We measured larval abundance, chlorophyll-a, temperature, and salinity from four depths at one location in Fidalgo Bay, WA, by boat each day from July 11 to July 14, 2017. Each day, we completed eleven sampling events. During each sampling event, we collected samples from four depths in the water column: surface (0.5 m below surface), bottom (0.5 m above seafloor), and two mid-depth samples, which evenly split the depth between surface and bottom samples. We planned each sampling event to begin at specific times relative to the predicted low tide with the goal of collecting approximately equal numbers of samples during ebb and flood tide. To collect each larval sample, we used a modified bilge pump to filter 100 liters of water from our targeted depths through a 102-µm mesh plankton net to ensure retention of Olympia oyster larvae. Each sample was stored on ice while in the field and then preserved in 70% ethanol.

We used an Olympus Optical Company SZ-ST stereoscope fit with polarized lens filters to hand sort Olympia oyster larvae from each sample. First, we narrowed down all the potential local species of bivalve larvae that might be in our samples based on reproductive season (Loosanoff et al. 1966, personal communication Julie Barber). We then distinguished Olympia oyster larvae from these other species by comparing morphological features relative to size based on identification keys (Loosanoff et al. 1966, Shanks 2001) and reference Olympia oyster larvae that we reared in the laboratory. Reference larvae were fixed and photographed under an Olympus CH-2 microscope to aid identification.We measured larva shell lengths perpendicular to the hinge of each larva digitally using a stereomicroscope equipped with a camera and ImageJ software (Leica MC170 HD and Leica Application Suite, Leica, Wetzlar, Germany).