Impacts of mosquitofish, Gambusia affinis, on endangered Mohave tui chub, Siphateles bicolor mohavensis, growth and population dynamics: A mesocosm study approach

Authors: Sujan M. Henkanaththegedara, Craig A. Stockwell

Year: 2008 (xl)

Abstract

In the western United States, many native fish species have been negatively impacted by the introduction of mosquitofish, Gambusia sp., however some species have co-existed with non-native mosquitofish for many decades. Such in the case with the Mohave tui chub, Siphateles bicolor mohavensis (Cyprinidae), a state and federally protected fish species that co-occurs with mosquitofish in two of its four habitats in California. We conducted a controlled mesocosm experiment to evaluate possible impacts of mosquitofish on Mohave tui chub (Lake Tuendae population) growth and population dynamics. We stocked mesocosms with 8 Mohave tui chubs (TL=80-120mm) and 75 mosquitofish (25 males and 50 females) and monitored the growth rates of tui chubs and population dynamics of two species over a 4-month period. Although the experiment yielded comparatively high numbers of Mohave tui chub larvae at the end of the experiment, there is no significant difference of number of chub larvae produced between control and treatments (t=0.566; P>0.05). However, there was relatively low survival and recruitment of mosquitofish in the mesocosms (F=1766.04; P<0.001). Both male and female mosquitofish populations declined significantly (F=262.42; P<0.001) possibly due to predation by adult tui chubs. Additionally there are significant interaction term between time of sampling and sex of mosquitofish (F=172.85; P<0.001). Most surviving mosquitofish were large females which incidentally are too large for the measured gape size of tui chubs. There was no significant treatment effect on tui chub condition (F=0.85; P>0.05). Mean instantaneous growth rate of individual chubs (g/year) was significantly higher in the presence of mosquitofish compared to the control (t=-1.7998; P<0.05); contrary to the expectations. These results suggest Mohave tui chub may impact non-native populations of mosquitofish which in turn may allow co-occurrence of these two species.