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Beginning in May 2014 and 2015, we initiated a study in Wells (Webhannet River) and in Portland (Fore River) to examine the interactive effects of tidal height and predator exclusion on the fate and growth of wild and cultured soft-shell clams.

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 through the Casco Bay Estuary Partnership.  In 2015, the study was funded by the University of Maine at Machias and the Downeast Institute.


Numerous people from the Wells Reserve, Gulf of Maine Research Institute, Casco Bay Estuary Partnership, the Waynflete School, and AmeriCorps all pitched in to help.  The photos below give a small sampling of the effort this type of project takes.  Kristin Wilson (Wells Reserve) kindly provided the photos except the last one.  You can read the Final Report from 2014 here.  The Final Report from 2015 is here.

 This was taken on May 11, 2014 near the upper intertidal at the Wells (Webhannet River) site.  We are looking at a schematic of the field layout of the treatments and holding onto the experimental units (6-inch plastic plant pots) used in this study.

Tim Dubay in sticky mud near the upper intertidal at the Wells study site in May 2014 dragging the clams and plant pots to the next location.

Tim Dubay and Caroline Casals help to set up a 2 x 5 matrix of pots (two replicates of each of five predator-exclusion treatments).  Pots were set out at both the low and upper intertidal in three blocks of 10 pots each.  Blocks were approximately 20 meters apart.

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

You can barely see the crew working in the lower intertidal from this shot taken near the upper intertidal in the Webhannet River.

During the summer, experimental units in each block at both tidal heights were checked to see if any problems existed.  Here, near the upper intertidal, the pots in this block are just fine.

Some of the experimental units in the lower intertidal, however, became scoured or buried under 2-5 cm of sand in the lower intertidal blocks in the Webhannet River.

In October all of the experimental units at both the Webhannet and Fore River sites were collected and the contents of each washed throug a 2 mm sieve.

We were thankful for lots of help from the AmeriCorps volunteers who were working at the Wells National Estuarine Research Reserve.

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

Most 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!

This is the size range of the 2014 year class of soft-shell clams (October 10, 2014)

How can you tell the difference between a hatchery-reared juvenile and a wild juvenile?  The cultured clam leaves a distinct disturbance line in both of its valves upon planting in any sediments (this is the clam on top).  Wild clams do not have as distinct markings near their hinge (bottom clam).

This is a good comparison of the biomass of clams from a protected vs. an unprotected (control) experimental unit (Wells; October 10, 2014).

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

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

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

One 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.

This 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).

 

Read the Final Report.

 

 

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