Population Biology of Intertidal Whelks in Wassaw Sound, Georgia

1
Population Biology of Intertidal Whelks in Wassaw
Sound, Georgia Alan J. Power, Mary
Sweeney-Reeves, Todd C. Recicar, Dodie M.
Thompson, Randal L. Walker Marine Extension
Service, Shellfish Research Laboratory,
University of Georgia, 20 Ocean Science Circle,
Savannah, GA 31411-1011 Tel [phone]
Fax [phone] Website www.uga.edu/maricult
ure Email alanpowr_at_arches.uga.edu
Lightning
Pear
Channeled
Knobbed
Introduction Objectives Four species of whelk
(family Melongenidae) are found within the
coastal waters of Georgia the knobbed whelk,
Busycon carica (Gmelin, 1781), the channeled
whelk, Busycon canaliculatum (Linnaeus, 1758),
the lightning whelk, Busycon perversum (Linnaeus,
1758), and less commonly, the pear whelk, Busycon
spiratus (Lamarck, 1816). These whelks are
commercially trawled and also captured
intertidally by oystermen, clammers, and sport
fishermen. To date the whelk fishery remains one
of the most economically successful molluscan
fisheries in the state. There is, however, very
limited information on the factors influencing
the distribution, biomass and population dynamics
of the various whelk species in southern US
waters. Consequently, the present study was
initiated to examine individual growth rates and
the temporal variability in the abundance and
population structure of each species in the
intertidal zone in Wassaw Sound, Georgia.
There was a large difference in the numbers
captured in October/November 2000 (N340) and
October 2001 (N1024), which may be due to the
refining of locating techniques. Pear whelks were
not observed intertidally, however the authors
have observed the species locally in baited pots
that were placed below the low tide level. The
female to male ratio was higher for each species
over the sampling period (knobbed from
2.20-25.15, lightning from 13.00-38.50, and
channeled whelks from 1.00-5.33). In addition,
there were no male lightning whelks captured in
May/June 2001. Figures 3, 4 and 5 illustrate the
pooled shell length (mm) frequency distributions
of males and females captured over the entire
sampling period for knobbed, channeled and
lightning whelks, respectively (excluding trawled
specimens). Female whelks dominated the larger
size classes for all species. The largest whelk
captured was a female lightning measuring 257mm
in shell length, 160mm in width, and weighing
1,440g. The largest sized knobbed and channeled
whelks were 228mm, and 173mm, respectively. The
smallest whelk captured was a male knobbed
measuring 39mm in length, 18mm in width and
weighing 42g. Mean shell lengths for males of
knobbed, channeled, and lightning whelks were
98mm, 92mm, and 120mm, respectively. Mean shell
lengths for females were 128mm, 107mm, and 148mm.
The difference in sizes between sexes may partly
explain the higher abundance of female whelks
noted in this study, the larger size increasing
their chances of being located. While the mark
and recapture component of the study is still in
its infancy, preliminary data indicates very slow
growth rates and very limited movement from
release sites for all species, with several
specimens displaying no growth and even negative
growth over the study period to date. From Table
1 (includes trawled specimens) it would appear
that channeled whelks have the fastest growth in
terms of shell size, increasing in length at a
rate approximately three times that recorded for
knobbed whelks (mean of 1.84 versus 0.61mm/mo.).
This is not that surprising due to the difference
in shell thickness between the species. For the
same reason channeled whelks had the lowest
growth in terms of weight (5g/mo.),
approximately one third of that recorded for both
other species (15g/mo.).


Figure 2
Figure 3
Figure 4
Figure 5
Methodology Whelks were collected at low tide
from mud flats and oyster reefs at six locations
(Map 1) throughout Wassaw Sound, on a seasonal
basis, over an annual period. One low tidal
period was used to collect specimens at each
location for each sampling period (Fig. 2). Shell
length, shell width, and individual weight were
recorded for all specimens after an identifying
label was glued onto the shell. Whelks were
relaxed by briefly placing into a 7 Magnesium
Chloride solution. Subsequently each one was
sexed by pulling on the operculum and noting the
presence or absence of a penis. All whelks were
then returned to their respective capture sites.
Size frequency distributions, sex ratios, and
sexual dimorphism in terms of body size were
determined. Individual growth rates and seasonal
migration patterns were monitored by attempting
to recapturing as many previously tagged and
released specimens as possible over each sampling
period. In order to supplement the numbers of
tagged whelks at each site, an approximate total
of 3,400 knobbed, 835 channeled and 23 lightning
whelks were also captured by trawling with the
the R/V Bulldog off the northeastern end of
Jekyll Island near Brunswick, Georgia. These were
processed as described above and released in
Wassaw Sound at the various intertidal study
sites.

• Future Research Needs
• Collect further growth data by continuing the
  mark and recapture component for several years
  Examine
  growth rates between male and female whelks
• Investigate earlier mortality or sex reversal
  in male whelks at larger sizes
• Examine growth rates for different size classes
• Investigate an ecological separation of sexes
• Determine the spawning season for intertidal
  versus offshore populations
• Investigate possible migration from the
  intertidal zone into deeper water during periods
  of extreme cold and heat by trawling in the Sound
  off the intertidal sampling sites during these
  months
• Investigate the alternative possibility of a
  vertical migration down into the mud during these
  extreme months by tagging with metal tags and
  subsequently searching with metal detectors
• Examine intertidal mud flats as potential
  nursery areas for juvenile whelks

Map 1
Results Discussion Monthly mean water
temperature values for the Skidaway River (Map 1)
between September 2000 and November 2001 are
given in Figure 1. Temperatures ranged from 8.74
o C in January 2000 to 29.84 o C in August 2001.
Red data points indicate the intertidal sampling
dates. Error bars reflect 1 standard error
from the mean. Figure 2 presents the frequency
of knobbed, channeled and lightning whelks
captured from the six study sites per sampling
period (excluding whelks that were originally
taken by trawl from Jekyll Island). Also shown
are the ratios of females to males for each
species and for each occasion. Knobbed whelks
(N2909) far outnumbered the other species
(combined N319), with the highest abundance
being recorded in the early spring months.


Figure 1
Table 1