Dumb question.

I'm hoping that somebody more familiar with commercial fishing methods than myself could provide some insight on the effectiveness of "minimum mesh size" requirements in creating semi-selective gill net fisheries.

I've gathered from a bit of research that minimum mesh size is typically defined by the "stretch measure" which counts the number of vertical inches (or cm) between each knot. 6" minimum mesh size seems pretty common in these parts. I assume in this case that juvenilles get through, jacks get through, bulls and cutthroat get through, etc., while fish around 4-5" deep are retained. I feel like I've seen a lot of fish in this size range over the years with some abrasion around the dorsal/midsection, and a few bigger fish with more extensive trauma. After a conversation with a biologist this week, though, I'm wondering (based on his claim) if there is an upper size limit for effectiveness as well (a slot limit enforced by the net, if you will).

I gather that once a fish is in past the gillplate, it's retained unless it can fit through the hole or gets lucky. Is that accurate? If so, I figure the only other fish that get "past" a net must be fish deeper than 4-5" (but probably more like 6-7") at the gillplate (fish that don't fit into the holes in the first place and swim around in circles until they find their way upriver). I've never taken a girth measurement at the gillplate on any fish, but it seems this would be a very large fish (especially if it was a steelhead), such that the "range" this biologist spoke of is mostly imaginary, particularly in fisheries where very large fish are rare.

Can anybody comment on this?

Thanks,
-IS