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Clam Tents: A New Approach
to Soft-Shell Clam Culture and Management
here to view this document as a PDF file.)
Soft-shell clams, Mya arenaria, are an enigma to
scientists, managers, and shellfish harvesters in southeastern Mas-sachusetts
and throughout the bivalve's range. One year, clams may settle in
very dense concentrations, while, the following year, there may
be no soft-shell clam recruitment at the same site. Why?
Recruitment refers to the naturally-occuring replenishment
of a population. To fisheries managers and biologists, recruitment
is generally gauged by the number of a specific year class that
survive until they reach a harvestable size. A female soft-shell
clam, two and one-half inches long, is capable of producing three
to four million eggs when she spawns. Of the millions of eggs generated
by a population of clams, Belding (1930) estimated that one clam
out of two million survive to harvestable size. In southeastern
Massachusetts, a female may spawn twice a year. The larvae live
in the water column for several weeks before settling to the sediment.
During the planktonic stage, larvae are subject to tides, tidal
currents, wind-driven currents, waves, and other natural and episodic
events that affect water movement. For example, if the wind blows
offshore for the week that the larvae are ready to set, all or most
of the larvae could be transported out to sea and the potential
intertidal population for that spawning event will be lost.
Mya arenaria larvae that survive the planktonic
stage metamorphose and settle on sediment or other substrates as
post-set juveniles. These newly settled clams are tiny, less than
0.25mm in length. Soft-shell clam juveniles can burrow quickly and
attach themselves to sand grains or other hard structures using
byssal threads. However, these tiny clams may attach to small particles
which themselves may be moved by wind and currents. Other potential
dangers are broken or detached byssal threads, which increase the
risk of these small clams being transported away from the tidal
As if physical forces weren't enough, the young
clams are also at risk from predators. Early post-settlement mortality
is typically around 80% in the first 100 days, primarily due to
predation. Moon snails, whelks, most species of crabs, lobsters,
ribbon worms, and many birds and fishes all eat tiny clams.
History of Soft-shell Clam Population
Historically, management of soft-shell clams was
limited to monitoring productive areas and harvesting legal sized
clams when and if densities were high enough to make it economically
feasible. In the early and mid-1900's, some attempts were made to
relay clams from areas of good recruitment and high density to areas
with good growing conditions but low recruitment. Those early efforts
were unsuccessful, primarily because predators had easy access to
the relayed clams.
Since the early 1990s, quahog (Mercenaria mercenaria)
farmers from lower Cape Cod observed that the nets they used to
protect their hatchery-reared quahogs were collecting soft-shell
clam seed. These nets were laid down over the seed quahogs to protect
them from predators and from washing out of the sediment. The soft-shell
clam seed apparently settled and attached to the nets with their
byssal threads. Later in the season, the quahog farmers found the
soft-shell clams growing rapidly in the sediment alongside their
quahogs. Initially, the quahog farmers considered the volunteer
soft-shell clams a nuisance, competing with their farmed quahogs
for space and food. Eventually, they saw it as an opportunity and
began collecting the soft-shell seed as an additional component
of their shellfish harvest.
Before long, quahog farmers were placing netting
on the flats with the sole intent of collecting wild soft-shell
clam spat as a commercial crop. This practice originated the concept
of the clam tent. Following their informal experimentation, Cape
Cod growers realized they could increase the rate of "catching"
soft-shell clam seed by modifying the way the net was placed over
the sand flat. They experimented with mesh size, form of structure,
height of netting off the bottom, and orientation of the tent to
The designs of modern clam tents vary depending
on the location it is used in and the grower's own preferences.
Generally, variations fall under two basic designs: free-standing
and raceway systems.
These designs, often with modifications, are currently
in use on the flats in southeastern Massachusetts. A newer innovation
has beendeveloped by Karl Rask and Richard Dickey from The Resource,
Inc. (Orleans, MA). Although the information contained in this bulletin
is the most up-to-date available, clam tent designs are constantly
changing as farmers and resource managers experiment with variations
that work in their specific locations. Each clam flat represents
a unique environment and may require modifications in clam tent
The following is an attempt to provide answers
to some of the most frequently asked questions about clam tents.
What are clam tents?
Clam tents are netted structures that are placed over native intertidal
sand flats to promote the recruitment of soft-shell clams. Their
design is highly variable and can be modified to meet site specific
Why are clam tents being used?
As described above, larval and newly settled soft-shell clams are
often lost to hydrodynamic perturbations or to predators. Clam tents
offer juvenile clams the opportunity to establish themselves in
the sediment and grow to a size less likely to be washed out of
the sediment or to be consumed by predators. Flats with clam tents
often yield higher densities of harvestable clams than areas without
In an economic analysis generated by WHOI Sea Grant
and the Town of Barnstable Division of Natural Resources, a tented
flat recruited 13.6 harvestable clams per square foot in an area
that had not had a clam set in twenty years. The tent promoted clam
recruitment resulting in an estimated gross harvest of $42,458 per
acre, at a landed value of $1.25 per pound of live clam. The estimated
cost of materials and labor per acre for deploying and retrieving
the tent was $4,533 per acre. The economic return for tenting the
site was an estimated net profit of $37,925 per acre of tent, not
counting the harvest cost.
Why do clam tents work?
Although growers and researchers are not sure exactly how clam tents
work, it is commonly believed that they provide a refuge from swift
currents by slowing down the movement of water as it passes through
the mesh. In turn, this may reduce sediment movement and may allow
the clams to attach more easily to the net or bottom sediment. Once
the clams have settled under the tents, the tents serve as a refuge
from many surface-active predators.
How do I build a clam tent?
The general concept of a clam tent is to suspend and anchor plastic
mesh netting over the clam flats to exclude predators or competitors.
Two common designs are included in this bulletin and can be used
as a general guide to their construction.
When do I put the clam tents on the
The idea is to get the nets onto the flats as the larvae are being
spawned. Soft-shell clam larvae spend about two weeks swimming in
the water column before settling to the sediment. If the nets are
on the flats when the clams spawn, the tents will be in place when
the larvae are ready to settle. The net will "season"
in the water before the larvae settle, but will not be exposed so
long that algae or other fouling organisms will settle on the nets
As timing is critical, it is important to know when
the soft-shell clams are spawning in your area. In southeastern
Massachusetts, the clams spawn in May and again in September. Further
north, the clams spawn only once a year, usually in June or July.
Spawning depends on local conditions such as air and water temperature
and the availability of plankton. Check with your Sea Grant Marine
Extension Agent or your local shellfish constable to get an estimate
of when the soft-shell clams will spawn in your area.
When do I remove the clam tents from
Clam tents function both as a substrate for larval attachment and
a trap that collects post-larval clams entrained in the moving tidal
currents. After settlement, the tents continue to act as barriers
to predators, so it is a good idea to keep the tents in place until
the clams have grown large enough to anchor themselves in the sediment
and to resist predators. Usually, tents are kept on the flats through
the summer and into the fall. Once the water temperature has dropped
below 10 o C, predators will be less active (and therefore less
of a threat), and clams should have reached 10-20mm in size. At
this point, tents can be removed from the flats. Tents must
always be removed before the flats ice up during the winter.
Ice will destroy the tents and could transport them off-site where
they may pose an entanglement threat to marine life and birds.
What kind of clam enhancement can I
expect from clam tents?
An experiment was conducted in Barnstable Harbor, Massachusetts
in 1995-1996 by the Barnstable Shellfishermen's Association, the
Town of Barnstable Division of Natural Resources, and WHOI Sea Grant
to test the effectiveness of clam tents. The study involved four
intertidal flats, of which approximately 150,000 square feet were
tented (3.43 acres). The Barnstable Harbor sites had a history of
harvestable populations of clams but hadn't recruited any clams
during the previous ten to twenty years. The 1995 summer season
yielded no appreciable recruitment of soft-shell clams within the
harbor and no recruits could be found in any the untented experimental
flats. At one 700 square-foot tented site, clams were recruited
at a density of over 100 clams per square foot (mean shell length
6.33mm or 0.25 in.) as recorded during Fall, 1995. The tents were
removed in November, 1995, and by the following year the mean clam
density was 13.6 individuals per square foot, at a mean size of
71mm (2.8 in.). If this recruitment success could be achieved routinely,
the result would be a return of approximately 566 bushels of soft-shell
clams harvested per acre of tents. As stated above, the estimated
net profit per acre of clam tent would be $38,000. This represents
a significant economic boost for the shellfish industry.
Who can use this technology?
Clam tents can be adapted for use by many different agencies interested
in natural resource management. Currently, tents are being deployed
by many shellfish aquaculturists on Cape Cod interested in diversifying
their harvest from primary crops of cultured quahogs and oysters.
In addition, some shellfish constables are investigating the use
of clam tents in an attempt to stabilize the recruitment of soft-shell
clams on their flats as a benefit to the wild fishery. Clam tents
also have been employed by environmental management groups to reclaim
flats that have lost soft-shell clam populations to adverse environmental
conditions, overfishing, or other, unex-plained factors. Essentially,
clam tents offer a low-cost tool for promoting soft-shell clam recruitment
to areas that can support them.
- Belding, D.L. 1930. The soft-shelled clam fishery of Massachusetts.
Marine Fisheries Series #1. MA Division of Marine Fisheries. Boston,
MA. 65 p.
- Marcotti, T. and D. Leavitt. 1997. The Barnstable Harbor Shellfish
Recruitment Enhancement Project. Final Report to Barnstable Health,
Safety, and Environmental Services Dept. -- Natural Resources
Division, Barnstable, MA. 47 p.
"Clam Tent" Marine Extension Bulletin written by: Dale
F. Leavitt, WHOI Sea Grant Marine Extension Leader & Aquaculture
Specialist, Barnstable County Cooperative Extension Service. June
For more information about the research or outreach
projects profiled in the Marine Extension Bulletin, contact WHOI
Sea Grant at the address listed below.