Soft-shell clams, Mya arenaria, are an enigma to scientists, managers, and shellfish harvesters in southeastern Massachusetts 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 flat.
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 Management
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 current.
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 design.
The following is an attempt to provide answers to some of the most frequently asked questions about clam tents.
What are clam tents?
Why are clam tents being used?
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?
How do I build a clam tent?
When do I put the clam tents on the flats?
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 the flats?
What kind of clam enhancement can I expect from clam tents?
Who can use this technology?
"Clam Tent" Marine Extension Bulletin written by: Dale F. Leavitt, WHOI Sea Grant Marine Extension Leader & Aquaculture Specialist, Barnstable County Cooperative Extension Service. June 1998.
For more information about the research or outreach projects profiled in the Marine Extension Bulletin, contact WHOI Sea Grant at the address listed below.
Woods Hole Sea Grant 193 Oyster Pond Road, MS#2, Woods Hole, MA 02543 (508) 289-2665 firstname.lastname@example.org