The study aimed to determine if invasive largemouth bass were negatively impacting the survival of native western pond turtles at the Bernard Field Station. The researchers analyzed the stomach contents of 21 bass caught from a lake at the field station. None of the bass stomachs contained turtle hatchlings. However, the bass' diets included other species like crayfish and amphipods, indicating they still impacted the ecosystem. Additional data suggested seasonal diet changes and that bullfrogs may be a more significant threat to the turtles. While the results did not support the initial hypothesis, they provided insights into species interactions and potential conservation methods.

1. Micropterus salmoides from pHake
Lake Do Not Affect Actinemys
marmorata Survival Rates
Kian Bagheri
Dr. Nina Karnovsky
Bio 41E: Evolutionary and Ecological Biology
Pomona College
8 May 2015

2. Bagheri 1
Abstract
Invasive species have detrimental impacts on the ecosystems they invade. They can
indirectly change food webs, decrease biodiversity, and alter ecosystem conditions by preying on
native species (Marchetti et al., 2004). Largemouth Bass (Micropterus salmoides), an invasive
species at the Bernard Field Station, have been shown to prey on native species within the BFS.
These Bass are amongst the most advantageous organisms in the study of invasive species
because they are relatively easy to catch, anesthetize, and dissect. This experiment aimed to use
this invasive species to see if they were directly affecting the survival of the Western Pond Turtle
(Actinemys marmorata). We hypothesized that the presence of the Largemouth Bass were
causing these declines in the Western Pond Turtle population by consuming their hatchlings. We
tested our hypothesis by catching twenty-one Bass from pHake Lake and analyzing the contents
of their stomachs in the laboratory to see if there was any indication of turtle hatchlings. We
were able to determine that our results were not consistent with our hypothesis, as none of the
Bass that we caught showed any indication of having consumed turtle hatchlings. Though our
results refuted our hypothesis, we were still able to compare them to data collected in 2011 -
2012 to find that there were seasonal changes to the Bass’s consumption patterns, that they still
affected the BFS ecosystem by preying on other species and competing for resources, and that
there was another potential species affecting the survival of the Western Pond Turtle.
Introduction
Invasive species severely affect native communities through interspecific interactions
such as predation and competition between invaders and natives, thereby risking the extinction of
native species (Miyake and Miyashita, 2011). Not only do invasive species compete with native
species for resources, but they also upset the natural balance of an ecosystem, greatly reducing
its biodiversity. Worldwide, 20 percent of endangered vertebrates are imperiled as a result of
invasive species (MacDonald et al., 1989), and 42 percent of the species on the Endangered
Species List are at risk primarily because of invasive species (Pimentel et al., 1999). Invasive
species are also the second greatest threat to biodiversity after habitat loss. Studying invasive
species is important because their numbers are dramatically increasing in the United States, and
thus, they pose a serious threat to the health of native wildlife species and ecosystems.
In this experiment, we examined the Largemouth Bass (Micropterus salmoides), an
invasive species at the Bernard Field Station (BFS) and a native species to Midwestern and Gulf

3. Bagheri 2
Coast regions in the United States, to see if they were affecting the survival of the Western Pond
Turtle (Actinemys marmorata). These Bass were introduced to the BFS as a non-native species in
1981, and have been associated with a decline in native prey species (Hayes and Jennings, 1986).
Western Pond Turtles, on the other hand, are important because they are the only native
freshwater turtle in California, and are more endangered than previously thought. Based on
preliminary observations by Dr. Nina Karnovsky, the Western Pond Turtle population at the BFS
appeared to be in decline. We hypothesized that the Largemouth Bass were causing these
declines in the Western Pond Turtle population at the BFS by consuming their hatchlings. We
predicted to find turtle hatchlings within their stomachs. To test this hypothesis, we caught
twenty-one Largemouth Bass, and subsequently analyzed the contents of their stomachs in the
laboratory to see if they contained any indication of turtle hatchlings. Using data from 2011 -
2012 in addition to the data that we collected, we characterized the diets of the Largemouth Bass
in pHake Lake. By testing this hypothesis, we hoped to illuminate key insights to species
interactions and develop new methods for preserving native species at the BFS.
Materials & Methods
In this experiment, we tested whether an invasive species, the Largemouth Bass, was
influencing the survival of the Western Pond Turtle population at the BFS. We collected all of
our data at the BFS in Claremont, CA. We visited the BFS twice to catch the Bass, on April 7
and 11, 2015. We caught the Bass from pHake Lake, a manmade lake within the BFS. Each
member had a fishing rod with Fat Ika Bait, a net, and a bucket filled with water. Once we
caught the Bass, one member would lure it into the net while the other would unhook it from the
line. After we caught three to four Bass, we would go back to shore to offload the Bass. Dr. Nina
Karnovsky humanely anesthetized the Bass using MS-222 (Sigma Aldrich). After being put to

4. Bagheri 3
sleep, we took preliminary measurements of each Bass (weight, total length, fork length, and
standard length). We transported the Bass back to the Seaver South Laboratory, and stored them
in the freezer until we were ready to dissect them the following week. We caught a total of
twenty-one Bass.
In the laboratory, we dissected each Bass to analyze the contents of their stomachs. First,
using a scalpel, we cut open the Bass. Using scissors, we cut into the Bass to make a flap about 4
cm x 4 cm. After taking a preliminary look at the inside of the Bass, we sexed it. Females
generally had a clear indication of eggs (gooey orange color), and males had indication of
gonads. After sexing the Bass, we identified the stomach, and cut it out. In a bowl, we cut the
isolated stomach and washed its contents with 80% ethanol. To identify the exact contents of
each Bass’s stomach, we put the bowls under a microscope. Once we confirmed our findings, we
recorded them. We went on to dissect the next Bass until all twenty-one were dissected.
After finishing our dissections, we set up two motion-sensor cameras at pHake Lake to
determine if there was any evidence of the Western Pond Turtle. We put the cameras on opposite
sides of the lake near the shore and tied them to a tree. We allowed the cameras to run for five
days without interruption (from April 24 – 28, 2015), and programmed them to take a picture
every 45 seconds or every time there was significant movement. After five days had elapsed, we
removed the cameras and obtained the data.
Results
The objective of this experiment was to determine whether the invasive species
Largemouth Bass was driving the turtle population at the BFS to extinction. We caught a total of
twenty-one Bass from pHake Lake over the course of two visits, and characterized their diets by
dissecting their stomachs. Of the twenty-one Bass that we analyzed, none of them showed any

5. Bagheri 4
indication of having consumed turtle hatchlings (Table 1). We did, however, find other notable
remains within their stomachs, and their percentages relative to the sum total that the Bass
consumed are as follows: crayfish (2.72%), amphipod (76.65%), snail (19.1%), small fish
(0.78%), bait (0.39%), and unknown (0.39%) (Table 1 and Figure 1).
We then added our data to an already existing file on the Largemouth Bass stomach
contents from 2011 – 2012. We created a bar graph displaying percent occurrence of anything
found in the Bass’s stomachs (Figure 2). This graph revealed the three most popular
consumption choices for the Bass in pHake Lake: amphipods (94.77%), snails (1.55%), and
dragonfly larvae (1.18%) (Figure 2).
Finally, we made a bar graph revealing what percent of their diet was composed by each
category (Figure 3). This graph was separated based on the four sampling periods: Fall 2011,
Spring 2012, Summer 2012, and Spring 2015. From this graph, it was readily apparent that
amphipods were the preferred food of the Bass, with the only exception coming in Fall 2011,
when large damsal larvae predominated at 96.41%. This figure also indicates that snails
comprised a large portion of the bass diet beginning in Spring 2015.

6. Bagheri 5
Table 1: Contents found within the stomachs of the Bass for Spring 2015. Occurrences refers to
the number of Bass that we found to have that content within its stomach, amount refers to the
sum total of all the occurrences, and percentage refers to each amount divided by the total (257).
Content
Turtle
Occurrences
0
Amount
0
Percentage
0%
Crayfish 5 7 2.72%
Amphipod 5 197 76.65%
Snail 3 49 19.1%
Small Fish 2 2 0.78%
Bait 1 1 0.39%
Unknown
Total
1
17
1
257
0.39%
100%
Figure 1: Pie chart depiction of Bass diets for our data only (Spring 2015).
2.72%
76.65%
0.00%
0%
0%
0%
0%
19.07%
0.78%
0.39% 0.39%
Crayfish
Amphipods
Damsal Larvae
Dragon fly Larvae
Dragon fly
Hemiptera
Ants
Snail
Small Fish
Unknown
Bait

7. Bagheri 6
Figure 2: Graphical depiction of Bass diets using our data (Spring 2015) and the data obtained
from 2011 - 2012. The x-axis represents prey consumption, and the y-axis represents percent
occurrence.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
PercentOccurance Largemouth Bass
Summer 2012
Fall 2011
Spring 2012
Spring 2015

8. Bagheri 7
Figure 3: Bar graph depiction revealing what percent of the Bass’s diet was composed by each
category for the pooled data (Fall 2011, Spring 2012, Summer 2012, and Spring 2015).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Total Summer
2012
Fall 2011 Spring
2012
Spring
2015
Percent of Diet Composed By
Categories
Bait
Unknown
Small Fish
Snail
Ants
Hemiptera
Dragon fly
Dragon fly Larvae
Damsal Larvae
Amphipods
Crayfish

9. Bagheri 8
Discussion
In this experiment, we were concerned with determining how much of an impact the
Largemouth Bass were having on the survival of the Western Turtle Population at the BFS
through a careful analysis of the Bass’s stomach contents. From our results, we were able to
conclude that the Largemouth Bass did not have an impact on the Western Pond Turtle at the
BFS, as none of them had any indication of turtle hatchlings within their stomachs. With that
being said, however, our results indicated that they were still affecting the BFS ecosystem in
other ways and that there was also another potential species affecting the survival of the Western
Pond Turtle.
Although our results did not support our hypothesis, they did reveal that the Bass are not
selective in what they choose to consume. This is supported by the fact that in five out of the
twenty-one Bass that we caught (23.81%), there was evidence of crayfish consumption. We
found one of these Bass to have even consumed three crayfish. In the data from previous years,
there was also significant evidence indicating that the Bass had consumed crayfish, as nineteen
out of the seventy-three (26.03%) Bass had shown evidence of crayfish consumption. Crayfish
are freshwater crustaceans that resemble small lobsters. Their hard, shell-like covering
exoskeleton reveals that the Bass have flexible diets. The fact that one of our Bass showed
evidence of having consumed three crayfish also indicates that the Bass can consume a
significant amount of food relative to their body size during one feeding period. In addition, in
one of the Bass that we caught, its stomach contained the Fat Ika bait that we used to catch it.
This finding again reaffirms that the Bass are able to consume large objects. All of these findings
suggest that they are more than capable of consuming turtle hatchlings, especially since turtle

10. Bagheri 9
hatchlings are anywhere between 2.5 – 3.0 cm, whereas crayfish can grow to be between 5.0 –
12.0 cm in length and the Fat Ika Bait that we used was about 10.0 cm in length.
We can attribute our result that none of the Bass had turtle hatchlings within them to our
small sample size. We only caught twenty-one Bass, which is not a substantial amount, given
that there are an estimated 1,000 Bass in pHake Lake according to Dr. Karnovsky. Our small
sample size makes our data less reliable. We thus cannot conclusively state that the Bass are not
consuming Western Pond Turtle hatchlings, because it is likely that with a larger sample size, we
would have found evidence of turtle hatchling consumption. In addition, we did see potential
evidence of the Western Pond Turtle (Figure 4), so we cannot confidently attribute our results to
a lack of turtles in the lake.
It has also been shown that the biggest threat to the endangered Western Pond Turtle is
the bullfrog (Durham, 2010). A voracious predator, this amphibian has been known to feed on a
wide range of prey including insects, fish, and even turtle hatchlings (Kiesecker and Blaustein,
1998). The hatchlings are particularly vulnerable due to their small size and limited mobility. It
is hard to control or eradicate bullfrogs because they have a high fecundity and broad diet. We
can thus rationalize our results by claiming that the bullfrog is perhaps the main predator of the
Western Pond Turtle at the BFS. Our motion sensor camera even caught an image of the bullfrog
(Figure 5) and we observed them a few times ourselves while we were out fishing for the Bass.
Finally, our results reveal that there is strong evidence showing that seasonal changes
drive Bass consumption patterns (Figure 2). In warm-weather climates, amphipods are more
abundant. Warmer temperatures make them more active, increasing their feeding and
reproduction rates (Lane, 2002). Our Bass consumption patterns support this fact (Figure 2), as
nearly 95% of their diet in the Summer 2012 comprised of amphipods. In Spring 2012 and 2015,

11. Bagheri 10
there were high numbers of amphipod consumption in the Bass diets as well. In addition, our
only results in colder temperatures were from Fall 2011. During this time, damsel larvae
comprised of nearly 100% of their diet (Figure 2). An explanation for this consumption pattern
can be attributed to the fact that the damselfly lays its eggs during cold temperature periods so
that they hatch in warmer temperatures. Since the larvae are immobile during the fall, the Bass
are much more readily able to consume the larvae.
By understanding the consumption patterns of the Bass, we are more readily able to
predict when they will consume certain type of prey, and this could have huge ramifications for
the conservation of certain species at the BFS. In the future, we recommend that the Bass be
completely removed from pHake Lake. We could also catch and dissect the bullfrogs at pHake
Lake to see if they are affecting the survival of the turtle population. All of these factors would
contribute to the preservation of native species at the BFS.
Figure 4: Image taken by our motion-sensor camera indicating possible evidence of the Western
Pond Turtle. The boxed image is what seems to be evidence of a turtle. The figure at the bottom
of the picture appears to be the back of a bird.

12. Bagheri 11
Figure 5: Image taken by our motion sensor-camera indicating evidence of a bullfrog.
Acknowledgements
We would like to thank Dr. Nina Karnovsky for her tireless work in helping make this
experiment a success. Dr. Karnovsky helped catch, anesthetize, and analyze the Bass, and
dedicated countless hours to making sure that the experiment was conducted in an efficient and
thorough manner. We would like to thank our laboratory TA’s, Mimi Starr and Molly Shallman,
for coming to the BFS to help us catch the Bass and also for helping us analyze the contents of
their stomachs in the laboratory. We would like to thank Max Karnovsky and Christian Settles
for also helping us catch Bass at the BFS. We would like to thank Yonghong Ren for driving us
to and from the BFS and for helping us set up our motion-sensor cameras. Finally, we would like
to thank the manager of the BFS for allowing us to use the facility in order to collect our data and
Pomona College for allowing us to use its facilities.

13. Bagheri 12
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