This article [PDF] provides an overview of sport clamming in Humboldt Bay, and historical trends over time:

McVeigh, Brooke A. B.,  John J. Geibel, and Peter E. Kalvass. 2008. Sport Clamming in Humboldt Bay, California During 2008: Comparisons with Historical Survey Data. California Department of Fish and Game, Eureka, CA