Size and Abundance of Pismo Clams in the Intertidal and Subtidal

Introduction The Pismo clam, Tivela stultorum, was once the foundation of a thriving commercial and recreational fishery in California (Bureau of Marine Fisheries, 1949). While there has been concern and interest in monitoring and management of the Pismo clam population, little is known about the biology and distribution of the clams themselves. For example, the characterization of the abundance and distribution of clams across a depth gradient is unknown. Furthermore, the anthropogenic impact on the distribution of Pismo clams at a particular site through recreational fishing has also never been described. The distribution of Pismo clams has been historically limited to sandy beaches from Monterey, California down to Baja, Mexico (Shaw & Hassler, 1989). These clams exist primarily in the intertidal and shallow subtidal but have been observed in depths up to 80 feet (Weymouth, 1923). Pismo clams exhibit continuous growth, with age estimated by growth rings on the shell (Coe & Fitch, 1950). These clams can grow approximately 20mm in length per year for their first four years, with growth slowing after this period (Shaw & Hassler, 1989). Pismo clams can grow to about 114mm (the minimum legal size limit for recreational fishing south of Monterey County) in 5 to 8 years (Shaw & Hassler, 1989). Pismo clams possess sturdy, thick valves and a strong foot, which are favorable adaptations to this high-energy environment (Coe, 1947). Settling approximately one foot or less below the sand's surface, the clams extend their short incurrent and excurrent siphons just above the sand, filtering out small food particles (Coe, 1947). Clams tend to orient themselves with their incurrent siphon facing towards the shore to filter organic matter from the surf as it rushes back out to sea (Weymouth, 1923). The high water turnover in the surf zone may provide favorable conditions for optimal nutrient circulation and availability (Knox, 2000). Because Pismo clams inhabit the easily-accessible intertidal region (intertidal) and shallow subtidal B 82.731 biology 117 region (subtidal), they are susceptible to human harvesting, which has altered the overall abundance of the species. Pismo clams have been harvested commercially and recreationally in California since the early 1900s (Bureau of Marine Fisheries, 1949). The commercial fisher's most common method of clam collection was running a plow parallel to the low tide line, exposing the clams to the fishers (Weymouth, 1923). The Pismo clam commercial fishery was ranked third in economic importance of all mollusks due …