As we wrap up 2018, we have a heartwarming story of a very special invertebrate making a return to our Point Pinole Reef: the Olympia oyster (Ostrea lurida).
2017 was a record rainfall year for California, and most of us rejoiced in the welcome moisture. However, the consistent rainfall between January and March 2017 brought so much freshwater into the San Francisco Bay that salinity at Point Pinole dropped to under 10 parts per thousand (ppt) for over two months. Olympia oysters are best adapted for salinities in area of 25ppt, and while they can close up and wait during inclement conditions, they were not able to survive such a long time without feeding. By the end of March 2017, we had lost 97% of the oysters on the reef, and other oyster restoration projects in the bay noted similar die-offs.
For the last year and a half we have diligently searched for oysters on the reef, without finding more than a handful. That all changed this fall, when a California Naturalists class joined us at the reef to hunt for oysters as part of their citizen science curriculum. We were overjoyed to find plenty of new oyster spat adhered to the reef balls! This means that enough mature oysters were present in the area to seed the reef during their spawning season this last summer. After looking at many parts of the reef and crunching the numbers, we’re finding upwards of 300 small oysters per reef ball, which is about where the population stood before the Winter 2017 die-off.
This is not the first time that rainfall has caused oyster die-offs in the bay and elsewhere, but it is something that may be happening more frequently in the future. Climate change models predict that we’ll see more extremes in California weather, including both very dry and very wet years. Research indicates that a suitable salinity is one of the most important environmental factors that support an oyster population, right up there with having hard substrate to grow on. We will continue to monitor the growth of oysters on our reef, and hope that this winter brings a few breaks in the rain to allow the bay salinity to increase again. If you’d like to join us and monitor the reef (we’re going out next in May 2019), please contact Helen at helen@thewatershedproject.org. To read a recent article about the reef, click here.
