eremy Miller is awfully proud of himself here at the low intertidal in the Webhannet River (May 11, 2014)

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 

2015 Final Report.

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. 

 

ome of the experimental units had hundreds of wild, 0-year class soft-shell clam recruits.

Some of the experimental units contained hundreds of wild, 0-year class soft-shell clam recruits.

ost of the wild clam recruits were associated with experimental units that were protected with Pet screen. In this sample, 7 live hatchery-reared clams (out of 12) survived; however, there were 467 wild clam recruits as well!

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 10, 2014)

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.

comparison of the biomass of clams from a protected (predator exclusion) vs. an unprotected (control) experimental unit

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)

his one is a recent settler. It is about 3 mm (one-eighth of an inch) in carapace width.

This green crab is a recent settler (about 3mm).

reen crabs also were found in some of the pots. This one is about 10 mm (about one-half an inch) in carapace width.

Green crabs were also found in some of the pots. This one is about 10 mm (about 1/2 inch) in carapace width.

ne data sheet was used for each sample (experimental unit). This is the one that had 467 wild recruits and seven live hatchery clams. The disturbance line allowed us to measure an initial length for each live clam. Only 20 of the 467 wild clams were measured. The 20 were sampled in such a way to provide a representative measure of the average length of the 0-year class clams.

All data was recorded.

ll clams (live and dead) were counted and measured to obtain survival and growth information.

All clams (live and dead) were counted and measured to obtain survival and growth information.

his is a sample from the low intertidal from the Fore River site (near the Waynflete High School playing fields in Portland). In this sample, all 12 hatchery-reared clams survived. The average initial shell length of these 12 clams was 12.0 mm (about one-half an inch), and the average final shell length was 23.4 mm (almost one inch). Growth of clams in experimental units protected with Pet screen vs. protected with a larger aperture flexible netting was depressed by 26% at this study site. A total of 214 wild clams occurred in this experimental unit. The average size of the wild clams was 8.5 mm (about one-third of an inch).

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.

 

 

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