Monitoring the Recruitment of Postlarval Caribbean Spiny Lobster (Panulirus argus) Using Witham Artificial Collectors Mark Butler Department of Biological Sciences Old Dominion University Norfolk, Va 23529-0266 USA mbutler@odu.edu RATIONALE Recruitment of marine species has been variously defined depending on the species and circumstances. In this instance, I specifically refer to recruitment of postlarvae, that is, the arrival or supply of planktonic spiny lobster postlarvae to coastal areas from the offshore oceanic plankton where they resided for many months as larvae. I also refer specifically to a single species of lobster the Caribbean spiny lobster, Panulirus argus because the methods used and their success vary among lobster species. The monitoring of postlarval lobster recruitment has seen many uses, including the estimation of future fishery stock, the experimental investigation of limits to recruitment, and the acquisition of postlarvae for aquaculture or wild stock enhancement. A variety of artificial collectors have been developed to estimate the relative abundance of new lobster recruits, and their general construction and widespread use has been reviewed elsewhere (see Phillips & Booth 1994). Several types of collectors have been used to collect P. argus in the Caribbean over the years, but three have been particularly successful and are currently in use (Fig. 1). The Phillips collector (developed in western Australia) is in use in Cuba, the GuSI collector is used in Mexico, and the Witham collector is used in Florida, Bermuda, the Bahamas, and in Belize. All are effective at capturing P. argus postlarvae, although their rates of capture and relative cost of construction and maintenance vary. The Witham collector is the only device whose catch has been shown to be correlated with planktonic abundance and with settlement of postlarvae in the region downstream of the collector (Herrnkind & Butler 1994). Witham collectors will be used as part of the Coral Reef Targeted Research Recruitment program because of their current widespread use and proven utility. Their use is described here. Figure 1: Three types of artificial collectors now used in Caribbean to capture postlarval spiny lobster (P. argus). At left is the Phillips collector, at center is the GuSI collector, and at right is the Witham collector. Drawings are not to scale and the design details of each collector can vary among users. 1
METHODS Site selection: The optimal site for Witham collectors is on the landward edge of a channel leading from offshore into a coastal lagoon, these areas tend to concentrate postlarvae as they are funneled into the channel before being dispersed into the lagoonal nursery (Fig. 2A). A second choice is to place them on the edge of an island closest to the lagoonal nursery, where the postlarvae may be concentrated by tidal eddies (Fig. 2B). Do not place them directly within a channel and its high currents, but in areas adjacent to these. Witham collectors should be placed in shallow water < 3m deep over sparsely covered hard-bottom or sandy substrates. Collectors placed over potentially good settlement habitat (e.g., algal-covered hard-bottom, dense seagrass, rubble reef) will not yield reliable estimates or good catches of postlarvae. Collectors should be placed no closer than 20m to another collector. Channel Island x x x x Figure 2: Shaded areas indicate shallow water, the arrow shows the direction of water flow on a rising tide, and X indicate alternative positions for placing the collectors (note: multiple collectors at any position are needed. (A) Channel situation and (B) Island situation. Three to five collectors are typically maintained at each monitoring site to provide a sufficient sample size for postlarval estimation. However, this number could be increased or decreased after preliminary sampling for one year and after proper sample size estimation using power analysis. One year is often required for preliminary sampling because of the extreme month-to-month variability in postlarval supply. For the TR project, we are initially using three collectors per site. Please maintain that number until we have sufficient data from all of the sites and have had the opportunity to analyze those data so as to assess our sampling effort at each core area. Construction: Detailed instructions for collector construction can be found in Appedix A. In brief, the frame of the collector is made of ¾ inch pvc pipe, pvc t fittings, and pvc elbow fittings glued together with pvc cement. A length of 3/8 inch rope runs through the frame and ends in a brass clip, which allows the collector to be easily attached and detached from its mooring. Strips of hogs hair air conditioning filter material hang over each rung of the ladder-like frame and are attached to the frame with cable ties. Four buoys or floats are attached via rope to the four corners of the frame. The mooring is simply an anchoring system of some type; often two concrete blocks are used (preferably with holes filled with concrete and stainless steel eyebolts cemented in). An appropriate length of rope is attached to the anchoring system with the other end tied to a surface buoy; a loop in the line near the surface serves as an attachment point where the collector is clipped to the mooring (Fig. 3). 2
Figure 3: Diagramatic representation of a deployed Witham collector. The mooring shown is a concrete block with a stainless steel eyebolt to which a rope with a buoy is attached. A brass clip connects the collector to a loop in the mooring rope. Sampling & Maintenance: For monitoring purposes, Witham collectors are sampled during the day once a month 7 days after new moon. This results in 13 sample periods per year (13 lunar periods). A small vessel capable of carrying at least two persons is needed, as is a large mesh bag with handles (one per core area provided to TR Project personnel). As the vessel approaches the collector from downstream, one person grabs the mooring buoy, unclips the collector from the mooring, and guides the collector in the water into the large mesh bag held by the second person. Both then lift the collector from the water, let the water drain from the bag, then bring the collector onboard. The collector is removed from the bag, and turned upside down on a flat surface, where each of the hogs hair leaves is sequentially turned (like pages in a book) and carefully scanned for postlarvae. The mesh bag is also searched. The data are then recorded for each Witham collector separately. The number of transparent postlarvae, pigmented postlarvae, and early benthic juvenile lobsters are recorded (see data sheet in Appendix B). A variety of other species will recruit to these collectors too, so one may desire to enumerate or collect these two this is not necessary for the TR Project. When completed, the collector is returned to its mooring. Not counting travel time to the site, sampling three Witham collectors typically requires about 30 60 minutes. The hogs hair filter material must season for 1 month in seawater before it becomes attractive to postlarvae. Also, the filter material must be replaced every 3 6 months because they become fouled and no longer attractive to postlarvae. After the first few months of use, a rotation schedule where one collector at each site is replaced each month during normal sampling with a spare collector with new hog s hair works well. A rotation schedule for the TR project is provided below as an example of how the collectors should be rotated for a single site. For the TR project, we will have one spare collector available per sampling site (i.e., two per core area). This extra collector is taken to the site to replace the collector that is scheduled to be rotated out of service. The filter material on the collector taken from the field is then removed and new filter material attached to the old collector frame so that it is then ready for the next month s rotation. The pvc frames are repaired or replaced as needed. 3
Witham Collector Rotation Schedule per Site Sample Month Collector Number Deployed in Field Extra Collector 1 3 1 2 3 4 4 4 2 3 1 5 4 1 3 2 6 4 1 2 3 7 3 1 2 4 8 3 4 2 1 9 3 4 1 2 10 2 4 1 3 11 2 3 1 4 12 2 3 4 1 4
Literature Cited: Herrnkind, W.F. & M.J. Butler IV. 1994. Settlement of spiny lobsters, Panulirus argus in Florida: pattern without predictability. Crustaceana 67: 46-64. Phillips, B. F. & Booth, J. D. 1994. Design, use and effectiveness of collectors for catching the puerulus stage of spiny lobsters. Reviews Fish Biology 2: 255-289. 5
Appendix A Detailed Instructions for Constructing Witham Postlarval Lobster Collectors Tools Needed (Fig. 1) pvc pipe cutter carpet knife or scissors measuring tape knife or screw driver pvc cement duct tape (to keep rope from fraying) pencil to mark pvc pipe for cutting Materials List per Collector (6) 15 inch x 30 inch pieces of hog s hair air conditioning filter material (¼ of the 30ft roll in which this material is typically sold) (6) 16 inch pvc pipe (3/4 inch diameter; schedule 40 thick walled) Fig. 1: Top from left: pvc cutter, pvc cement, (10) 1 ¾ inch pvc pipe carpet knife, scissors. Bottom from left: (10) ¾ inch T fittings measuring tape, duct tape, pencil. (2) ¾ inch 90 degree elbow fittings (24) 8 or 10 inch long cable ties (1) brass, aluminium, or stainless steel spring clips with swivel (5) bouys (6 8 inch diameter) 25ft of garden hose (for rope chaffing protection) (2) concrete blocks; 2 or 3 holes sufficient concrete mix to fill holes in concrete (2) eye bolts stainless steel, aluminium or galvanized steel approximately 10m of 3/8 inch rope (depending on depth of water where collector is deployed) Instructions (1) cut pvc pipe to required dimensions and acquire t and elbow fittings (Fig. 2). Precise cutting of pvc pipe lengths and proper seating of pipe in each fitting will make construction much easier. Fig. 2: Top: 16 inch long pvc pipe. Bottom: t fitting, 1 ¾ inch long pvc pipe, 90 degree elbow fitting. All ¾ inch pvc. 6
(2) glue middle of pvc t fittings onto ends of 16 inch pvc pipes; these form a rung in the collector frame ladder. There will be five of these. (3) glue two 1 ¾ inch pvc pipes into two opposing ends of t fittings, then glue the two pvc rungs together. Do this so that you have the 5 rungs connected; the first rung will not have a 1 ¾ pipe protruding and the last will have a 1 ¾ pipe protruding from each t fitting (Fig. 3). Fig. 3: pvc pipe ladder with 5 rungs glued together. (4) run the end of the rope through one end of the ladder with the open t fitting, then out through the 1 ¾ pvc pipe end, then through an elbow fitting, a 16 inch pvc pipe, an elbow fitting, and then back into the opposite side of the ladder with the 1 ¾ inch pipe and out the ladder through the t fitting end (Fig. 4a). This is necessary so the rope can make the complete loop through the pvc frame. Now glue the elbows to the last 16 inch pvc pipe and then to the end of the frame (Fig. 4b). Fig. 4: (a) rope is strung through frame, elbow, 16 inch rung, elbow, and then back through the frame. (b) elbows cemented to 16 inch rung before rest of frame. 7
(5) Tie the tag end of rope to standing line of rope using a bowline knot, then secured with an overhand knot. At the end of the standing line, tie the spring clip (Fig. 5). Fig. 5: Rope tied through completed frame and then to spring clip. (6) The hogs hair filter media comes in 30ft long x 30 inch wide rolls (Fig. 6a); cut the roll into 15 inch long x 30 inch wide pieces (Fig 6b). One roll will make enough for 4 collectors. Fig. 6: (a) At left: two rolls of hog s hair filter material, the one at right still in wrapping. (b) cutting the 15 inch sections from the roll. 8
(7) Fold a piece of filter material lengthwise with the mesh webbing side to the outside and the hairy side inside, then slip over one of the pvc rungs of the frame (Fig. 7a). Secure with 3 or 4 cable ties wrapped around material and rung. Poking a whole through the filter material first with a knife or screw driver makes it easier to push the cable tie through the material. Repeat this for each of the 6 rungs on the collector frame (Fig. 7b). Fig. 7. (a) At left, one piece of filter material attached to frame. (b) At right, all 6 pieces of filter material attached to collector frame with cable ties. (8) Attach 4 buoys to 4 corners of frame using rope; make sure buoys protrude from top of collector the top is what you see in Figure 7b. The collector is complete. Now for the mooring. (9) Mix concrete and pour into holes of concrete blocks; into one hole insert the eyebolt so that it protrudes above block. Insert into same hole orientation in each block. For example, center hole of each block as shown in Figure 8. Figure 8. A series of concrete blocks filled with concrete and an eyebolt cemented into the center hole of each. In this case, the eyebolts were quite large but smaller eyebolts are OK as long as the rope can pass through the eye. 9
(10) When ready to deploy collectors. Take collectors, concrete block anchors, remaining buoys (one per collector), rope, and 2 3 ft lengths of garden hose (one for each mooring) in boat to site. At site, determine depth so can assess how long rope of mooring needs to be. Cut rope to appropriate length (approximately depth plus 2 meters. Pass tag end of rope through garden hose then both hose and rope through eyebolts in two concrete blocks. Tie tag end of rope to standing line with bowline knot secured with an overhand knot. Tie one buoy to end of standing line then add an overhand knot loop to line 1-2 feet below buoy. This constitutes the mooring for the collector. The collector clips to the mooring loop via its spring clip (Fig. 9). Figure 9: At left, a completed mooring. At right, a collector (without its buoys) is clipped to the mooring. (11) Clip the collector to the loop in the mooring line. Drop concrete blocks over the side of the boat making sure that the blocks are upright, the eyebolts point toward the surface, and the blocks do not chafe the line. Then pay the collector line out so that it deploys upright in the water (Fig. 10). Figure 10: A collector deployed in the field as viewed from below the surface. 10
Appendix B Example Data Sheet for Witham Postlarval Lobster Collector Postlarval Lobster Collector Data TR Project Core Area: Site: Date: Observer: Lat: Lon: Collector Date Number of Individuals Remarks Number Deployed Trans* Pig* Juv* Total Core Area: Site: Date: Observer: Lat: Lon: Collector Date Number of Individuals Remarks Number Deployed Trans* Pig* Juv* Total 11