This issue of survival rate in salt water and estuary areas is not limited to coho. Any of the salmonid species that appear in the estuary and river tidewater areas with their scales loose are vulnerable to high incidental mortality rates. I think Eyefish's gut feeling is too high based on my personal experience using both purse seines and gillnets to capture bright salmon for radio tagging. We never experienced 80% mortality, but never got radio tag signals from anywhere from 25 to 35% of fish captured in salt water bay or tidewater river areas.

The only fish I've seen that could be reliably handled in salt water and get fresh water survival rates were chum salmon that were already showing signs of sexual maturity while in salt water - that is, they were coloring up and developing the thick slime layer that is common once fish are well above tidewater.

If one wants to release salt water and tidewater fish with the highest probability of survival, the key is to not net the fish or handle it. Use one of those hooks on a stick, grab the leader, slide the hook down the leader and flip the hook out of the fish's mouth without ever touching the fish. Do that and mortality rates, whatever they are, will be the lowest possible outside of not fishing.

Sg