Wait on, back up a sec, you do realise the seals, despite the common name, are a native species which was nearly hunted to extinction in the early 1800s and has still not recovered to the previous population level? My understanding (reconfirmed by mr google) is that their diet is primarily mass schooling fish and squid species found off the continental shelf like arrow squid, slimies and tunas, probably because they are more plentiful and easier prey than more dispersed shallower water species.
Put simply the seals are not the problem and my money would be on the primary cause for inshore