Response of mosquitofish (Gambusia affinis) populations to seasonally unpredictable perturbations

Many questions remain unresolved about the linkages between life history attributes of fishes and the tactics that these organisms employ in response to environmental uncertainty. Such questions include 1). If a perturbation affects the entire ecosystem, what are the consequences for a given population of fish? 2). What tactics can a fish employ to increase its chances of leaving offspring? 3). Do fish respond differently to such perturbations depending on the season? 4). How do these changes relate to the overall resilience of the population? The research reported here was designed to address such questions. Mosquito fish (Gambusia affinis) populations in thirteen experimental ponds at Oak Ridge Nat'l Laboratory were sampled ten times between June 1988, and July 1989 in response to a series of chemical disturbances. During each sampling period the population size and total biomass of Gambusia in each pond was estimated using photographs and a length weight regression. Size-frequency histograms were used to examine seasonal and dose-related changes in population structure. Lipid content and reproductive allotment were measured for a series of fish from each pond on all dates to explore the energy allocation patterns at the individual level. Fish populations declined significantly in each dosed pond. The intensity of the response was related to season. A dose administered on October 29, 1988 eliminated all fish from five of the experimental ponds. In the fall fish from the treated ponds retained a higher rate of reproduction which resulted in pulses of recruitment in the experimental ponds that did not occur in the reference ponds. Fish in the dosed ponds entered the winter with lower lipid levels, however over winter these populations lost less biomass, entering the spring in better condition which resulted in higher growth rates, and greater fecundity. In the spring the phenolic treatment group was the only group that showed recruitment. I attributed the results to reduced intraspecific competition: the dosed ponds had lower population densities of Gambusia. Although Gambusia populations appear very resilient to disturbances the effects of the fall treatment were still evident in July 1989, nearly eight months after the last dosing had been administered.