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Evaluation Of A New Funnel Trap for Sampling Amphibian Larvae

David M. Mushet, Ned H. Euliss, Jr.

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

and

Sally G. Zodrow

College of Natural Resources
University of Wisconsin-Stevens Point
Stevens Point, WI 54481

Although several well-tested methods for sampling populations of adult amphibians exist, methods that sample their larvae in the variety of habitats found in depressional wetlands of the Prairie Pothole Region (PPR) are lacking. Seining has commonly been used to sample salamander larvae; however, in habitats with dense hydrophytes it may be impossible to pull the seine and simultaneously maintain contact with the substrate. Dipnetting is another commonly used technique (Heyer 1976, 1979; Berger 1984), but dense vegetation may also restrict its usefulness. In addition, strong-swimming salamander larvae may avoid dip-nets, making them difficult to sample with this method. Modified minnow traps often are used to sample larval amphibians because they are commercially available or can be easily constructed and they work well in a variety of lentic waters, including those choked with vegetation (Shaffer et al. 1994). Minnow traps function in both sparsely and densely vegetated wetlands, however they have other limitations. Minnow traps are generally submerged in the water, and nighttime hypoxia characteristic of pothole wetlands during summer frequently suffocates captured amphibians. In addition, except in very shallow wetlands, multiple traps must be set to sample the entire water column. A further disadvantage of minnow traps is that the capture rates are low and variable, requiring large sample sizes to satisfy statistical considerations.

In response to our need to sample amphibian populations in the diverse wetland habitats of the PPR, we designed and evaluated a new funnel trap. Herein, we describe our evaluation of this funnel trap that eliminates many of the problems associated with minnow traps, dip-nets, and seines. We also report on use of 2 phases of the deepmarsh vegetative zone by tiger salamanders (Ambystoma tigrinum).

STUDY AREA

We conducted our evaluation on the Cottonwood Lake Study Area (CLSA), located approximately 32 km NW of Jamestown, Stutsman County, North Dakota. CLSA is situated on the eastern edge of a region of hummocky, collapsed, glacial topography known as the Missouri Coteau (Bluemle 1991). The mean annual temperature for the study area is approximately 4o C with mean monthly extremes of -14o C and 21o C occurring in January and July, respectively (Winter and Carr 1980). Average precipitation for the study area is 45 cm with most occurring during the summer. Snowfall averages 87 cm per year (Winter and Carr 1980). Long-term data indicate that the annual potential evaporation at CLSA is about twice the annual precipitation (Rosenberry 1987). Detailed descriptions of CLSA's location (Swanson 1987), vegetation (Poiani 1990), hydrologic setting (Winter and Carr 1980), and hydrology and chemistry (LaBaugh et al. 1987, Swanson 1990) are available.

We sampled larval amphibians in 5 semipermanent wetlands (Stewart and Kantrud 1971) (Wetlands P1, P6, P7, P8, and P11; Winter and Rosenberry 1995; LaBaugh et al. 1996) at CLSA. Biotic communities, water depths, and water chemistry vary by wetland class, by season, and in response to long-term drought cycles. The wetlands were generally less than 2 m deep and contained water throughout the year; they often go dry during periods of prolonged drought and were dry as recently as July, 1992. The central zones of the wetlands were dominated by cattails (Typha sp.), hardstem bulrush (Scirpus acutus), bladderwort (Utricularia vulgaris), and sago pondweed (Potamogeton pectinatus). The density and diversity of vegetation and the wide range of depths of this wetland class complicate and often compromise efforts to sample their larval amphibian populations.

METHODS

We sampled each of the 5 study wetlands 3 times during 1994 (July 18-29, August 1-12, and August 15-26) using our funnel traps, minnow traps, dip-nets, and seines. During each sample period, we used funnel traps and minnow traps first, followed by dipnetting, and finally seining. We alternated the timing of funnel trap and minnow trap deployment after an initial random selection (e.g. if funnel traps were deployed first during sample period 1 than minnow traps were deployed first during sample period 2, etc.). For both funnel and minnow traps, we deployed 10 traps in the deepmarsh zone of each wetland. To compare normal emergent and openwater phases of the deepmarsh zone, the number of traps we placed in each phase was proportional to the surface area covered by that phase. Within each phase, we placed traps randomly. We left the traps in place for 24 hours after which we identified to species, recorded, and released all captured animals. We then removed the traps from the wetland and waited a minimum of 24 hours before sampling with the next trap type or technique to allow populations to redistribute and resume normal activities in study wetlands.

After we completed sampling wetlands with both passive trapping methods, we actively sampled wetlands using a 22.0 cm x 47.5 cm rectangular dipnet (Shaffer et al. 1994). We made 30 sweeps of 1 m length at random locations throughout the deepmarsh zone. The day after sampling with dipnets, we seined each wetland using a 0.6 cm-mesh, 4.0 x 1.3 m seine. The seine was weighted at the bottom and pulled through the water along a 10 m random transect at a speed slow enough to maintain contact with the wetland substrate but fast enough to avoid losing captured animals (Shaffer et al. 1994).

We compared effects of trap type (our funnel traps vs. minnow traps), zone phase (normal emergent and open-water), and sampling period (July 18-29, August 1-12, and August 15-26) on counts of captures using a General Linear Model analysis of variance (ANOVA) (SAS 1989). We used a split-plot randomized block design with repeat measures and subsampling. The wetlands served as blocks (i.e. whole-plots), with zone phases as sub-plots, and with sampling period being the repeated measures factor. We considered sampling locations within period, phase, and trap type as subsamples and used a Fisher's Least Significant Differences (LSD) test (Milliken and Johnson 1984) to isolate mean differences. We did not compare the dipnetting and seining techniques statistically because we encountered difficulties using these techniques in the field.

RESULTS AND DISCUSSION

During this study, tiger salamanders were the only species sampled by the 4 methods we evaluated. Tiger salamanders are the most abundant larval amphibians at CLSA and commonly prey on other amphibian larvae. We did not evaluate predation of salamanders on other larval amphibian species potentially caught by traps.

Our funnel trap captured more tiger salamanders (3.25/trap/24 hours) than minnow traps (0.5/trap/24 hours) (F=7.30; df=1,8; P=0.027). Our trap design also improved survival of captured animals relative to minnow traps. Only 1 tiger salamander out of a total of 555 captured (531 larvae and 45 adults) died in our funnel traps compared with 10 mortalities (5 larvae and 5 adults) out of 51 salamanders (45 larvae and 6 adults) captured with minnow traps. We believe the increased mortality associated with minnow traps was due to the inability of individuals to reach the wetland surface to gulp air (Lannoo and Bachmann 1984a) assuming the amount of dissolved oxygen was insufficient to satisfy their physiological requirements.

Tiger salamander capture rates were similar within the normal emergent and openwater phases of the deepmarsh zone (F=2.34; df=1,4; P=0.20). Although suggesting equal use of the 2 phases by tiger salamanders, the similar capture rates could also be an artifact of varying activity patterns within the 2 phases. If tiger salamanders spent the majority of their time relatively inactive in the central, openwater phase of the wetlands but were very active as they passed through the normal emergent phase during nocturnal foraging excursions to shallow water zones, similar capture rates could result even though use of the 2 phases by tiger salamanders was different.

Although only marginally significant, the number of captures tended to decline seasonally (F=3.15; df=2,32; P=0.056) during our study. These differences may reflect removal of individuals due to departure of metamorphs, predation, cannibalism (Lannoo and Bachmann 1984b), or increased use of shallow water zones not sampled. It is also possible that seasonally increasing plant density limited salamander movement.

Dipnet samples did not prove to be a satisfactory sampling technique in our study area. We failed to capture a single individual in any of the 90 dipnet sweeps from each of 5 study wetlands. Dense vegetation coupled with the quick swimming responses of the tiger salamanders may have caused this result. Our efforts to seine wetlands met with similar failure. The dense vegetation, both living and dead, made it impossible to effectively seine any of the 5 study wetlands. The inability of these techniques to work in dense vegetation severely limits their use as a quantitative tool, especially in wetlands with well-developed plant communities.

The funnel traps we designed have effectively sampled tiger salamanders at CLSA in a related investigation. We have routinely sampled 17 wetlands (9 semipermanent and 8 seasonal) at CLSA with water depths ranging from 10 cm to over 2 m and with diverse vegetative characteristics. The funnel traps we designed have been in use at CLSA since 1992. An especially severe drought ended in July, 1993. Water levels rebounded and by August 1995 water levels at CLSA had reached their highest levels since studies were initiated in 1967. Since 1992, vegetation within the wetlands has changed greatly due to the drastic change in water levels. During 1992, many wetlands were dominated by dense stands of emergent vegetation. As water levels rose, emergents were flooded out and dense mats of dead vegetation formed. By 1994 dense stands of submergent vegetation dominated the centers of most semipermanent wetlands at CLSA. Throughout these changing conditions, we have not encountered any problems using our traps to monitor tiger salamanders at CLSA. Because our traps perform well in a wide variety of habitats and do not significantly enhance mortality of captured animals, we believe they have potential as a standard sampling method to facilitate comparative studies among a variety of wetland types.

LITERATURE CITED

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Patuxent Wildlife Research Center
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Last Modified: June 2002