1. Introduction
Mosquitofish (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.
Methods
To 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.
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.
Discussion
The 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).
Acknowledgements
I 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 Cited
Berra, 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
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Figure 3. The average number of mosquitofish/trap (+1SE) in Olde Minnow and Wood
Duck Ponds during summer 2015. N=10 traps/pond
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
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
Figure 6. The average number of snails (+1SE) found in mosquitofish stomachs
from Olde Minnow and Wood Duck Ponds during summer 2015. N=30 fish/pond
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