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.
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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