Investigation into the Cause of the Clam Decline: 2017

Downeast Institute Field Studies – Freeport, Maine


Clammer and research technician Clint Goodenow and Dr. Brian Beal hold the bottom of a large recruitment box.

Clammer and research technician Clint Goodenow and Dr. Brian Beal hold the bottom of a large recruitment box.

In 2017 DEI and local clammers conducted six different experiments at eleven locations which built on our research findings in previous years.  We continued testing possible bioremediation techniques to reduce predation, such as using blood worms to reduce the presence of milky ribbon worms, and also using different sized quahogs (hard-shell clams) to deter predation on soft-shell clams.  We measured clam recruitment and recruit survival throughout the season, along with green crab recruitment, to determine the timing of both animals’ reproduction and how they interact, and built and deployed larger-scale versions of predator protective recruitment boxes to see if they were scaleable to collect even more seed. We tested two different types of clam impoundment covers to increase the success of that technique and continued an experiment that was conducted in 2014 in Freeport (as well as Stockton Springs in 2007) to determine if clams settle gregariously. 

These experiments resulted in a series of discoveries. Through the staggered deployment of recruitment boxes, we were able to determine that clam settlement is protracted, rather than a single event, that lasts until the early fall. In southern Maine, the bulk of clam settlement occurs in the spring with green crab settlement occurring in late summer.  Our research showed that the clams that settle later in the season have the highest chance of surviving and are the clams that will make up future years’ harvests.

We demonstrated that it is possible to scale-up clam recruitment boxes to increase the number of clams that the boxes protect from predators. This has practical applications for shellfish management as it can be used to enhance flats by capturing wild seed, overwintering it, and planting it in the spring before the ocean temperatures heat up. 

DEI’s 2017 clam impoundment study uncovered a new record for the amount of clams recruiting to a flat. We found an amazing 2,562 clams per sq. ft. in a cove that clammers do not dig, due to it being “unproductive”. DEI’s research found that the cove is certainly not unproductive; it is just that the clams are being eaten before they reach legal size. 

In order to ensure as much confidence in our results as possible, experiments were designed to be statistically valid.  The details, experimental design, and results can be found below.

Experiments & Results

#1: Bioremediation to Reduce Milky Ribbon Worm Predation
#2: Understanding the Timing of Clam Recruitment
#3: Scaled-up Recruitment Boxes
#4: Using Adult Clams to Enhance Settlement
#5: Testing Clam Impoundments
#6: Bioremediation Using Quahogs to Deter Predation on Soft-Shell Clams

#1: Bloodworm Bioremediation

Bioremediation to Reduce Milky Ribbon Worms

In 2017 we refined our study design based on what we learned in 2016 and continued to test the hypothesis that the presence of bloodworms would result in a decrease in clam deaths caused by milky ribbon worms.  This is part of our work to determine if results of laboratory experiment (Bourque et al., 2001) could be duplicated successfully in the wild. As in the previous year, clammers from the Maine Clammers Association (MCA) assisted with deployment and sampling.

The experiment was set up in the same location, Collins Cove, and tidal height (mid-intertidal) as 2016.

In 2017, we used one hundred and twenty protective wooden boxes (40 more than the previous year). The boxes were smaller than those used in 2016 (1-ft x 2-ft x 6-in deep).

In order not to introduce worm and crab predators to the boxes by filling the boxes with ambient sediments from the mudflat, we filled the boxes with terrestrial sand from a local (North Yarmouth) gravel pit. Approximately fifty pounds of terrestrial sand were added to each box by MCA clammers and DEI scientists. The sand was wetted with seawater before planting clams. MCA clammers dug 120 6-in deep x 1-ft x 2-ft depressions in the mud so that each box was able to sit nearly flush on the mudflat, providing as natural conditions as possible.

Three different densities of cultured juvenile soft-shell clams (60, 120, or 180) were placed in each box along with four different densities of bloodworms (0, 4, 6, or 8). 14,400 juvenile clams were used for this experiment along with 540 commercial-sized bloodworms.

The boxes were predator-protected with various mesh bottoms designed to allow or deter ribbon worms, and to retain the blood worms. One-half (60) of the boxes had a PetScreen® bottom (that excludes milky ribbon worms), and half (60) of the boxes had a 4.2-mm flexible netting bottom (that allows milky ribbon worms to enter the box). Both types of bottoms were reinforced with a 1-ft x 2-ft piece of vinyl-coated trap wire. Each box has a top that is made of a rigid (extruded) mesh with a 2.1-mm (0.083-inch) aperture.

All boxes were sampled in November 2017, and the entire contents of each was washed through 2 mm sieves. This size sieve allowed most of the sand to pass through the sieve, leaving only clams and worms. Number and size of live and dead cultured and wild soft-shell clam seed was measured, along with the number of living worms, to determine the success of the bioremediation technique.

The samples from each box were frozen, and some of the 120 samples have been processed. We will be completing the analysis of the data during 2020.

Images (click to view slideshow)

#2: Recruitment Timing

2. Measuring Clam Recruitment Through the Season

Building upon the Downeast Institute’s recent discovery that clam spawning is not a single event, a large-scale study was conducted in the Harraseeket River in 2017 to determine when clams begin and cease recruiting to intertidal mudflats. Results of DEI’s previous experiments showed that flats without clams are not “dead mud”, but that juvenile clams are settling, yet not surviving due to high levels of predation. Understanding the timing of clam recruitment informs when best to deploy predator protection.

To determine the local timing of wild clam spawning and settling, along with the settlement patterns of green crabs, we used the recruitment boxes developed over the previous two years.  Recruitment boxes were deployed every two weeks from 5 May to 22 September.  This research was designed to answer the questions: when do clams and their green crab predators recruit to the flats and what is their pattern over time? For example, is the pattern short and intense? Broad and protracted? Skewed and intense? Skewed and protracted? We also wanted to learn the size of clams and green crabs that are able to survive through the fall and winter.

To answer these questions we deployed  recruitment boxes every two weeks at five locations on the eastern side of the Harraseeket River starting in early May. These locations are called “blocks” and each block was 250 ft. away from other blocks. From previous experiments (2014, 2015, and 2016) we knew that this side of the Harraseeket River typically receives a lot of recruits. 

This study used recruitment boxes with different types of netting, one with Petscreen ® top and bottom (1/13-in aperture), and one with a 1/8-in aperture flexible netting top and Petscreen ® bottom. 

Every two weeks, beginning on May 5, 2017,  DEI researchers and clammers deployed four recruitment boxes (two of each type) at each of the five blocks (i.e. 20 boxes were deployed every two weeks). At the same time, five sediment cores were sampled (0.2 sq. ft) at each location (n=25). These core samples help us determine the amount of clam settlement and survival that is occurring in the unprotected mud flats. Our last deployment of recruitment boxes occurred on September 21, 2017 (i.e. 11 deployments through the field season). In total, researchers and our clammer partners deployed 220 boxes. To ensure we captured data about the entire year’s clam recruitment levels, we continued to collect core samples for three more weeks after September 21 (October 4, October 20, and Nov. 9, the date the recruitment boxes were removed).

Because we know that the clam reproductive cycle is strongly influenced by seawater temperature extremes, we recorded temperatures, using a HOBO temperature recorder, at the intertidal site (Across the River) for the duration of the experiment. From this we know that seawater temperature was 48°F at the beginning of May, when we first deployed the boxes. From DEI’s work spawning clams for 30 years, we know that clam spawning commences when seawater temperatures reach 50°F. Analysis of the 2017 temperature data found that 2017 was cooler than previous summers in the Harraseeket River, with the site only reaching its high temperature of 68°F during the first week of August. By the time the boxes were removed from the mud in November the water had cooled to 50°F. 

An examination of the core samples taken through the field season showed clam survivorship was up to 4 and 6 animals per sq. ft. from May 5-June 1. Our research found that when seawater temperatures rise above 12oC (53.6oF), which they did after May 20, 2017, clam densities begin to fall precipitously. When the core data is superimposed on the temperature data, it shows that the highest clam mortality occurred when seawater temperature was between 54°F and 59°F.

Core samples that were taken prior to June contained soft-shell clams that had settled the year before (2016) and overwintered. Their sizes ranged from ¼ inch to ¾ inch. Recruits from 2017 started showing up in the June 1, 2017 samples (⅛ inch to ¼ inch). 

Recruitment boxes were removed from the flats on Nov. 9. The entire contents of the boxes were processed through a 1-mm mesh sieve and all clams and crabs were counted. This is the same process that researchers and clammers used on the 25 core samples after each of the season’s 14 sampling dates. 

Clams found in the boxes (i.e. clams that were protected from predators) averaged about 200 clams per box (about 100 clams per sq. ft). This density found in Freeport corresponds to recruitment densities found in 2017 clam protection studies conducted by DEI with community partners in eastern Maine and Searsport. Clams from the first three Freeport deployments (May 5- June 1) had two distinct size classes of clams: ones that settled early and were about an inch in size and those that settled late and were about ¼ to ½ inch (6-12 mm). We found a few clams that were an inch and a half, which is a phenomenal growth rate. No clams that size were found at the end of the season in the unprotected core samples. This is because the clams died before they could grow. 

The contents of the boxes confirmed what we saw in the contents of the core samples. In addition to shellfish recruitment, we also learned more about green crab recruitment.

Inspection of the content of the boxes revealed that clam spawning began in mid-May and continued until at least early to mid-August. A peak in clam recruitment occurred during the first half of June (June 1-June 15, 2017), when seawater temperatures ranged from 54°F to 60°F. Recruitment continued through mid-August and perhaps into September. This experiment revealed that clam recruitment is skewed and protracted, not a single event. 

The experiment found that clams recruiting to the flats before mid-July are subjected to intense predation, and most die. Green crab recruitment appeared to begin sometime between August 10 and August 23, when the largest influx of small crabs occurred in the boxes. The crabs likely continue to settle to flats throughout October and perhaps early November and can overwinter at sizes smaller than the aperture of PetScreen® (1.6mm). The success of these overwintering crabs is the key to survival success of clams the following year.

The green crabs’ later settlement, along with the size difference at settlement (0.2 mm for clams; 1 mm for green crabs) gives green crabs a life strategy advantage over clams. 

While fewer clams settle to the flats after mid-August, these animals represent the bulk of those that make it to survive into the fall and winter. The clams that settle to the flats late in the summer will become, for the most part, the commercial fishery in 1-2 years. 

The results of this groundbreaking experiment were presented at the 2018 Maine Fishermen’s Forum:

Images (click to view slideshow):

#3: Scaled-up Beal Boxes

3. Scaled-Up Recruitment Boxes

Building on recruitment studies from previous years, we scaled up the size of the recruitment boxes in an attempt to protect larger numbers of clam seed. We deployed 15 large-scale recruitment boxes at a single site that we had determined, through previous studies, receives a large number of clam recruits. 

The scaled-up recruitment boxes were of two sizes: Ten were 4 ft (48 inches) x 8-ft x 3 inches deep, and 5 were 26 inches x 8 ft x 3 inches deep. All 15 boxes had the same bottom, PetScreen® held in place by vinyl-coated lobster trap wire. 

Of the ten 4-ft x 8-ft x 3-inches deep boxes, half of them were covered on top with a piece of PetScreen®. The other half were topped with a piece of flexible netting (3.2 mm aperture). These boxes were about 16 times larger than “regular” size recruitment boxes. 

The remaining 5 smaller boxes had a larger size mesh (2.1 mm extruded netting) covering their tops. The reason these boxes were narrower is because the 2.1 mm mesh roll is only available in a 26-inch width. 

Because the boxes were so large, the bottoms were taken to the mudflats and researchers and clammers hammered the pre-made tops onto the recruitment box frames while on the mudflat. 

At the end of the season researchers and clammers pulled off the tops and used a coffee can corer to sample the boxes. We found that the boxes contained 16 times more clams in the large boxes than we typically do in the same area with the regular size recruitment boxes! In other words, we collected the same density of clams in the regular boxes as the scaled-up boxes. This shows that recruitment boxes are indeed scaleable.

The scale-ability of recruitment boxes is useful for shellfish managers who are trying to sustain harvests through the impacts of warming waters and increased predation. Managers and others can deploy these scaled-up recruitment boxes to collect and protect clam seed, overwinter the seed, then plant the seed in the early spring before the waters warm, as a method of enhancing clam flats.

Images (click to view slideshow)

#4 Gregarious Settlement

4. Using Adult Clams to Enhance Clam Recruitment

Many marine invertebrates settle gregariously near their own kind, especially adults.  This has been shown in barnacles, ascidians (sea squirts), tubeworms, oysters and other bivalves, but has not been shown definitively in soft-shell clams. At the same coves, Spar and Recompense, as in 2014, we continued to test whether the presence of adult clams attracts settling juvenile clams (i.e. gregarious settlement). The 2014 experiments were compromised by milky ribbon worm predation.

If it is found that adult clams do attract settling juveniles, planting adult clams could be a way to locally enhance clam populations. The studies were designed to examine the combined effects of adult clams and predator-deterrent netting on the number of juvenile wild clams.

In the lower intertidal, we repeated the experiment with protections in place that we thought would protect clams from milky ribbon worms. One of the more important findings from the investigation into the cause of the clam decline is that in order for clams to survive they need to be protected from milky ribbon worms. Many of our experiments in 2014 relied on using nets to protect clams, as DEI has done in many coastal communities since the 1980s. Milky ribbon worms live in the sediments, thus nets that cover the top of the flat do not stop their predatory activities.

With low survival under nets, we moved to using experimental units (EU) that protect clams from the top and bottom as well as the sides. For this experiment in 2017 we planted clams inside 8-in diameter x 6-in deep plastic plant pots. 

We deployed these 240 plant pots in May, with half being placed in Spar Cove and half at Recompense Flat. At each site, half of the plant pots (n=60) had a piece of window screening placed on the bottom of the plant pots (because plant pots have rather large drainage holes) to keep out milky ribbon worms, the other half had no protection against milky ribbon worms. Ambient sediment was placed in the pots. Adult clams were planted inside the EUs in six different densities: 0, 1, 2, 4, 8, and 16. We used 1,240 adult clams, sized 50-65 mm SL (shell length), for this experiment.

After digging the EUs into the mud, half of the plant pots were protected across the top by a piece of Pet Screening®, while the other half were protected by a 3.2-mm flexible screening.  All pieces of top mesh were 20 inches x 20 inches in size and held in place by a rubber band.

The experiment was set up prior to clam spawning season and continued until November, well after clam larvae had settled to the flats.

In November, the 120 pots from each site (N = 240) were removed, their contents washed through 1-mm sieves, and all adult clams (live and dead) and clam juveniles (recruits) counted and measured. Upon processing the samples, we discovered that the window screening inserts we had placed in the bottom of the plant pots were not effective in stopping milky ribbon worms from reaching the adult clams inside. The milky ribbon worms were able to push and slide through the small spaces between the holes in the bottom of the plant pot and the window screening. Again, most of the adult clams were dead, making it impossible to assess whether their presence enhanced juvenile clam recruitment.

Images (click to view slideshow)

#5: Clam Pounds

5. Testing Different Covers for Clam Impoundments

We continued to test an original prototype for protecting and growing clams in pounds.  Impoundments were being tested for their ability to protect and store adult clams, enabling clammers to sell their previously harvested clams in the late summer when prices are as much as 50% higher.

Last year the impoundments we planted in two mid-intertidal locations (Recompense and Winslow) were impacted by predation and anoxia. With so many clams dying in the pounds, it was clear that this specific pounding needed to be redesigned. 

During 2017, in the mid-intertidal at Winslow Park, one of the same flats used in the previous year, we planted 16 pounding cages and planted them with a half bushel of clams each, as in 2016, but using a different design to protect the clams inside the impoundments. We placed the impoundments in a specially-made covering with either a zippered top or a Velcro top. Two Styrofoam floats were placed inside the bag so that the screening billowed up in the water column during tidal inundation. This ensured maximum water flow to the legal size clams protected inside.

Though collecting data on clam recruitment was not the purpose of the study, the zippered clam impoundment covers had the effect of protecting the recruited clams from predators.

Again, we sampled the impoundments in August, when the clam price was highest.  When we opened the zippered clam impoundments we made a major discovery. 

While survival of the impounded adult shellfish was somewhat disappointing, zippered impoundments provided enough protection from predators to allow over 2,500 soft shell clams per square ft. to settle and survive. Analysis of 6-inch diameter sediment cores showed that the zippered impoundments had 2,562 clams per square foot, or 27,583 clams per square meter. This density of juvenile clams is the highest ever recorded by scientists. This density is almost double the previous record of 1,377 per square foot found on the east side of the Harraseeket River (on a flat called Across the River) in 2014. 

The video below shows what we found:

Outside the protected areas – in the open, unprotected areas – clam densities were drastically smaller. Sediment cores taken in this area showed a density of only 8.5 juvenile clams per square foot, which are unlikely to survive as predation rates continue to intensify during September and October when water temperatures remain high. 

This experiment, along with others conducted as part of the Freeport Clam Experiments, has begun to reveal the true productivity of the mudflats. For example, the location of the recent discovery had been deemed “unproductive” (meaning that it has been without any clams for local clammers to dig) as it had not been commercially productive for over 30 years. However, if we could ask the green crabs that inhabit the area, they would tell us that the flats have and continue to provide an incredible amount of food.  These mudflats are, in fact, highly productive, and the clams that settle here are providing the energy for the predators of the intertidal ecosystem.

Images (click to view slideshow)

#6: Quahog Bioremediation

6. Examining a Novel Bioremediation Technique Using Large and Small Quahogs to Deter Predators from Juvenile Soft-Shell Clams

A possible bioremediation technique local clammers wanted to try used large and small quahogs in an attempt to deter predators from eating soft-shell clams. This study was conducted at one site (Collins Cove). 

The study included a number of treatments that were designed to test whether or not adult and/ or juvenile quahogs reduce the effect of predator foraging on juvenile soft-shell clam growth and survival.

Eighty-four experimental units (6-inch plastic plant pots) were seeded with various combinations of large and small quahogs and live clams (14 different treatments replicated 6 times). 

The pots received either a “sham” treatment (2-5 adult quahogs’ shells with the meat removed and hot-glued shut), 24 or 48 juvenile quahogs (864 for the total experiment), or 0-5 live adult quahogs. In each pot we also planted 12 hatchery-reared juvenile soft-shell clams (1,008 total). Half the pots were covered with predator-deterrent netting (PetScreen®).  The other half were not protected from predators (see the field schematic below).  

Samples from this experiment are being processed during the summer of 2020 and results are hoped to be determined soon.

Bourque, D., Miron, G. & Landry, T. Predation on soft-shell clams (Mya arenaria) by the nemertean Cerebratulus lacteus in Atlantic Canada: implications for control measures. Hydrobiologia 456, 33–44 (2001).

Images (click to view slideshow)

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