Ickstream Steel,

Gillnets are the most selective common commercial fishing gear this side of fish traps. As Carcassman mentions, gillnets are selective for fish size, and as FNP points out, 6" of stretched mesh = 12" of girth that is being fished for. Gillnets select for the narrowest size range of fish among species with a fusiform (torpedo) like shape, such as salmon and steelhead. A gillnet of a given mesh size will catch a wider size range of fish like the rockfish species, although gillnetting for those species is generally impractical.

Salmonids that are significantly smaller - juveniles, jacks, or under-sized adults - and significantly larger are typically excluded from a given gillnet. Monofiliment gillnets are more size restrictive than multi-strand (Seven-strand, Miraclestrand) nets that stretch more than mono. Hanging a gillnet also affects its selectivity. Hung "even" in the conventional diamond-shaped openings is more size selective than a net that is hung slack. Slack hung multi-strand nets are the choice for making the so-called tangle nets in the lower Columbia sorta' selective spring chinook gillnet fishery. A slack hung multi-strand net in a small size that cannot gill a chinook will tangle chinook of various sizes in the teeth and external mouth parts. A fair amount drop out of the net, but most are tangled well enough to be landed. The flaws with tangle nets in that fishery is that the nets tangle chinook, allowing them to be sampled for adipose fin presence is that the fish are extremely sensitive to net handling in tide water, so a significant number of unmarked fish die anyway, and the tangle nets are perfect for gilling and killing steelhead present in the river at the same time.

So yes, gillnets are selective for size, but they are not selective between marked and unmarked, hatchery and wild fish.

Sg