Quantitative Comparison Between the Chesapeake Bay Conch Pot and the Standard Crab Trap for Harvesting Whelks in Wassaw Sound, Georgia

Transcription

1 Quantitative Comparison Between the Chesapeake Bay Conch Pot and the Standard Crab Trap for Harvesting Whelks in Wassaw Sound, Georgia Randal L. Walker Todd Recicar and Alan J. Power Issued by the Georgia Sea Grant College Program The University of Georgia, Athens, Georgia Marine Extension Bulletin No 26, April 2003

2 Quantitative Comparison Between the Chesapeake Bay Conch Pot and the Standard Crab Trap for Harvesting Whelks in Wassaw Sound, Georgia Randal L. Walker Todd Recicar and Alan J. Power Shellfish Research Laboratory Marine Extension Service University of Georgia 20 Ocean Science Circle Savannah, GA

3 Acknowledgments The authors wish to thank Mr. T. Shierling for constructing the crab traps for our use, Mr. J. Lambrix for his aid in the field collection of data, and Ms. D. Thompson and Ms. M. Sweeney- Reeves for measuring and tagging of whelks in the laboratory. The authors wish to thank Mr. G. Davidson for his editorial services and Ms. C. Ingram for her graphic services while publishing this document. Funding for this project was from the University of Georgia s Marine Extension Service.

4 Abstract A quantitative comparison between the Chesapeake Bay conch pot and the standard crab trap for harvesting whelks was performed in Wassaw Sound, Georgia in late March-early April The efficiency of different bait (blue crabs -Callinectes sapidus, menhaden -Brevoortia tyrannus, and a combination of both) was also investigated. Five hundred sixty-one whelks were captured of which 89.1% were channeled (Busycotypus canaliculatus), 6.1% were knobbed (Busycon carica), 4.3% were pear (Busycotypus spiratus) and 0.5% were lightning (Busycon sinistrum) whelks. A two-way ANOVA found little difference in the efficiency of either the gear or bait. However, crab traps consistently caught more channeled whelks (N=296) than conch pots did (N=205), and a bait combination of crab and fish generally caught more whelks. Crab traps also caught 100%, 88% and 78% of the lightning, knobbed and pear whelks, respectively. This was not however, statistically different from the pot catch due to a high variance. We recommend the use of standard crab traps baited with fish and crab over the Chesapeake Bay conch pots for harvesting whelks in coastal Georgia. Keywords: Whelk, conch pot, crab trap, bait, Busycon, Busycotypus

5 List of Figures Figure 1. Standard East Coast crab trap and the Chesapeake Bay conch pot used in the whelk study in Wassaw Sound, GA. 9 Figure 2. Number of whelks per species caught on each sampling interval from March 23 to April 6, 2001 at Dead Man Hammock, Wassaw Island, Georgia. 11 Figure 3. Mean number of channeled whelks caught by trap or pot per time interval at Dead Man Hammock, Wassaw Island, Georgia. 12 Figure 4. Mean number of channeled whelks caught by crab, fish or crab and fish bait per timeinterval at Dead Man Hammock, Wassaw Island, Georgia. 13 List of Tables Table 1. Combined number of tagged and untagged whelks caught in crab traps and conch pots per time interval from March 25 to April 6, 2001 at the Dead Man Hammock area of Wassaw Island, Georgia. Number of untagged whelks represents whelks caught in crab traps or conch pots which had no previous tags; number of previously tagged whelks represents animals gathered from intertidal collections and tagged for a population study. 15

6 Table of Contents Acknowledgements 3 Abstract 4 Introduction 7 Materials and Method 8 Results 10 Discussion 14 References 18

7 Introduction Georgia s whelk fishery is a relatively new industry. The first recorded landings totaled 5 kg in Initially, whelks were landed as by-catch of the winter cab trawl fishery, and until 1997 they were managed under crab trawl regulations. The recurrence of poor shrimp harvests in the area, coincident with favorable market conditions and the recognition of abundant whelk populations offshore, led to an intensification of effort and a dramatic increase in landings during the late eighties and early nineties. In the winter months after the shrimp season has closed, many shrimp boats swap their gear to fish for whelks in offshore areas using nets with larger mesh and heavier chains. Lesser numbers of whelks are gathered inshore as by-catch in blue crab, Callinectes sapidus, traps or are collected by hand from intertidal areas by clammers and oystermen (Walker, 1988). Landings peaked in 1990 with 462,197 kg. In more recent years, landings have averaged 254,243 kg (Georgia Department of Natural Resources Landings data 2002). At present the whelk fishery remains one of the state s most economically successful molluscan fisheries. Current market prices average cents per pound for total weight including the shell, which translates to 66 cents per pound for raw meat (pers. comm. Julie Califf, Georgia Department of Natural Resources). As in other places, Georgia whelks are marketed as conchs; however whelks from the Family Melongenidae are not true conchs (Family Strombidae). Four whelk species inhabit the coastal waters of Georgia: the knobbed whelk, Busycon carica; the lightning whelk, Busycon sinistrum; the channeled whelk, Busycotypus canalicalatus; and the pear whelk, Busycotypus spiratus. The bulk of the offshore fishery targets knobbed whelks while inshore blue crab fishermen harvest channeled whelks that enter their traps. These two species dominate in terms of abundance. Fewer numbers of lightning whelks are landed, and the pear whelk occurs infrequently (Walker, 1988). Elsewhere in the world, baited traps are commonly used to specifically target whelk species e.g., Buccinum undatum in Northern Europe and Canada (Hancock, 1967; Himmelman, 1988), and the genus Neptunea in Japan (Ito et al., 1981). In Georgia, channeled whelks represent an abundant and largely unexploited commercial resource, which could potentially evolve into a directed fishery. Georgia crabbers use the standard crab trap that was patented by Benjamin F. Lewis in the 1920 s and perfected ten years later. This square trap is constructed of PVC-coated wire and weighted with a steel rebar frame. These traps have two internal chambers, an upper and a lower one. The lower consists of four funnel entrances ( throats ) and a bait box. As soon

8 as a crab enters, it feels threatened and instinctively swims to the upper chamber through oblong funnel entrances, the design of which make a return to the lower chamber unlikely. This design works well for Georgia s crabs, however, alternative traps should be evaluated for harvesting whelks. To that end, the present study examines the catch effectiveness for whelks of the standard blue crab trap versus the Chesapeake Bay Conch Pot. This report also examines the effectiveness of different baits for whelk catches for each type of gear. Materials and Methods The standard crab traps measure 2 x 2 x 2 feet with a mesh size of 1.5 inch (Figure 1). Four funnel entrances, each of which measures 6.5 inches in diameter, surround the base. Chesapeake Bay Crab Pots are wooden structures measuring 23 x 23 inches in length and width and inches high, with an open top surrounded by a taught line to prevent the escape of captive whelks (Figure 1). A total of seven 1.5-inch gaps were formed by the positioning of vertical wooden slats on each of the four sidewalls. These pots do not have a bait box; instead there is a large steel spike (6 x 0.25 inches) in the center upon which horseshoe crabs (Limulus polyphemus) are impaled. We constructed a simple wire cage around this spike to keep the bait enclosed and away from swimming predators. Three lines of alternating crab traps and conch pots were placed intertidally on a shallow mud/sand bottom just offshore of the Dead Man Hammock area of Wassaw Island in Wassaw Sound, Georgia (N W to N W ). The site was chosen for its uniformity in tidal fluctuation, depth, bottom type and current structure. Each line consisted of four traps and four pots, alternating at an average distance of 10 meters. A polyethylene line was threaded through all traps and pots and anchored at each end. A buoy was fastened to each end that allowed the traps to be checked by boat at high tide when necessary. Each line consisted of one of three combinations of bait: Atlantic Menhaden (Brevoortia tyrannus), blue crab (Callinectes sapidus), or a combination of both. Pots and traps were deployed between March 23, 2001 and April 6, 2001 and were sampled at twoday intervals (except for the third date, which was a four day interval due to weather conditions). Bait was replaced on each sampling period. Catch per pot or trap was recorded by species and all captured whelks were measured for total weight per species. Each whelk was measured for shell length and shell width, and sexed, tagged and released subtidally near an intertidal oyster reef along the shore of Dead Man Hammock. Occurrence of blue and spider, Libinia emarginata, crabs was also noted per pot or trap.

9 Figure 1. Standard East Coast crab trap and the Chesapeake Bay conch pot used in the whelk study in Wassaw Sound, GA.

10 Whelk catch data was analyzed by a two-way Analysis of Variance (ANOVA) ( = 0.05) and Tukey s Studentized Range test ( = 0.05). Factors for the two-way ANOVA were sampling gear and bait. Data was analyzed for all whelk species combined, and individually by species for channeled, pear, and knobbed whelk data. Results A total of 561 whelks were caught during the two-week trial period (Figure 2.). Channeled whelks represented 89.1% of the catch with knobbed, pear, and lightning whelks making up 6.1%, 4.3%, and 0.5% of the catch, respectively. Lightning whelks (N=3) were only caught once on the final day of the experiment. No pear whelks were caught on the second or fifth sampling dates. The slight increase in numbers of channeled whelks caught during the third period was presumably due to the two extra days that traps and pots were not checked due to bad weather. Two-way ANOVA reveal that little differences occurred in catch regardless of the gear (Figure 3) or bait (Figure 4) used. Out of the 24 tests conducted on gear efficiency (combined data, and individual channeled, knobbed, and pear whelks X 6 sample dates), crab traps caught significantly more whelks than conch pots on five occasions. For all species combined, crab traps caught significantly more whelks than conch pots on the third (mean=4.2 vs. 2.6) and fourth (mean=3.7 vs. 2.0) sampling dates (p= and , respectively). Crab traps caught significantly (p=0.0172) more channeled whelks than conch pots on the fourth date (mean = 3.2 vs. 1.8) and more knobbed whelks (p=0.0456) on the sixth date (mean = 0.4 vs. 0). Traps also captured significantly (p=0.0077) more pear whelks than conch pots (mean=0.92 vs. 0.25) on the first sampling date. The three lightning whelks caught on the last day of the experiment were all caught in crab traps. Of the 24 tests run for bait efficiency, significant differences occurred 7 times. For all catch data combined, the fish and crab bait combination captured significantly more whelks than either the crab or fish bait alone on the third and sixth sampling dates (p= and , respectively) and also for channeled whelk numbers on these dates (p= and , respectively). On the fourth sampling date, the crab and combination bait treatments captured significantly more whelks than gear baited with fish alone for all whelk data (p=0.0022) and also for channeled whelks (p=0.0172). For knobbed whelks, there were no significant differences in the catch rates between different baits. Significantly (p=0.0023), more pear whelks were captured on the first

11 Channeled Knobbed Pear Lightning Number of Whelks Caught g Sampling Interval Figure 2. Number of whelks per species caught on each sampling interval from March 23 to April 6, 2001 at Dead Man Hammock, Wassaw Island, Georgia.

12 35 30 Number Channeled Whelks Caught g Conch Pot Crab Trap Interval Figure 3. Mean number of channeled whelks caught by trap or pot per time interval at Dead Man Hammock, Wassaw Island, Georgia.

13 12 Mean number Channeled Whelks Caught (SE) Fish Crab Crab/Fish Sampling Interval Figure 4. Mean number of channeled whelks caught by crab, fish or crab and fish bait per time interval at Dead Man Hammock, Wassaw Island, Georgia.

14 sampling date with the combination bait treatment than with the fish treatment. The three lightning whelks were caught in crab traps baited with crab at interval 6. During this experiment, 226 previously tagged whelks from a nearby intertidal mark and recapture study were caught in either crab traps or conch pots (Table 1). Of these, twenty-seven were recaptured a second time (11.95%). Only three of these individuals were knobbed whelks, and the remainder was channeled whelks. Of the 335 whelks captured that were not previously tagged, thirty-five channeled whelks were recaptured a second time (10%). It required an average of 7.9 ± 0.03 (SE) (range 4 to 10 days from release) before the thirty-five channeled whelks that were tagged during this experiment were recaptured. Ninty-two crabs were caught during this experiment: 75 spider crabs and 17 blue crabs. All blue crabs were caught in crab traps baited with fish. The majority of the spider crabs (60%) were caught in crab traps baited with fish, while 33% were caught in crab traps baited with crab and fish. Only 5% of spider crabs were obtained in crab traps baited with crabs. No spider crabs were gathered from conch pots. Discussion The results of this experiment show that there is little difference in using the standard crab trap versus the Chesapeake Bay Conch Pot for harvesting whelks in coastal Georgia. Overall, crab traps caught consistently, but not statistically (only in 1 of 6 sampling intervals) more whelks than the conch pots (Figure 2). Channeled whelk was the main species caught (89%). Apparently, neither pots nor traps were very effective at catching the knobbed or lightning whelks. A large population of knobbed whelks is known to occur at the intertidal oyster reef area located in this area. Yet only thirty-four knobbed whelks were harvested in the course of this experiment, 88% of which were caught in crab traps. In addition only three lightning whelks were caught, also in crab traps. This may indicate an ecological separation in terms of diet between these sympatric species. Channeled whelks are primarily scavengers. Knobbed and lightning whelks are predators and they rarely eat carrion. The knobbed and lightning whelks have thicker shells and are notorious for preying on commercially important shellfish species (e.g., oysters, Crassostrea virginica, and clams, Mercenaria mercenaria). They penetrate bivalve shells by chipping and wedging the valves open (Colton, 1908; Warren, 1916; Magalhaes, 1948; Carriker, 1951; Paine, 1963; Davis, 1981; Walker, 1988), and then they insert their radula and remove the meat.

15 Table 1. Combined number of tagged and untagged whelks caught in crab traps and conch pots per time interval from March 25 to April 6, 2001 at the Dead Man Hammock area of Wassaw Island, Georgia. Number of untagged whelks represents whelks caught in crab traps or conch pots which had no previous tags; number of previously tagged whelks represents animals gathered from intertidal collections and tagged for a population study. Time Number of Number of Number of Number of Untagged Previously Recaptures Recaptures of Whelks Tagged Whelks of Untagged Previously Tagged Whelks Whelks March March March April April April

16 Pear whelks have a thin and lightweight shell similar to channeled whelks. Therefore, it might be expected that they share a similar attraction to carrion; however, only twenty-three were collected (78% from crab traps) over the course of the study period. It should be noted though that this number represents more live pear whelks than have been seen by one of the authors (Walker) in his 25 years of fieldwork in Wassaw Sound. While the other three species of whelks frequently move into the intertidal areas where oyster beds occur, pear whelks have never been observed close to shore in Georgia. In an ongoing population study of intertidal whelks gathered from seven different sites throughout Wassaw Sound, no live pear whelks have been found among the 15,000 whelks tagged to date. In an earlier study in Wassaw Sound, knobbed whelks were observed to move farther up into the intertidal zone during spring and fall than lightning or channeled whelks (Walker, 1988). Lightning whelks will move up into the mean low water area; whereas, channeled whelks generally stay near the spring low water mark. Observations by Kent (1983) in Florida show lightning whelks prevail over the lighter-shelled pear whelks in competing for shellfish resources. It is possible that in Georgia, pear whelks cannot compete with the other three species of whelks and consequently are restricted to a subtidal habitat. On the other hand, since pear whelks are nocturnal (Paine, 1962), we may not observe them in intertidal areas since we only collect during daylight hours. In a United Kingdom study of the common whelk, Buccinum undatum, pots baited with shore crabs and salted skate caught more whelks (mean=33.8 per pot) than those baited with either crab (mean=22.5 per pot) or salted skate alone (mean=10.2 per pot) (Hancock, 1963). This was due to the fact that the presence of crab bait deterred crabs from entering the pots, but did not deter the whelks. In addition, this bait combination lasts longer since crabs have no interest in it. The results were less clear in our study, which showed that the type of bait did not appear to play a significant role in catch rates of whelks. Only seven cases (out of 24) showed significant differences in catch rates relative to the type of bait used. However, no clear preferences emerge. In four of these cases, the fish/crab combination caught significantly more whelks than either of the other baits. In the other three cases, significantly more whelks were caught with either fish or the combination of fish and crab bait than the crab bait alone. Blue crabs were only caught in traps baited with fish during this experiment as were the majority of spider crabs. It should be noted that the capture of only 17 blue crabs during this experiment was presumably the result of prolonged drought conditions in Georgia that caused the salinity of coastal waters to be elevated. As a coincidence of elevated saline conditions, Hematodinium, a protozoan parasite of crabs, devastated the coastal blue crab population. The blue crab fishery is

17 the second most important fishery in Georgia, but during the course of this experiment no commercial crabbing occurred in Wassaw Sound, since stocks were decimated by disease. Both species of crabs can easily exit the conch pot. The catch rate of untagged whelks decreased with time, while the catch rate of tagged whelks increased. The slight increase in numbers of channeled whelks caught at time three was presumably due to the extra two days that traps and pots were not checked due to adverse weather. This continual decrease in catch would imply that the sampling gear was very efficient at catching whelks within an area. Tagged whelks were moving back into the trapping area from their release point approximately 200 meters away at the intertidal oyster reef located in front of Dead Man Hammock. It has been shown that whelks in Georgia and South Carolina do not migrate far from their release location (Walker, 1988; Anderson et al., 1985). Off the north coast of France, Sainte-Marie (1991) showed that for Buccinum undatum, >97% of tagged whelks move away from their release point at a speed equal to or exceeding that of undisturbed animals. As is evident from similar trapping studies, whelk catches decline over time (Ito and Tachizawa, 1981). Whelks move slowly and have no planktonic dispersal phase, resulting in small local populations. The possibility of quickly recolonizing overharvested areas is very much reduced (Caddy, 1989). Villemure and Lamoureux (1975) showed that B. undatum populations are drastically altered after a few years of low fishing intensity. In Georgia, some fisherman want to harvest whelks with the Chesapeake conch pots; whereas, many fishermen, especially the blue crabbers, are opposed to this. Blue crab fishermen have been criticized by the public for having too many crab trap floats in the water. Floats interfere with recreational boat traffic and are aesthetically displeasing. Blue crabbers have voluntarily reduced the number of floats or traps that an individual crabber may deploy and have started a cleanup campaign for removing old crab traps from the waters. Their concern is that if conch pots are introduced into Georgia, the number of floats will increase and again become the focus of public complaint. Based on the results of this study, the Chesapeake conch pot does not catch more whelks than the standard crab trap. Crab pots consistently caught more whelks of all species than the conch pots. Population studies aimed at estimating the abundance and distribution of the channeled whelk in our coastal waters would also be necessary in order to develop an adequate management plan prior to the commencement of a whelk-trapping fishery in Georgia.

18 References Anderson, W.D., A.G. Eversole, B.A. Anderson and K.B. Van Sant A biological evaluation of the knobbed whelk fishery in South Carolina. South Carolina Wildlife and Marine Resources Department, Marine Resource Division, Charleston, SC, Final completion project R, 72 pp. Caddy, J.F Recent developments in research and management for wild stocks of bivalves and gastropods. In: Caddy, J.F. (ed.). Marine Invertebrate Fisheries: their assessment and management. New York: John Wiley & Sons, Inc. 752pp. Carriker, M.R., Observations on the tightly closing bivalves by Busycon and other predators. Ecology 32: Colton, H.S., How Fulgur and Scotypus eat oysters, mussels, and clams. Proceedings of the National Academy Sciences in Philadelphia, 60: Davis, J.P Observations of prey preference and predatory behaviour in Busycon carica (Gmelin) and B. canaliculata (Linnaeus). Biological Bulletin Mar. Biol. Lab. Woods Hole 161: Hancock, D.A Marking experiments with the commercial whelk (Buccinum undatum). Special Publications of the International Commission of N.W. Atlantic Fisheries Vol 4: Hancock, D.A Whelks. Ministry of Agriculture, Fisheries and Food. Laboratory Leaflet No. 15. Fisheries Laboratory, Burnham on Crouch, Essex, U.K. Himmelman J.H., Movement of whelks (Buccinum undatum) towards a baited trap. Marine Biology, 97: Kent, B.W Patterns of the coexistence in Busyconine in whelks. Journal of experimental Marine Biology and Ecology 66:

19 Ito, H. and S. Tachizawa An estimation of the density of the available stock of a sea snail, Neptunea arthritica, by trap fishing. Bull. Hokkaido Reg. Fish. Res. Lab. No 46: Ito, H.T., T. Wakui, Y. Tateuchi & S. Tachizawa, Observations on the behaviour of a sea snail, Neptunea arthritica toward a trap. Bull. Hokkaido Reg. Fish. Res. Lab. 46: Magalhaes, H., An ecological study of snails of the genus Busycon at Beaufort, North Carolina. Ecological Monograph 18(3): Paine, R.T Ecological diversification in sympatric gastropods of the genus Busycon. Evolution 16: Paine, R.T Trophic relationships of 8 sympatric predatory gastropods. Ecology 1: Sainte-Marie, B Whelk (Buccinum undatum) movement and its implications for the use of tag-recapture methods for the determination of baited trap fishing parameters. Canadian Journal Fisheries and Aquatic Science 48: Villemure, L., and P. Lamoureux, Inventaire et biologie des populations de buccin (Buccinum undatum L.) sur la rive sud de l estuaire maritime du Saint-Laurent en Min. Ind. Com. Cah. Inf. 69: 41p. Walker, R.L Observations on intertidal whelks (Busycon and Busycotypus) populations in Wassaw Sound, Georgia. Journal of Shellfish Research 7: Warren, S Feeding habits of Busycon. Nautilus 30: