Bryan, C.F.

Hartman, R.D.

Korth, J.W.

Northeast Gulf Science

English

Aquila Digital Community

Northeast Gulf ScienceVolume 10Number 2 Number 2 Article 108-1989An Adjustable Macroplankton Gear for ShallowWater SamplingC.F. BryanLouisiana State UniversityR.D. HartmanLouisiana State UniversityJ.W. KorthLouisiana State UniversityDOI: 10.18785/negs.1002.10Follow this and additional works at: https://aquila.usm.edu/gomsThis Article is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Gulf of Mexico Scienceby an authorized editor of The Aquila Digital Community. For more information, please contact Joshua.Cromwell@usm.edu.Recommended CitationBryan, C., R. Hartman and J. Korth. 1989. An Adjustable Macroplankton Gear for Shallow Water Sampling. Northeast Gulf Science 10(2).Retrieved from https://aquila.usm.edu/goms/vol10/iss2/10Northeast Gulf Science Vol. 10, No. 2 August 1989 159 AN ADJUSTABLE MACROPLANKTON GEAR FOR SHALLOW WATER SAMPLING Plankton gear are ideal for sampling fishes and crustaceans early in their life histories when they are perhaps more randomly distributed, and their densities more quantifiable than at any other life stage. The late Dr. Elbert H. Ahlstrom described the plankton net as a "simple and thorough" sampling device (Lasker 1984). The gear are used to estimate cohort size and potential recruits to adult fish populations, and are adaptable to a variety of field settings (Aron 1962, Dovel 1964, McGowan and Brown 1966, Tranter and Smith 1968, Hempel and Weikert 1972, Bagenal and Nellen 1980, and Snyder 1983). Principal among the concerns of macroplanktologists have been: 1) net clogging before the sampling effort is completed; 2) net avoidance by larvae due to gear visibility or bridle deflection; 3) sufficient volume filtered to accom-modate for sparseness and patchiness of the organisms; 4) adequate estimates of volume filtered; 5) sampling near the shoreline or structure; and 6) nets that open and close for sampling discrete depths. Unfortunately, gear modifica-tions designed to accommodate those concerns often produce one or more of the following shortcomings: 1) small eggs and larvae are extruded through meshes coarser than 0.5 mm; 2) long nets with length to diameter ratios >5:1 are expensive and cumbersome to use and clean; 3) large nets (>1.0 m in diameter) or net series that open and close require either a large research vessel, or modifi-cations which make the vessel less suit-able for other collection activities; 4) sta-tionary and passive gear are inefficient in slack currents; and 5) centrifugal pumps filter small volumes, or harvest from fairly localized regions and conse-quently yield inaccurate estimates of abundance and distribution. In an effort to reduce some of the shortcomings of standard macro-plankton gear, we designed an apparatus for sampling in a shallow tidal channel near Sabine Lake in southeast Texas. MATERIALS AND METHODS An inexpensive, removable frame was bolted to the gunwales near the bow of a shallow-draft vessel from which plankton nets were suspended (Fig. 1). Any aluminum welding shop should be able to construct the frame. Aluminum plates (13 mm thick) and pipes (up to 55 mm outside diameter with 5 mm walls) proved sufficiently sturdy for our 0.5-m nets fitted with 0.505-mm mesh, and should be able to withstand forces developed by nets up to 1.0 m in dia-meter. Angular bracing (6 mm thick) was used throughout the frame to reinforce aluminum welds. The net frames could be any size and adapted for any shallow-draft vessel, so long as hinges extend the net support plates outside the gunwales of the boat. We attached marine plywood (A, Fig. 1) inside the frame to provide Figure 1. An adjustable shallow water sampling assembly for collecting macroplankton. 1Bryan et al.: An Adjustable Macroplankton Gear for Shallow Water SamplingPublished by The Aquila Digital Community, 1988160 Short papers and notes extra working surface area. Triangular plates (B) were hinged to each end of the frame. A pipe sleeve (C) was welded vertically to the triangular plate. The pipe sleeve was 55.0 mm in diameter, with a wall thickness of 5.0 mm. Net poles (D) are 45 mm in diameter, with 3.0-mm wall thickness. They slide up and down and twist in the sleeves to pro-vide for opening and closing net mouths for fishing specific depths. Net frames (E) are welded to the bottom of each pole. Plankton nets were tied to these frames. Horizontal pipe sleeves (F) mounted on the frame near the hinges guide support poles (G) that slide to the stern when the nets are not fishing. Ropes from the frame through the eyes (H) of the support poles were tied above the nets for rein-forcement. Set screws or pins (I) hold the poles in position while sampling. Approx-imately 30 minutes were required to assemble the apparatus. After zeroing the flowmeters (Fig. 1, J), net poles were submerged to appro-priate depths and twisted so that the net mouth was open for sampling at the specified depth for the specified length of time. Support poles were extended for-ward until the ropes were taut. The boat was anchored in the current to fish passively, or moved ~ 1.0m/sec upstream. After sampling, the net poles were twisted 90° to close the mouth of the net and pulled up and across the bow. The net mouth was then twisted facing upward with the net suspended over the water where it was washed using a bilge pump fitted with a garden hose and nozzle. RESULTS We collected over 1000 samples in a 30-m wide x 4-m deep tidal channel near Sabine Lake, Texas, from July 1984 through April1986 (Hartman eta/. 1987). Although we encountered no underwater obstruction, we cracked aluminum welds on two occasions when pushing the nets at speeds > 1.0 m/sec. After reinforce-ment with wedge-shaped braces welded to the vertical pipes and hinged plates, the gear proved sufficiently strong for simultaneously pushing two 0.5-m nets of 0.505-mm mesh at depths up to 4 m and speeds up to 1.3-m/sec. With respect to the above-listed con-cerns of planktologists, we witnessed the following: 1) The length-to-diameter ratio of the nets was 3:1 and the 0.505 mm mesh clogged only when volumes filtered exceeded 150m3 or the water was particularly high in suspensoids. During calibration there was linearity of response in revolutions per tow length registered on the flowmeters at current speeds from 0.1 to 1.1 m/sec. 2) Densities of late larval stages and juveniles were greater at night than during the day, sug-gesting gear avoidance; however, fishes approaching the nektonic habit ap-parently avoided light and were captured near the bottom during the day (Hartman eta/. 1987). 3) The sample volume filtered averaged 89 m3; however, nets up to 1.0 m in diameter, or longer pushes, could have been made to increase sample volume. 4) Organisms could be simul-taneously collected from two depths near the boat bow, thus avoiding the bow wake and prop wash. 5) Net poles and the vessel were easily maneuvered to avoid obstructions. Another attribute was that the poles could be folded inboard when travelling between collecting stations, and stored in the boat when trailing to and from the laboratory. LITERATURE CITED Aron, W. 1962. Some aspects of sampling the macroplankton. Conseil lnt'l Expl. 2Gulf of Mexico Science, Vol. 10 [1988], No. 2, Art. 10https://aquila.usm.edu/goms/vol10/iss2/10DOI: 10.18785/negs.1002.10Northeast Gulf Science Vol. 10, No.2 August 1989 161 Mer, Rapp. Proc. Verb. Reunions. 158:29-38. Bagenal, T. B., and W. Nellen. 1980. Sampling eggs, larvae and juvenile fish. PP. 13-36. InT. Backiel and R. L. Welcomme (editors). Guidelines for sampling fish in inland waters. FAO Tech. Pap. 33. European Inland Fish. Adv. Comm., Rome, Italy. Dovel, W. L. 1964. An appraoch to samp-ling estuarine macroplankton. Chesa-peake Sci., 55:77-90. Hartman, R. D., C. F. Bryan, and J. W. Korth. 1987. Community structure of fishes and crustaceans in a southeast Texas estuary. Final Report. U.S. Fish and Wildlife Service, Albuquerque, NM., iii + 116 pp. Hempel, G. and H. Weikert. 1972. The neuston of the subtropical and boreal northeastern Atlantic Ocean - a review. Mar. Bioi. 13:70-88. Lasker, R. 1984. Dr. Ahlstrom. p. ix. In Moser, H. G., W. J. Richards, D. M. Cohen, M.P. Fahay, A. W. Kendall, Jr., and S. L. Richardson (editors). Onto-geny and systematics of fishes. Spec. Pub I. Number 1, Amer. Soc. lch. Herp. Allen Press Inc., Lawrence, Kansas. McGowan, J. A. and D. M. Brown. 1966. A new opening-closing paired zoo-plankton net. University Qf California. Scripps Institute of Oceanography. Reference 66-23, LaJolla, California. Snyder, D. E. 1983. Fish eggs and larvae. pp. 165-197. In Nielson, L.A. and D. L. Johnson (editors). Fisheries Tech-niques. Amer. Fish. Soc., Bethesda, Maryland. Tranter, D. J., and P. E. Smith. 1968. Filtration performance. pp. 27-56. In D. J. Tranter, and J. H. Fraser (editors). Zooplankton sampling. UNESCO, Monogr. Oceanogr. Meth. 2, Paris, France. Bryan, C. F., R. D. Hartman, and J. W. Korth, Louisiana Cooperative Fish and Wildlife Research Unit*, School of Forestry, Wildlife, and Fisheries, Louisiana Agricultural Experiment Station, Louisiana State University, Baton Rouge, LA 70803, (504) 388-4184 *Jointly supported by the Louisiana State Univer-sity and Agricultural and Mechanical College, Louisiana Department of Wildlife and Fisheries, and U.S. Fish and Wildlife Service; The Wildlife Management Institute cooperating. 3Bryan et al.: An Adjustable Macroplankton Gear for Shallow Water SamplingPublished by The Aquila Digital Community, 1988

An Adjustable Macroplankton Gear for Shallow Water Sampling

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