"But some derbies have found creative ways around that restriction. This summer’s Vancouver Chinook Classic, for instance, will be strictly catch and release. When an angler catches a fish, an official chase boat will pull alongside and estimate the fish’s weight based on its length and girth to determine the derby winner."
Frankly the idea of keeping a fish in a net alongside the boat, thrashing around, while a derby official comes alongside to measure it (in maybe 5 minutes in the best case scenario) is pretty stupid. And there's a chance the tired fish gets swallowed by a seal when it's released.
If we're all that concerned with conservation, why not just donate directly to the PSF?