Analyzing the stomach contents of invasive American
bullfrogs (Lithobates catesbeiana) in the West Kootenay
Region of British Columbia

Miranda Hark
Faculty Advisor: Doris Hausleitner
RFW 271, School of Environment and Geomatics, Selkirk College
April 7, 2020

Data collection done in cooperation with Becca Merenyi
Samples provided by Khaylish Fraser, Central Kootenay Invasive Species Society

Page 5 of 11

Disclaimer
The contents provided on the following pages are student work produced as part of the written
evaluation in the diploma program in the School of Environment and Geomatics, Selkirk
College, Castlegar, British Columbia, Canada. This work obtained a minimum assessment grade
of “C” (60%) and met the requirements of the assignment for which it was completed.

The author has provided written permission to include the following academic work in the
Selkirk College Online Repository (SCOLR) and retains full copyright over their work. Canadian
Copyright law applies to use of this work.

1. Abstract
What are American bullfrogs (Lithobates canesbeiana) at Lomond Lake in British Columbia
eating? American bullfrogs are large, aquatic frogs that eat almost anything in front of them.
After they were introduced to the province of BC, they began to spread and displace native
amphibians. In some instances, they prey directly on the native amphibians. This research was
done to gain an understanding of the diets of American bullfrogs through literature, to determine
the diet trends and key prey species of bullfrogs at Lomond Lake, and to determine if the
Lomond Lake population was preying on native amphibians. Bullfrog specimens from Lomond
Lake were dissected and their stomach contents analyzed and organized in Microsoft Excel. Of
the 34 bullfrogs dissected, 78% of the stomachs with contents contained insects. Almost 90% of
the total stomach contents were insects. Other contents included arachnids, mammals, and
vegetation. No native amphibians were identified within the stomach contents. This may be
because of a lack of native amphibians at Lomond Lake, the availability of other more suitable
prey options, or due to the small size of our specimens. Dissecting a larger number of Lomond
Lake bullfrogs may help to further define the trends in their diets and help to confirm that our
analyses are relevant to the whole population. Conducting similar research on the diets of
amphibians native to Lomond Lake may deepen our understanding of why the bullfrogs do not
appear to be preying on them currently and assess their risk of displacement and predation in the
future.

2. Acknowledgements
My research partner, Becca Merenyi, a second-year Recreation, Fish and Wildlife student at
Selkirk College, completed the data collection portion of this research with me. Doris
Hausleitner, M.Sc, R.P. Bio, was my faculty advisor. She teaches two second year RFW courses
at Selkirk College. Brenda Beckwith, PhD, assisted with the structure and editing of my applied
research project and report. She teaches the Applied Research Project (RFW 271) course within
which this work was completed. Valerie Huff assisted with the data analysis in Microsoft Excel.
Khaylish Fraser of the Central Kootenay Invasive Species Society donated the American
bullfrogs used for our research to Selkirk College.

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3. Introduction
American bullfrogs (Lithobates catesbeiana) are one of the world’s 100 most invasive species
(Anthony 2013). They are brown or green aquatic frogs with golden eyes and are known for
eating “anything they can fit in their mouths,” from insects to snakes to mammals (Factsheet 7
n.d.). They are distinguishable from other frogs because of their distinct tympanum (ear) and
their lack of dorsolateral folds. The males have yellow throat patches and the females have
whitish-cream coloured throats. The females are larger than the males (Factsheet 7 n.d.).
A threat to other amphibians in British Columbia (BC), American bullfrogs were introduced to
the province in the early 20th Century to be farmed for meat due to their large size of up to 20
centimeters, not including legs (Factsheet 7 n.d.). Since this time, they have displaced other
amphibians by taking over their habitat and, in some instances, they prey directly on native
amphibians (Anthony 2013), including species at risk (Crestonwildlife c2011). For example, the
northern leopard frog, red-listed in BC and endangered federally, inhabits only two areas in BC
(Crestonwildlife c2011). One of these areas is the Creston Valley Wildlife Management Area in
the West Kootenay, a wetland that has experienced an invasion of American bullfrogs
(Crestonwildlife c2011).
Information on the impacts of American bullfrogs in the West Kootenay region is limited
currently and management efforts are somewhat constrained due to funding. Although some
analysis of the bullfrogs’ stomach contents has been done, it has not been the primary effort in
their surveillance and management (Vogel et al. 2017). At present, bullfrogs are being managed
only in the Creston and Nelway areas in the West Kootenay region where surveillance has been
ongoing since 2014 (K. Fraser, pers. comm., 2019 November 12). This management has
included passive and active acoustic and visual surveillance, capture and euthanization, and
public outreach and education to date (K. Fraser, pers. comm., 2019 November 12). A bullfrog
risk assessment is being developed by the American Bullfrog Action Team (ABAT) for the
Kootenay-Boundary region to aid in future management decisions (K. Fraser, pers. comm., 2019
Nov 12). ABAT conducts the American bullfrog surveillance and eradication program to gather
more scientific information on American bullfrogs in the region. The Central Kootenay Invasive
Species Society (CKISS), an ABAT partner, has donated samples from their bullfrog euthanasia
efforts at Lomond Lake near Nelway for students to use for research at Selkirk College.
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I believe that with further research on the diet of these bullfrogs, better management practices
can be created to minimize their potential effects on native amphibians. My research goal is to
analyze the stomach contents of American bullfrogs from Lomond Lake to assess their diet, with
an emphasis on the identification of native amphibian species. To meet my research goal, the
specific objectives of this research are:
•

To conduct a literature review to gain an understanding of how American bullfrogs have
affected BC, their general habits, and common prey species.

•

To determine the key species in the diet of American bullfrogs sampled from Lomond
Lake through the analysis of stomach contents.

•

To determine if American bullfrogs are eating native amphibians.

•

To analyze and describe the trends in the diets of American bullfrogs, including
male/female comparisons.

If American bullfrogs are consuming not only the prey of native amphibians, but the amphibians
themselves, it is important to develop best management practices to conserve the native species.
The results of my research will be sent to Khaylish Fraser, Aquatics Program Coordinator, at
CKISS, to contribute to the ABAT bullfrog risk assessment.

4. Methods
4.1 Samples Background
Lomond Lake is located 1.3 km from Nelway, BC, and is accessed via Pend D’Oreille Road off
of Highway 6. It is 27 km south of Salmo (Figure 1). CKISS captured, anesthetized, and
euthanized, by freezing, American bullfrogs at Lomond Lake in 2016. Some specimens were
given to Selkirk College by Khaylish Fraser to use for research. They have been kept frozen in
storage in a freezer outside the maintenance building at Selkirk College and some specimens
were used in 2016 and 2017 for student projects in the Recreation, Fish, and Wildlife Program.

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Figure 1. Location of Lomond Lake, circled in pink, south of Salmo, BC.

4.2 Data Collection and Potential Analysis
I conducted this research with Becca Merenyi, a fellow Selkirk College student. Data collection
and analysis occurred between December 1, 2019 and February 27, 2020 in a biology laboratory
at the Selkirk College campus in Castlegar. We dissected 34 bullfrogs. We recorded length
(mm), weight (g), and sex of each individual before dissection began. We identified yellow
throat patches as male, white throat patches as female, and labelled those we were unsure of as
“Unknown.”
4.2.1 Dissection
Each frog was laid on its back and an opening was created with a scalpel on the underside of the
body between the hind legs. Scissors were used to cut through the skin and muscle in the middle
of the body perpendicular from the original opening up through the mandible. The liver and fat
bodies were then removed to make the stomach visible and accessible (Hansler 2014). After
using scissors to snip above and below the stomach, it was removed and sliced lengthwise
around the outer curve with a scalpel to open it and empty the contents into a metal tray (Frog
2010). Anticipated contents included insects, invertebrates, and, depending on the size of the
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bullfrogs, birds, amphibians, and small mammals. Once the dissections were complete, the frogs
were disposed of in the garbage since they were not preserved with any chemicals. Our data for
each frog was recorded and sorted in Microsoft Excel.
4.2.2 Analysis
The stomach contents were analyzed based on the sex (male and female) of the bullfrogs and the
frequency of occurrence of the different items found in the stomachs. We identified trends in the
diets of males versus females, as well as within the Lomond Lake bullfrog population in general.
The most common species preyed upon by bullfrogs was expected to give insight into which
native species, if any, are being threatened, by which group of bullfrogs (male, female) and to
what extent. This could include the number of different species of insects, invertebrates, birds, or
mammals being consumed, and what percentage each group made up of the whole diet. For
example, insects might have included eight different species and made up 70% of all the
identified species in the diet of adult male bullfrogs. If native amphibians were identified in the
stomach contents it would confirm that the bullfrogs were threatening them more directly than in
the competition for resources. The diet trends of our bullfrogs were compared with results from
similar studies done on bullfrogs in other locations in order to assess the normalcy of the
Lomond Lake population.

5. Results
We dissected 34 American bullfrogs, including 13 males, 13 females, and 8 whose sex we could
not determine (“unknown”). Our largest frog was 140mm; the smallest was 54mm. Of the
bullfrogs sampled, 21% had no stomach contents, made up of females and “unknown” bullfrogs.
We did not find any native amphibian species in the stomachs.
Of the stomachs that had contents, most contained insects (78%), which made up almost 90% of
the total contents (Table 1). The identified insect species included wasps (Formicidae family),
bees (Apoidea family), dragonflies (Aeshnoidae family), beetles (Coleoptera order),
grasshoppers (Acridoidea family), and moths (Lepidoptera order). In addition to insects, we
observed spiders (Arachnida class), mice (Mammalia class), snails (Gastropoda class), and
general vegetation.

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Table 1. Frequency of occurrence of stomach contents in American bullfrogs from Lomond Lake.

Insecta
Vegetation
Gastropoda
Mammalia
Arachnida

Number of Stomachs
Containing
21
5
3
2
1

Frequency of
Occurrence
77.8%
18.5%
11.1%
7.4%
3.7%

Total Number Found
in All Contents
81
5
4
2
1

% of All Stomach
Contents
88.0%
5.4%
4.3%
2.2%
1.1%

In general, we found the male bullfrogs to be larger both in length and weight than the females,
and the bullfrogs in which we could not determine sex tended to be the smallest of our samples
(Figure 2). In comparing males and females, we found that the males’ stomachs more frequently
contained vegetation, while females’ stomachs contained the only arachnids and mammals
identified in the study (Figure 3).

Figure 2. Weight and length distribution among American bullfrogs from Lomond Lake.

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90.0%
80.0%

Frequency (%)

70.0%
60.0%
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
Insecta

Gastropoda

Mammalia

Occurrence in Males

Arachnida

Vegetation

Occurrence in Females

Figure 3. Frequency of occurrence of stomach contents in male and female American bullfrogs from Lomond Lake.

6. Discussion
The diet preference for American Bullfrogs in Lomond Lake consists largely of insects, a result
mirrored by similar research from Vancouver Island, BC (Jancowski and Orchard 2013) and
Argentina (Quiroga et al. 2015). Also consistent with Jancowski and Orchard (2013), we found a
low percentage of arachnids in the stomach contents of our specimens, the male bullfrogs were
larger than the females, and the smallest of the bullfrogs tended to have less in their stomachs.
This may indicate that the Lomond Lake bullfrog population is growing and feeding as average
bullfrogs would be. The range of contents found in the stomachs was within the ranges of these
studies; although, not every species found in other studies, such as microbenthic invertebrates,
was found in our Lomond Lake bullfrogs.
There may be a number of reasons why we did not observe amphibians in our bullfrog stomach
contents. In contrast to the 200 mm length that American bullfrogs can grow to, even our largest
(140 mm) was notably smaller. It could be that our specimens were not large enough to be
consuming the amphibians that are native to Lomond Lake. It is also possible that there is
enough other food for the bullfrogs to eat that they haven’t yet taken an interest in eating other
amphibians. Additionally, perhaps there are few native amphibians at Lomond Lake available to
be eaten in the first place.

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Because the male bullfrogs’ stomachs did not contain either of the mice identified in the stomach
contents, maybe it could be suggested that the males tend to eat less mammals than the females;
however, I would not draw that conclusion unless I got the same results with a repeat sample of
Lomond Lake bullfrogs. With further research and a larger sample size, perhaps more reliable
comparisons of male versus female diets could be made.
6.1 Limitations
If we had had more students involved in our dissections, we could have analyzed more bullfrogs
which would have helped to define our trends further. With only 34 stomachs emptied, it is
possible that our contents did not define the average trends of the whole Lomond Lake
population. Since our bullfrogs were smaller and likely younger than some populations, our
findings may not be indicative of the real risk to native amphibians at Lomond Lake. If
remaining bullfrogs at the lake are left to keep growing, it is possible that their diets will change,
and native amphibians will be preyed on by these larger bullfrogs. If I were to do this research
again, I would try harder to involve more people so that we could have more data to analyze.
6.2 Recommendations
I would recommend research be done on the native amphibians currently populating Lomond
Lake. The species present, their diets, and their population sizes could be helpful in determining
why they are not being preyed on by the bullfrogs and may give insight into their level of risk of
displacement and predation. If possible, obtaining bullfrogs from the eradication efforts by
ABAT in the Creston area and analyzing their stomach contents would be a good next step. That
research could be compared with the Lomond Lake bullfrogs and together the diet information
could be used to more widely describe the bullfrogs in the West Kootenay region. In addition,
because the endangered Northern-leopard frog resides in the Creston Valley Wildlife
Management Area (Crestonwildlife c2011), information on the diet trends of the Creston
bullfrogs could be key in managing this at-risk species.
Finally, we observed pink colouring on several of our bullfrogs’ thighs and stomachs, which our
faculty advisor, Doris Hausleitner, previously taught us is a sign of chytrid fungus – a fungus that
has significant negative effects on amphibians. Studying the remaining Lomond Lake bullfrogs
to identify how many of them may have chytrid fungus and comparing the diets of those with

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and without the fungus, could yield some interesting information that may also have
management implications for ABAT.

7. Literature Cited
Anthony L. 2013. A frog for the killing. Canadian Geographic. [accessed 2019 Oct 22];
133(2):46-60. http://web.b.ebscohost.com/ehost/detail/detail?vid=7&sid=d908713e-b4d34878-85a2-96a25c47e3b4%40pdc-v-sessmgr05&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ
%3d%3d#AN=88006701&db=a9h.
Crestonwildlife.ca. c2011. Creston Valley Wildlife Management Area. [accessed 2019 Nov 18].
https://www.crestonwildlife.ca/research/northern_leopard_frog.
Factsheet 7: Bullfrog. Victoria BC: BC Frogwatch, Conservation Data Centre, Wildlife Branch,
Ministry of Environment, Lands, and Parks; [accessed 2019 Oct 22]. https://www2.gov.bc.ca/
gov/content/environment/plants-animalsecosystems/wildlife/wildlifeconservation/amphibians
-reptiles/amphibians-in-b-c/frogs-toads.
Frog stomach dissection [streaming video]. 2010. Kimdepue. 1:02 min. [accessed 2019 Nov 9].
https://www.youtube.com/watch?v=ylInpFAGd2A.
Hansler B. Bullfrog dissection “basic” [streaming video]. 2014. 22:04 min. [accessed 2019 Nov
9]. https://www.youtube.com/watch?v=cPaHbNEbtNo.
Jancowski K, Orchard SA. 2013. Stomach contents from invasive American bullfrogs Rana
catesbeiana (= Lithobates catesbeianus) on southern Vancouver Island, British Columbia,
Canada. NeoBiota. [accessed 2019 Oct 22]; http://web.b.ebscohost.com/ehost/pdfviewer/
pdfviewer?vid=4&sid=d908713e-b4d3-4878-85a2-96a25c47e3b4%40pdc-v-sessmgr05.
Quiroga LB, Moreno MD, Cataldo AA, Aragon-Traverso JH, Pantano MV, Olivares JBS,
Sanabria EA. 2015. Diet composition of an invasive population of Lithobates catesbeianus
(American bullfrog) from Argentina. Journal of Natural History. [accessed 2019 Oct 23];
49(27/28):1703-1716. http://web.a.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=5&sid=
2292fe39-8710-42be-a28f-d0ffb80c3648%40sessionmgr4006.

Vogel J, Sternberg M, Fraser K, Bates E. 2017. American bullfrog surveillance and eradication
program. Central Kootenay invasive species society; [accessed 2019 Oct. 22]. http://a100.gov.
bc.ca/appsdata/acat/documents/r52849/COL_F17_W_1222_1507657171664_7649412937.pd
f.

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