As with all things biological, the answer is yes and no. Back in the 80s, Skagit coho were chronically under escaped. So much so that there was no bay/river fishing for them by netters. It got complicated by the (then) fairly robust chum runs as the incidental catch was pretty high. So, WDF and the Tribes put in something like a 6" or 6.5" minimum mesh requirement on gill nets during the chum fishery.

Coho by catch essentially disappeared. When salmon management ended (WDF conservation control) on 11/30 a 5" minimum was put in place for steelhead harvest. Suddenly, all sorts of coho showed up.

While gill nets are not perfect they can be used in a size-selective manner and significantly reduce by catch if the two species are of reasonably different sizes.

Still, it would need to be enforced and users would need 3 or 4 nets for a year rather tan just one.