On October 9, the first-year cohort went on a field trip to Stanley Park to do intertidal sampling and learn about experimental design. We assisted Dan Esler from the USGS to gather data on the distribution of Pacific blue mussels (Mytilus trossulus) in the intertidal off the northwest side of Stanley Park as part of a larger project on trophic relationships in nearshore marine ecosystems.
Blue mussels are fed on by diving seabirds including the sea duck Barrow’s Goldeneye (a mussel specialist). A long-term study has quantified Barrow’s Goldeneye numbers in this stretch, but gathering data on mussels allows us to relate this to variation in their prey numbers. Since mussels are sessile and easy to sample their abundance, this data can then be related to their predator densities.
After hearing Dan’s introductory presentation, we headed over to the shoreline and divided into groups. It was a gorgeous, sunny day but we couldn’t stop for too long to take it in – we were fighting against time, as the tide would only be low enough for us to access the intertidal zone for about 2 hours.
We messed around with our GPS units until they led us to the start of our transects – 16 transects in total, which were evenly spaced but with a randomly chosen start point. We laid out a measuring tape at a right angle from the shoreline, and sampled from randomly spaced quadrats within the lower, middle, and upper intertidal zones. The quadrats varied in size depending on mussel density – we needed at least 20 mussels for measurements, but in some places they were so packed in that we could gather enough in 6.25 x 6.25 cm, while other locations needed to expand to 25 x 25 cm or larger. The rocks the mussels use for substrate were slippery and the mussels themselves liked to hide in nooks and crevices.
We did our best to be thorough, even as the waves began to splash at the far ends of our transects. Then we headed back along the seawall to turn in our data sheets and mussel samples.