Walker Creek flows into Tomales Bay

Walker Creek flows into Tomales Bay

We saw an incredible amount of rain in February and while we are happy to have it, what does all this freshwater mean for oysters? Oysters like brackish water with salinity levels around 25 ppt (for reference the ocean is around 35 ppt, and freshwater is 0 ppt). When it rains, a lot of that freshwater flows into the Bay, lowering salinity levels and stressing our Olys. After repeated serious rain events, the saltiness of the water is so diluted it causes an incredible stress to the oysters and could possibly lead to massive die offs. And in our urban paved environment, a lot of the freshwater comes from streets via storm drains that tie into our creeks and empty into the Bay without any treatment or filtering. carrying trash and pollutants.

California received over 18 trillion gallons of water in February. However, that water doesn’t do us any good if it hits impermeable pavement, collects a bunch of cigarette butts, pesticides, and heavy metals, runs into a storm drain and then shoots out into bay waters. The more rainwater we can capture in gardens, allow to infiltrate permeable surfaces, and divert from storm drains is not only good for recharging our groundwater resources, it keeps our oysters less stressed.

A recent UCLA study has found our rainy season may become even more concentrated into a smaller period of the year then it already is, with fewer spring and autumn storms. Periods of extremely low bay salinity may become regular events that our oysters must contend with every winter.

Interestingly, studies have found Olympia oysters are locally adapted within the bay. An oyster from Richmond might have a better tolerance of lower salinity levels than an Oyster from Fremont. Relief also comes in the form of strong tides, “In February, Tomales Bay Oyster beds were inundated by over 17” of rain. Fortunately, we had nice high tides that month that replenished the salt water in the oyster beds” says Martin Seiler of Tomales Bay Oyster Company. The more healthy and rigorous Oly populations we have spread through the Bay, with high genetic variation and locally adapted tolerances, the better chance we have to keep a viable population of Olympias in the bay. By establishing oyster beds now and with better rainwater management, we can help Olys weather the storm.