Harpswell: Tidal upweller experiment to enhance natural quahog population

Contacts

Paul Plummer – Marine Resource Administrator
pplummer@town.harpswell.me.us

David Wilson – Marine Resources Committee
WilsonDavid0051@gmail.com

short project summary (PDF)

Background

Harpswell is one of several towns shifting its attention from soft-shell to hard clams. For several years, harvesters grew quahog spat in floating nursery trays. In 2020, a grant gave the Harpswell Marine Resources Committee the opportunity to raise quahogs in a tidal upweller and transplant the seed into unproductive areas.

Tidal upweller

The upweller was built in 2020 based off a design from Signature Oyster Farm in Martha’s Vineyard.

In May 2021, the team launched the upweller in Card Cove at a site permitted for limited purposes aquaculture. The Marine Resources Committee placed approximately 400,000 quahog seed, 4mm in size, in six of the eight upweller silos. From May to October, the upweller was cleaned once a week with a pressure washer and pump. In October, the quahog seed, now averaging 4-8mm, was removed and transplanted into Card Cove, Hen Cove, and Ridley Cove. The planting sites will be revisited in 2022 to assess the effectiveness of the transplant.

Upweller design (Photos courtesy of David Wilson)

Takeaways

If relying on volunteers for construction and maintenance, prepare to adjust the project timeline and expectations.

Purchasing smaller quahogs can save money; however, the costs associated with growing the quahogs to size (staff time, boat costs, fuel, etc) might negate those savings.

Providing access to the upweller can be challenging: volunteers need transportation.

The silos experienced significant algae and marine growth. In the future, the town may cover the silos to restrict sunlight, or experiment with periwinkles that eat the algae.

Cleaning and maintenance of the upweller could happen more often than once a week.

Permitting for a municipal upweller is a challenge due to the aquaculture review process.

The Marine Resources Committee is looking to move the upweller to areas with stronger tidal flow to enhance quahog growth.

Harpswell hopes to continue using the tidal upweller to expand the local quahog resource. By participating in this project, harvesters gained knowledge about shellfish farming, and felt that the upweller added purpose to the shellfish program. Harvester David Wilson commented at 2021 Shellfish Focus Day that upwellers could be a useful tool in every town’s management plan if the town intends on seeding—and, “it’s fun watching them grow.”

Timeline

2020

Marine Resources Committee members and harvesters build the upweller based on a design from Signature Oyster Farm
May 2021

The tidal upweller is placed in Card Cove and loaded with quahog seed
May – October 2021

Regular maintenance on the upweller
October 2021

Quahog seed planted into Card Cove, Hen Cove, and Ridley Cove
Spring 2022

Sites revisited for a shellstock inventory