During 2018 the Gouldsboro shellfish committee and the CSI-Maine project set out “experimental units” — our “plant pot” experiments — in two coves that were once productive but had become so overrun by crabs that they were considered to be “dead mud.” In the spring of 2019, a team of students in the Pathways program at Sumner Memorial High School analyzed the data so that they could be presented to the shellfish committee and others on March 20.  In this post we describe the design of the experiments and the coves where they were set out. A subsequent post describes what the students found. 

Locations of the Sites

The Gouldsboro shellfish committee and its clam warden, Mike Pinkham, decided on the experimental sites in collaboration with Kyle Pepperman of the Downeast Institute (DEI). Both sites are close to other areas that are known to be productive, so it was reasonable to think that there might be potential to bring these sites back to productivity.  The map at the top of this post shows the two sites in blue.

Names of bays and coves sometimes have local names rather than “official” names. Bunker Cove is actually named on some maps.  The other site is not officially named; we called it “Lesko Lane” because that is the name of the small private road to the site. The other places named on this map in yellow are the high school (SMHS); John Small Cove, where the town and students have been working with the Maine Department of Marine Resources (DMR) on other research and where Gouldsboro is actively using protective netting; Jones Cove, a site that Gouldsboro is seeding, netting, and managing with help from DEI, and the old lobster pound that is available for other experimental work.

The Experimental Plots

On May 22 of 2018, Mike Pinkham, Kyle Pepperman, and Bill Zoellick set out the experiments at each of these sites.  The experiments were laid out according to the design pictured below.

Each circle represents a 6″ plastic flower pot.  Each pot is filled with mud from the site and 12 juvenile clams, each measuring approximately a half inch (12.5 mm) in shell length that are placed on top of each pot and covered with a thin layer of fine mud.

Half the pots are covered with protective netting (represented here by shading) and half are not covered with nets (but do have a net “collar” around the edge of the pot that keeps the small clams from moving or washing out of the pots).

Each small rectangle in this diagram represents a “block” of pots that is about 1 meter on each side.  There are 3 blocks at each tide level.  The horizontal distance between the blocks varies with the terrain but is generally in the range of 8-12 meters.  The vertical distance between each row of blocks depends greatly on the slope of the mudflat (the “tidal gradient”).  For a cove where the tide goes out a long way, the distance is greater than it is in a cove with a steeper slope.  The goal is to place the rows so that they provide information about clam growth and survival at locations that are underwater only when the tide is pretty far in (High), at locations that are underwater most of the time (Low), and in-between.

For the details of HOW to actually set these pots out, see the post titled “Setting up and Deploying the Plastic Plant Pot Experiment.“

The Thinking Behind This Design

This design was developed by Dr. Brian Beal at DEI and has been used there in a number of studies over the years. Because the clams are in the pots over a full growing season, this experiment enables us to collect information about growth rate.  Because we know that each pot started out with 12 clams, we can learn something about what happens to clams even when they don’t survive and grow. (Here is a link to an articlle I wrote last year that shows how we can measure clam growh tne learn something about their fate.)

Use of nets on half the pots provides us with a picture of how important use of nets is at a particular site. Placing the pots at different locations along the tidal gradient gives us information about how predator activity and growth might change from the shore to the low tide line. If we do find that growth or survival changes along the tidal gradient, that information can help a shellfish committee in making decisions about where to focus its seeding and netting effort.

Finally, one very important feature of this design is that it does not take long to put the pots out and to retrieve them.  Apart from transportation and set up time, it takes an hour or less to set up a complete experimental site.  (Here is a link to a story about doing that with middle school students.)  Retrieving the pots takes even less time.  Because this work can be done by a few adults or students and done pretty quickly, it is possible for towns to use this approach to gather information about a number of bays and coves that the town might want to return to productive use.

Retrieving the Experiments

Students and shellfish harvesters retrieved the experiment on October 23, 2018 after five months in the mud on the flats. The next day they washed the mud off the clams, using sieve boxes to collect any new, small recruits in addition to the clams that had been placed in the pots in the spring. All the clams were placed into plastic bags with labels identifying the site and pot in which they were found and were then placed in a freezer. Later that month, we retrieved the clams from the freezer so that students could count the number of survivors and recruits in each pot and collect data about the amount of growth for each survivor.

Notes on the Two Sites

As the following photos show, the two sites differed in terms of site structure and the texture of the mud on the site. The mud on the Lesko site (top photo) contained a lot of gravel and clay. The site itself contained a lot of rocks and small boulders and the gradient from high tide to low tide was moderate, not shallow.

In contrast, the Bunker site had a shallow gradient, extending out a long way at low tide. The mud was soft and silty, almost soupy.

As the students analyzed the data collected from the experiments on these two sites, they found that even though the sites were not very far apart on the map, there were striking differences in clam growth and survival. It seems likely that some of those differences are related to differences in the makeup of the mud and other features of terrain on the sites.