Mosquitofish (Gambusia) in Guion Pond

     The little pond behind Guion is one of the richest natural habitats on campus. It is full of amphibians, insects, crayfish, and microscopic crustaceans, protists and algae. Each year, the only population of spotted salamanders known on campus returns to the pond to breed.

     In late summer 2002 Jeff Janovetz noticed mosquitofish (Gambusia) in the pond. I had never seen fish in the pond, and it became clear that they had been introduced by humans in recent months. There is no natural means for the fish to have dispersed into the pond. They might have been dumped by a fisherman, or by someone using mosquitofish for teaching or research who thought they were being "kind" to the fish by releasing them, or by someone nervous about mosquitos as vectors of West Nile Virus. 

     Mosquitofish are hardy, prolific little fish capable of producing large populations in a short time. A female produces a brood of up to 300 young less than a month after mating, and she may produce 3 or 4 broods in a season. By late October there were over 1000 mosquitofish in our tiny pond.

     Why are we concerned about mosquitofish in Guion Pond?   Mosquitofish are surface feeders on a wide array of crustaceans, aquatic insects, and small vertebrates. They can reduce the abundance of species they feed on, and of species they compete with.

     The inhabitants of Guion Pond include many animals, both vertebrate and invertebrate, that are primarily found in fish-free habitats. Most permanent water has fish in it, but temporary ponds that dry out completely during part of the year do not. (These are sometimes called ephemeral ponds or vernal pools.) The animals in temporary ponds have life cycles that allow them to leave the water before it dries out completely.  They are capable of exploiting the temporary habitats; in permanent waters, however, they often cannot survive fish predation and/or competition. Guion Pond is waterfilled year-round, but fish are unable to move upstream from the upper lake and leap over the pond's dam.

     What will be the effects of the arrival of Gambusia? I posed this question on an internet listserve of ecologists, and received twenty-seven responses in two days. Some ecologists felt that the effects would be minimal or non-existent, but the majority were quite concerned. There is evidence that salamander larvae can be killed by Gambusia, and that salamander adults avoid breeding in ponds with fish. This poses a problem for the spotted salamanders, which return to the same breeding site every year. Our spotted salamanders probably all grew up in Guion Pond, and this is probably their only available breeding site.

     So what are our options? If we had several ephemeral ponds on campus, we might have decided to leave the Gambusia alone as an interesting learning opportunity for our students. Since this is our only pond, we (Jeff Janovetz and Linda Fink) feel that the Gambusia have to be removed. The three approaches to this would be introducing a predator, chemical poisons, or draining the pond. Our ecologist colleagues overwhelmingly favored draining as the best solution.

     Summer 2002 was one of the driest years on record. Through August, Guion Pond was as low as it has ever been, and the spring feeding it was entirely dry as well. It would have been relatively easy to pump out the pond and let it stay dry for several weeks. Most of the animals have left the pond by then, so there would have been little mortality other than the mosquitofish.

     Unfortunately, by the time we decided to take action it was mid-fall and the drought had been broken by several magnificent, mammoth rain events. Guion Pond was filled again, and its inflow was restored.  Nonetheless, we rented a pump and with a fire hose and help from Physical Plant we pulled the water level down. On 14 November, Dr. Fink's Introductory Biology Lab and Dr. Janovetz's Vertebrate Diversity students converged on the pond with nets, boots, buckets, and a plan to remove and census all of the mosquitofish before draining the last of the water.

     Our failure was muddy, messy, and overwhelming. The pump and hose were unable to pull out the last water once it was saturated with mud; the pond bottom was near quicksand so that we had to throw down plywood boards to stand on; and the fish were far better at evading our nets then we were at maneuvering our plywood and boots. Even if we could have removed most of the fish, there was no possibility that we would get them all. To keep the pond drained completely enough and long enough to kill the remainder, we would have had to divert the inflow. After several hours it was clear that we were beaten. Temporarily.   One summer, during a long dry spell, we will drain the pond again. If the weather is kind to us, the pond will stay dry long enough to kill all of the fish.

     Several biology majors have studied the effects of fish and crayfish predators on salamander eggs and larvae.  Their studies show that the eggs are well protected, but larvae are at risk from both predators.  

     Finally, readers, study local natural history, don't disrupt it. Please do not release any captive-bred organisms into the wild, or transfer wild organisms from one area to another. If you get tired of the frogs you buy at the pet shop, don't dump them in the nearest creek. If you bring live snails home from a river field trip, don't let them go in a different river. If you raise a local caterpillar, go ahead and release the butterfly that emerges; but please don't rear painted lady or monarch butterfly caterpillars obtained from commercial suppliers and then release the adults.

Read more about effects of mosquitofish on amhibians in:
Gamradt, S.C. and L.B. Kats. 1996. Effect of introduced crayfish and mosquitofish on California newts. Conservation Biology 10(4): 1155-1162.

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URL: //nature.sbc.edu/animals/gambusia.html       email:lfink@sbc.edu

This site is maintained by Professor Linda S. Fink  (434) 381-6436
Department of Biology
Sweet Briar College, Sweet Briar VA 24595
Last updated: July 2007
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