Can I still fish catch-and-release for salmon in a river section that's closed to salmon fishing?

No. It is illegal to fish for salmon in areas closed to salmon fishing - including catch-and-release. You can fish for other species in waters that also have salmon. But any unintentionally hooked salmon must be released immediately. If you fish with salmon gear in a closed area, a conservation officer may think you are fishing for salmon and cite you for it.