Summary
DEI conducted studies in Wells (Webhannet River) and Portland (Fore River) in 2014 and 2015 to examine the interactive effects of tidal height and predator exclusion on the survival and growth of wild and cultured soft-shell clams.
In order to ensure as much confidence in our results as possible, experiments were designed to be statistically valid. 2014 results from Portland and Wells showed that deterring predators increased average juvenile clam densities by 23 to 95 times. Results from 2015 showed that deterring predators resulted in a statistically significant enhancement of juveniles (21 times more in Wells and three times more in Portland).
Our research found that 71%-77.9% of planted clams survived in units protected from predators, while only 0% to 3.1% percent survived when not protected from predators.
Results were also documented here:
Beal, Brian F., “Interactive effects of tidal height and predator exclusion on growth and survival of wild and cultured juveniles of the soft-shell clam, Mya arenaria L., at two intertidal flats in southern Maine” (2015). Maine Sea Grant Publications. 40.
https://digitalcommons.library.umaine.edu/seagrant_pub/40
Results concerning the effects of tidal height, predation, and spatial variation on clam recruits were published in the Journal of Shellfish Research in 2018.
Funding for the study in 2014 came from the U.S. Fish & Wildlife Service via the Maine Department of Environmental Protection through a grant written by Beth Bisson (Maine Sea Grant). Funds were administered by the Casco Bay Estuary Partnership. In 2015, the study was funded by the University of Maine at Machias and the Downeast Institute.
DEI’s field research is a team effort. Our scientists were assisted by volunteers from the Wells Reserve, Gulf of Maine Research Institute, Casco Bay Estuary Partnership, the Waynflete School, and AmeriCorps.
The photos below provide a hint at the amount of effort needed to execute rigorously-designed field experiments and show some of the details of the experimental design. Kristin Grant (Maine Sea Grant and Wells Reserve) kindly provided most of the photos.
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- Near the upper intertidal at the Wells (Webhannet River) site, researchers examine the field layout schematic showing research treatments, while holding the experimental units (6-inch plastic plant pots) used in this study (May 2014).
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- Jim Dubay, in sticky mud near the upper intertidal at the Wells study site in May 2014, drags the clams and plant pots to the next location.
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- Jim Dubay and Caroline Casals help install a 2′ x 5′ matrix of pots, which will contain two replicates of each of five predator-exclusion treatments. Plots were set out at both the low and upper intertidal in three blocks of 10 pots each. Blocks were approximately 20 meters apart.
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- One can barely see the crew working in the lower intertidal from this shot taken near the upper intertidal in the Webhannet River.
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- During the summer, experimental units in each block at both tidal heights were checked for problems. Here, near the upper intertidal, the pots in this block are just fine.
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- Some of the experimental units in the lower intertidal of the Webhannet River became scoured or buried under 2-5 cm of sand.
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- In October, all of the experimental units at both the Webhannet and Fore River sites were collected, and the contents of each washed through a 2 mm sieve.
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- AmeriCorps volunteers from the Wells National Estuarine Research Reserve helped sieve the samples.
Some of the experimental units contained hundreds of wild, 0-year class soft-shell clam recruits.
Most of the wild clam recruits were found in experimental units that were protected with Pet screen. In this sample, 7 out of 12 hatchery-reared clams survived, as well as 467 wild clam recruits.
This is the size range of the 2014 year class of soft-shell clams (October 2014).
How can you tell the difference between a clam that began life in the hatchery and one that is wild? The cultured juvenile clam leaves a distinct disturbance line in both of its valves upon planting in any sediments, resulting in the hatchery mark seen on the clam in the top of this photo. Wild clams (as seen at the bottom of the photo) do not have such distinct markings near their hinge.
Compare the biomass of clams from a protected experimental unit (right) with that from an unprotected (control) experimental unit (left). These results are typical of our findings. (Wells; October, 2014)
This green crab is a recent settler (about 3mm).
Green crabs were also found in some of the pots. This one is about 10 mm (about 1/2 inch) in carapace width.
All data was recorded.
All clams (live and dead) were counted and measured to obtain survival and growth information.
This is a sample from the low intertidal of the Fore River site in Portland, in which all 12 hatchery-reared clams survived. Their average initial shell length was 12.0 mm (about 1/2 inch), and the average final shell length was 23.4 mm (almost 1 inch). Growth rates of clams in experimental units protected with Pet screen vs. those protected with a larger aperture flexible netting were depressed by 26% at this study site. A total of 214 wild clams with an average size of 8.5mm (1/3 inch) were found in this experimental unit.
