Gouldsboro has been awarded $20,000 from the Maine Shellfish Restoration and Resilience Fund (MSRRF) to continue the work it began last year in its Shellfish Resilience Lab. This second round of support from MSRRF will allow the town to develop techniques, conduct experiments, and collect data to address the green crab predation that reduced survival in last summer’s cohort of juvenile clams.
Last summer, we placed 100,000 juvenile clams in nursery trays that we floated in the old lobster pound behind the lab. Each tray held about 10,000 clams that were a little more than a month old and had a shell length of about 2 mm. The plan was for the clams to grow to about a half-inch by fall. Drawing upon the Downeast Institute’s experience with this process, we expected 80% to 90% of the clams to survive through the summer.
As we reported last fall, things didn’t work out that way. There were at least a few small green crabs in most of the trays along with the crushed clam shells they had eaten. A few of the trays had been turned into a single crab’s kingdom where it devoured most of the clams and other crabs. We estimated that only 31,000 to 35,000 of the original 100,000 clams escaped the hungry crabs.
After talking and thinking with the folks at the Downeast Institute about how this could have happened, our best guess is that the relatively mild 2020-21 winter enabled many more egg-bearing females to survive with viable eggs, resulting in a “baby boom” of crabs hatched in 2021. Female green crabs release viable eggs as larvae in May and June. Starting out as very small plankton, the larvae spend most of their first summer going through a series of stages until they finally become small versions of adult crabs, except that they still have a tail that they use in swimming. Then, each larva goes through one more molt where it loses its tail and finally becomes a juvenile crab that has a carapace width (cw) of about 2 mm.
Nursery trays are covered by screen material — much like window screen material but stronger — that allows water and nutrients to cycle through the tray. Crab larvae are small enough to pass through the screen and get into the tray. With a much larger “crop” of 2021 crab larvae floating about, a greater number of first-year crabs got into each tray. In addition, the milder winter allowed a greater number of second-year crabs (hatched in 2020) to survive their first winter. Smaller individuals from this cohort might have been able to squeeze through the screening.
How did the larvae from 2021 and small crabs from 2020 manage to turn into such large crabs in one summer? The Gulf of Maine Research Institute reported that Gulf experienced marine heat wave conditions over most of 2021 and that summer sea surface temperatures were the second warmest on record. Crabs grow faster when its warmer, and a lot faster when its a lot warmer and they have a captive food supply they can eat without expending much energy.
We’re not sure that we’ve got this story completely right. Experments we will conduct this summer provide an opportunity to better understand the details.
What We’re Going to Do
This winter feels less mild than the last one, so it might be that larval green crabs will not be as numerous this summer. But hope is not a strategy, and we know that the Gulf of Maine will continue to get warmer. So, our request to the MSRRF focused on comparing two approaches to getting more of our juvenile clams through their first summer.
Improved, Tended Nursery Trays
The first approach focuses on improving clam survival in nursery trays. We will divide the trays into smaller compartments with a wooden barrier that crabs cannot climb over. Last year, any crab getting into a tray had access to all the clams in the tray. We know that we cannot keep crabs out of the trays, but expect that compartmentalizing the trays will limit the damage that any single crab can do. Our plans include experimenting with different compartment sizes.
The second change to our nursery tray operation will be that tending the trays will involve more than just periodically cleaning off seaweed and other algae. Starting in July, we will periodically open up the nursery trays to find and pick out green crabs. This will not only reduce the number of clams eaten by crabs, but will also give us a better understanding of when crabs are getting into the trays, how big they are, and, perhaps, how quickly they grow.
Growing Clams in Upwellers Inside the Lab
Moving the summer growth operation into the Shellfish Lab is the second strategy we will explore this summer. If you look closely at the photo of our seawater tank at the top of this page, you will notice short, white plastic pipes sticking out of the darker “manifold” pipe that runs down the center of the tank. The short pipes and the manifold are how seawater leaves the tank to return back to the ocean. This summer we will attach a plastic, five-gallon bucket to each of the short white pipes. We’ll replace the bottom of each bucket with mesh that will allow water to enter from the bottom, flow up through the bucket, and then exit to the manifold through the pipe attached toward the top of the bucket’s wall. We will put the newly hatched clams into buckets where they can feed on plankton that “wells up” around them as water moves up and out the bucket.
Below is a picture of an upweller used outdoors at the Downeast Institute that might help you understand all this description about pipes and buckets. The buckets are covered with black plastic to keep seagulls from using the buckets and clams as a cafeteria. We won’t need to cover the buckets in our upweller because it will be inside the lab.
Because the pump will be actively moving water past the clams in the upweller, we expect the clams to grow more quickly. Small crabs will come in with everything else in the water, but watching for crabs in the upweller should be easier than in the nursery trays. On the downside, we have to run the pump all summer. Keeping track of the costs of labor and electricity for each approach will be as important as measuring growth and survival.
We thank the Maine Shellfish Resilience and Restoration Project for providing Gouldsboro with the financial support that will make this work possible. We also thank the Downeast Institute and Schoodic Institute who are important partners in this work, the Maine Department of Marine Resources for technical support, and the residents of Gouldsboro for their support in showing up to help with the work.