect their stocks, fisherman arent.