There have been a number of papers and books recently that discuss the whole issue of shifts in diets/predators. One example is the removal of wolves, which led to a lot more coyotes, and the problems they cause. When the absolute top carnivore (wolf, bear, Orca, Great White) is removed, what replaces them are more generalist in their feeding, which means that, ultimately, they can do more damage to prey because they switch among prey items.

Ultimately, the problem may not be too many seals, but too few seal predators. Or, as has been postulated for Orca and sea lion, the primary food source has been removed and they have shifted to less desireable ones.