Date

Summer 2026

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Environmental factors and physiological tolerances shape species abundance and distribution across their geographic ranges. Species inhabiting coastal systems are subject to a range of conditions driven by both geology and oceanography. For example, infaunal organisms are influenced by sediment characteristics and beach morphology, nearshore wave dynamics, and sea surface temperature (SST). An infaunal resident of the sandy beach intertidal, the Pismo clam (Tivela stultorum), was once a valuable commercial resource along the California coast, ranging from Santa Cruz County, CA to Magdalena Bay, BCS, Mexico. Pismo clams experienced considerable population declines due to overexploitation in the early 20th century. Populations are highest in the center of their range—Pismo Beach, CA—while substantially lower at the northern edge in Monterey Bay, CA. Despite their persistence, no long-term monitoring programs have been established at this northern edge; this study is the first to do so in this region. This project examined how the distribution, abundance, and size structure of Pismo clam populations relate to sedimentary and oceanographic habitat conditions within Monterey Bay. To achieve this, intertidal transects spanning across Monterey Bay were conducted over a five-month period to assess clam distribution, abundance, and size at beaches with different sediment grain size distributions, wave energy, and thermal regimes. Clams were generally found throughout the bay but with some beaches having greater abundances or sizes of clams. Out of all predictors, greater clam. occurrence was linked with beaches with less skewness in sediment grain sizes, flatter beach slope, and warmer SST. Clam. density was associated with decreased sediment skewness and increased kurtosis—i.e., sediments that were more uniform in size with relatively fewer large particles. Larger clams were generally found at beaches with flatter slope and longer period swells. In addition, clam biomass was also driven by more uniform sizes of sediments, smaller median grain sizes, and beach slope. Together these results suggest that Pismo clam populations across Monterey Bay were best supported at beaches where favorable sediment characteristics overlapped with benign oceanographic conditions, implying that both geologic and oceanographic factors contribute to habitat suitability at the northern edge of their range. Through identifying these physical factors that most strongly influence Pismo clam populations, this study contributes to a better understanding of the habitat conditions necessary at the margins of their geographic distributions, and for their successful recovery in Monterey Bay.

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