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Status Report on the Calico Scallop, Argopecten gibbus, Fishery in Florida

Management considerations for calico scallops include minimum size limits, appropriate fishing areas and techniques, and the impact of fishing activities on essential calico scallop habitat.

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INTRODUCTION
The calico scallop, Argopecten gibbus, supports a small but locally important commercial fishery that is centered at Port Canaveral, Florida. Although substantial harvest may be realized from the Florida panhandle, and some harvest occurs along the southwest coast of the state, most of the harvest comes from the calico scallop beds occupying the continental shelf between Ft. Pierce and St. Augustine.

Until recently, the fishery was essentially self-regulated and functioned under the auspices of an organization of calico scallop processors called the Calico Scallop Conservation Association (CSCA). The primary mission of the CSCA was to maintain a mutual agreement among the processors to refrain from harvest until at least 75% of the target scallop stock had achieved a minimum shell height of 1.5" (38 mm) (Arnold, 1995). Because the fishery relied on efficient processing of the landed product, and CSCA members owned the necessary processing equipment, this agreement proved quite effective in preventing initial harvest of economically sub-optimal scallop beds. However, the CSCA was less effective in terminating harvest when the supply of large scallops had been depleted. Then, harvesting effort would be shifted to sub-optimal beds within the fishery zone in an effort to keep the processing plants operating at or near capacity.

The calico scallop fishery originally developed in North Carolina in the early 1960s, but the focus of the fishery shifted to the Cape Canaveral beds during the early 1970s as the extent of those beds was realized and the equipment necessary for large-scale processing was developed (Cummins, 1971). Peak landings from the fishery were recorded during the early 1980s, but landings have never again reached even 50% of the 1984 zenith.

It has been suggested by some fishery participants that the activities of the fishery, specifically removal of the shell bed necessary for successful scallop recruitment, may have reduced or eliminated the possibility of recruitment events similar to the recruitment event responsible for the early-1980s landings (Anonymous, 1998). Other fishery participants argue that, since the mid-1980s, there have been many recruitment events similar to that observed during the early 1980s (Keith Smith, pers. comm.). The lack of equivalent success, they argue, is due to post-settlement factors that prevent the recruits from achieving adulthood. Both arguments have their merits. Settling calico scallops do require shell or other hard substrate to provide an anchor for byssal attachment (juvenile scallops anchor themselves with thin, strong, filaments called "byssal" threads much as a mussel anchors itself to a rock). Successful settlement does not, however, ensure survival to adulthood.

The Cape Canaveral region is hydrodynamically complex (e.g., Leming, 1979), and those hydrodynamic forces have the potential to disperse concentrated patches of juvenile scallops. Additionally, predators (Wells et al., 1964) and parasites (Moyer et al., 1993) may take a considerable toll on juvenile and adult scallops. It is probable that a combination of suitable biological and physical factors conspired to produce the large number of calico scallops available for harvest during the early 1980s. As with many other molluskan fisheries (e.g., hard clams in southwest Florida), such an event may not be observed again for many decades, if ever. Nevertheless, the fishery continues to operate, albeit at a much lower level of landings than was realized during the 1980s.

In 1999, the Florida Marine Fisheries Commission (now the Florida Fish and Wildlife Conservation Commission [FWC]) adopted a rule outlawing the harvest of calico scallops if the average number of adductor muscles ("meat") in the catch exceeded 250 per pound (550 per kilogram), an amount equivalent to a minimum average shell size of about 1.5" (38 mm) maximum disk diameter. The intent of that rule was to limit the harvest of smaller, less valuable scallops. Rules were also adopted to define allowable gear design and usage for calico scallop harvest and to delimit those areas within state waters that were closed to the harvest of calico scallops. The gear design and usage rule was necessary so that trawls used within state waters met the requirements of the net-limitation amendment and calico scallop nets could be exempted from requirements for turtle excluder devices. The definition of allowable areas was required to prevent the use of calico scallop trawls in areas where other trawl fisheries were excluded. During 1998, the South Atlantic Fishery Management Council completed a calico scallop management plan that addressed concerns of overfishing and habitat destruction for the fishery in federal waters of the south Atlantic region (Anonymous, 1998). The federal management plan did not address calico scallop size limits; those limits were essentially set by FWC for all calico scallops landed within Florida.

BACKGROUND
With a maximum life span of 24 months but a typical life span of approximately 18 months (Roe et al., 1971), the calico scallop is a relatively short-lived animal. During that time, a healthy scallop will usually spawn 3–4 times. Scallops begin spawning at a size as small as 19 mm shell height and as young as four months (Miller et al., 1979). Spawning and recruitment occur throughout the year (Allen, 1979), but maximum reproductive effort occurs in the late fall and in the spring (Porter and Schwartz, 1976; Moyer and Blake, 1986). The fall spawn, though typically less intense than the spring spawn, may be critical to the maintenance of standing stock in the following year (Moyer and Blake, 1986). After a 14–16 day pelagic larval phase, the settling scallops attach to hard substrata, commonly the disarticulated shells (shells that are separated or broken) from previous generations of scallops. Scallops that settle in spring generally reach a size of 30–35 mm shell height by the following fall and are fully able to reproduce. As a result of this rapid growth and early maturation, scallop groups, or cohorts, may overlap, and many different size classes of scallops may occupy a scallop bed.

Both hydrodynamic and biological processes may influence year-class success. Hydrodynamically, coincident with scallop distribution along the Florida east coast, coastal upwelling events have been documented on the Florida continental shelf between Ft. Pierce and St. Augustine (e.g., Green, 1944; Leming, 1979). Similar, less well-documented upwelling events have been observed on the continental shelf near Cape San Blas where occurring calico scallop populations are exploited. Upwelling injects cold, nutrient rich water onto the shelf; the potential effects of this process include increased food availability for adults and larvae (Yoder, 1985), induction of spawning due to water temperature changes (Costello et al., 1973; Barber and Blake, 1983; Wolff, 1988), and larval transport and export in upwelled water masses (Nelson et al., 1977; Bailey, 1981; Yoder et al., 1983; Roughgarden et al., 1988; Krause et al., 1994). Biologically, infection by a protozoan parasite of the genus Marteilia may have been responsible for a calico scallop population crash recorded off Cape Canaveral during 1991 (Moyer et al., 1993). The parasite appears to infest the digestive gland of scallops to such an extent that the scallops starve to death. During the 1991 event, infestation was first detected in January in an otherwise healthy and abundant scallop population. By February the population had been essentially obliterated. While similar devastation was observed during 1989, it is unknown whether Marteilia is a natural feature of the calico scallop population or a recent introduction.

MANAGEMENT ISSUES
There is considerable debate among calico scallop fishing industry participants concerning the need for fishery management. In essence, this debate pits the owners of Type I vessels, those vessels equipped to process the scallops at sea, (Anonymous, 1998) against the owners of Type II vessels, shell-stock vessels that return the entire catch to shore for processing. The debate focuses on two issues. First, is the size limit effective in increasing stock abundance or the overall economic return to the fishery? Second, does the removal of shell material from the scallop beds negatively impact recruitment success? Unfortunately, the biological data necessary to answer these questions are not available. Thus, the following discussion presents the two sides of each argument based upon interviews by the author with Mr. Bill Burkhardt (Type I vessel operator) and Mr. Keith Smith (shore-based processor). Both men have considerable experience in the calico scallop industry. Arguments on either side of each issue are then considered within the context of available information on scallop biology and life history.

Most of the vessels involved in the calico scallop fishery use the same techniques for harvest. Calico scallops are harvested by means of otter trawls, and each vessel generally deploys two trawls simultaneously. Scallop trawls have a maximum headrope length of 40’ (12 m), and the nets are constructed with 3” (7.6 cm) stretch webbing that cannot exceed 500 ft² (46 m²) in total area. Tow time is limited to 25 minutes. The resultant harvest is then run through an automated, multi-step processing operation that produces a shucked and cleaned adductor muscle ready for packaging. While the harvesting and processing procedures are essentially the same among processors, the location of the processing operation differs between Type I and Type II fishing vessels. This difference has important implications for the design and implementation of rules and regulations governing the calico scallop industry.

There is concern among some calico scallop fishermen that the size limits implemented by the FWC are ineffective. As noted, "undersized" scallops are ever-present on the beds, but the proportion of undersized scallops varies among beds. During the fishing operation, it is very difficult to determine the average size of scallops in the catch, and that average may change from tow to tow. This is a minor concern for Type I vessels that process at sea because scallop meat size can be monitored and fishing activities adjusted accordingly. However, for Type II vessels the determination of concurrence with the size limit regulation is made at the processing plant after the scallops have been offloaded from the fishing vessel and processed. Thus, average meat count is not determined until the catch is landed; by then it is too late to return the catch to the beds. The undersized scallops are either discarded or combined to form scallop "medallions." In either case, the effectiveness of the size limit regulation is compromised because the undersized scallops are harvested and lost from the population.

The size limit also may be counterproductive. Scallops suffer natural mortality throughout their life span, and considerable mortality may occur before a scallop cohort reaches harvest size. Under conditions of high survival, these losses may be minor and would be offset by increased growth and the harvest of a more valuable product (as is the intent of the size regulation). However, it is not uncommon for a group of scallops to suffer considerable mortality (up to 100%) prior to reaching the minimum harvest size, in which case a potentially valuable resource cannot be legally harvested and is lost.

Some fishery participants also have expressed concern that the process of fishing for calico scallops actually destroys the very habitat upon which the fishery is dependent (Anonymous, 1998). As noted, settling scallops commonly attach to the empty shells of their predecessors. Harvesting by otter trawl results in the removal of 1) the relict shell bed and 2) the shells of living scallops, each of which is lost from the system as the entire harvest is transported to shore for processing. For example, based upon available biological data (e.g., Arnold, unpubl. data), it is a valid assumption that a scallop shell is approximately equivalent in weight to the adductor muscle removed from that shell. Applying that assumption, well over 100 million pounds of scallop shell has been removed from the Cape Canaveral beds since 1978; that would be a minimum estimate based upon the weight of scallop meat reported landed since that time. The actual value could be considerably greater when the weight of relict scallop shell and other shell by-catch is considered. However, considering that the Cape Canaveral scallop grounds cover an area of approximately 500 mi² (1300 km²), that weight of shell removed over the past 20+ years may constitute a very small proportion of the total available habitat.

Type I fishing vessels do not remove shell from the sea; instead, they process the scallops and return the shell overboard. Obviously, this technique eliminates the problem of shell removal, but it does not necessarily eliminate the impacts of harvest. The shell bed is still disturbed and may be redistributed to areas unsuitable for scallop settlement. Type II vessels do remove shell from the sea, but as stated above there is no available estimate of the significance of this removal to the overall availability of suitable substrate for scallop settlement. This issue cannot be effectively resolved without a better understanding of the scale of scallop shell removal and the relative importance of relict scallop shell versus other available substrates for calico scallop settlement.

Visit the Species Accounts Section for more information.

To view the most current fishing regulations for the Calico scallop in state of Florida, please visit the Florida Administrative Code (FAC) Web site, Chapter 68—FISH AND WILDLIFE CONSERVATION COMMISSION located at: http://fac.dos.state.fl.us/

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REFERENCES
Allen, D.M. (1979). Biological aspects of the calico scallop, Argopecten gibbus, determined by spat monitoring. The Nautilus 94: 107–119.

Anonymous (1998). Final Fishery Management Plan for the Calico Scallop Fishery in the South Atlantic Region, Including a Final Environmental Impact Statement, Regulatory Impact Review, and Social Impact Assessment/Fishery Impact Statement. South Atlantic Fishery Management Council Award Number NA87FC0004, 124 pp. + appendices.

Arnold, W.S. (1995). Summary report on the calico scallop (Argopecten gibbus) fishery of the southeastern United States. Florida Department of Environmental Protection, Florida Marine Research Institute, 22 pp.

Bailey, K.M. (1981). Larval transport and recruitment of Pacific hake Merluccius productus. Mar. Ecol. Prog. Ser. 6: 1–9.

Barber, B.J. and N.J. Blake. (1983). Growth and reproduction of the bay scallop, Argopecten irradians (Lamarck) at its southern distributional limit. J. Exp. Mar. Biol Ecol. 66: 247–256.

Blake, N.J. and M.A. Moyer. (1991). The calico scallop, Argopecten gibbus, fishery of Cape Canaveral, Florida. In: Shumway, S.E. (ed). Scallops: Biology, Ecology and Aquaculture. Elsevier Science Publishing Company, Inc., New York, N.Y., pp. 899–911.

Costello, T.J., J.H. Hudson, J.L. Dupuy, S. Rivkin. (1973). Larval culture of the calico scallop, Argopecten gibbus. Proc. Natl. Shellfish. Assoc. 63: 72–76. Cummins, R., Jr. (1971). Calico scallops of the southeastern United States, 1959–69. NOAA NMFS Spec. Sci. Rpt.—Fish. 627: 22 pp.

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Moyer, M.A., N.J. Blake, W.S. Arnold. (1993). An ascetosporan disease causing mass mortality in the Atlantic calico scallop, Argopecten gibbus (Linnaeus, 1758). J. Shellfish Res. 12: 305–310.

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Porter, H.J. and F.J. Schwartz. (1976). Seasonal variation in tissue weight and total solids of the calico scallop, Argopecten gibbus (Linne) and their relationship to changes in gonad condition. Proc. Natl. Shellfish. Assoc. 66: 104–105.

Roe, R.B., R. Cummins, Jr., H.R. Bullis, Jr. (1971). Calico scallop distribution, abundance, and yield off eastern Florida, 1967-68. Fish. Bull. 69: 399–409. Roughgarden, J., S. Gaines, H. Possingham. (1988). Recruitment dynamics in complex life cycles. Science 241: 1460–1466.

Wells, H.W., M.J. Wells, I.E. Gray (1964). The calico scallop community in North Carolina. Bull. Mar. Sci. 14: 561–593.

Wolff, M. (1988). Spawning and recruitment in the Peruvian scallop Argopecten purpuratus. Mar. Ecol. Prog. Ser. 42: 213–217.

Yoder, J.A. (1985). Environmental control of phytoplankton production on the southeastern U.S. continental shelf. In: Atkinson, L.P., D.W. Menzel, and K.A. Bush (ed.). Oceanography of the southeastern U.S. continental shelf. Am. Geophysical Union, Washington, D.C., pp. 93–103.

Yoder, J.A., L.P. Atkinson, S.S. Bishop, E.E. Hofmann, T.N. Lee. (1983). Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf. Continental Shelf Res. 1: 385–404.