The Effects Of An Artifically Elevated Thermal Environment And Seasonal Acclimatization On The Thermal Tolerance Of The Western Mosquitofish, Gambusia affinis
Gambusia affinis, the western mosquitofish, is a very hardy, live-bearing fish that has invaded freshwater habitats worldwide. The success of the western mosquitofishs' invasion is due to its reproductive capabilities and ability to tolerate a wide range of temperatures. The mosquitofish can inhabit waters as low as 0°C and higher than 40°C. Because of this species' eurythermicity, questions have been raised regarding the impact of artificially heated environments on its thermal tolerance limits and whether its thermal tolerance limits are consistent across seasons. Past studies have shown that mosquitofish populations inhabiting hot ponds receiving thermal effluents at steam-electric power stations had higher upper thermal tolerance limits than populations inhabiting the associated lake's main reservoirs. This higher thermal tolerance limit was found to be heritable, pointing to fitness differences between two populations. This led to the suggestion that directional selection was occurring in the population exposed to thermal effluents, leading to a more thermally tolerant population of moquitofish. This study examined whether the previously documented increase in upper thermal tolerance of the mosquitofish population affected by thermal effluents has continued to increase over the past five years, supporting the hypothesis of directional selection and whether a continued increase in the population's upper thermal tolerance has impacted its lower thermal tolerance limits as well. In order to determine whether thermal effluents and seasonal acclimatization affects the thermal tolerance limits of G. affinis, the upper and lower thermal tolerance limits were established for two populations and compared to previous research; temperature tolerance polygons were established to examine whether thermal effluents influence the degree of eurythermicity of this species; and seasonal comparisons of the upper and lower thermal tolerance limits were assessed to determine whether seasonal acclimatization has influenced the thermal tolerance limits of this species. The upper thermal tolerance limits were found to be inconsistent. Depending on the season, the population exposed to thermal effluents did not always exhibit a higher thermal tolerance than the population exposed to ambient lake waters. The lower thermal tolerance limits were more consistent, showing that the population exposed to thermal effluents were consistently less cold tolerant than the population exposed to ambient lake waters. Only female individuals exposed to thermal effluents exhibited an increase in upper thermal tolerance compared to individuals tested five years previously. Due to the inconsistency in the upper thermal tolerance limits and the continuously varying upper and lower thermal tolerance temperatures among seasons, seasonal acclimatization was shown to heavily influence the thermal tolerance limits of G. affinis. Temperature tolerance polygons demonstrated that the population exposed to thermal effluents and the population exposed to ambient lake waters had similar areas of thermal tolerance, suggesting that these populations generally inhabit the same thermal niche. These polygons also supported the fact that G. affinis has a wide range of thermal tolerance.