Geiger, SP, JC Cobb, B Pittinger. 2006. Is Habitat availability
limiting recruitment of calico scallops (Argopecten gibbus)?
National Shellfisheries Association Annual Meeting, Monterey CA. J.
Shellfish Res. 25: 729-730.
|Figure 1 Annual harvest of calico
scallop adductor muscle meat in Florida.
The Florida calico scallop fishery has declined from a peak of
42.7 million pounds of adductor muscle meats landed in 1984 to none
landed in 2005 (Figure 1). Removal of scallop and other shell as
bycatch may have depleted the essential fisheries habitat for
settling veligers, or larva. Between 1952 and 2003 71 million
kilograms (kg) of meat was landed, 19.4 million kg in 1984 alone.
This harvest translates into an estimated 1.3 billion kg of scallop
shell and 2 billion kg of bycatch removed from the habitat.
The goal of this study was to compare current abundance and
distribution of scallops and shell with available historic data. We
also are investigating the association between spat (attached
juvenile bivalves) and its preferred substrate on two of the
historic scallop fishing grounds. Substrate may include calico
scallop shell, other mollusc shell, or live rock.
Based on historic records and verbal accounts of fishermen, we
established a grid of 240 stations on each coast, divided into four
zones of 60 stations each (Figure 2). Historically the fishing beds
were based along the 20-fathom line around Cape Canaveral, but were
said to occur much shallower along the Gulf of Mexico coastline,
within one to five miles of shore. On each coast we planned to
sample 15 randomly selected stations from each zone during both the
spring and fall of two consecutive years. At each station, we
pulled a 0.6 meter (m) dredge for five minutes. All live organisms
were counted and weighed. We also weighed dead shell of both calico
scallops and all other molluscs. Observations of spat were noted,
and when possible, the substrate they were attached to was
recorded. Historic data was obtained from a NOAA cruise reports
(from research vessels Silver Bay, Oregon, Oregon II, Pelican,
Combat, and Delaware II). Early sampling was conducted using
trawls. During the 1970s, most of the surveys were conducted using
video systems. Vessel logs and other written descriptions have been
located, and often include summary maps, but limited trawl net
data. There is also very limited survey data from the peak period
of commercial harvest, 1980-1990.
Figure 2 Sampling sites on Florida's west and east coasts.
The abundance of mollusc shell from Atlantic Ocean stations was
roughly comparable between current samples and those from 1956-1964
and both had more calico scallop shell than current samples from
the Gulf of Mexico (Figure 3A). The peak of shell abundance in the
Atlantic seems to have occurred at 52 m around 1960, and at 40 m in
recent samples (Figure 4). In the current study we find similar
amounts of calico scallop shell and other shell in Atlantic Ocean
samples, but in the Gulf of Mexico shell from other molluscs was 20
times as abundant as scallop shell (Figure 3B).
Figure 3. A. Total shell was higher in current
east coast samples (EC) than historic (SB) or current Gulf of
Mexico (WC) samples (mean, std. dev., std. error). B. Shell of
other molluscs (WCO) was much more abundant than calico scallop
shell (WCC) on the Gulf coast, but on the east coast, calico
scallop shell (ECC) was slightly more abundant than other shell
(ECO) (median, 25 percent - 75 percent, range).
Figure 4 Shell from historic and current collections.
Scallops were collected at 31 percent of the Atlantic stations,
spat at 9 percent, and both calico scallop shell and other shell at
89 percent. The general distribution of live scallops and shell
have not changed since surveys began in the 1950s (Figure 5). They
occur in 25-30 kilometer-long patches parallel to the shelf
break. Scallops were collected at 10 percent of the Gulf of
Mexico stations, spat at 8 percent, scallop shell at 63 percent,
and other mollusc shell at 71 percent. Rocks and hard bottom were
common (Figure 6). No comparable historic data is available.
Figure 5 Live scallop collections from a variety of sources:
1956-1964, trawl data from R/V Silver Bay and others; 1980's trawl
data R/V Bellows and R/V Delaware II; 2004-2005, trawl data R/V
Figure 6 A. Calico scallop distribution in Gulf of Mexico
samples. B. Total catch in Gulf of Mexico samples.
Most spat found at Atlantic stations were on scallop shells (46
percent), but they were also found loose and on other shells, rocks
and trash. Gulf of Mexico spat were most commonly found on other
mollusc shells (64 percent) but were also found loose and on rocks,
and calico scallop shells (Figure 7). Scallop shells also serve as
valuable settlement substrate for a wide variety of other
organisms, especially as they age on the seafloor (Figure 8).
Figure 7 Calico scallop spat appear to use any available
substrate for settlement, but prefer the inside surface of
Figure 8 Calico scallop shell serves as settlement substrate for
a wide variety of organisms, creating valuable habitat.
* The Cape Canaveral calico scallop bed currently has a similar
spatial extent to the extent indicated by historic records.
* Calico scallops are seasonally abundant and are associated
with shell deposits.
* The southwest Florida scallop bed is limited in extent and
* Calico scallops can utilize a variety of substrates for
* Scallop shell provides a valuable habitat for a wide variety