I believe that much of the by catch occurs in fisheries directed at other species of flatfish/flounder. As such, to net one catches the other. Longlining would be hook fishing and that would take halibut as bycatch. Might have to forego the flatfish harvest. At the same time, biologically, the halibut can "take" the bycatch if other fisheries are constrained.

Kinda like we can have ocean fisheries for salmon if we keep the bays and rivers closed.