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A New Funnel Trap for Sampling Amphibian Larvae in Wetlands

 

David M. Mushet, Ned H. Euliss, Jr., and Bruce A. Hanson

U. S. Geological Survey
Biological Resources Division
Northern Prairie Science Center
8711 37th Street Southeast
Jamestown, ND 58401-7317

In response to our need to sample amphibian larvae in diverse wetland habitats of the Prairie Pothole Region (PPR), we designed and evaluated a funnel trap that overcomes many of the problems associated with conventional sampling methods (Heyer 1976, 1979; Berger 1984; Shaffer et al. 1994).

Our funnel trap has a welded rectangular frame constructed of 0.63-cm-diameter steel rod and is covered with 0.32-cm-mesh galvanized screen (Figures 1 and 2). We constructed a funnel opening on one side of the trap to direct animals through a 5 cm-wide opening, raised the bottom of the funnel opening 7.62 cm above the trap bottom to retain captured animals, and attached a hinged mesh lid to the top of each trap to facilitate removal of captured amphibians. We extended 4 steel rods 30 cm beyond the bottom of the trap to securely anchor the trap when the device was pushed firmly into the substrate. These legs can be omitted and weights may be substituted and used as anchors in wetlands with rocky substrates. An additional feature of our trap is a 200-cm drift fence we attached to the opening of the funnel to direct free-swimming larvae to the trap. The drift fence is constructed of 0.32-cm-mesh nylon, 91.4 cm-wide with the ends folded over to form end loops. We attach the drift fence to the trap by inserting a threaded steel rod through a small opening at the top of the trap and through the end loop in the drift fence. We then screw the rod into a 0.63-cm nut welded to the bottom of the funnel opening. The opposite end of the drift fence is supported by inserting a length of 2.54-cm PVC pipe through the other end loop and into the wetland substrate in an upright position. Lead weights attached to the bottom edge of the drift fence ensure maximum contact with the substrate. Floats attached to the top of the submerged portion of the drift fence help maintain its vertical orientation. Where water depths exceed 91 cm, we increase the height of the fences by adding a second length of 0.32-cm-mesh nylon to an existing drift fence.

Figure 1. A funnel trap for sampling amphibian larvae in wetlands. Overall height of trap and drift fence should be determined by the maximum depth anticipated in the wetland being sampled. The trap should be set 10 cm above the water surface to allow sampling of the entire water column while simultaneously providing oxygen to captured animals.



Figure 2. Construction detail of a funnel trap designed to sample amphibian larvae in wetlands.

These funnel traps have proven effective in all wetland vegetative zones and in water depths ranging from 10 cm to over 2 m. Our funnel traps cost approximately $45 per trap for materials; minnow traps are commercially available for $13.50 per trap. Labor to install funnel and minnow traps is similar. While the higher cost of our trap may limit their use to some extent, greater capture rates combined with the ability to sample the entire water column with reduced mortality may justify these higher initial costs. In addition, when sampling rare taxa, our trap may identify sample populations other sampling techniques overlook.

LITERATURE CITED

Berger, T. J.  1984.  Community ecology of pond-dwelling anuran larvae.
     Unpubl. Ph.D. dissertation, Univ. of Kansas, Lawrence.

Heyer, W. R.  1976.  Studies in larval amphibian habitat partitioning.  
     Smithsonian Contrib. Zool.  242:1-27.

Heyer, W. R.  1979.  Annual variation in larval amphibian populations 
     within a temperate pond.  Jour. Wash. Acad. Sci.  69:65-74.

Shaffer, H. B., R. A. Alford, B. D. Woodward, S. J. Richards, R. G. Altig, 
     and C. Gascon.  1994.  Quantitative sampling of amphibian larvae.  
     p. 130-141, in W. R. Heyer, M. A. Donnelly, R. W. McDiarmid, L. 
     C. Hayek, and M. S. Foster (eds).  Measuring and monitoring biological
      diversity:  standard methods for amphibians.  Smithsonian Instit. Press, 
     Washington, D.C. and London England.

U.S. Department of the Interior
U.S. Geological Survey
Patuxent Wildlife Research Center
Laurel, MD, USA 20708-4038
http://www.pwrc.usgs.gov/naamp3/naamp3.html
Contact: Sam Droege, email: Sam_Droege@usgs.gov
Last Modified: June 2002