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Massachusetts Aquaculture White Paper - Shellfish Bottom and Off-Bottom Culture
This chapter will discuss the basic biology and production techniques for shellfish (both bottom and off-bottom culture). In addition, this chapter addresses problems inherent to shellfish aquaculture, including environmental, economic, business, legal, etc.
The exclusive form of commercial aquaculture engaged in Massachusetts to date, both privately and by several cities and towns and the Division of Marine Fisheries, is mollusk culture, employing several methods of cultivation. This chapter provides significantly more detail on shellfish than other forms of aquaculture and this is a simple reflection on the dominance of shellfish in the Massachusetts aquaculture industry. Massachusetts is not alone in its predominant interest in mollusk culture. Globally, bivalve mollusk culture is second to only crustaceans as the most economically successful forms of marine animal aquaculture. While bivalve mollusk culture is well established in the state, recent proposals indicate commercial interest in finfish (cod, salmon, and steelhead trout) as well.
Hard clams (quahogs), soft clams (steamers), oysters, bay scallops, and mussels all reproduce by spawning; the discharge of microscopic eggs and sperm into the surrounding water, triggered by increased water temperature. The number of eggs released by a single female is impressive; a mature oyster may release up to fifty-million eggs in one season.
The eggs are fertilized in the surrounding water and within a matter of hours, the embryos develop into motile, swimming larvae. The duration of the larval period varies with water temperature and generally ranges between one to three weeks (these are requirements that may be artificially manipulated in commercial hatcheries to maximize fertilization and shorten developmental periods). The larval stage is terminated by metamorphosis; the transition to the early juvenile stage, which resembles a miniature of the adult form. Because shellfish larvae are essentially planktonic organisms that drift about and are subject to dispersal by currents, winds, and wave action, they may undergo metamorphosis, or set, far from the site of initial fertilization.
The behavior of the various species begins to differ at this stage of development. The early juveniles of quahogs and soft clams are capable of crawling on the bottom sediments. Upon finding an acceptable substrate, they dig in by means of a muscular foot. While still small, they are capable of emerging and wandering, searching for greater food sources, more desirable locations, etc. However, the remainder of their life is sedentary and spent beneath the sediment surface. Until they reach legally marketable size, clams at this stage are referred to as seed.
All bivalve mollusks (bivalves) are filter feeders, extracting their food from the surrounding water. Because they dwell in the bottom substrate, clams are equipped with siphon tubes that enable them to draw in the water from above. Unfortunately for bivalves, natural predators are numerous, including several species of crabs, snails, starfish, drills, finfish and waterfowl, many of which patrol the bottom.
Quahogs are hardy and long-lived. Soft clams, oysters, and mussels are more vulnerable to age and predation. Bay scallops are comparatively short-lived. All of these species are capable of reproduction by the end of their first full year of life.
The natural habitat of these mollusks varies considerably. The oyster and the soft clam can tolerate conditions of varying salt concentrations, adapting them to estuarine as well as ocean environments. The quahog, bay scallop and mussel prefer higher salinity and are more likely to occur in salt ponds and coves without appreciable inflows of fresh water. The soft clam, oyster, and mussel can all thrive intertidally. While the quahog may live in the tidal flats, its preferred location, along with the bay scallop, is sub-tidal.
The nature of the bottom sediments is critical to normal development and reproduction. Soft bottom muds are generally inhospitable, due to the potential for siltation and hypoxia. These soft muds might be indicative of situations where contaminants may be a problem.
Phytoplankton, an essential component of shellfish diet, depend upon the availability of inorganic nutrients as well as radiant energy for photosynthesis. Run-off from upstream sources, the addition of point-source pollutants, narrowing of embayments, and reduced water flow all impact upon these organisms. Excessive artificial enrichment (i.e. introduction of partially treated or untreated sewage or other fertilizing agents) may result in eutrophication, which degrades food sources and reduces viable shellfish habitat. The addition of nitrogenous compounds can stimulate the growth of several species of planktonic algae undesirable for shellfish food, which compete with desirable food species. In high concentrations, these algal cells can shade out natural stands of benthic plants such as eel-grass and, as they die, contribute to the accumulation of organic matter on the bottom. As this material decomposes, the oxygen concentration in the bottom waters drops. Alternatively, bottom vegetation may be stimulated by excessive nutrients to grow in such dense concentration that tidal circulation near the bottom is reduced or eliminated, thereby inhibiting the growth of bivalves.
Despite the enormous reproductive potential of shellfish, many factors contribute to a very high mortality rate during their life cycle, most particularly during the larval and juvenile stages. Shellfish aquaculture techniques are designed to reduce these losses, by protecting shellfish at early, vulnerable stages from predation and other adverse natural phenomena as long as economically feasible.
The common hard shell clam, or quahog, is well adapted to life in the sea, particularly the sand and mud flats of the subtidal and lower intertidal zone. The northern quahog, Mercenaria mercenaria, belongs to the class Bivalvia, easily identifiable by two, somewhat rounded, hinged shells, protruding burrowing foot, and the purple or dark blue border found on the inside of the shell. The variety known as M. Mercenaria notata is widely grown in Massachusetts. It is distinguished by a chestnut brown zig-zag line of the outside of the shell. Some cultivators prefer this variant because of its faster growth rate and natural identifiability. The name Mercenaria comes from the historic use of the shell for making Indian money, or wampum. Beads made from the purple part of the quahog shell were the most valuable form of wampum.
The quahog spends most of its life (which can last for up to 20 years) buried into the sediments of the subtidal and lower intertidal zone, with its two siphons reaching just above the surface to feed and discharge wastes. It feeds by filtering phytoplankton from water that it pulls in over its gills with one siphon and then pumps it back out through the other. Quahogs reproduce in the same manner as most bivalves, by shooting vast quantities of sperm and eggs into the water. Here the eggs are fertilized and dispersed by the currents. Hard clams spawn when the water temperature reaches approximately 60 degrees F. Of the millions of eggs shot out by female clams, only a small percentage survive to maturity in an uncontrolled environment.
Quahogs typically grow up to four inches and sometimes larger. Commercially, there are three names, based on size; the littleneck (48mm valve length or 1.5 inches), the cherrystone (60mm valve length or 2 inches), and the chowder ( greater than 75mm or 3 inches or more). Some individuals can attain sizes of up to 130mm or 5 inches.
Quahogs are the preferred food of several predator species, notably green crabs, starfish, moon snails, and horseshoe crabs.
Two kinds of oysters are cultured in Massachusetts, the Eastern, or American oyster, Crassostrea virginica, and the European oyster, Ostrea edulis. Oysters, like other epifauna, live on the sediment surface. The larvae metamorphisize and search out hard clean surfaces (cultch), which may include live oysters, to which they cement themselves. Recently-set oysters are known as spat. Setting oysters, depending upon conditions, may result in heavy local concentrations in the form of bars or reefs, where competition for space is fierce. Once attached, an oyster never moves again, unless dislodged by external forces.
Oyster shells, or valves, differ in size and weight, with the right valve (the top valve) forming flat and the left (bottom valve) being heavier and cupped. The valves are joined by a ligament and the shell shape can vary widely. The size and shape of an oyster's shell is significant in aquaculture; the raw market demands a full, shapely, aesthetically-appealing shell. Shell growth largely depends upon water temperatures and culturing times will vary according to the size to be attained. For example, in Massachusetts, it can take two - four years to attain market size while in Florida, where oysters are grown year round, eighteen months is usually sufficient.
American oysters are divided into separate male and female individuals and spawn by spewing millions of eggs and sperm into surrounding waters, where external fertilization takes place. European oysters are hermaphroditic; each individual possesses male and female germ cells, and fertilization occurs inside the inhalant chamber of the oyster. The larvae then incubate in the chamber for up to ten days prior to being ejected into the surrounding waters. Immediately before setting, both types of oysters develop eye spots and a foot and begin to search out cultch to attach to. Satisfactory cultch includes any clean, hard surface (usually empty bivalve shells).
The color, texture, and quantity of the meat in the shell is very important to the raw oyster market and those qualities will vary with age, water quality, and time of harvest. Oysters will accumulate and/or metabolize almost every element contained in the water around them. The quality of the meat drops during the spawning season. After spawning, oysters build up glycogen, which provides the appearance of a "fattening" or "fat" oyster, the most desirable kind.
Oysters are subject to a variety of natural predators, an environmental hazard that may be substantially reduced by cultivation and predator-control methods. Depending upon salinity, Atlantic oyster drills, thick-lipped oyster drills, rock shells, knobbed whelks, channeled whelks, and starfish all prey on oysters.
Oysters are also subject to a variety of diseases and parasites, the most recently notorious being MSX, discussed infra. More recently, Juvenile Oyster Disease (JOD) has been blamed for some oyster mortalities in the Northeast and mid-Atlantic aquaculture facilities. The severity of impacts by disease and parasites on oyster populations is thought to be related to water quality; higher salinity, high temperatures, and nutrient loading appear to make oysters more susceptible to disease.
Oysters provide a welcome habitat for commensal and competing organisms. Several live in or on the shell; the boring sponge, the boring clam, and the mud worm are examples. Barnacles, tunicates, and mussels (as well as other oysters) will attach themselves to the outside of the shell, as will as several species of algae. One algal species, Codium fragile, produces aveoliated branches filled with gas that may actually lift the oyster and carry it off with the tide.
Bay scallops, Aequipecten irradians, differ from clams and oysters in that they retain the ability to move actively throughout their lives. The juveniles will attach themselves to underwater eel-grass or algae through extruded filaments known as byssal threads. The Eelgrass offers protective isolation from many predators crucial during early life stages, and then settle-out within or adjacent to the vegetation. Their ability to move and remain above the sediment as they mature makes them less prone to bottom predation. Bay scallops eventually lose the tendency to attach themselves; they dwell on the bottom as adults, manipulating themselves through "jet propulsion" by forcefully closing their valves. Scallops less than one year old (determined by the annual growth ring) are considered seed and may not be harvested.
The bay scallop, as with other shellfish, has two shells, or valves, with the "laying" shell (the one the scallop rests on)being lighter in color and having a byssal notch or foot groove. The outer surface of a scallop shell exhibits prominent ridges and furrows, which radiate from the beak to the free margin. During the winter months, the scallop does not grow and a heavy concentric line, similar to that of a tree's annual ring, forms. Growth begins again in the spring and it is this ridge that determines the age of a scallop.
Scallops, unlike quahogs and soft clams, have only one adductor muscle, commonly referred to as the "eye." Bay scallops are hermaphroditic; all individuals possess male and female sex organs and produce both sperm and eggs.
Bay scallops spawn in response to increases in water temperature in the spring, beginning at approximately 61 degrees F., although spawning may be induced artificially in the hatchery by increasing water temperatures; 68 - 84 degrees is the spawning range, with 74 - 76 degrees inducing spawning within fifteen minutes after placement. Bay scallops have the capacity to spawn repeatedly over a period of weeks, depending upon external conditions.
Scallop larvae go through two distinct developmental stages, during which the mortality rate is at the highest point in the life cycle. The embryonic, or sub-veliger stage is the period prior to development of a shell; the post-embryonic, or veliger stage is the period during which the larvae have developed shells but prior to attainment of full development. During this pre-adult stage, the larvae are capable of both digging and swimming.
Once the scallops "set" by way of attachment to an available surface with byssal threads, they have reached the dissoconch ("two shelled") stage and are approximately 1/3 " in size. The scallop does not reach its full internal and shell development until the plicated stage, where the radiating ridges characteristic of the adult scallop develop. Bay scallops have a much shorter life-span than do oysters, clams, or mussels. Most bay scallops do not live beyond two years and they may reach sexual maturity within six months. They also retain their free-swimming (through jet-propulsion, expulsion of water through the "ears" coupled with shell closure) and locomotive abilities, which mandates that cultivation be off-bottom, in enclosed nets, bags, or cages.
Bay scallops, like other shellfish, are filter feeders. Unlike clams, who feed from below the substrate, scallops use their gills to siphon diatoms and other planktonic matter from the surrounding water.
Scallops are not as likely to act as a vector for disease as other shellfish because the adductor muscle is the most common food product. Pathogens are typically transferred through the food chain as the result of eating the whole shellfish, including the gut, where pathogens typically accumulate. However, consumption of whole and roe-on scallops has increased dramatically over the last two years, leading to increased risk of food product contamination.
Mussels also initially fasten themselves by byssal threads to substrate or vegetation at metamorphosis. Like scallops, recent evidence has revealed a close relationship between blue mussels and eelgrass during early life stages. Unlike scallops, maturing mussels reattach themselves to a hard substrate, often other mussels, continuously for the remainder of their lives. If stressed, matured mussels can detach themselves and subsequently re-attach their byssal threads after finding a more desirable location.
All stages of mussel development appear to be influenced by water temperature. Mussels will spawn at temperatures between 62- 80 degrees F., but will spawn almost continually when the temperatures remain in the 75 degree vicinity. The larvae are free-swimming, and may attach several times by means of byssal threads until a desirable location (one that is abundant in food and maintains water temperatures above 65 degrees seem to be favored) is located. Mussels grow most quickly when continually immersed in water since they can almost constantly feed.
Once permanently set, the competition for space is vigorous. It is common to find mussels growing in layers of other mussels, both naturally in intertidal and sub-littoral levels, and when cultured on lines. Mussels grown in water temperatures above 65 degrees mature more quickly than those grown in colder waters, maturing in less than a year. Below that range, mussels may take up to two years to reach market size.
Bottom Culture - Overview
The practice of cultivating bottom-dwelling shellfish (most commonly hard shell clams and oysters) in nursery trays and pens from juvenile stages to maturity is collectively known as bottom culture. Since at least 1974, intensive quahog culture has been ongoing in Barnstable (62 acres), Provincetown (45 acres), Wareham (90 acres), and Wellfleet (80 acres), utilizing bottom culture on the intertidal flats.
Oysters and Quahogs are commonly cultured using bottom culture techniques.
The practice is conducted in three stages:
1. Production of Seed - very small clams (1 - 25 mm) are obtained from hatcheries (or other sources, including natural collection) where clams are cultured from initial spawning through the larval metamorphosis to the juvenile stage. Once juveniles reach the size of 2mm, they can be marketed as "seed," although the preference is for seed greater than 8 mm, which better resist predation.
2. Field Planting - the seed are "planted" in net-covered boxes measuring approximately 4 feet x 8 feet x 6 inches, that are filled with "clean" sand (sand that does not contain predator species; i.e. green crabs), and are slightly elevated above the intertidal flats on legs. These boxes, known as "nursery boxes" or "cages", may contain up to 10,000 seed clams. Smaller, more manageable boxes sized 4'x 4' are being used more commonly.
3. Grow Out - the seed are allowed to remain in the nursery trays until a size of 19 - 24mm, although all clams between 19 and 176 mm are considered field plant size by the industry. At this point, they are transferred to narrow, net-covered plots ("pens," or sometimes "runways") for grow out. When the clams reach 51 - 63mm (2 - 2.5 inches) they are harvested.
There are approximately 646.5 acres of tidelands currently licensed for shellfish cultivation (commercial and research) in twenty-two Massachusetts cities and towns (Barnstable, Brewster, Chatham, Dennis, Duxbury, Eastham, Edgartown, Essex, Fairhaven, Falmouth, Gosnold, Mashpee, Mattapoisett, Nantucket, Oak Bluffs, Orleans, Plymouth, Provincetown, Truro, Wareham, Wellfleet, and Yarmouth). Cultivated species include, in order of economic importance, (1) quahogs, (2) American oysters, (3) bay scallops, (4) soft shell clams, (5) European oysters, (6) surf clams, and (7) blue mussels.
Due in large part to the extent of privately held tidelands in the state, much of the shellfish culture in Massachusetts is conducted on private property. Often the culturist is not the owner of the flats under cultivation. The recent Massachusetts Supreme Court decision on Pazolt vs. Massachusetts has ruled that culturists must obtain permission from the owners of privately owned tidelands prior to establishing aquaculture facilities. For a discussion of the problems associated with the public use of private property, see Legal/Regulatory.
The cost of hatchery seed (6-8 mm) varies from $25 to $35/100. Gross sales of $25,000 -$35,000/acre have been reported at harvest of mature clams, 2-3 years after field planting. However, severe winters and intense cyclical predation can significantly reduce sales and prolong the maturation period.
According to the reports submitted by municipal shellfish constables, the landed value of cultured shellfish totaled $620,000 in 1988, $1,933,000 in 1989, $906,000 in 1990, and $1,884,000 in 1991. These values are questionable at best. Not all towns with acreage under active cultivation submitted reports and chronic under-reporting is an acknowledged problem. The consensus opinion of the Massachusetts Aquaculture Association and the Massachusetts Shellfish Officers' Association is that the reporting value represents approximately 30-40% of the actual landed value.
The successful growth and harvesting of food-quality shellfish requires high water quality, quality that is very vulnerable to the effects of competing coastal uses.
Bottom-dwelling shellfish are filter-feeders, obtaining nutrients (phytoplankton, heterotrophic micro-organisms, minerals and dissolved organic particles) by straining them from the surrounding water when ambient temperatures are above 10 degrees C. Conditions that affect either the availability of food (i.e. toxic blooms of certain flagellates, "brown" or "red" tides that repress certain diatom populations) or the quality of the water (i.e. excessive turbidity) affects the set of spat and the growth of seed.
Shellfish are capable of filtering, along with their traditional nutrients, various pollutants and wastes from the surrounding water and substrate. Coastal development and recreational uses supply these pollutants in abundance, either through direct (point-source) discharges or from run-off (non-point source) discharges. The chronic discharge of pollutants systematically degrades the quality of the water. The results of such degradation may vary from death, mutanogenic or teratogenic effects upon shellfish, and sickness and/or death of consumers further up the food chain.
Degradation of water quality (and associated substrate degradation) includes characteristics such as lower dissolved oxygen levels, changes in temperature, changes in salinity, and changes in turbidity. All of these parameters have a direct bearing upon the quality and quantity of shellfish propagated and harvested.
Maintaining and upgrading water quality presents a host of problems. Coastal development, industrial, and recreational use are the primary causes of near-shore water quality degradation. A balance of the competing uses is extremely difficult to achieve and is still evolving.
Culture on tidal flats requires access to work areas that may not be accessible by boat. Off Road Vehicles (ORV) traffic over dunes, coastal banks, tidal flats, and through sensitive resource areas could place endangered species and fragile ecosystems at risk. Use of the intertidal area for bottom culture also raises concern over the potential loss of resting and feeding areas for migratory birds. Some environmentalists see the use of nursery trays, netting, and pens on the flats as developing a "monoculture" that threatens the intertidal ecosystem.
Bottom-Culture - Problems
The following are both real and perceived problems associated with bottom culture techniques:
Shellfishing, both recreational and commercial, is viewed as a Massachusetts' birthright. The notion of "domesticating" it into what is viewed as a form of rather unaesthetic farming, is not popular; it is inconsistent with the way Massachusetts has traditionally perceived itself and its natural resources. Despite some promise as a beneficial alternative to traditional commercial fishing, it also is perceived as a source of competition with wild fisheries (although the limited data available demonstrate little basis for such concern) for the same market.
The reservation of tidelands for shellfish licensed-areas and aquaculture licenses should not, in theory, limit areas used by commercial fishermen engaged in gathering wild stock. The fear of this happening exists. The opposite is true in some areas of the United States where wild stock represents a strong market resource and significant political pressure is employed to limit aquacultural endeavors; i.e. Alaskan salmon fishery. A related conflict is the contamination of wild stocks through the importation of seed from other areas providing a risk to native populations through disease introduction and gene pool homogeneity.
The fear of competition has a long history in the Northeast. Within the last century, violence has erupted over the existence and placement of shellfish licensed-areas. Maine, Massachusetts, New York, and Connecticut have all experienced their own versions of the "Quahog Wars", violent clashes between culturists and commercial fishermen resulting in shootings, boat-burnings, and the seizure and/or destruction of product on both sides of the argument.
The New England states have resolved many of these conflicts in a number of ways. The states of Maine and Connecticut have established procedures designed to limit aquaculture to areas that do not intrude upon the wild fishery. Massachusetts, who has, for the most part, delegated the licensed-area procedure to local authorities, requires that any area proposed for a shellfish licensed-area be a non-productive tideland; a shellfish desert, and some towns have created shellfish management plans that prohibit the siting of licensed-areas in any area where a potential conflict over access to wharves, moorings, or productive bottom may develop. Rhode Island, with its insistence upon a very old and very literal interpretation of the "free and open fishery," has forced most aquacultural activities out of the state.
Misconceptions exist about bottom culture providing a livelihood or really being a viable business endeavor in Massachusetts, fueled to some extent by the failure on the part of many aquaculturists to conduct their activities in a consistent, businesslike manner. It has been reported to Massachusetts aquaculturists and to various state agencies that coastal aquaculture should not be viewed as a real form of sustainable fishing nor as an industry that justifies or deserves the assistance available to many other small businesses, funded through taxpayer dollars and assisted by state and federal programs and subsidies. The perception exists and persists that the people engaging in bottom culture do it as a hobby, or to pick up a few extra dollars "under the table" or to supplement their "real" jobs. For many serious bottom culturists, this is simply not the case. Given the productivity that can be achieved with a carefully managed licensed-area, bottom culture is engaged in as a serious, full-time occupation.
The nature of the work involved to sustain a profitable licensed-area is not something most people would do to "moonlight." Even those considered part-time aquaculturists typically spend at least a portion of each day on the licensed-areas. For intertidal licensed-areas, all work has to be done on ebb tides, which vary from day-to-day. It means going out to secure the licensed-area in foul weather. During harvest season, it means harvesting every day, no matter what the conditions may be. The work is manual, extremely labor-intensive, and technical assistance and tools are very limited. Immediate responses to emergencies: floods, storms, etc., are necessary if a year to two years' worth of work and investment is going to be saved. Bottom aquaculture is not, nor should it be considered, a hobby.
Although the work and investment risk are substantial, the returns may be significant, as pointed out elsewhere in this report. However, it is acknowledged by nearly everyone involved with coastal aquaculture: shellfish officers, harbormasters, Division of Marine Fisheries personnel, and several aquaculture trade organizations, that bottom culturists do not, by and large, conduct their operations in a professional manner. Record-keeping is poor and in most cases, insufficient to establish a clear financial picture. Reporting of harvest quantities and prices received is chronically and severely skewered on the low end; average annual reports, required by Massachusetts General Law Chapter 130, contain amounts and prices that are just sufficient for a licensed-area-holder to maintain his/her right to the licensed-area. The under-reporting problem is so generally known that the Division of Marine Fisheries assumes that actual quantities of some species of shellfish harvested and prices obtained are as much as 40 - 60% higher than what is reported.
There has traditionally been little public interest or support for programs that encourage aquacultural development over other uses of highly desirable coastal property. Banks and other lending institutions find little justification for creating small business loan packages for aquaculturists. State and federal government agencies are reluctant to devote thin resources to endeavors which do not appear to provide an adequate return for the investment. In addition, the risk of losing the harvest to disease, predation, destruction by storms, algal blooms, theft or other causes leads to a perception that aquaculture is a high-risk investment, causing banks and lending institutions to proceed carefully.
Shellfish have, despite public education and awareness campaign efforts by the industry and the state, a mixed reputation for wholesomeness. Shellfish have in some instances been linked with disease transmission to humans, causing many consumers to avoid any form of shellfish. Aquaculture and public health are discussed in greater detail in Seafood Safety/Public Health.
Water Column (or Off-Bottom) Culture - Overview
The use of water column suspension techniques, including enclosures (cages, lantern nets) and lines (longlines and strings attached to rafts or racks anchored on the bottom) for the production of bottom-dwelling and sedentary shellfish, as well as for motile species of shellfish (i.e. scallops) is commonly referred to as water column or off-bottom culture. These techniques are designed to minimize bottom predators, contain mobile species (i.e.scallops) in one place until harvest, and maximize the use of a three-dimensional space for cultivation.
Oysters have been cultivated at least since Roman times, by growing them off of the bottom on rafts and strings. Today, oysters are grown all over the world, using a variety of methods, predominantly off-bottom. In Japan, one of the largest oyster producers, cultivation methods include rafts, lantern nets, and longlines. In Australia, another large oyster-producer, sticks and trays are used. In France and England, oysters are cultivated on off-bottom posts, in mesh nets, and on longlines. Off-bottom cultivation has uniformly produced (1) more oysters than in the same area on the bottom, (2) accelerated growth rates, (3) improvement in meat quality, and (4) significant reductions in predation.
Two different kinds of oysters are cultured in Massachusetts, using off-bottom and bottom methods: the Eastern, or American oyster, Crassostrea virginica, and the European oyster, Ostrea edulis. In Massachusetts, there are currently seventy-eight oyster cultivation licensed-areas, representing both bottom and off-bottom culture in seventeen cities and towns (Barnstable, Chatham, Eastham, Edgartown, Essex, Falmouth, Gosnold, Mashpee, Mattapoisett, Nantucket, Orleans, Plymouth, Provincetown, Truro, Wareham, Wellfleet, and Yarmouth).
Mussels are one of the easiest species of shellfish to grow. Although extremely popular in Europe, on the U.S. West Coast, and in Maine, mussel culture is limited in Massachusetts. There had been as many as eleven blue mussel cultivators in operation in Massachusetts during the "mussel boom" years of the late 1980's, but there is very little, if any commercial production of mussels today. Recent advances in cleaning and grading wild harvest mussels has resulted in a drop in price for the cultured mussels which had previously been marketed as a superior product (Brooks, 1993).
The blue mussel, Mytilus edulis, is simple to culture off-bottom, due to its prodigious reproductive capabilities. The spat is abundant and sets easily on longlines and ropes, precluding the need for hatchery support. Mussel seed in Massachusetts comes entirely from natural populations. Empty mussel strings are placed in the water in June and July (depending upon temperature) to catch seed. Following collection of the seed, the strings may either be left in place or transferred to another growth area. The mussels are grown for 6 - 10 months until reaching harvest size and may remain on the strings for almost two years until the last of a given set is harvested. It is necessary throughout the culture period to lift the strings periodically to remove fouling organisms and to thin the growing mussels.
Cultured mussels produce a shell that is much thinner and flexible than that of a naturally occurring organism, creating some unique harvest problems. Layers of mussels have to be removed from the lines carefully, to avoid damaging the shellfish. Although no one answer can account for the difference in shell structure, the most popular theory has it that mussels react to intertidal pressures and wave action by growing much more rigid, thicker shells than do those suspended in the water column.
Mussels cultured off-bottom do not experience the predation problems of bottom-dwelling mussels, who are a favorite food for many varieties of starfish. Off-bottom culture has been the target of a commensal organism; the pea crab, who is found to co-exist in the shell of mature, cultured mussels. Cultivators have taken two approaches to the pea crab; it is either marketed as a desirable companion product, or it is destroyed by adding extra carbon dioxide to the water around the culture strings.
Although not imported for aquacultural purposes, the zebra mussel provides a interesting lesson on the potential risks associated with farming exotic species. The zebra mussel, Dreissena polymorpha, reached the Great Lakes region inadvertently in the bilge water of large ships from Asia. In the Great Lakes, the mussels breed prodigiously and have no natural enemies to keep their populations in check. This mussel has become so prolific that routine maintenance must be employed to clear intake ducts, sewer pipes, drains, and boats.
The bay scallop, Argopecten irradians, differs from the soft clam, the quahog, the oyster, and the mussel in several respects of significance to the aquaculturist. It has more rapid growth, a shorter life-span, and is less likely to transmit disease to human consumers if taken from contaminated areas. It is also harder to adapt to cultivation.
Scallops are subject to attack from several predatory bottom species, notably the starfish and the oyster drill, again making off-bottom techniques the culture of choice for scallops.
Lantern Culture utilizes a cylindrical container fashioned from nylon netting, divided into sections and hung from floats and is used in Massachusetts to culture bay scallops and oysters. As juveniles metamorphosize and attach themselves to a substrate, they are maintained in a hatchery or other natural environment until they attain approximately 3 millimeters in size. At that point, they are transferred to natural growing areas, equipped with fine mesh nets suspended from rafts on the surface and anchored to the bottom. This method utilizes the three dimensional growing space for continuous feeding and limits predation.
Raft culture utilizes strings suspended from a surface raft which is anchored to the bottom. In many places, suspended culture from rafts or longlines has proven to yield the greatest production per unit area of the possible culture methods. Mussels and oysters may be cultured in this manner. In Spain, where mussels are grown suspended from rafts, a maximum production of 500 metric tons per hectare (250 tons per acre) has been achieved. This figure is 1,000 times greater than any other form of aquaculture in which animals are grown without supplemental feeding. Currently there are no raft culture operations in Massachusetts.
Rafts may measure up to approximately 30 x 300 feet, with 180 - 270 ft. longlines, The mussel strings are suspended from the rafts and extend about 21 - 30 feet below the surface. Up to 400 strings may be hung from a single raft or longline and the raft can support up to 9 metric tons of mussels.
The term "rack" may be used in several ways. Depending upon what is meant by the word, alternative culture methods are available.
A rack for the growing of scallops and oysters is a device constructed from wood, wood and plastic, and/or various hybrids of non-corroding metals and plastics, that resembles a bureau, complete with pull-out drawers, suspended several feet off of the bottom. Seed shellfish are set out in the drawers and allowed to mature in the water column, safe from bottom-crawling predators.
A rack may also be used in concert with longlines and/or string and lantern culture. When used in this way, a rack is nothing more than a hollow square or rectangular frame, resting on the bottom as an anchor to provide stability and separation for the lines or lanterns suspended in the water column.
There are approximately twenty-eight licensed off-bottom aquaculture operations on 226.5 acres of surface water and water column off of fourteen Massachusetts cities and towns (Barnstable, Chatham, Dennis, Eastham, Edgartown, Essex, Fairhaven, Gosnold, Mashpee, Orleans, Plymouth, Provincetown, Wareham, and Yarmouth). Few of these licensees are operational. Cultivated species include American and European oysters, bay scallops, and blue mussels.
The authority to regulate and maintain near shore areas has traditionally been local; harbormasters and shellfish officers routinely allocate mooring space, issue permission for temporary uses, assign public marina slots, and oversee shellfish licensed-areas.
Water column aquaculture requires greater regulatory involvement than bottom culture generally due to the exclusionary nature of this technique. Off-bottom culture may require a Massachusetts General Law Chapter 91 waterways license, issued by the Department of Environmental Protection or a Chapter 91 permit issued by the municipal harbormaster, because it uses "navigable" area: the water column and/or the surface, as well as the bottom and may conflict with navigation or other uses. Off-bottom culture also requires a license authorized through Massachusetts General Law Chapter 130, Section 57. Local authorities (selectmen, mayor) issue the Section 57 licenses and to some extent, they intrude upon the traditional authority granted to harbormasters to assign moorings and designate areas for scuba diving. Which use takes precedence, how the uses should be distinguished, how the uses may accommodate each other, have the potential for controversy and the procedures employed in the licensing of the various uses are a subject for further legal scrutiny and clarification. Coordination efforts between state and local entities concerning information exchange and reporting requirements also need to be reviewed.
Off-bottom culture carries with it its own set of demanding water quality requirements, as well as its own effects upon surrounding water, microfauna, macrofauna, submerged vegetation, and sediments.
Phytoplankton: Mussels and oysters feed by filtering suspended particulate matter, both plankton and detritus; the biomass of phytoplankton downcurrent of mollusk culture therefore may be expected to be reduced. Mollusks are effective in removing particulate matter. In one study, particulate concentrations were measured after passage through oyster rafts, each supporting 50,000 - 90,000 oysters. The particulate concentration was reduced 76-95% after passage through 11 rafts (although some of this may be attributed to passive settling rather than removal by the oysters.)
There is also anecdotal evidence of the filtering efficiency of mussels. Growers have found that mussels on the upcurrent side of a raft grow faster than those on the downward side, presumably because of the reduction in food concentration as the water passes through the raft. Divers have reported the water clarity in the midst of mussel strings to be substantially improved over conditions beyond the perimeter of the raft, presumably because the mussels have reduced the concentration of suspended particulates.
Waste Loading: Shellfish culture is associated with nutrient-loading, but nutrition is obtained from indigenous phytoplankton and particulate matter, rather than from any external food source. Thus, the culture operation does not introduce any "new" nutrients into the marine environment, but promotes the recycling of those which are already present. There is actually a net decrease in nutrient levels in the system, since about only 40% of the total nutrients removed by mollusk culture is released directly to the water column; 30% fall to the bottom as particulate material and 30% are removed in the harvest.
Shellfish generate solid waste in the form of feces and pseudofeces. Shells which fall from the culture structure also accumulate on the bottom immediately under the raft or longline. Studies of oyster culture in Japan indicate that the amount of solid waste produced by shellfish culture can be considerable. A raft of oysters in Hiroshima Bay holds 350,000 - 630,000 oysters. During a nine month culture season a single raft will produce 16 metric tons of feces and pseudofeces, with an additional 4.5 tons attributable to feces of fouling organisms growing on the rafts. Approximately 20 - 30% of this material is deposited on the bottom. Additionally, the shading effects of a large raft over an eelgrass bed could lead to considerable degradation.
As noted earlier, off-bottom culture has the potential to provide some of the highest yield figures for shellfish culture, particularly if some of the other problems associated with the growth of the off-bottom industry can be adequately resolved. The actual dollars generated by off-bottom culture are not precise; current figures do not distinguish between culture practices and many figures do not distinguish between cultured product and wild stock.
Most of the traditional oyster grounds in Massachusetts have now either been closed due to pollution or have been harvested to the point of extinction. Significant natural sets still occur in some of the salt ponds on Martha's Vineyard and in the Towns of Wellfleet and Wareham, but much of the current production is aquaculturally-derived, utilizing both naturally produced and hatchery seed. The most favorable growing areas and licensed-areas are found in the waters around lower Cape Cod; virtually no oysters are harvested north of Cape Cod.
Catch statistics from 1889 to the present indicate that oyster production in Massachusetts has steadily declined since the turn of the century. Production fell during the 1950s and has remained relatively constant since that time. The sharp decline in the 1950s occurred during a period of set failures in Long Island Sound that prevented Massachusetts growers from obtaining seed. The brief resurgence in the industry in the 1980s resulted from the increase in the availability of seed from Connecticut in the 1970's.
The importation of seed had to be discontinued in the mid-1980's after the identification of the oyster pathogen - Haplosporidium nelsoni, commonly known as MSX Disease, among the imported oysters. Recently, oysters selectively bred for resistance to MSX at Rutgers University have shown encouraging results and offer considerable hope to the industry. More recently, however, oyster growers have suffered heavy losses among juvenile oysters from a cause other than MSX, presumed to be a pathogen, but as yet unidentified. These recent mortalities, which produced heavy losses in 1990-1991, occurred among hatchery-reared seed in Massachusetts, New York, Rhode Island, and Connecticut.
Reported total landings on Cape Cod in 1989 were approximately 7,000 bushels, of which more than half were cultured on private growing grounds. In 1990, the wholesale price for oysters was slightly over $10.00 per pound (shucked), or more than twice that for quahogs.
Massachusetts has always been a major producer of bay scallops, although predominantly from the harvest of wild, rather than cultured, stock. This species rarely occurs in commercial quantities north of Cape Cod, exceptions being Plymouth and Duxbury Bays, but the comparatively warm water of the numerous and shallow coves and salt ponds of southern Massachusetts have provided an ideal habitat in the past. For many coastal communities, the bay scallop fishery has produced a steady source of income and has been an important contribution to the economy during the winter.
The price for bay scallops varies greatly from year to year, varying indirectly with abundance. In 1990, which was a lean year, the ex-vessel price was $6.61 per pound of shucked meats. In more productive years, the price may be considerably less. A consistent characteristic of the bay scallop fishery has been fluctuations in abundance from year to year, with relatively productive years often immediately followed by general scarcity. The annual bay scallop harvests in Massachusetts near the turn of the century were not much larger than in good years today. However, during the 1970s and 1980s, bay scallops virtually disappeared from many areas in which they were quite abundant, and good years occurred infrequently. It is suspected that the disappearance of the bay scallop in some areas may be linked to increasing boat traffic, as larger amounts of hydrocarbons - a toxin to this species - are released into the water. Some research has linked the loss of eelgrass and eutrophication of coastal waters to the decline in bay scallop productivity through the reduction in preferred scallop habitat. A blight in the 1930's which desroyed much of the eelgrasses on the Atlantic Coast resulted in the total collapse of the bay scallop fishery. Today, a variety of non-point source pollutants have impacted the abundance of eelgrass and therefore poses a threat to bay scallops.
In the past, only the adductor muscle of the bay scallop has been marketed for consumption. Pathogens do not accumulate in the adductor muscle. Therefore, scallops may be taken from areas closed to the harvest of other bivalves if only the adductor muscle is to be marketed. However, increases in whole and roe-on scallop consumption has increased, and these scallops must come from approved areas. The legal proscriptions against the harvesting of scallops less than one year old has provided a practical means for protecting the natural stock. Bay scallops, however, are highly sensitive - particularly in the embryonic and larval stages - to a variety of chemicals frequently associated with industrial and domestic wastes and the adults cannot survive in areas of heavy siltation or reduced oxygen conditions.
Although the bay scallop is readily cultured through its early development in hatcheries, to date it has proven impractical to rear to maturity in large numbers in the wild due to its mobility, as well as its vulnerability to predators. This is where aquacultural techniques such as lantern nets have the potential to become so valuable.
Off-Bottom Culture - Problems
The following are problems, both real and perceived which are associated with Off-Bottom Culture.
Water column culture suffers from the same lack of common understanding afflicting bottom culture. The cultural identity of Massachusetts in its wild shellfish industry is at odds with the notion of a domestic shellfishery, particularly one that intrudes upon traditional shore and near-shore activities. Off-bottom culture also intrudes upon the water surface and near-shore bottom, affecting mooring availability, recreational boating, navigation, and commercial fishing activities.
Due to the exclusionary nature of this aquaculture technique, other activities (including navigation) in the area of a water column aquaculture facility are displaced. Siting of these facilities is therefore subject to considerable controversy from other users.
There have been reports of marine mammals and reptiles, including endangered species, becoming entangled in off-bottom facilities particularly in Cape Cod Bay. Measures need to be taken to reduce such interactions.
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Published: September 1995