Before answering the question, one needs to frome the question clearly.

Gillnets are very selective. The target a size of fish. This was clearly shown in the Shagit in the 1980s. Because of concerns for coho, the in-river fishewry for chum was consuted with 6 or 6.5 inch minimum sytretcg mesh. Lots of chum and very few coho, were caught. On December 1, when management changed to steelhead and minimum mesh dropped to something like 5", coho showed up in the catch.

They select for size, not for species. As such, fishing for sockeye comes with a maximum, in order to reduce catch of larger Chinook. The selectivity is also controlled by how the net is hung.

As fished now, gillnets in a river are way more selctive for the stocks that can sustain harvest than any hook and line (sport or commercial troll) operating in the ocean. They are probably more selctive than purse seines, espceially whn you dump 500-1000 fish on the deck and try and sort through for the non-target species.