What are the physical conditions that influence the growth of clams?
There appear at least three essential conditions for rapid growth of clams: (1) a good current; (2) low and level flat; and (3) a tenacious soil, relatively free from decaying matter.
A low flat gives the clams longer feeding periods, as the water remains over them longer, therefore there is a greater growth. This has been experimentally shown by Dr. A. D. Mead.
According to Prof. J. L. Kellogg, clams cannot do well in a soil which contains much decaying organic matter, as the acids eat away the shells. Soils of this description also facilitate the spread of infection from one clam to another.
Current is the chief essential for successful clam culture. The term "current" does not imply a rapid flow of water, but rather a good circulation of water over the flat. In the Essex and Ipswich rivers the clam flats have a continuous current. On such flats the growth is more rapid than on flats which have no circulation of water, in addition to the mere rise and fall of the tide. The current performs the work of (1) keeping the flats clean and carrying away all contamination, but its most important work is as (2) food carrier.
Value of a Clam Farm.—The value of an acre of clam flats, if properly cultivated, is about $450 per year for the average clam flat. Many of the more productive flats will yield a far greater amount, while others will not yield as much. It has been often erroneously stated that an acre of clam flats would produce $1,000 per year. This is a decided overestimation, as it would be hardly possible for the most productive flat to yield that amount. It is possible, however, for a good flat to yield about $750 per year, but this is only under the most favorable conditions. Such yields as these are large for the clammer, whose average yearly income is only $400 (a few of the more expert clammers make possibly $700 to $750), and a man possessing a clam farm of 1½ to 2 acres would make a good living.
Method of operating a Clam Farm: choosing the Ground.—In choosing a grant, the planter should have in mind three things: (1) the accessibility of the grant, for his own convenience, and nearness to the market, as much of the success of clam farming depends upon the expense of marketing the product, and the ease with which it can be disposed of; (2) the length of time allowed for labor by the exposure of the flat (flats vary greatly in the amount of time exposed each tide, the low flats being submerged nearly all the time, and the high flats having a much longer exposure),—a high flat possesses the advantage of allowing a longer working period for the clammer; (3) the natural facilities of the flat itself as regards the growth of clams. Moreover, the flat should be chosen in regard to (1) soil; (2) current; (3) tide. A good flat should have a soil which is tenacious and compact, affording at the same time easy digging. Probably the best soil is a mixture of fine sand and mud in a ratio of one-third mud to two-thirds sand, as this amount of mud gives the right degree of tenacity.
The growth of a clam depends upon the circulation of water over the flat, as the current carries the food, and, therefore, the more current the more food for the clams. Current also keeps the bed clean, and prevents contamination and disease from spreading among the clams. Then, again, the growth of a clam depends upon the amount of water over the bed; i.e., length of time covered. The clam can only feed when the tide is over the bed, and thus the feeding time is limited for the higher flats. While experiments have shown that clams grow faster when continually under water than when exposed part of the time, the question of tide is not so great a factor as that of current in regard to clam growth, and can be almost disregarded.
The best flat for clam planting is a fairly high flat with a good current over it, as it gives nearly as rapid growth and a much longer period to dig than a flat which is exposed only a short period. This flat must have the right kind of soil, which must not be shifting sand or too soft mud, but a compact, tenacious mixture.
The Seed Clams.—Nature has provided the means of stocking these farms. The set of clams is usually restricted to certain localities, which, however, vary from time to time, and heavy sets are found in limited areas. These sets run as thick as 2,000 per square foot of surface, occasionally covering an area of 3 acres. From these natural set areas the natural clam flats are partially restocked by the washing out of the small clams. More often these whole sets are wasted, as the clams, instead of washing on the good flats, are carried to unproductive places and consequently perish. Thus there are areas of heavy set which are of no use to any one, as practically all the clams perish before they become adults. These areas of heavy set occur in nearly every harbor of the coast to a greater or less extent, and are available for nearly every town.