Age Determination and Validation of Rainbow Darter (Etheostoma caeruleum)

The accurate determination and validation of age is an important tool in fisheries management. Population age profiles allow insight into population dynamics, mortality rates and growth rates. Many studies in the Grand River, Ontario have focused on the impact of municipal wastewater effluent (MWWE) on fish population health. The Rainbow Darter (Etheostoma caeruleum), a small-bodied, benthic fish, has been used as a sentinel species in these studies on population level responses and reproductive health, including the high occurrence of intersex. The objective of the current study was to develop a method to accurately age Rainbow Darter, validated by use of marginal increment analysis and population length-frequency histograms. Rainbow Darter were collected on a monthly basis and size (length, weight) and the formation of annuli on both otoliths and scales, were assessed. Darters spawn in late April-early May and young-of-year darters reach a catchable size by June. A distinct young-of-the-year cohort was apparent until November, after which it is difficult to separate them from other age classes. The ability to accurately age Rainbow Darter will allow further analysis of size-at-age and growth in this species and allow a better understanding of how other endpoints, such as intersex, respond to effluent exposure.