Status of intertidal populations of Pismo clams (Tivela stultorum) across their range in California

Abstract

Long-term monitoring programs are critical to understanding a species’ population dynamics through time and to inform policy decisions. Pismo clams, Tivela stultorum (Mawe, 1823), are an iconic species in California and once supported commercial, recreational, and subsistence harvest for many decades. Despite their economic and cultural importance, little is known about their current population status and monitoring efforts in California are spatially and temporally limited. This project aimed to: (1) collect standardized baseline data on Pismo clam abundance across their range in California; (2) identify factors associated with Pismo clam presence to inform future monitoring efforts and to begin developing testable hypotheses about population drivers; and (3) synthesize historical surveys of Pismo clam populations in California. We conducted intertidal field surveys for Pismo clams at 38 sites in California during 2018 and 2019. We observed no legal sized clams (4.5 in/114 mm) and only 2% of individuals were half of legal size across all survey locations. Clam populations are in lower abundance and comprise smaller sizes than historical accounts. We detected small individuals at most sites, suggesting that successful recruitment has occurred on the majority of beaches where clams were present; however, population recovery may require many years. Finally, we found that median sediment grain size is an important predictor for clam presence on open coast beaches in California. Future research will need to examine a number of factors and their interactions across spatial scales to disentangle Pismo clam population drivers.