COS 130-6 - Using Eastern Mosquitofish (Gambusia holbrooki) Populations to Assess Waterhyacinth (Pontederia crassipes) Management

Abstract: Waterhyacinth (Pontederia crassipes) is a prolific invader in many parts of the globe, but little is known about how this invader directly affects fish populations. Current management involves using herbicides, physical removal, and established biological control agents. In South Florida, waterhyacinth costs taxpayers millions of dollars annually for removal. Due to the time required for physical removal and changing attitudes towards pesticide use, we examined an integrated approach to management focused on better utilization of biological control. In 2020, we established large outdoor mesocosms to simulate ponds. Each mesocosm was stocked with waterhyacinth and native aquatic plants [Frog's Bit (Limnobium spongia), Spatterdock (Nuphar advena), Water lily (Nymphaea mexicana), Spikerush (Eleocharis palustris), Duck-potato (Sagittaria latifolia), Pickerelweed (Pontederia cordata), and Arrow arum (Peltandra virginica)], and small, native fish. Fish populations mainly consisted of Eastern Mosquitofish but also included Bluefin Killifish (Lucania goodei) and Least Killifish (Heterandria formosa).



After the tanks had been established for a year, we began management treatments. Our management treatments included a "true" control using insecticide to control biological control, an herbicide-only (no biological control 2-4, D only) treatment, a biological control-only treatment, and an integrated treatment that combined both biological control and 2-4, D herbicide. We sampled tanks quarterly using minnow traps to assess the populations in each tank through November 2022. We found that, as long as we managed waterhyacinth cover, fish populations in the mesocosms remained stable, regardless of the treatment type. Due to public perception of pesticides and the cost and time associated with manual removal, we suggest the expanded use of biological control with only targeted herbicide applications. Broadly, this study addresses a knowledge gap that exists about non-commercial fish populations. While our example is location dependent, it can be applied more widely to many ecological examples in a variety of locations.