My understanding is that no vertebrates can digest cellulose. Some are able to obtain nutrients from cellulose through the aid of microbes in their digestive systems; think cows. The microbes help the host animal to break down the cellulose.

All the woody like material I have found in the stomachs of trout/steelhead/salmon appear to have been unchanged by any digestive activity. Just one example it is not unusual to find a stick building caddis in a trout's stomach; while the case remained unchanged the larvae inside often appears to have been digested.

Not sure what is the reason behind the question but once ingest salmonids have shown the ability to "pass" some significant items (rocks, etc.).

Curt