As much as I promote a strong release ethic, coho really don't make for a very good C&R target until they have hardened off in the river.

In my experience, they tolerate the stress of a hooking encounter much worse than any other salmon species. Out in the open salt is bad, and it's even worse in transition water (intertidal zone as new fish are physiologically acclimating to fresh water). Handling mortality alone at that vulnerable stage in their migration is 40% without a hooking encounter.

Once the fish harden off (scales set, slime coat fortified), they survive handling MUCH better.

From the fish's perspective, if hundreds of fish are to handled in a days fishing, further upriver is probably a better place to do it.