(06/1/23 Note: Data in the clam survival table has been updated to correct an estimation error.)

As March 2023 began, people from all along Maine’s coast gathered at the Maine Fishermen’s Forum (MFF) for the first time since 2020 to discuss the state of Maine’s fisheries. Mike Pinkham and I jumped on the opportunity to tell people about what Gouldsboro has been doing in its Shellfish Resilience Lab over the past two years. This year’s plans for the Lab grow directly from what we learned last year, and the presentation shared our thinking with the 100 or so folks attending the Shellfish Forum to get ideas and feedback. The audience did not disappoint us. They asked on-target questions, offered new ideas, and some will visit Gouldsboro this summer to continue the conversations started at MFF. We share the presentation here with the same spirit and goals. We hope to bring you up-to-date, get your ideas, and encourage you to visit Bunkers Harbor to see what’s happening and share your thoughts.

I have organized this blog post as a quick walk-through of what we presented in the slides. A downloadable version of the entire presentation is attached at the bottom of the post.


Starting in 2016, Gouldsboro used seed clams to restore clam flats to productivity. Each spring, clam harvesters distributed 100,000 or more clams on mudflats and covered them with nets to protect them from seagulls, crabs, and other predators. A clam is ready for use as seed stock when it starts the second summer of its life and measures between 8 and 12 mm (about 1/3 to 1/2 inch). Once seed clams are in the mud; if all goes well, they reach harvestable size by their third summer.

One consideration when seeding clam flats is that one-year-old clams are expensive. Currently, a thousand one-year-old clams cost $30 or more, with prices rising. However, a town can purchase clams that are only a few weeks old for between $5 to $7 per thousand. The question motivating our work in Gouldsboro was:

Can we save money and end up with more seed clams if we build a community-scale system that lets us start with inexpensive, small clams, grow them through their first summer, and keep them alive through the winter for use as seed the following spring?

One goal was to answer that question for Gouldsboro. But another way to keep track of what we did, what it cost, and what worked and what didn’t so we could share that information with other coastal communities. Gouldsboro learns a lot from other communities such as Brunswick, Harpswell, Waldoboro, Bar Harbor, the “Seven Towns” communities, Milbridge, Lubec, and many others. If we could figure this out, we needed to share it.

Initial Plan

For years, the folks at the Downeast Institute (DEI) have had success growing clams outdoors during their first summer, keeping them in floating trays like the ones pictured below. We planned to do the same thing. The trays shown below were floating in the abandoned lobster pound behind the Lab from July through mid-November of 2021. Each tray started with 10,000 clams that we hoped would grow enough for use as seed. Once we took the clams out of the trays in the fall, we would keep them alive over their first winter in the tank inside the Lab until we put them in the mud in the spring. Because clams’ growth (and feeding) nearly stops in the winter, we figured we could make do with a modest-sized pump that just slowly circulated seawater, supplemented by an air pump and bubblers to be sure that there was plenty of oxygen in the water.

4 foot wide shallow wooden trays covered with plastic and tied together while floating in still water
Nursery trays floating behind the Lab in 2021

Simple, right? Put them out in trays where they grow all summer, keep them alive in bags in a tank over the winter (which only requires monitoring to make sure that the pump is running and bubblers are bubbling), and use them as clam seed in the spring. What could go wrong?

2021 Did Not Work Out as Planned

Let’s start with what DID work: We got the growth we had hoped for. As you can see in this graph, the clams grew to an average shell length of 12.6 mm, most larger than 8 mm.

A box plot showing the distribution of the shell lengths. The mean value is 12.6 mm and the median is just over 12 mm.
Shell lengths for a sample of 353 of the clams grown in nursery trays in 2021.

What DIDN’T work was keeping the clams alive: Green crabs got into all the trays and ate more than two-thirds of the clams. About half the clams survived in some of the trays, but in other trays was more like 10%. We estimated the overall survival rate to be about 30%. Below is a picture of one of the crabs in the trays, surrounded by broken clam shells.

A green crab in a tray of broken clam shells attacking an orange glove

Other than the carnage, what amazed us was how large many of the crabs were. Most of the trays were undamaged and still sealed tightly. The only way that crabs could get in was through the screening that allowed water and nutrient movement through the tray. The holes in the screen are rectangles measuring 0.9 x 1.7 mm, meaning that a crab needs to be smaller than about 2.2 mm to squeeze through a hole diagonally. As you can see from the photo, some of these crabs grew to a carapace width of an inch (25.4 mm) or more. That is a LOT of growth between June 30, when the trays were set out, and November 16, when we brought them in and opened them up. We learned that crabs grow pretty fast when surrounded by food that takes almost no effort to catch.

The 2022 Plan

We knew that we would be “out of business” if we couldn’t improve on the 2021 results; we would be unable to make the cost vs. benefit numbers work if we continued to lose two-thirds or more of our clams. We decided to attack the problem from two directions:

  • Improve the structure of the nursery trays to make it harder for crabs to get in and more challenging for the ones that did get in to move around.
  • Raise some of the clams in upwellers inside the Lab over the summer to compare the growth and survival rates for the upweller clams and the nursery tray clams.

What’s an upweller? It is a seawater tank holding plastic buckets that have had their bottoms cut out and replaced with fine mesh. Water in the tank moves up through the mesh and out through a pipe at the top of the bucket. Water and food moves up and past the clams sitting on the mesh.

Three five gallon pails holding water and clams, each attached to an outlet pipe. Some of the pipes are more in the water than others
Clams in buckets in our upweller

How It Went in 2022

We purchased 300,000 very young (several weeks old) clams for the 2022 season. We wanted to have enough to divide between the nursery trays and upweller trays and, if they grew fast enough, we also hoped to run some experiments to explore trade-offs between clam density (how many we put in a single upweller bucket) and growth rate. The question of how many we can grow at once is important because the cost of running the pump is relatively constant, regardless of how many clams we have in the upwellers. If we can put more clams in the upweller without greatly affecting the growth rate, the cost per thousand clams will go down.

Below, the photo on the left shows what 300,000 clams look like. The scale is just a small kitchen scale. Three hundred thousand clams that have just formed their shells do not weigh much or take up much space. The photo on the right is a magnified view of what those clams look like when spread out on a plastic screen. Each hole in the screen measures 1 mm on a side. Most of the clams had shells a bit longer than 1 mm but shorter than 1.5 mm. We took these photos on May 26, 2022.

A lump of tiny clams sitting on a kitchen scale and other picture of these clams on a 1 mm sieve
What 300,000 very young clams look like

Problem #1: Slow Growth

The first problem we encountered was that the clams grew very slowly, as seen in the photos below. The image on the left shows the clams on the same 1 mm screen a month after we took the picture above. You can see most had grown only a half millimeter or so. The photo on the right was taken a month and a half later. The clams had grown a bit more–some growing faster than others–but they were still much smaller than we had hoped they would be as summer was ending.

Two pictures of small clams an a 1 mm sieve, one taken at the end of June and the other in mid-August. The June picture shows clams ranging in size from 1.5 to 2.0 mm. In the August photo, sizes range between 2.0 and 4.5 mm.

We purchased a flow meter and found that our pump delivered only about 7 gallons per minute. The number of upweller buckets we used varied over the summer, but it was often about 10. Four gallons is a reasonable estimate of the amount of seawater in each bucket. Assuming the flow was evenly distributed across the buckets and all the water was exiting through the buckets, the rate of new water coming into each bucket was less than 3 quarts per minute. That’s not much. It translates into a complete water change every 5.7 minutes (about 10.5 changes per hour) and likely accounts for at least some of the slow growth.

Although the clams were smaller than we had hoped, they were large enough to put into trays outside by August. At the end of the first week of August, we put 50,000 of them into five nursery trays (10,000 clams per tray) in the harbor, where they stayed through the rest of August and into November. The rest of the clams remained in the upweller. We retrieved the trays on November 16 and measured the lengths of clams sampled from the trays and the upweller. The graphs below show that clams in the harbor grew more quickly than the ones we kept in the upweller, and the difference is statistically significant. However, even with the extra growth, most of the clams were smaller than we would like for use in seeding. (Larger clams have a better chance of burrowing and escaping the many small green crabs on the flats in late spring / early summer when we use them as seed.)

Two box plot graphs. One shows end of season shell length for clams that spent the entire summer in the upweller. The other shows the length of clams that grew in the harbor since early August. The ones in the harbor were larger by about 1.5 mm
Comparison of clam growth in nursery trays and the upweller tank. Clams started with lengths between 2.0 and 4.5 mm in early August when some were placed in trays, and all were retrieved and measured on November 16.

Problem #2: Continued Predation

So, growth was better out in the nursery trays. What about survival? That was, once again, a different story. As the following graph shows, there were LOTS of crabs in the improved trays. And many of them grew quickly, reaching carapace widths greater than 3/4 inch just from early August to mid-November.

Five graphs, each showing the count and size distribution of crabs in each of the 5 nursery trays. Overall, there were 190 crabs with median sizes between 9 and 13 mm.

Not surprisingly, the crabs ate many clams. We weighed and counted samples from each tray and each upweller bucket to produce the estimates of the number of surviving clams you see below.

A table comparing clam survival in the upweller and out in the harbor. Survival in the trays in the harbor was 49% and was 76% in the upweller
(06/19/23 Note: An earlier version of this article and table incorrectly used 250,000 as the “starting N” for the upweller, resulting in an underestimation of the survival rate.)

Nursery tray survival was better in 2022 than in 2021 but still too low for economic viability. We also note that we lost about a quarter of the clams in the upweller. We share our thinking about possible explanations for these results in the Analysis section below.

Problem 3: Labor Requirements

As I wrote in the “Goals” section at the start of this post, we hope to develop a system that we can keep using and that other towns can use. Doing that depends not only on getting good growth and survival but also on out-of-pocket costs and labor, which is sometimes an out-of-pocket cost but also — and often, in our case — a gift from volunteers.

Upwellers require maintenance. As the clams grow, they produce waste products that, together with silt and other material in the seawater, clog the screens in the bottoms of the buckets, which, in turn, interferes with feeding and growth. Noah Milsky, our 2022 intern, cleaned buckets as part of his daily routine. Once Noah left to return to school, we relied on volunteers to clean the buckets and scrub down the tank every week. Our volunteers were GREAT — faithful and careful. But by the time November rolled around, they were really looking forward to being DONE. The cost of hiring that much help would be prohibitive, and it is not something one can expect volunteers to do over the long term, year after year.

Analysis and Plans for 2023


Based on conversations with others growing shellfish indoors, it is clear that we need to circulate more water through the upwellers than we did last year. Our pump, which might have sufficed to keep clams alive in the winter, did not move enough water to support our upweller. It was also the wrong pump for the job for other reasons that we will describe in a future article.

This year we will install a new pump capable of moving more than 50 gallons per minute and will also increase the size of our piping from 1 inch to at least an inch and a half, and if we can do it, 2 inches. Finally, we will do away with as many plumbing elbows and other flow restrictions as possible. Stay tuned … we will document what we do.

Another way to increase growth would be to move clams into nursery trays earlier in the summer, which should be possible if we can get them growing more quickly in the upweller in May.

Survival / Predation

Moving clams out into nursery trays earlier in the summer is not attractive if it continues to be primarily a way to feed crabs. Last year, we thought that we might be able to reduce predation by dividing the nursery trays into four compartments. (The 2021 trays were just a large open space under the screening.) Our thinking grew from our observation in 2021 that when we had a tray with one large crab, there seemed to be fewer other crabs — presumably, the big crab ate the smaller crabs along with our clams. We thought that dividing the trays into separate spaces might at least protect a portion of the clams.

However, the data we collected this year about crab counts in the trays do not support this line of thinking. We had LOTS of crabs. The volunteers opening the trays noticed that small crabs could squeeze underneath the compartment dividers, which might be part of the problem and one we might be able to fix, at least to some degree. But the numbers from 2022 also strongly suggest that we have crabs making their way into the tray all across the screening rather than just in one or two compartments. If that is the case, dividing the tray into compartments will not help keep crabs out and may only serve to protect some smaller crabs from the larger ones.

So … what then? At this point, we have come up with four ideas to explore.

  1. It’s the Harbor. Last year, as in the past, DEI grew clams in nursery trays and achieved survival rates greater than 80%. Could it be that Bunkers Harbor has a much denser population of small crabs that can settle into trays?
  2. Finer Screening. Can we reduce the number of crabs getting into the trays by using screening with smaller holes or by using more than one layer of screening?
  3. Timing. Research at DEI (and elsewhere) indicates that crab settlement happens in waves rather than evenly across the summer. If we could put larger clams out sooner in the year, might they get big enough to be less vulnerable to the small crabs that can squeeze through the screening? Or, if we put them out later, might we be able to miss the worst of the crab settlement?
  4. Pick Out Crabs. We could open up the trays two or three times over the summer to pick out crabs. That might also help us get information about the timing of crab settlement. The downside is that it increases the need for labor.

We plan to explore each of these ideas this summer, so stay tuned. We will also once again try growing clams all summer and fall in the upweller, working on the assumption that a bigger pump and more flow will help address the growth problem.

Finally, what about the loss of about 26% of the clams who remained in the upweller? The first, most important answer is that we don’t know. Here are some possible explanations:

  • Mortality due to low flow. This seems likely. We know that we were not circulating nearly enough water and nutrients past the clams. Very small clams are fragile and die quickly when they don’t get what they need. Once they die, their tiny, empty shells flow up and out of the upweller. If this is the cause, fixing our flow problem might take care of it.
  • Loss of small, live clams. We know from observation that some of the smaller clams would sometimes float up toward the top of the buckets. Were they all alive and well? Maybe. It seems likely that we did have live clams that were sucked into the pipes leading back to the ocean. But 40%? That seems unlikely, but for those who did make the trip, we wish them well.
  • Mortality due to competition with other creatures in the upweller. Last summer, intern Noah Milsky collected data and talked to experts about the other filter feeders we found growing on the clams. He concluded that filter feeders could increase mortality by reducing the food available to the clams, a problem our inadequate flow rate made worse. He focused on larger, one-year-old clams that were in the tank along with the very small clams, but it is possible that competition from other feeders could affect clams of all sizes. Again, we don’t know for sure, and again, more flow and food might solve the problem.
  • Problems in estimating numbers. As Noah reported last summer, getting good estimates of large numbers of very small clams by counting and weighing samples, especially when much of the weight in the sample is water, is notoriously difficult. We know for a fact that Noah eventually gave up on doing estimates by weight and just ended up counting thousands of clams, one at a time. But why would estimates always err in the direction of underestimating the count? If others have advice on this one, please let us know!


We have come up with two potential ways to reduce the amount of labor required for this operation. One depends on our figuring out how to improve survival in nursery trays. Even if we have to open the trays and pick out crabs every month or six weeks, that would probably take less time (and perhaps be more interesting for volunteers) than the daily grind of cleaning upweller buckets.

The other potential solution came out of conversations with others at the Maine Fisherman’s Forum. Our takeaway from those conversations is that we could have moved our clams off the 1mm mesh in the upwellers to a larger mesh much sooner — perhaps after only a few weeks if we can speed up their growth. Moving clams to successively coarser screens would improve water flow and reduce the amount of silt and gunk caught on the screens, perhaps making it possible to clean the clams and buckets just once a week.

Closing Thoughts

When we began this project in 2020, it seemed like the largest challenges might be dealing with the occasional low tides and significant vertical distance between the water and the Lab. But things got more complicated, and then more complicated still. But we have also learned a lot and believe that this year will be when we begin to sort some of these issues out and see a way forward.

The final slide in the presentation expressed thanks to the many organizations and people that made this work possible. Gouldsboro depends on their contributions of financial support and time,

Below are the slides that we presented at the 2023 MFF.

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