Abstract
Environmental chemicals are just one mechanism by which anthropogenic actions influence populations and individuals. Perhaps one of the most widely reported effects of pollutants on wild populations, in both the academic literature and the media, is that of chemicals on fish sexual development. Plasticity in the sex-determination systems of fish has long been exploited in aquaculture and by researchers working in the field of sex-determination. Such research has typically focused on inducing a reversal in the functional sex of individuals via an environmental manipulation, often exposure to exogenous chemicals; termed environmental sex reversal. In the 1990s British ecologists demonstrated that pollutants in rivers were altering the sex of wild fish. Those findings broadened the focus of research on sex reversal in fish to include ecological and evolutionary themes. Now, almost twenty years since those initial findings were reported, multiple studies and reviews on the subject of piscine sex reversal have been published.
The main aim of this thesis is to address a number of questions that have been proposed in the field of environmental sex reversal. These questions comprise the over-arching theme for this thesis: what does environmental sex reversal mean for fish and fish populations?
In Chapter 2, I used a comparative approach to test whether differing taxonomic groups vary in their susceptibility to chemically induced environmental sex reversal. I found that most fish species are uniformly susceptible to chemical sex reversal. In Chapter 3, numerous published estimates of the effects of environmental sex reversal on morphology were combined using meta-analysis. My results suggested that both exposure to sex reversal inducing chemicals and sex reversal itself can have negative effects on reproductive fitness and alter growth patterns. In Chapter 4, I used computer simulations to assess how sex-reversed individuals might be used to affect the population dynamics of two invasive species (Gambusia affinis and G. holbrooki). Those models demonstrated that sex-reversed fish can alter the dynamics of populations and may be used as a bio-control. However, impaired reproductive fitness of those sex-reversed individuals may dampen their population-level effects. In my final data chapter (Chapter 5), I induced sex reversal in the western mosquitofish (G. affinis) via hormonal exposures and observed the behavioural and morphological effects of that sex reversal. Chapter 5 showed that both sex reversal and exposure to exogenous chemicals can have independent effects on the behaviour and morphology of G. affinis. Additionally, I found that individuals not exposed to sex-reversing chemicals may modify their behaviour in the presence of sex-reversed fish.