Micropterus salmoides from pHake Lake Do Not Affect Actinemys

marmorata Survival Rates

Kian BagheriDr. Nina Karnovsky

Bio 41E: Evolutionary and Ecological BiologyPomona College

8 May 2015

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

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interspecific interactions such as predation and competitionbetween invaders and natives, thereby risking the extinction ofnative species

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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

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 -

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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

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

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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

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).

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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.

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%

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Unknown

Total

1

17

1

257

0.39%

100%

2.72%

76.65%

19.07%

0.78% 0.39% 0.39%

CrayfishAmphipodsDamsal LarvaeDragon fly LarvaeDragon flyHemipteraAntsSnailSmall FishUnknownBait

Figure 1: Pie chart depiction of Bass diets for our data only (Spring 2015).

Crawdad

Amphip

ods

Damsa

l lar

vae

Drago

n Fly la

rvae

Drago

n Fly

Hemip

tera

AntsSn

ail

Small

fish

UknownBait

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

Largemouth Bass

Summer 2012Fall 2011Spring 2012Spring 2015

Per

cen

t O

ccu

ran

ce

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.

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Total Summer 2012

Fall 2011 Spring 2012

Spring 2015

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Percent of Diet Composed By Cat-egories

BaitUnknownSmall FishSnailAntsHemipteraDragon flyDragon fly LarvaeDamsal LarvaeAmphipodsCrayfish

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).

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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

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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,

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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.

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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.

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