Available at: https://digitalcommons.calpoly.edu/theses/2172
Date of Award
MS in Biological Sciences
College of Science and Mathematics
College of Science and Mathematics
Marine shellfish play a vital role in intertidal ecosystems and coastal communities, but many of these fisheries are small-scale and lack the necessary monitoring to ensure long-term sustainability. Effective management often requires information on key demographic parameters, such as population status, reproduction and growth. Pismo clams (Tivela stultorum) are a culturally important and iconic species in California, which supported a thriving commercial and recreational fishery throughout much of the 1900’s. However, Pismo clam populations have declined statewide in recent decades and are attributed to human harvest and predation by California sea otters (Enhydra lutris); However, no studies have examined their populations, population drivers, or life history for at least 40 years. Managers require updated and expanded information on populations, habitat associations, reproduction and growth rates to effectively manage, regulate, and recover Pismo clam in California.
In Chapter 1, we investigated current Pismo clam population levels in California and examined the role of abiotic and biotic factors as correlates of clam abundance. We quantified Pismo clam presence, density and biomass at 38 sites in California during 2018 and 2019. Our results indicate that while human population density does not appear to drive clam populations, median sediment grain size is an important predictor for Pismo clams on open coast beaches. As median grain size increases, the probability of clam presence, density, and biomass decreases, suggesting that the composition of beach habitat is a critical factor regulating Pismo clam populations. Additionally, clam density and biomass are significantly higher on beaches north of Point Conception compared to beaches south. This suggests that Pismo clam population declines are more complicated than conventional wisdom suggests. Overall, Pismo clam densities are lower and size structures are shifted towards smaller sizes than historical accounts. This study is the most comprehensive set of population surveys to date and identifies key factors associated with Pismo clam abundance, which may be used to inform management and guide restoration and recovery of this once iconic species.
In Chapter 2, we examined life history characteristics of Pismo clams in California. Specifically, we investigated the annual reproductive cycle of Pismo clams in California, pairing multiple metrics within a single study to describe the sex ratio, gonad development stages, body condition index, and length at sexual maturity. Further, we examine age-length relationships across California to provide estimates of age structure and growth rate, which will better inform recovery timelines for the recreational fishery in California. Our results indicate that the sex ratio is 1:1, peak spawning occurred in late summer, and clams can spawn in their first year (<20 >mm). Cycles of body condition were influenced primarily by mean monthly sea surface temperature, but mean monthly chlorophyll-a concentration, photoperiod, clam size, and year were also important. Body condition was significantly correlated with the proportion of clams in the Ripe stage. Thus, body condition has the potential to be a rapid, inexpensive proxy for monitoring reproduction in Pismo clams, potentially providing useful information about changes in reproductive patterns. Finally, examination of age-length relationships for Pismo clams suggest that clams may require over 13 years to reach a legally harvestable size (114 mm across most of California). The estimated age at legal size is substantially older than historical estimates, which suggested that Pismo clams could reach legally harvestable size in as few as 6 years.
Collectively, this work represents a significant advance in our knowledge of the biology and ecology of this iconic and culturally important species. Furthermore, it provides vital information on the current population status, reproduction, and growth rates to inform management, regulation, and potential recovery of Pismo clams in California.