1. Life History of the Knobbed Whelk, Busycon carica, in Georgia
Walker, R., A. Power, M. Sweeney-Reeves, E. Covington, C. Sellers & M. Mitchell Shellfish Research and Aquaculture Laboratory,
University of Georgia, Marine Extension Service, 20 Ocean Science Circle, Savannah, GA Website:
Introduction
Knobbed whelks (Gmelin, 1781) (Family Melongenidae) are an important commercial prosobranch species in Georgia. The whelk fishery is presently Georgia’s leading molluscan fishery. Whelks are fished primarily in winter, but industry wants to fish year around. The fishery has exhibited declining landings (591,000 lbs. in 1999 to 63,000 lbs. in 2002) which is indicative of overfishing. In 2000 a field research project was initiated to determine the various life history parameters needed by the Georgia Department of Natural Resources to wisely manage this fishery. Aquaculture potential was also investigated.
Analysis of annual bands in the operculum show that female whelks grew to a larger size (N=397, range 32 to 167 mm) in a shorter time period than males (N=310, range 30 to 128 mm). In Georgia males reach sexual maturity at 85 to 90 mm in shell length and at an age of approximately 4 years, while females achieved sexual maturity at 100 mm and at an approximate age of 6 years. The von Bertalanffy growth parameters for each sex are as follows: Lmax = , k = , and to = for females and Lmax = , k = , and to = for males (Fig. 1). Slow growth precludes aquaculture development of this species.
Knobbed whelk’s main spawning period is in spring with a lesser spawning period in fall (Fig. 2). In intertidal areas numerous small-sized males can be observed attempting to spawn with a single larger-sized female. In Spring mating occurs from March to April, with egg laying starting in April. Genetic studies with Drs. John Avise and DeEtte Walker (University of Georgia) show that multiple males do contribute their gametes to the fertilization of eggs within a single egg case string. An equal sex ratio of male to female embryos occur within the egg case string. This tends to dispute earlier claims that whelks are protandric. Females may lay from 1 to 23 capsules per day (0 = 7.69 ± 2.12 S.E.) on strings containing 40 to 157 capsules (0= 89.8 ± 3.6). The number of developing embryos per egg capsule ranged from 0 to 99 with the greatest number of eggs per capsule occurring in the middle section of the string. Hatching was mostly complete by early June, although some juveniles were still observed within the capsules. Juveniles emerged from the capsules at a mean length of 5.6 mm (range 3.31 to 6.96). Cannibalism was observed in laboratory growth studies.
Methodology
Whelks were collected monthly from trawls off St Simons Island, GA. Each whelk was identified to species and measured for shell length and width, weighed, sexed, and tagged. Thirty knobbed whelks per month were sacrificed for histological examinations of the gonads. Whelks (N=707) were sacrificed to determine age and size at sexual maturity with operculums being analyzed to determine growth rate.Tagged whelks were released on intertidal sand/mud flats adjacent to oyster reefs at eight inshore sites.
To determine migration patterns and egg laying patterns at six of the intertidal sites, whelks were collected seasonally. All whelks were measured, weighed, sexed, tagged and released at their collection site. Field observations were made of mating and on egg laying activities.
In the laboratory egg case strings were analyzed for number of egg capsules per string, number of eggs per capsule, and size of embryos.
Results
Whelks (N=18,878) from offshore trawls and from inshore sites were collected, tagged and released at one of eight inshore intertidal stations. A total of 2,595 channeled, Busycotypus canalicalatus; 16,007 knobbed, Busycon carica; 256 lightning, Busycon sinistrum; and 20 pearwhelks, Busycotypus spiratus were tagged and released between February 2000 and April Recapture efforts will continue through Spring In general tagged whelks remained in the location that they were released. A few knobbed whelks were found to move across Wassaw Sound with the maximum straight line distance covered of m in days. Seasonal whelk movement occurred on the intertidal flat where whelks were present in spring and fall and generally absent in winter and summer. Female knobbed whelks move to the spring low mark to subtidal areas of the flats to lay their egg case strings.
Females of all species were larger in size than males and in most cases females significantly out numbered males. Channeled and knobbed whelks gathered offshore exhibited a slightly higher ratio of females to males; whereas, in inshore intertidal areas, females greatly out numbered males 9 to 1. It was observed that when offshore whelks at an approximate equal sex ratio were tagged and released inshore, males were generally not recaptured.
Figure 1. Growth of B. carica in Georgia
Discussion
Whelk fishermen in Georgia wish to fish year around. We do not believe that the whelk population can support a year round fishery. In fact, we do not believe that the whelk population can support the present seasonal fishing that occurs. Whelks are long-lived, slow growing, require six years for females to reach sexual maturity, and thus are a poor fishery species. The fishery targets the larger-sized females. Although there are two spawning seasons in Georgia, the peneaid shrimp fishery removes the whelks’ Fall egg-case strings from the commercial nearshore fishing grounds. Offshore reproductive success is dependant upon the Spring breeding and egg laying event. A closed fishing season during the Spring spawning season should continue to be enforced. Closure of nearshore whelk beds on a rotational basis is recommended.