IntroductionMosquitofish (Gambusia affinis, Figure 1) have been introduced to various regions across the world to control mosquito populations (Berra 2001), including Olde Minnow and Wood Duck ponds in the Denison University Biological Reserve (Burger 2009). Normally mosquitofish are an invasive species and they are not native to Ohio. Longear sunfish (Lepomis megalotis, Figure 2) are traditionally native to Ohio and they feed mainly on insects and invertebrates ( Berra 2001). Sunfish have historically been present in Wood Duck Pond but not Olde Minnow. However in the summer of 2014 juvenile and adult sunfish were observed in Olde Minnow Pond. This creates an unusual situation where a native fish (sunfish) is invading into an established population of an invasive species (mosquitofish). For this project we wanted to see if the introduced sunfish influence the population of mosquitofish. The impact of sunfish on mosquitofish might occur through changes in population size or in the diets of the mosquitofish, so we explored both these ecological features in the two ponds.

MethodsTo estimate the size of the mosquitofish population, we trapped Olde Minnow and Wood Duck in the Denison University Biological Reserve weekly during the summer of 2015. Ten traps were set in each pond, eight along the shoreline and two in the center of the pond. We collected the traps after 24 hours and measured the first ten male and female mosquitofish in each trap, along with any sunfish in the traps. The remaining fish in each trap were sexed and counted, and all fish were then released back into the pond.

In Olde Minnow in early July we conducted a mark-recapture survey of sunfish to estimate population size. We collected fish from five seines across the whole pond. We also set fish traps in the eight perimeter locations used for mosquitofish surveys. Any fish we caught were sexed, measured, marked by a fin clip, and released back into the pond. As a side project we stomach pumped the sunfish and removed a scale to help age the fish before releasing them. The following week we repeated the process of seining and trapping, collecting both marked and unmarked fish. We used our counts of marked and unmarked fish to find the estimated population size using the Chapman modification of the Lincoln-Petersen model for mark recapture studies (Zale et al. 2012).

To sample fish diets we collected 15 random male and female mosquitofish every two weeks from each pond by using a dipnet to catch fish. The fish were immediately put on ice and then placed in the freezer once back in the lab. I dissected the stomachs of each fish and used a microscope to identify and count the different prey items found in the stomach.

Figure 1. Female (top) and male (bottom) mosquitofish. Image from http://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=846

Figure 2. Sunfish from Olde Minnow Pond. Photo credit= J.Rettig

Figure 3. The average number of mosquitofish/trap (+1SE) in Olde Minnow and Wood Duck Ponds during summer 2015. N=10 traps/pond

June 2-3 June 10-11

June 16-17

June 23-24

June 30- July 1

July 7-80

10

20

30

40

50

60

70

80

Olde Minnow

Date

Num

ber o

f Mos

quito

fish

per T

rap

May 26-27 June 2-3 June16-17 June 30-July 1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Olde MinnowWood Duck

Date

Num

ber o

f Cop

epod

Figure 4. The average number of copepods (+1SE) found in mosquitofish stomachs from Olde Minnow and Wood Duck Ponds during summer 2015. N=30 fish/pond

May 26-27 June 2-3 June16-17 June 30-July 1

0

50

100

150

200

250

Olde Minnow

Wood Duck

Date

Num

ber o

f Chy

doru

s

Figure 5. The average number of Chydorus (+1SE) found in mosquitofish stomachs from Olde Minnow and Wood Duck Ponds during summer 2015. N=30 fish/pond

Results• The abundance of mosquitofish (average number/trap) changed across the summer

and was higher and more variable in Olde Minnow (Figure 3. rmANOVA Pond x Time interaction: F=3.195, p=0.039).

• In the Fall of 2014 lab records indicate that there were no sunfish in Olde Minnow (Johnston and Rettig unpublished data). Our mark recapture study estimates there are 167 adult longear sunfish as of July 2016. Wood Duck had very low numbers of sunfish during summer 2016 with only 7 encountered in all of our traps.

• There was a significant Pond x Sample Date interaction for the number of adult copepods in the mosquitofish diets (Figure 4. 3-way ANOVA Pond x Sample Date interaction: F=2.705, p=0.046). Mosquitofish in Wood Duck ate many copepods initially but then consumption declined. Mosquitofish in Olde Minnow ate more copepods in the midsummer than early and late summer.

• There was a significant Pond x Sample Date interaction for the number of Chydorus in the mosquitofish diets (Figure 5. 3-way ANOVA Pond x Sample Date interaction F=12.569, p=<.0001). The mosquitofish in Olde Minnow ate more Chydorus in midsummer than in early and late summer. In Wood Duck Chydorus were far less abundant and less variable in the mosquitofish diets.

• There was a significant Pond x Sample Date interaction for the number of snails in the mosquitofish diets (Figure 6. 3-way ANOVA Pond x Sample Date interaction F=2.833, p=0.039). The mosquitofish in Wood Duck ate more snails in the beginning of summer then consumption dropped throughout the summer. In Olde Minnow there were never many snails eaten.

Results Continued

DiscussionThe population size of mosquitofish in Olde Minnow had lot of variation and was very high at the beginning then had up and down weeks throughout the summer (Figure 1). These fluctuations could be due to the presence of sunfish in the pond. From pumping the stomachs of the sunfish we have seen that sunfish can be predators of mosquitofish. The sunfish could also be dictating what the mosquitofish are eating by consuming zooplankton or other food items that the mosquitofish would normally eat.

The diets of mosquitofish vary greatly between the two ponds. The mosquitofish in Olde Minnow eat a large number of Chydorus and copepods along with many larval insects such as chironomids and diptera pupa. The mosquitofish in Wood Duck eat copepods and Chydorus as well, but in much smaller numbers. They also feed on snails and an unknown early egg embryo. It should be noted that there are sunfish present in Wood Duck, just in much smaller numbers and sizes than in Olde Minnow.

If I were to continue this project I would take a closer look at the diets of the sunfish in Olde Minnow. I have 32 gut samples from sunfish in Olde Minnow. Looking closely at them would allow a better understanding of the feeding behaviors of the sunfish and we could see if they were directly competing with the mosquitofish. I would also like to compare the diets of mosquitofish from this summer (2015) with diets collected by Margaret Gaglione who sampled Olde Minnow mosquitofish in 2010 when sunfish were absent (Gaglione and Smith, unpublished data).

AcknowledgementsI would like to thank Dr. Jessica Rettig, Dr. Geoff Smith, Maggie Jones, and Wesley and Logan Smith for all their help on this project. This research was supported by the Laurie Bukovac and David Hodgson Endowed Fund of Denison University.

Literature CitedBerra, T.M. 2001. Freshwater Fish Distributions. Academic Press, San Diego, California.Burger, A. 2009. Mosquitofish population dynamics in Olde Minnow Pond Granville:

Research poster (unpublished)Gaglione, M. 2010. Factors affecting the diets of mosquitofish (Gambusia affinis) in the

DUBR ponds Granville: 2010. (unpublished manuscript)Johnston, M. 2004. Comparison of plant and fish communities in two small ponds in

central Ohio. (unpublished senior thesis)Zale, A.V., D.L. Parrish, and T.M. Sutton, editors 2012. Fisheries techniques, 3rd edition.

American Fisheries Society, Bethesda, Maryland