Blue , , Trap Selectivity Studies: Mesh Size

Item Type article

Authors Guillory, Vincent; Prejean, Paul

Download date 25/09/2021 22:19:07

Link to Item http://hdl.handle.net/1834/26450 Blue Crab, Callinectes sapidus, Trap Selectivity Studies: Mesh Size

VINCENT GUILLORY and PAUL PREJEAN

Introduction sublegal (<127 mm carapace width jacent to emergent vegetation were (CW)) to legal ratio. Unacceptable sampled at four different sites: Houma Crab traps first appeared in Louisi­ numbers of sublegal blue will be Ship Channel (HSC), Bay Chaland ana in the early 1950's and by the retained if the mesh is too small; con­ (BC), Crooked Bayou (CB), and Lake middle 1960's had replaced drop nets versely, catch of legal blue crabs will Mechant (LM). and trot lines as the dominant gear in be reduced if the mesh is too large. All traps were 60.9 cm (24 inches) the commercial blue crab, Callinectes A 10% tolerance of sublegal blue in width and depth, 50.8 cm (14.5 sapidus, (Guillory et aI., 1996). crabs is allowed in Louisiana (Guillory, inches) in height, and constructed of Vinyl-coated 3.81 cm (1.5 inches) hex­ 1996). The catch and subsequent sale black vinyl-coated wire. Traps with 2.54 agonal or square mesh traps are cur­ of sublegal blue crabs have historically cm (1.0 inch) square mesh (2.54 SQ), rently used almost exclusively by com­ been a major enforcement problem in 3.81 cm square mesh (3.81 SQ), 5.08 mercial crab fishermen in Louisiana and Louisiana. This problem has probably cm (2.0 inches) square mesh (5.08 SQ), along the Gulf of Mexico. There are, become even more prevalent in recent 2.54 cm by 5.08 cm rectangular mesh however, other commercially available years because of several factors (Guil­ (RECT), and 3.81 cm hexagonal mesh wire meshes that could be used for crab lory, 1996): 1) increased effort; (HEX) were compared. traps. Mesh size selection is crucial be­ 2) expansion offishing areas into fresh­ Traps were baited with approxi­ cause it determines the size composi­ water or shallow marsh habitats where mately equal portions of fish and tion of catch, and consequently the sublegal blue crabs dominate; 3) adop­ grouped in replicates when placed in the tion oftraps constructed with 3.81 cm (1.5 water. Traps were hauled approximately inches) square mesh wire, which retains 24 hours after baiting. All blue crabs The authors are with the Louisiana Department of Wildlife and , P.O. Box 189, Bourg, more sublegal blue crabs than the tradi­ were measured in 10 mm CW size LA 70343. tional hexagonal mesh wire traps; and, 4) groups with reference to the minimum removal of liability to dealers and pro­ legal commercial size of 127 mm CWO ABSTRACT-Catch rates and sizes of cessors of sublegal blue crab violations Size group designations were the mini­ blue crabs, Callinectes sapidus, were com­ due to a change in legislative statutes. mum value within the size range. pared in traps with 2.54 cm (/.0 inch), 3.81 Excessive undersize blue crab reten­ The inclusive dates, number of sam­ cm (1.5 inches), and 5.08 cm (2.0 inches) square mesh, 2.54 by 5.08 cm rectangular tion has been recognized since the in­ pling runs (i.e. all traps fished for 24 mesh, and 3.81 cm hexagonal mesh. Catch troduction of traps (Green, 1952); how­ hours), and number of replicates ofeach of legal blue crabs by number was signifi­ ever, research on blue crab trap effi­ trap for each area were: HSC, 3/17/95 cantly greater in the traditional hexagonal ciency (Isaacson, 1962; Eldridge et aI., to 3/30/95, nine trips, six replicates; BC mesh trap than in all other trap types. Sublegal catch by number was highest 1979; Guillory, 1989, 1990; Guillory and CB, 4/27/95 to 6/17/95, 16 trips, (34./-63.3% of total) in the 2.54 cm and and Merrell, 1993) has not considered two replicates; and, LM, 6/29/95 to 7/ 3.81 cm square mesh and rectangular mesh mesh size. The objective of this study 21195, 11 trips, four replicates. traps and lowest in the 5.08 cm square mesh was to compare catch rates and sizes of All data summaries and statistical trap. The hexagonal mesh trap had signifi­ blue crabs collected in traps with vari­ cantly lower catch rates of sublegal blue analyses were conducted using SAS crabs than all other trap types except the ous mesh sizes and shapes. (1988). Catch rates by number (blue crabs/trap-day) and mean carapace 5.08 cm square mesh. Mean size of blue Methods crabs by trap type exhibited an inverse pat­ width by trap type were examined for tern to that shown by catch of sublegal The study was conducted in the significant differences (P<0.05) using crabs. The most effective trap to maximize legal catch and minimize sublegal catch was Terrebonne estuary, Terrebonne Parish, the General Linear Models procedure. the 3.8/ cm hexagonal mesh trap followed south-central Louisiana from March A carapace width-weight regression by the 5.08 cm square mesh trap. through July 1994. Shallow waters ad­ equation developed by Guillory and

59(1), 1997 29 Hein (1995)1 was used to convert blue Table 1.-Mean sublegal (5L), legal (L), and total (TOT) catch per effort by number (CPENO) and mean size (mm carapace width) by trap type and probability levels between trap types (2.5450, 3.8150, and 5.0850 = 2.54, 3.81, crab size to weight. The average catch and 5.08 cm square mesh; RECT = 2.54 by 5.08 cm rectangUlar mesh; HEX = 3.81 cm hexagonal mesh). rate by number in each 10 mm size Trap Type group was multiplied by the calculated weight at the midpoint of the size group Item Mean 3.81S0 RECT 5.08S0 HEX to yield the average catch rate by weight. SL CPENO 2.54 SO 10.95 0.0001 0.0001 0.0001 0.0001 This method was considered adequate to 3.81 SO 5.38 0.2370 0.0001 0.0001 compare pooled data without statistical RECT 4.68 0.0001 0.0001 5.08 SO 0.16 0.0066 evaluation for significant differences. HEX 1.86 L CPENO Results 2.54 SO 3.43 0.6666 0.2970 0.6628 0.0174 3.81 SO 3.56 0.1306 0.3729 0.0432 Number and Weight of Crabs RECT 3.10 0.5315 0.0006 5.08 SO 3.29 0.0043 Catch rate of legal blue crabs by num­ HEX 4.23 TOTCPENO ber was greatest in HEX traps and low­ 2.54 SO 14.38 0.0001 0.0001 0.0001 0.0001 est in 5.08 SQ and RECT traps (Table 3.81 SO 8.93 0.1477 0.0001 0.0007 RECT 7.78 0.0001 0.Q385 1). Legal catch rate was significantly 5.08 SO 3.12 0.0009 greater in HEX traps than in other traps; HEX 5.99 no other trap combinations were signifi­ SIZE 2.54 SO 112.0 0.0001 0.0001 0.0001 0.0001 cantly different. 3.81 SO 122.9 0.5027 0.0001 0.0001 RECT 122.1 0.0001 0.0001 Sublegal catch rate by number was 5.08 SO 146.0 0.0001 significantly greater (P<0.0001) in the HEX 133.8 2.54 SQ trap and significantly lower (P<0.0001) in the 5.08 SQ trap than in other traps (Table 1). Sublegal catch rate Table 2.-Percent sublegals and catch per effort by weight (CPEWT) in grams by trap type (5L=sublegal, L=legal, TOT=total, 2.5450, 3.8150, and 5.0850 = 2.54, 3.81, and 5.08 cm square mesh, respectively; RECT = 2.54 by 5.08 was significantly lower (P<0.0001) in cm rectangular mesh; HEX = 3.81 cm hexagonal mesh). the HEX trap than in all traps except Trap Type the 5.08 SQ trap. Sublegal catch rate in the 2.54 SQ, 3.81 SQ, and RECT traps Item 2.5450 3.81S0 RECT 5.08S0 HEX ranged from 63.1 to 76.1 % of the total CPEWT·SL 735.5 607.2 382.2 14.3 169.9 CPEWT·L 519.2 550.5 478.3 560.0 672.2 (Table 2). The 5.08 SQ trap was the only CPEWT-TOT 1.252.7 1.157.7 860.5 574.3 842.1 trap with less than the allowable Percenl SL 76.1% 63.3% 65.1% 5.0% 34.1% sublegal tolerance of 10%. The catch rate of sublegal blue crabs in the hex­ agonal mesh trap was reduced 46.2% found at LM, the density of blue crabs Catch per effort per size group for from the 3.81 cm square mesh trap; may have been too low for trap satura­ each trap are tabulated in Table 3. The these two traps are commonly used by tion effects attributed to high densities modal peaks for each mesh type were commercial fishermen. of sublegal blue crabs (Guillory and 97 mm CW for the 2.54 SQ trap, 107 Overall catch rate by number varied Merrell, 1993) to influence catch rates mm CW for 3.81 SQ and RECT traps, significantly among all traps except the of legal blue crabs. 127 mm CW for the HEX trap, and 137 3.81 SQ and RECT traps (Table 1). The catch rates by weight by trap type mm CW for the 5.08 SQ trap. The size Highest numbers were taken in the 2.54 are given in Table 2. Weight of legal range of retained blue crabs was identi­ SQ trap and lowest numbers in the 5.08 blue crabs was highest (20% greater cal in all traps except the 5.08 SQ trap. SQ trap. than any other trap) in the HEX trap, The HEX trap, the most common trap Statistical differences in sublegal and followed by the 5.08 SQ and 3.81 SQ historically, was considered as a con­ legal catch rates between traps within traps. Overall weight was greatest in the trol to compare carapace width fre­ areas were similar to the pooled data, 2.54 SQ and 3.81 SQ traps because of quency distributions. Except for the with the exception of LM, where no sig­ high numbers of sublegal blue crabs. 5.08 SQ trap, the carapace width fre­ nificant difference in legal catch rate quency distributions for the experimen­ was found between any trap. Since the Size tal traps reflected higher retention in lowest overall catch rates were also Mean size was significantly lower size groups below 127 mm CW and (P<0.0001) in the 2.54 SQ trap and sig­ decreased catch rates above 127 mm nificantly higher (P<0.0001) in the 5.08 CWo The 5.08 SQ trap yielded de­ 1 Guillory, v., and S. Hein. 1995. Lateral spine variability and weight-size and carapace width­ SQ trap than other traps (Table 1). Mean creased catch rates of sublegal and small size regressions in blue crab (Callinectes size in HEX traps was significantly legal (127-136 mm CW) blue crabs and sapidus). Unpubl. Manuscr. on file at La. Dept. Wild!. and Fish., P. O. Box 189, Bourg, LA greater (P<0.0001) than other traps ex­ increased catch rates in larger blue crabs 70343. cept the 5.08 SQ trap. (~137 mm CW) than in HEX traps.

30 Marine Fisheries Review Table 3.-Catch rates by number by 10 mm carapace-width size groups by trap type (HEX = 3.81 cm hexagonal Brown, C. G. 1982. The effect of escape gaps on mesh; 2.54 sa, 3.81 sa, and 5.08 sa =2.54, 3.81, and 5.08 cm square mesh, respectively; RECT =2.54 by 5.08 cm trap selectivity in the United Kingdom crab rectangular mesh). (Cancer pagurus L.) and lobster (Homarus [L.]) Catch rates(no.) by trap types gammarus fisheries. J. Cons. Cons. Int. Explor. Mer 40: 127-134. Size group HEX 2.54 sa 3.81 sa 5.08 sa RECT Brown, R. S., and N. Caputi. 1983. Factors af­ fecting the recapture of undersize western 67-76 0.01 0.85 0.02 0 0.04 rock lobster (Panulirus longipes cygnus 77-86 0.Q1 1.30 0.06 0 0.02 George). Proc. Indo-Pac. Fish. Counc. 14: 87-96 0.06 1.94 0.26 0.01 0.19 139-154. 97-106 0.17 2.76 1.25 0.03 1.23 107-116 0.55 2.40 2.21 0.06 1.73 Davis, G. E. 1981. Effects of injuries on spiny 117-126 1.03 1.69 1.58 0.07 1.47 lobster, Panulirus argus, and implications for 127-136 1.57 1.58 1.46 0.58 1.35 fishery management. Fish. Bull. 41(1):21-27. 137-146 1.16 0.94 1.06 1.12 0.81 Eldridge, P. J., V. G. Burrell, Jr., and G. Steele. 147-156 0.85 0.51 0.61 0.86 0.53 1979. Development of a self-culling blue crab 157-166 0.36 0.28 0.28 0.48 0.27 167-176 0.18 0.09 0.12 0.16 0.10 pot. Mar. Fish. Rev. 41(11-12):21-27. 177-186 0.06 0.Q1 0.02 0.08 0.03 Fogarty, M. J., and D. V. Borden. 1980. Effects 187-196 0.04 0.01 0.01 0.Q1 0.01 of trap venting in gear selectivity in the in­ shore Rhode Island American lobster, Hom­ arus americanus, fishery. Fish. Bull. 77:925­ 933. Discussion sublegal blue crabs that may occur dur­ Green, J. C. 1952. Effectiveness of crab traps in ing trap confinement or during onboard South Carolina. Bears Bluff Lab., Contrib. No. The carapace width frequency distri­ 14,12 p. culling would be reduced. Over half of butions reveal that the size range ofblue Guillory, V. 1989. An evaluation of different es­ blue crabs caught in traps have damaged cape vents in blue crab traps. Proc. La. Acad. crabs retained by each trap type was body parts (Eldridge et aI., 1979); inju­ Sci. 52:29-34. similar, although the relative abundance ___. 1990. Effects ofescape vents on catch ries may result in smaller molt incre­ by size group differed between traps. rates of premolt blue crabs. Proc. La. Acad. ments in various decapods (Van Engel, Sci. 53:20-28. Significant differences were found in 1958; Davis, 1981). Physiological _:-:--,---' 1993. Ghost fishing in blue crab traps. catch rates of legal and sublegal blue N. Am. J. Fish. Manage. 13:457-466. stress, dehydration, and gill damage due crabs and mean sizes between traps. As _-;-;-__ . 1996. A management profile of the to air exposure during onboard culling blue crab, Callinectes sapidus. La. Dep. Wildl. compared to the standard HEX trap, the may result in delayed mortalities to de­ Fish., Fish. Manage. Plan Ser. 8, Pt. 2, 34 p. 2.54 SQ, RECT, and 3.81 SQ traps re­ capods (Lyons and Kennedy, 1980; Guillory, v., and J. Merrell. 1993. An evaluation tained excessive numbers of sublegal of escape rings in blue crab traps. La. Dep. Brown and Caputi, 1983; Hunt et aI., Wildl. Fish., Tech. Bull. 44, 29 p. blue crabs while the 5.08 SQ trap had a Guillory, V., P. Prejean, M. Bourgeois, J. Burdon, significant reduction in catch oflegal blue 1986). Second, ghost fishing mortality and J. Merrell. 1996. A biological and fisher­ crabs. The HEX trap was the most effi­ ies profile of the blue crab, Callinectes would be reduced because of decreased sapidus. La. Dep. Wildl. Fish., Fish. Manage. cient trap in terms of maximizing legal catches of sublegal blue crabs. Mortali­ Plan Ser. 8, Pt. 1,210 p. catch and minimizing sublegal catch; ties in ghost traps averaged 25.8 blue Hunt, J. H., W. G. Lyons, and F. S. Kennedy, Jr. however, the unculled catch still exceeds 1986. Effects ofexposure and confinement on crabs/trap for one year (Guillory, 1993) spiny lobster, Panulirus argus, used as attrac­ the 10% sublegal tolerance. The 5.08 SQ and 17.3 blue crabs/trap for 3 months tants in the Florida trap fishery. Fish. Bull. trap was the next most efficient trap. 84(1):69-76. (Arcement and Guillory, 1993). Mortal­ The reduction in sublegal catch, Isaacson, P. A. 1963. Modifications of Chesa­ ity in vented traps was about one-third peake Bay commercial crab pot. Commer. while maintaining legal catch, through that of unvented traps because of a re­ Fish. Rev. 25(1): 12-16. optimum mesh size selection would Krouse, J. S. 1978. Effectiveness of escape vent accrue many benefits to the fishery. duction in sublegal blue crab catch shape in traps for catching legal-sized lobster, First, catch rates oflegal blue crabs may (Arcement and Guillory, 1993). Homarus americanus, and harvestable-sized Third, fishing efficiency of commer­ crabs, Cancer borealis and Cancer irroratus. increase immediately because of trap Fish. Bull. 76:425-432. cial fishermen would be increased be­ saturation effects that occur due to ex­ Krouse, J. S., and J. C. Thomas. 1975. Effects of cessive retention of sublegal blue crabs cause of a reduction in onboard culling trap selectivity and some population param­ time. eters on size composition of the American (Guillory and Merrell, 1993) and later lobster, Homarus americanus, catch along the because of decreased fishing and han­ Acknowledgments Maine coast. Fish. Bull. 73:862-871. dling/trap confinement mortalities on Lyons, W. G. and F. S. Kennedy, Jr. 1980. Ef­ The following individuals assisted fects of harvest techniques on sublegal spiny smaller blue crabs. Increased catches of lobsters and on subsequent fishery yield. Proc. legal-sized decapods have been demon­ with field work or trap construction: Gulf Caribb. Fish. Inst. 33:290-300. Willard Dupre and Jerry Merrell. SAS, Inc. 1988. SAS introductory guide for per­ strated in traps where escape vents were sonal computers, release 6.03 edition. SAS used to reduce sublegal catch (Krouse Literature Cited Institute Inc., Cary, N. C., III p. and Thomas, 1975; Fogarty and Borden, Van Engel, W. A. 1958. The blue crab and its Arcement, G., and V. Guillory. 1993. Ghost fish­ fishery in Chesapeake Bay. Part 1. Reproduc­ 1980; Brown, 1982; Guillory and ing in vented and unvented blue crab traps. tion, early development, growth, and migra­ Merrell, 1993). Injuries or stress to Proc. La. Acad. Sci. 56:1-7. tion. Commer. Fish. Rev. 29(6):6-17.

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