There's a little sleepy seaside burg debating whether to add a new cannery.
Life is slow there, and the fishing is dying.
It turns out that the reason the salmon are disappearing is that the owners of the cannery company have been doing some DNA experiments on salmon. They accidentally released genetically altered salmon into the ocean, and those altered salmon were eaten by predator fish. Then there is a silly subplot, added after the fact by the filmmakers. The monsters are super-evolved and need to propagate their species. They don't seem to have any females in their race.