There are numerous irrigation and drainage ditches that flow into the Pitt. Those will warm up quickly in spring and I've checked them out where they join the Pitt. While I didn't see anything it does look like good bass habitat.
Jeff Welch, posts 'Professional' reports under the name Newsman, on various boards and some local papers reports he caught a 7lb fish out of Hatzig and has photos as evidence. Also good sized fish (3lbs+) reported out of Deer Lake in Burnaby.
Still I'd think most bass in the Fraser would be too small to get caught in the typical gill net used in the Fraser. Just my guess. If some do get caught in the Albion and PSF test fisheries I doubt anyone makes an attempt to count them.
FOC/DFO also doesn't have to follow the Province in setting limits in the river below Mission. I don't think there ever was a limit in tidal waters nor were black bass ever listed as a sport fish.