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Hydrobiologia vol. 65, I, pag. 65-68, 1979
A SIZE SELECTIVE UNDERWATER LIGHT TRAP R. B. AIKEN Department of Zoology, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L IC6 Received August 19, 1978
Keywords: light trop, size selective, aquatic insects
Abstract A size selective underwater light trap is described. Trap records indicate that the trap is effective in taking a wide variety of organisms within size limits that can be set by the experimenter.
Introduction Hungerford et al. (1955) described the construction and operation of an underwater light trap. Their records indicate that such traps are very efficient in the capture of large numbers of aquatic insects. Data from Engelmann (1974) and Engelmann & Tobisch (1972), indicate that such traps take virtually all orders of aquatic insects as well as mites, spiders, fish and salamanders. Traps with such catholic capture results make it necessary for the experimenter to sort through great numbers of organisms to select those he may want. The following paper described an underwater light trap that allows the experimenter to be at least size selective in the creatures taken.
Materials and methods Trap Design
The trap (Fig. I) is a rectangular box 62 cm high and 22 cm on a sice. The top (and uppermost 15 cm) of the sides are made of solid 6 mm plexiglass, enclosing an air space to allow animals with a physical gill type of respiration
Dr. W. Junk b.v. Publishers- The Hague, The Netherlands
access to fresh air. Three sides of the trap consist of two elements. The first is a fixed wall 6.5 cm wide x 44 cm tall that angles back slightly (250) toward the centre of the trap. The second element is a moveable gate-4.5 cm wide-that is hinged on the fixed portion of the wall. The fourth side of the trap is not angled and has no moveable gates. This side consists of two 7.5 cm wide plexiglass panels separated by a I6 and 5 cm area of fibreglass screen. The panels each hold two L-shaped brackets that accept the light chamber. The bottom 8.5 cm panel of the fourth side is a sliding plexiglass door through which the trap contents could be removed. The light chamber (Fig. ) is a solid plexiglass box 9.5 cm wide x 3.5 cm deep x 45 cm tall. One side of the chamber was 3.5 cm wide, the additional width forming a flange which fits into the brackets mentioned above. The top of the light chamber was fastened by wood screws and lined with a rubber gasket. The light was turned on by depressing a flexible rubber sheet which covered a hole in the side of the chamber over the switch. The light was a 12 volt 8" fluorescent tube that was connected to an automobile battery. Field Testing The trap was tested in a small pond (formerly a gravel pit) near Campbellville, Ontario on Io August, 1977. Previous collections yielded a wide variety of aquatic macroinvertebrates. For testing, the trap was set in the pond so that the air cavity and the top 5 cm of the gates were above the surface of the water. The gates were then set at the desired width and the light connected to the battery. After 30 minutes, the gates were closed and the trap carefully lifted from the 65
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I: pond. The contents were emptied into a sorrting tray and preserved in Kahle's fluid. Three gate widtlls-I.5, 5 and io mm.-were tested. The animals collectedI were sorted, ed taxon. the measured and identified. For each identified largest transverse distance in the both dorsi o-ventral and lateral plane were measured. The smallest of these two distances was labelled the least transversse dimension (LTD) and recorded.
Results and discussion The results of the trapping experiment are presented in Tables I and 2 and Fig. 2. The trap is indeed size selective. Results show that the animals taken had an LTD less than the trap gate width. While it may seem a trivial point that no animal larger than the gate width was taken, it does demonstrate that larger animals could not force their way past the hinged gate. It is also true that there were no fish present in the pond tested. Previous trials in areas with fish present show that Lepomis gibbosus (L.) could force the gates open so that some type of fastening device would be necessary. In several cases (e.g. Acarina, Oreodytes, Gerris nymphs, Ephemerella) the size of animal within a taxon increased with the gate width although the trend was not consistent. It is also evident that both the numbers and mean size of the animals increased as with gate width (Table 2). The variety of animals taken is in agreement with re66
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suits reported by Hungerford et a (I955) and Engelmann & Tobisch (I972). Virtually every aquatic insect order is represented- except Megaloptera and Plecoptera. Setting the trap with 5 cm of the gates above the surface proved successful in attracting large numbers of surface dwelling Hemiptera -especially Velia and Microvelia. The distance from which animals were attracted is unknown. In clear water (i.e. bottom clearly visible at 1.5 m), the lighted area extended for about 2 m in front of the trap. In several instances, I observed Anax larvae enter this lighted area and orient toward the light. Observations at the time of testing suggest that a 30 min. trial was not long enough to allow the full potential of the trap to be realized. Several adult Ranatraand Lethocerus were seen approaching the trap but did not get near enough in the allotted time to attempt to enter.
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