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Funnel Trapping Larval Amphibians
in Prairie Pothole Wetlands

Erin Johnson, Shawn Jones, Todd Gunderson, Kenneth Klang, and Michael J. Lannoo

The Muncie Center for Medical Education
Indiana University School of Medicine, Muncie, IN 47306

Iowa Lakeside Lab, Milford, IA 51351

Department of Biology, Humboldt State University
Arcata, California 95521
The Muncie Center for Medical Education
Indiana University School of Medicine, Muncie, IN 47306;
ph. 317-285-1050; fx. 317-285-1059; ØØMJLANNOO@BSUVC.BSU.EDU

(Address all correspondence to MJL at the Muncie Center)

[ Abstract ]

Key Words: funnel trapping, population, monitoring, survey, Rana pipiens, Ambystoma tigrinum, wetlands, sampling design

Recent concerns over reported amphibian declines (Barinaga 1990, Blaustein and Wake 1990, Wake 1991, Phillips 1994, Lannoo 1996) have prompted a renewed interest in methods that can be employed for amphibian sampling and monitoring (Heyer et al. 1994, Mac 1996). One such approach is to monitor aquatic life history stages (Shaffer et al. 1994). Advantages of aquatic surveys are that animals to be sampled are restricted to aquatic habitats, which are frequently discrete ponds and wetlands, and that animals are accessible in these habitats -- that is they can be relatively easily captured, unlike terrestrial stages in which animals frequently burrow, climb into the canopy, and otherwise disperse. Furthermore, aquatic sampling can be employed for most species of amphibians including those with complex life histories (most frogs), those that are completely aquatic (4 of 9 salamander families), but not for those relatively few species that lay terrestrial eggs and exhibit direct development. These several attributes make aquatic sampling desirable for many species and across many landscapes. Unbaited funnel (or minnow) trapping has several advantages over other aquatic sampling techniques, including reduced bias due to disturbance of animals (compared to dip netting or seining), less habitat destruction (compared to seining), repeatability, and good capture success. Also, traps are portable, inexpensive, commercially available, and easily employed by a volunteer work force (although identifying larvae can be difficult). Finally, data are easily quantified and can be expressed as a rate (captures per trap per unit of time).

We report funnel trapping success of 4 common midwestern United States amphibians, the eastern tiger salamander (Ambystoma tigrinum), the northern leopard frog (Rana pipiens), the American toad (Bufo americanus) and the western chorus frog (Pseudacris triseriata) in 17 prairie pothole wetlands, ranging in type from seasonal to permanent, located in northwestern Iowa. Our goals were to: 1) document the usefulness of this technique for sampling aquatic amphibians; 2) determine potential biases in this technique, including biases based on size, species, trap placement, trap orientation, and time of day; 3) determine effects of wetland type on trap success; 4) determine the effort needed to statistically document a change in the population (either a decline or an increase). In addition, we document differences in larval size across wetlands and the variation in species abundance and composition across years.

Study site
The study site is located in northwestern Iowa, near the Iowa Lakeside Laboratory in Dickinson County. The landscape is recent, about 11,500 years old, knob and kettle terrain and represents the farthest advance of the Altamont Lobe of the Des Moines Lobe of the Wisconsin Glaciation (summarized in Lannoo 1986). The site is located 2 1/2 half miles south of Lakeside Lab and encompasses a rectangle, one mile north to south by three miles east to west (Lakeville Township sections 34, 35, and 36). Seventeen wetlands, ranging in water regime from seasonal to permanent, were sampled within this rectangle and the adjacent southwestern corner. Sampling took place during May and June of 1995 and June of 1996. The summer of 1996 was drier than 1995, and indeed every summer dating back to 1990. In early June 1996, 3 seasonal wetlands were too dry to sample. By late July 1996, 9 of the 17 study wetlands had dried.

The amphibians of this area have been well studied (Blanchard 1923, Kuntz 1924, Lannoo and Bachmann 1984a,b, Lannoo et al. 1994, Lannoo 1996). Four species are usually found in these pothole wetlands: the northern leopard frog eastern tiger salamander, western chorus frog, and American toad.

Data collection
Amphibians were sampled using standard gee funnel (minnow) traps (MT2, Nylon Net Company, Memphis, Tennessee). Traps were made out of galvanized steel and were cylindrical in cross section with a funnel opening on each end. They were constructed of 0.64 cm (1/4 inch) square mesh screen and measured 42 cm (16 1/2 inches) long with a 22.8 cm (9 inch) maximum diameter and a funnel opening of 2.5 cm (1 inch). Traps were set unbaited with a portion of the mesh out of the water to allow animals to breathe atmospheric oxygen through air gulping (e.g., Lannoo and Bachmann 1984b).

In 1995, trapping took place between June 1 and June 14 in 16 basins, including 4 permanent, 8 semipermanent, and 4 seasonal wetlands. In 1996, trapping was attempted on 2 occasions, June 1 - 3rd and 25 - 27th. In early June 1996, a total of 10 basins were sampled before suspending the effort (see below), including 1 seasonal and 2 semipermanent wetlands which had dried by late June. Due to the drying of wetlands, only 11 basins, including 7 semipermanent and 4 permanent, were sampled in late June 1996. During the trapping period, between 4 and 12 traps, depending on the size of the wetland, were set in each site at about 2000 hours and were checked at 0800 hours the next morning. Contents were identified, counted, measured, and released and the traps were reset to be checked again at 2000 hours.

Traps were most commonly set along the shore, but were also set in the middle of shallow wetlands or suspended from poles placed in deeper water. Trap placement was influenced by biological constraints. During the summer when amphibian larvae are present, these wetlands become hypoxic overnight, and it is critical that animals have access to the water surface, and therefore for these traps to be partially out of the water (see above). Traps were grouped into couplets with one trap oriented parallel and one perpendicular to shore.

A 4 letter system of naming wetlands was designed to identify wetland type and location with respect to U.S. Highway 86, which bisects the study area from north to south. The first two letters identify wetland type: seasonal (SS), semipermanent (SP), or permanent (PP). The third letter indicates the wetland's position east or west of highway 86. The fourth letter was arbitrarily assigned (beginning with "A") to differentiate between wetlands of the same type located in the same area. In Garlock Slough (PPEA), small case letters were assigned to 2 different sampling sites within this large wetland.

Data analysis
Data were collected on numbers of animals per trap and their snout vent lengths. The data on number of animals collected per trap exhibited a Poisson distribution (see below) and were log transformed (e.g., Krzysik 1997). All analyses conducted on these data used the transformed data.

MONITOR is a public domain software program (Gibbs 1995) available at
ftp://, designed to evaluate the ability of a monitoring program to detect population trends. MONITOR evaluates relationships between several components of a monitoring program and their relative effects on the statistical power of the monitoring program. These components include number of plots monitored, number of counts per plot, duration and frequency of monitoring, count variation, and significance level associated with trend detection. For our purposes, standard deviation was used as the measure of count variance in determining the number of traps required to detect a 5% population trend (increase or decrease) over a period of 15 years of annual trapping with an 80% significance level.

In 1995, 248, 12 hour trapping periods in 15 wetlands captured a total of 177 amphibian larvae as follows (mean rates per wetland summarized in Table 1): 17 leopard frogs, 148 tiger salamanders, and 12 chorus frogs, but no American toads. This equals a leopard frog tadpole caught once in every 175. 1 trap hours, a tiger salamander larva caught once every 20.1 traps hours, and a chorus frog tadpole caught once every 248 hours. One male adult tiger salamander was captured. Average body sizes (SVL) varied across wetlands for both leopard frogs (range 21.6 to 29.3 mm, p = 0.001) and tiger salamanders (range 31.7 to 66.8 mm, p < 0.001).

During June 1 - 4th of 1996, 19, 12 hour trapping periods in 10 wetlands captured a total of 44 amphibian larvae as follows (mean rates per wetland summarized in Table 1): 10 leopard frogs and 34 tiger salamanders. This equals a leopard frog tadpole caught once every 22.8 trap hours, and a tiger salamander larva caught once every 6.7 traps hours. No chorus frog tadpoles were seen although adult choruses were heard in 2 wetlands (SPEA and SSEA). No American toad tadpoles were captured although schools, some of them large and estimated at over 1,000 animals, were seen in 4 wetlands (SPEA, SPWD,

SSWB, SPWF). These tadpoles were seen swimming directly through the mesh of the traps. On a few occassions, when traps were lifted from the water, tiger salamander larvae were observed escaping from the space between the two trap sides. Average body sizes (SVL) varied across wetlands for both leopard frogs (range 12.8 to 15.5 mm, p < 0.01 ) and tiger salamanders (range 23.5 to 26 mm, p = 0.68 ). Sample sizes were small: leopard frogs (n = 10) were only found in 2 wetlands; tiger salamanders (n = 34) were found in 5 wetlands, but in 2 of these we trapped only 1 animal. Trapping efforts were suspended at this time because we considered larvae to be too small to be effectively trapped.

During June 25 - 27th of 1996 trapping was resumed. In total, 212, 12 hour trapping periods in 11 wetlands captured a total of 1039 amphibian larvae as follows (mean rates per wetland summarized in Table 1): 852 leopard frogs, 186 tiger salamanders, no chorus frogs, and 1 American toad. This equals a leopard frog tadpole caught once in every 3 trap hours, and a tiger salamander larva caught once every 13.7 traps hours. Four wetlands sampled in 1995 and on June 1 - 3rd (SPEA, SSEA, SPEB, SPWB) were too dry to sample at this time. Of the remaining wetlands that were sampled on June 1 - 3rd, traps were set along the same area of shoreline. At this time (June 25 - 27th) average body size (SVL) varied across wetlands for both leopard frogs (range 28.3 to 44.0 mm, p < 0.001) and tiger salamanders (range 40.5 to 66.1 mm, p < 0.001).

Species bias: Tiger salamanders and leopard frogs were the predominate species captured by trapping. In 1995, the ratio of tiger salamanders, leopard frogs, American toads and chorus frogs captured in funnel traps was 12:1:0:1 respectively, even though American toad tadpoles were seen (unstandardized visual encounter survey) at 2 wetlands (SSWA, SPEA) and chorus frog tadpoles were seen at three wetlands (SSWA, SPWF, SPEA). Adult American toads and chorus frogs were heard calling at 2 wetlands (SPWB, PPWA) where tadpoles were not trapped. In the late June survey of 1996, the ratio of these animals was 852:186:1:0 respectively, although American toad tadpoles were seen in 6 wetlands, and in 4 of these wetlands (SPEA, SPWD, SPWE, SSWB) toad numbers at each site were estimated to be in the thousands. Furthermore, chorus frog tadpoles were observed in 1 wetland (SPEA; unstandardized seining survey) but were not trapped. Adult chorus frogs were heard chorusing in 2 wetlands (SPEA, PPEC) where tadpoles were not trapped.

Diurnal variation: Trap sets were timed to sample either daytime or nighttime activity. The relationship between time of day and capture rate varied by species and by year. Trapping success was not correlated with time of day in 1995 for leopard frogs (p = 0.04) or tiger salamanders (p = 0.06), or in 1996 for leopard frogs (p = 0.90). In 1996, more tiger salamanders were captured at night than during the day (p < 0.001).

Trap orientation: During both 1995 and 1996, the trap orientation (either parallel or perpendicular to the nearest shoreline) was not related to trapping success for leopard frogs (p = 0.3; p = 0.4) or for tiger salamanders (p = 0.4; p = 0.2).

Wetland type: There was a difference in trapping success between semipermanent and permanent wetlands. In 1995, the ratio of animals trapped in seasonal or semipermanent wetlands to those trapped in permanent wetlands was 148:0 for tiger salamanders (p < 0.001) and 8:1 for leopard frogs (p = 0.05 ), although this leopard frog sample included just 17 animals in 1 permanent and 3 seasonal or semipermenant wetlands. In 1996, the ratio of animals trapped in seasonal or semipermanent wetlands to those trapped in permanent wetlands was 3:1 for leopard frogs (p < 0.001) and 12:1 for tiger salamanders (p < 0.001).

Our data indicate that minnow trapping can be an effective method for sampling amphibian larvae in discrete wetlands. Unbaited traps passively capture animals as they move around feeding and avoiding predators within the basin. In our data set, factors such as trap orientation and diurnal variation do not generally influence trapping success, however this may not be true when sampling other species, or the same species in other areas. We therefore suggest setting traps for at least 24 hours and checking them at least once every twelve hours. We also suggest using different trap orientations. Once it can be shown in any given study that samples from specific populations exhibit no bias with regard to diurnal patterns and trap orientation, sampling effort can be reduced and standardized. We will return to a discussion of trap orientation below.

Several biases arise with trapping amphibian larvae. These biases must be considered when comparing trapping data between species to insure proper interpretation of the data. When trapping is limited to a single species, knowledge of biases can minimize trapping effort but it should be recognized that biases still exist (see below).

Size bias: The 2 1996 trapping efforts were conducted within the larval period (i.e., between post-hatching and pre-metamorphosis life history stages) for both northern leopard frogs and tiger salamanders. We attribute the increase in trapping success of northern leopard frogs from 1 animal caught per 22.8 trapping hours to 1 animal caught per 3 trapping hours to a bias against smaller tadpoles. At this point we cannot say whether this bias is due to smaller tadpoles escaping through the mesh of the trap, or to the reduced locomotory ability of smaller tadpoles (Wassersug and Hoff 1985, Hoff and Wassersug 1986, Lannoo et al. 1987). Indeed, while both explanations may be possible, we have evidence for the former. In 1995, toad tadpoles in SSWA were seen swimming through the trap and small tiger salamanders in SPWD were seen falling through the gap between trap sides when the trap was lifted from the water. Funnel trapping may also be biased against large animals such as pre-metamorphic tiger salamander larvae. The largest larva caught in our traps was 80 mm SVL; these animals approach in head width and body diameter the size of the funnel opening. Reduced numbers of animals captured over time may also reflect a reduction in the population due to predation.

Species bias: We feel that much of the species bias we observed in 1996 was due to size bias. In early June, leopard frog tadpoles and tiger salamanders were collected in a ratio of 0.3 leopard frogs to every 1 tiger salamander. Later in June, the ratio was 4.6 leopard frogs to 1 tiger salamander, an increase of 15.3 times. This reversal of trapping ratios probably reflects the growth of leopard frogs (see above), and therefore their increased catchability. By late June, American toad and chorus frog tadpoles are much smaller than either leopard frog tadpoles or tiger salamander larvae. The small size of these animals may explain their absence from our traps despite their being observed in several wetlands.

In-trap predation by tiger salamander larvae or fishes may produce a size or a species bias in amphibian samples. In fact, tiger salamander populations in our region contain cannibal morph larvae which specialize on larger prey such as tadpoles and conspecific larvae (Lannoo and Bachmann 1984a). Smaller animals are probably at increased risk. However, even small American toad tadpoles have anti-predatory features (Brodie and Formanowitz 1987), which may make them less susceptible to in-trap predation than either chorus frog or leopard frog tadpoles, or small salamander larvae.

When using metal traps, an additional species bias may arise from a difference in sensory systems available to salamander larvae compared to anuran tadpoles and fishes. Salamander larvae have electroreceptors as a component of their lateral line systems while anuran tadpoles, and most teleost fishes, do not (Fritzsch and Wahnschaffe 1983). While we and others assume that animals do not perceive trap presence and are trapped passively, if animals are aware of traps, approach or avoidance responses become relevant variables. In particular, an animal's behavior could be altered based on trap experience -- perhaps trap aversion if the experience was stressful, crowded, or hazardous, or trap attraction if the trap provided easy access to prey.

Wetland type: To the extent that our samples accurately reflect population size, amphibians are found in greater abundance in seasonal and semipermanent wetlands than they are in permanent wetlands (Table 1). An alternative explanation is that similar numbers of animals are present between permanent and more ephemeral wetland types, but because permanent wetlands are larger, amphibians are dispersed and therefore more difficult to capture, even when using an increased number of traps.

The relationship between amphibian abundance and wetland type may be tied to amphibian-fish interactions, which are generally detrimental to amphibians (e.g., Sexton and Phillips 1986). These interactions may be competitive in the case of Pimephales sp. and Culaea sp. (e.g., Peterka 1989, Lannoo 1996) or predatory in the case of game fish (e.g., Lannoo 1996). Seasonal and semipermanent wetlands generally do not support fish because they summer or winter kill either by drying completely or becoming hypoxic at night (e.g., Lannoo 1996). Permanent wetlands can support fish populations except during prolonged droughts, when water levels are reduced and these wetlands take on the dissolved oxygen characteristics of semipermanent wetlands (Lannoo 1997).

Year to year variation: The trapping data support our subjective impression of these trends that 1996 was: 1) a productive year for amphibians, at least up until wetlands began to dry; 2) a "leopard frog year"; and 3) a poor year for chorus frog.

Minnow traps as monitoring devices: One goal of periodic sampling ( monitoring) is to infer long term trends in population status. Aquatic sampling for amphibians provides such data, but once again these data must be interpreted cautiously. One reason for caution is that amphibian reproductive success varies from year to year (Pechmann et al. 1991), and therefore numbers of larvae need not reflect numbers of breeding adults ( Wassersug 1997). Secondly, early pond drying can completely eliminate previously healthy larval populations. In 1996, 9 of our 17 study wetlands dried. Three of these wetlands were followed closely. In one (SSWB) all larvae died. In 2 others (SPWD, SPWE) American toad tadpoles metamorphosed but most leopard frog tadpoles and tiger salamander larvae died. Finally, metamorphosis is considered to be the time when amphibians are most vulnerable to predators (Arnold and Wassersug 1978). When healthy larval populations are decimated they provide little recruitment into the adult breeding population.

Trap orientation was either not, or only weakly, related to trapping success. This, to us is a surprising result. First, we would expect bias in trap orientation if the predominant movement patterns of animals were either parallel or perpendicular (from deep to shallow water or vice versa) to the shoreline. Secondly, even with no movement bias we would expect orientation bias if we assume as did Eggers et al. (1982) that trapping success is representative of a discrete surrounding water volume. Water volumes sampled by each end of parallel traps represent a symmetrical pattern confined on one side by the shore, while opposite ends of perpendicular traps sample asymmetrical water volumes strongly biased toward the deep water end. In other words, both openings of a parallel trap catch animals while it is likely that only one opening (the open water side) of perpendicularly oriented traps catches animals.

Funnel trapping may be constrained by a maximum trap capacity. The highest numbers of any species caught in a single trap were: 56 leopard frogs (SPWG 1996), 10 tiger salamanders (SPWD 1995, SPWF 1996), 29 European carp (Cyprinus carpio), 62 Brown bullheads (Ameriurus nebulosus), and approximately 600 Pimephales sp. (PPEA 1996) in a 12 hour period. Trapping in wetlands with high animal densities may necessitate shorter trapping periods.

Larval sampling does have one advantage over the more commonly used adult breeding call surveys; the presence of larvae documents reproduction. Some species of amphibians, for example the chorus frog, will call when conditions are favorable but do not breed (Minton 1972).

Using trapping data from 1996 as a measure of means and variances (as measured by standard deviation), we used the software program MONITOR (see Methods) to evaluate the number of traps we would need to employ in each wetland to detect population trends over 15 years of annual sampling (see Table 2). Our goal of detecting positive and negative 5% population trends (growing or declining populations) was attainable for both leopard frogs and tiger salamanders using only 3 traps per year. In both species, positive population trends can be detected with fewer traps. This difference probably reflects the right skew in the distribution of animals; in small populations there is more room for population growth than for population decline. Means and variances from late 1996 were used in MONITOR calculations, because means were so small in 1995 that negative trends could not be sufficiently detected (80% significance) with a feasible number of traps. When possible, the statistical power of a survey can be enhanced by increasing the number of plots, number of traps, or number of monitoring years.

This work was supported by a grant from the Northern Prairie Science Center; thanks to Douglas Johnson, Diane Larson, and Brian Smith for their cooperation. Thanks to Mark and Judy Wehrspann, the Lakeside Lab managers, and Arnold van der Valk, the Lakeside director, for their support. Sam Droege made us aware of the availability and usefulness of the MONITOR software program.

Literature Cited
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Barinaga, M. 1990. Where have all the froggies gone? Science 247:1033-1034.

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Herpetologica 43:369-373.

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Table 1: Mean number of animals captured per trap by wetland in 1995 and late June1996. (See Methods for description of wetland designation)

Ambystoma tigrinum Rana pipiens

1995 1996 1995 1996

Wetland Rate Rate Wetland Rate Rate

West West


A 0.0 -- A 0.0 --

B 5.4 -- B 0.0 --


A 0.4 1.8 A 0.0 2.9

B 0.0 1.9 B 0.0 1.7

C -- 2.4 C -- 2.0

D 2.3 1.6 D 0.0 0.0

E 0.9 1.4 E 0.0 14.8

F 0.4 1.4 F 0.4 1.3

G 0.0 1.3 G 0.4 23.2


A 0.0 0.0 A 0.0 0.1

B 0.0 0.0 B 0.1 9.3

East East


A 0.0 -- A 0.0 --

B 0.0 -- B 0.0 --


A 0.6 -- A 0.1 --

B 0.0 -- B 0.0 --


Aa 0.0 0.0 Aa 0.0 0.0

Ab 0.0 0.0 Ab 0.0 0.0

B 0.0 0.7 B 0.0 1.1

mean 0.6 0.9 mean 0.1 4.0

mean PP 0.0 0.1 mean PP 0.0 * 1.9

mean SS/SP 0.9 1.7 mean SS/SP 0.9 6.2

-- water level below funnel entrance; -- water level below funnel entrance;

could not be sampled could not be sampled
* 2 animals in 100 traps

U.S. Department of the Interior
U.S. Geological Survey
Patuxent Wildlife Research Center
Laurel, MD, USA 20708-4038
Contact: Sam Droege, email:
Last Modified: June 2002