|
Article Excerpt
Introduction
The major harvesting area for bay scallops, Argopecten irradians irradians and Argopecten irradians concentricus, in eastern North America has been coastal bays from Massachusetts through Long Island, N.Y., and in North Carolina (Fig. 1). From the 1870's to the mid 1980's, the fishery was important economically and culturally to communities on the shores of southern Cape Cod, Buzzards Bay, and the islands of Martha's Vineyard and Nantucket in Massachusetts; Peconic Bay in eastern Long Island, N.Y.; and Bogue and Core Sounds, N.C. At times in the past, smaller bay scallop fisheries existed in Rhode Island, Connecticut, New Jersey, and Virginia.
In the years of large landings, the fishery provided local communities with considerable economic vitality in the fall and winter when business activity was often otherwise slow. Bay scallop landings over this range averaged nearly 300,000 bushels of live scallops/year from 1950 to 1985, but only about 40,000 bu/yr from 1986 to 2005 because the abundance of scallops declined sharply after 1985. A bushel of scallops in Massachusetts and New York contains about 350 scallops that yields about 6 pounds of meats (adductor muscles). A bushel of scallops in North Carolina contains about 250 scallops that yields about 5 pounds of meats.
This paper includes a review of the biology, autecology, and habitats of the bay scallops, Argopecten irradians irradians and A. i. concentricus, a description of scalloping gear and uses, the characteristics of the fishery including marketing, a history of each state's fishery and landings, the environmental changes in the habitats during and since the 1980's, and attempts made to restore the fisheries and landings. A description of the habitats provides some details on how the recent rises in temperature and pollution are affecting aquatic environments. Besides bay scallops and other commercial bivalves, the habitats are nursery and feeding areas for coastal and oceanic fishes besides a wide variety of other biota. Most biological and ecological information described relates to A. i. irradians from Massachusetts to New York. A substantial amount of the information reported here was collected from personal observations and from interviews of long-time bay scallop fishermen, town shellfish officers, and state shellfish resource management personnel. The names of the principal informants are listed in the footnotes. The landings data are assumed to be a rough reflection of the abundance of bay scallops in each locality, because the fishermen have consistently harvested nearly all the older year class of scallops in bays each year throughout the history of the fishery.
Biology and Ecology
Bay scallops are distributed widely over the bottoms of bays and harbors. Adult scallops are spaced randomly at least 2-6 in (5-15 cm) and usually much further apart from one another. They lie flat, unless lodged among dense grasses, usually eelgrass blades. Adults do not attach to one another or to any substrates. Individuals are oriented in different directions, rather than perhaps toward or against the water current, and each commonly lies in a slight depression, perhaps 0.3 in (1 cm) deep in the sand sediment. The scallops can be concentrated in shallow channels, but they are absent on both intertidal and extremely shallow sand bars.
The following description of the biology of the bay scallop is quoted almost entirely from the outstanding report on the bay scallop of Massachusetts written by D. L. Belding and published in 1910. The report contains a description of its shell and anatomy of its soft parts, mode of reproduction, feeding, and growth in the region from Massachusetts to Long Island, N.Y. It will not be otherwise cited in this section; only additional material will be cited. Descriptions of the biology and ecology of A. i. concentricus are also included.
[FIGURE 1 OMITTED]
Shell and Growth Line
The bay scallop has two valves (shells) that are nearly round. They are joined on a straight hinge line by a thin ligament. The lower valve on which the scallop rests on the bottom has a paler color, is more convex, and also differs from the upper in having a byssal notch. The valves are slightly wider than long. The northern bay scallop, Argopecten irradians irradians, is slightly smaller and is lighter in weight than the southern scallop, Argopecten irradians concentricus.
The average dimensions of a fully grown adult northern scallop (from Martha's Vineyard) are: length, 2.4 in (6 cm); width, 2.5 in (6.6 cm); thickness of whole scallops, 1.1 in (2.8 cm). The average weight of an individual valve is 11 grams. The average dimensions of a fully grown adult southern scallop (from Bogue Sound, N.C.) are: length, 2.7 in (7.2 cm); width, 2.9 in (7.6 cm); thickness of whole scallop, 1.5 in (3.7 cm); the average weight of a valve is 21.3 grams (Fig. 2). The valve hinge lines of both are straight to the end of well-developed "ears."
The outer surface of the shell of A. i. irradians has prominent ridges and furrows that radiate from the beak to the margin. Not counting those that extend onto the "ears," they number from 14 to 19 in different scallops, young and old, the average being 16. The furrows fit closely together at the margin when the valves close. Crossing the radiating ridges are thin concentric growth lines. Growth ceases during winter, and when it resumes again in spring a distinct growth line is formed by the thickened edge of the shell (Fig. 3). The presence of this growth line defines an adult scallop, according to regulations in the states. The scallop then is about 10 months old. The location of this line usually is 1.15-1.5 in (30-40 mm) from the hinge, but it varies between 0.4 and 2.5 in (10-65 mm), depending upon the size of the scallop when it ceases growing in the fall.
The inner shell surface is smooth and somewhat vitreous. Ridges and furrows exist, but they are not as conspicuous. The scallops' eyes are located near the outer edge of each valve.
The light shell is suitable for movement through the water, and its rounded thin form offers the least resistance for swimming. When the scallop swims, streams of water are forced by the aid of the mantle through small openings in the mantle near the "ears." The two subspecies, Argopecten irradians irradians and Argopecten irradians concentricus swim using the same method.
Internal Organs
The inner side of the bay scallop's valves is lined with a thin ciliated mantle that is attached to other living tissues (Fig. 4, 5a, b). Its free edge possesses numerous guard tentacles or tactile organs and blue bead-like eyes (Fig. 6a, b). The margins of the mantle are thickened lobes. When the scallop is resting, the lobes are held slightly apart. The mantle secretes shell and ligament and is also involved in water circulation and particle movement within the pallial cavity. Beninger and Le Pennec (1991) believe the mantle may be a site for the exchange of gases.
The guard tentacles vary in size and form and have a sensory function. When the scallops lie undisturbed, the tentacles lengthen and wave slowly in the water currents. The bay scallop eyes, blue and about one-fifteenth of an inch (1-1.5 mm) in diameter, are at the tips of short stalks which extend outward from the middle fold of the mantle lining the circumference of the valves and are among the tentacles; each usually is located within one of the furrows of the shell. The eyes, found in association with both the upper and lower valves, can detect movements in the environment at distances greater than required to cast a shadow. The guard tentacles will be withdrawn immediately at the passing of a shadow or at any slight disturbance (Wilkens, 1991). Bay scallops also appear to have an olfactory sense, because they swim away from starfish, Asterias forbesi, placed near them.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The adductor muscle (called the "eye," or "heart" in the fishery) is posterior to the center of the shell, and consists of a large anterior section and a smaller posterior section (Fig. 7a, b). This posterior section is tougher for a person to chew than the anterior section. When the muscle is cut from a valve, the valves immediately gape open, being forced apart by a V -shaped cartilaginous elastic pad in the middle of the hinge.
Toward the hinge of an adult scallop is a small muscular foot extending from the upper part of the visceral mass dorsally for about a quarter of an inch (8 mm). A byssal gland, on the proximal end of the foot, secretes a bundle of threads, termed the byssus by which the juvenile scallop anchors itself to an object.
[FIGURE 4 OMITTED]
Reproduction
Male and female reproductive organs are found in each adult bay scallop. The testis usually is cream-colored and lies just ventral to the digestive gland and foot and extends down the side of the ovary, which during the spawning season becomes bright orange. The gonad's surface is nearly solid black during some months. The ovary is larger than the testis. During the early part of the spawning season when the testis and ovary are full of spermatozoa and eggs, the reproductive organs are well-rounded. After completion of spawning, they become smaller and paler.
In the early part of the spring, the sex products begin to ripen. Once gametogenic maturity is reached, the most important stimulis for spawning is temperature (Barber and Blake, 2006). Sastry (1966) reported that in A. i. concentricus, gonad growth is timed to when phytoplankton is most abundant, so the scallops can accumulate reserves in the body for maintenance and growth of the gonads, and also food is available for the scallop larvae. Photoperiod also may influence reproduction. Laboratory experiments showed that an increase in reproduction condition (ripeness) will occur when the daily light hours are increased each day while the temperature is maintained constant. This observation has been made in A. i. irradians and other scallops (Couturier and Aiken, 1989; Devauchelle and Mingaut, 1991; Paulet and Boucher, 1991; Villalejo-Fuerte and Ochoa-Baez, 1993; Saout et al., 1999; Desanctis et al., 2006).
The final ripening of A. i. irradians takes place during May, when the water temperature has reached 45.5[degrees]-50[degrees]F (7.5[degrees]-10[degrees]C), and the scallop is prepared to spawn during the first part of June (also cited in Bricelj et al., 1987). The spawning season lasts from mid June to mid August; the greater part of the spawning may be at any time during this period (Fig. 8). Though not documented, bay scallops probably behave similarly to oysters, Crassostrea virginica, to maximize fertilization of their eggs. In the oyster, males initiate the spawning of females by releasing sperm into the water. Females draw in the sperm and this triggers them to release eggs immediately into the water to mix with the sperm for fertilization (Galtsoff, 1964). If so in bay scallops, the male portion of their organ would spawn and then other scallops take in the sperm and the female potions of their gonads would spawn. Each scallop probably spawns a few million eggs a season. Year-class success apparently depends upon the numbers of sperm and eggs produced (Barber and Blake, 2006), fertilization success, and survival of the larvae and juveniles.
[FIGURE 5 OMITTED]
[FIGURE 6a OMITTED]
[FIGURE 6b OMITTED]
[FIGURE 7a OMITTED]
Pennington (1985) has made some observations of the sea urchin, Strongylocentrotus droebachiensis, that may have direct application to bay scallops. He found that when the urchins spawn, their sperm becomes exhausted quickly. The sperm lost potency so rapidly that less than 10% fertilization resulted when 20-minute-old sperm were added to fresh eggs. Thus, the urchins' sperm must fertilize eggs minutes after they are spawned or they will not have the capacity to fertilize them. He suggested that adult urchins probably group together and then spawn synchronously, so their sperm and eggs will be concentrated and result in a relatively high fertilization rate. Egg fertilization would be far less successful if the adults were spaced far apart, especially if their sex products were scattered by currents.
[FIGURE 7b OMITTED]
Future tests involving bay scallops may show that when adults spawn within eelgrass meadows their sperm remains concentrated and has a good chance of being detected in sufficient quantity by other scallops to stimulate them to spawn eggs and more sperm. If the sperm were released on otherwise plain bottoms, it is more likely to be so scattered and its concentration so diluted that fewer scallops could detect it and spawn. Moreover, a mass spawning by a given number of adults within the meadows would likely result in a higher rate of fertilization than a spawning on plain bottoms because the sperm and eggs would remain close together.
The fertilized eggs develop into free-swimming larvae that feed and respire as they grow. After about 14 days, the larvae settle and attach to substrates, such as eelgrass (Fig. 9a). Where eelgrass is absent, other surfaces provide a substrate for bay scallop larvae to attach, but they do not offer the small scallops the cover protection that eelgrass offers. The alternate surfaces include pebbles and stones (Fig. 9b); the shells of dead scallops, mussels, and common Atlantic slippersnails (quarter-decks), Crepidula fornicate; glass bottles; concrete blocks; and, since the late 1950's, codium, Codium fragile, that was introduced to U.S. waters (Fig. 10). Codium is a dark green siphonous alga. In the fishery, it now goes by its generic name, but has also been called "dead man's fingers" and "sponge weed."
The larvae of A. i. irradians in eastern Long Island are 150-190 [micro] long when they attach to substrates (Tettlebach, 1986; Eckman, 1987). The optimal temperature for growth and survival of A. i. irradians larvae lies between 68[degrees] and 86[degrees]F (20[degrees]-30[degrees]C), and the optimal salinity lies between 20 [per thousand] and 30 [per thousand]. Scallops grow well at constant temperatures of 59[degrees]F (15[degrees]C) and at 90[degrees]F (30[degrees]C), but not at 50[degrees]F (10[degrees]C), and they do not survive a temperature of 95[degrees]F (35[degrees]C). They also do not survive a salinity of 10 [per thousand] (Tettlebach and Rhodes, 1981).
In a study conducted on Long Island, N.Y., in the 1990's, Tettlebach et al. (1999) reported on the spawning of bay scallops in late September, October, and maybe even early November following an earlier June/early July spawning. Then, Conant (1), in a study in 2000 and 2001, described a similar fall (September) spawning in Nantucket Harbor. A similar fall spawning also occurs in Cape Poge Pond, Martha's Vineyard. Referred to as "nubs" in Nantucket, the juvenile fall recruits pass through the winter at 0.2-0.8 in (5-20 mm) long, and in spring have a growth line on their shells a distance of 0.2-0.8 in from their umbos, rather than the 1.2-1.6 in (30-40 mm) distance that Belding (1910) had reported for bay scallops on southern Cape Cod.
[FIGURE 8 OMITTED]
The biology of A. i. concentricus is somewhat similar to A. i. irradians. As noted, A. i. irradians in the northeast spawns mostly in the spring, but A. i. concentricus in North Carolina spawns mostly in the fall (Bishop et al., 2005). The North Carolina fishermen and biologists notice the juvenile scallops ("spawns") in April but not before. They believe there is some spawning in the spring and most is in the fall (Smith (2)).
Erratic Annual Quantities
The quantities of bay scallop seed (juveniles) and subsequently market-sized scallops produced in various bays has varied widely from year to year, ranging from small, to medium, to large. The seed quantities bear little relationship to the quantities of adult scallops present that had spawned them. The principal controlling factors are: 1) production of the number of larvae by the adults (this is discussed below in Bay Scallop Habitat), 2) survival of larvae while swimming, feeding, and growing in the water to setting size for roughly two weeks, and 3) survival of the post-set juveniles from the time they set and attach to substrates to a size of perhaps 10-12 mm when most release themselves from substrates.
Survival of the swimming larvae probably depends mostly upon the quantities and species of phytoplankton available for them and also water temperature and predation. In bivalve hatcheries, workers have found that bay scallop larvae will grow only when fed specific species of phytoplankton. In contrast, the larvae of oysters and northern quahogs, Mercenaria mercenaria, will grow when fed a far wider range of species. Various scientific studies have shown that the presence of various phytoplankton species in natural waters is highly variable, by hour, by day, by week, by season, and by year. Their variability is commonplace: species populations often appear and disappear within a week; groups of various species replace each other; and the biomass levels and blooms vary widely by season and by year. The fluctuations can be caused by temperature, cloud cover, nutrients, grazing by zooplankton, and probably additional factors (Veldhuis et al., 1997; Gaard et al., 1998; Smayda, 1998; Thompson, 2001, Thomas et al., 2003; Dagg et al., 2006; Smetacek and Cloern, 2008). Such an irregular pattern of species occurrences suggests that the foods that bay scallop larvae require for survival are only sporadically present.
[FIGURE 9a OMITTED]
[FIGURE 9b OMITTED]
[FIGURE 10 OMITTED]
Little study has been made of the mortalities of...
|
