SCIENCE IN ACTION
By Kerstin Wasson
For the past decade, the Elkhorn Slough Reserve and Foundation have been committed to restoration of the native Olympia oyster (Fig. 1) in Elkhorn Slough. We want to ensure that this iconic bivalve that was harvested by Native Americans at the Slough for seven thousand years continues to thrive in this estuary as a legacy for future generations.
What limits the populations of this species? Some areas of the estuary that used to harbor oysters now have such degraded water quality that they can no longer survive there – this applies to places like Moro Cojo, Bennett Slough and North Marsh. Near the mouth of the estuary in Moss Landing, the water quality is fine, but the strong ocean flushing leads to overly cold waters and makes it hard for larvae to avoid being swept out to sea.
From the middle of Elkhorn Slough, around Moonglow Dairy, to the top of the estuary at Hudson Landing, adult oysters seem to survive quite well most of the time, so long as they can find hard substrate to settle on that keeps them from being buried in the mud. However, these adults seem to have difficulty producing baby oysters. It’s not clear where the problem lies – whether adults don’t reproduce in the first place, or whether the larvae they produce don’t persist long enough to settle. In the decade that we’ve been monitoring, we’ve only seen really good recruitment of a new juvenile generation in two years, 2007 and 2012 (Fig. 2).
The lifespan of wild oysters is not well known, but 5-10 years seems typical. Now that it has been 7 years since we had a recruitment event, we are getting concerned about the viability of the population. We monitor adult densities in a permanent transect at Kirby Park (Fig. 3), and it is quite clear that numbers are gradually dwindling (Fig. 4). Adult numbers decreased fairly gradually from 2013-2016, but we had more rapid decreases in 2016-2017. We guess that this is due to some of the oysters not surviving periods of decreased salinity in this rainy Spring. At Kirby this Month, Chela Zabin and I found that more than 1 out of 3 adult oysters had recently died (Fig. 1). (Oysters are cemented to the substrate by their bottom shell. After they die, the top shell remains attached for a while, then falls off. We assumed that dead oysters that still had the top shell attached, with no signs of predation, were killed during the Spring rains).
On the Reserve itself, the die-off due to the high rains seems to have been more severe. A cursory survey of the largest population on the Reserve, found on the south side of the walkway between South Marsh and Whistlestop Lagoon, revealed about 90% mortality! It’s not clear why this site should have fared worse than Kirby Park, but perhaps local freshwater seeps or local farm run-off provided lethal water quality conditions. Salinity data from our nearby NERR system-wide monitoring station shows that this past spring definitely had some fresher period than the previous years (Fig. 5).
This new mortality on top of years of poor recruitment has spurred us to action to try to save the Elkhorn oysters. Brent Hughes and I have received a generous grant from the Anthropocene Foundation. We will use the funds to try to rear a new generation of Elkhorn juveniles and bring them into the field to revitalize the population.
This venture will be hosted at the new Center for Aquaculture at Moss Landing Marine Laboratories, with support from faculty members Mike Graham, Scott Hamilton and Jim Harvey. A graduate student, Dan Gossard, and an aquaculture expert from the Monterey Abalone Company, Peter Hain, will mastermind the Olympia oyster aquaculture, with some guidance from Jill Bible of UC Davis, who successfully reared thousands of oysters for her thesis research.
Raising baby oysters is a lot of work. While some invertebrates, like abalone, can be tricked into spawning and settling with specific cues, so that they can be hatched and raised at the convenience of the aquaculturists, Olympia oysters are not so easy. The plan is to bring about 50 adults into the lab, feed them well and keep them warm, and hope that they spawn sometime soon. But the tanks have to be checked daily with a flashlight to see when larvae are released – could be in a week or in three months. Then, the water needs to immediately be filtered so the larvae can be grown separately from the adults (who might eat them, or poop on them).
The larvae are then grown in their own tank and provided with settlement substrates –in our case, pre-drilled clam shells. When they settle is a bit of a guessing game too – could be a few days to a few weeks. The shells have to be checked daily so that you don’t get too many juveniles settling on them – we only want 5-10 adults per shell so they can grow nice and big eventually without bumping into each other.
The juveniles will be kept in the aquaculture facility for a few months so they are a big enough size to plant out. They, like the adults and larvae, have to be fed daily, and have their water changed regularly. A lot of work for a long time, but luckily the grant funding and willing partners should make all this possible!
Finally, we will attach the clam shells to wooden stakes to put out into the field (Fig. 6). We can’t wait to bring a new generation of healthy juveniles to these sites that haven’t had any in five years, and hopefully give the Elkhorn Slough oyster population a big boost! We will monitor how the new juveniles do, and their effect on increasing local recruitment and population size in the area.