I just listened to the KCRW show Good Food (9/4) episode and they were talking about mussels! You can download the podcast from KCRW or iTunes. It's a great show.

Mussels are one of the safest, sustainable shellfish that you can buy. Essentially the farm raised variety are grown attached to ropes. Until recently I lived near a fish market in Redondo Beach where they would let me pick them out of the tank myself. Try to buy ones with their "beards" still attached. They're easy to remove. Personally, I have no appetite for New Zealand Green Lips.

I asked my friend who owns a restaurant in Sonoma and he said that in CA, they are best 6 months prior to spawning which would be Fall-Spring. I also found similar info here:
http://www.sallybernstein.com/food/columns/harlow/mussels.htm