Relative Body Size Infl uences Breeding Propensity in Fathead Minnows: Implications for Ecotoxicology Testing Procedure

Numerous factors affect the ability or choice of fi shes to breed. For example, studies demonstrate that the appropriate amount of light, temperature, and food must be present before many species will breed. For some species, we are also aware of social factors that affect breeding, such as the size or colour of one’s potential mates. Although studies on mate choice (i.e., choice of one potential mate over another) and factors affecting breeding are extensive, there remain signifi cant gaps in our knowledge with regard to scientifi cally important species. For instance, the fathead minnow (Pimephales promelas), used by numerous researchers as a test subject in reproductive toxicology and behavioural ecology, has well established physical parameters known to facilitate breeding. Conversely, there is very little data describing social factors which may infl uence breeding. The purpose of the current study was to examine some of the factors affecting mate choice in the fathead minnow. Results indicate a consistent relationship between male and female size (length and mass), which can be used to predict the probability of a couple’s breeding potential. Specifi cally, we found that female minnows prefer larger males. In successful pairs there was a greater difference in size between the male and female as compared with unsuccessful pairs. The fi ndings of this study could substantially improve methods for reproductive studies in laboratories or artifi cial streams by decreasing both the number of pairs tested against baseline performance criteria and the time needed to establish actively breeding individuals. This will decrease the cost and increase the effi ciency of future studies, as well as add ecologically interesting knowledge to the literature regarding a scientifi cally important, ubiquitous, and representative North American fi sh species.