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Exploring Alternative Methods for Determining Fishery Characteristics
Exploring Alternative Methods for Determining Fishery Characteristics

​​​​​​​​Led By

Jesse ​​Lepak​

Study Area

Four Colorado Reservoirs (Blue Mesa, Granby, Shadow Mountain, Williams Fork)

Project Status


Research Objectives

  • To explore less time-consuming and more reliable methods for determining fish age.

Project Description

Understanding lake and reservoir food web structure plays a crucial role in determining how best to manage a sport fishery. In order to inform management decisions, man​agers use a variety of techniques to characterize important aspects of lake and reservoir fisheries.

For example, managers can use otoliths, calcified structures in a fish's inner ear that grow like tree rings, to determine the age of the fish. This information helps managers understand things like fish growth rates, the age structure of a fish population and if certain age classes of fish are missing from the population. This information can help managers identify and potentially address problems in a fish population like disease, poor spawning or recruitment, and overpopulation

Currently, researchers use thin sections of the otoliths to determine the age of the sample fish. However, this method is labor-intensive, requires extensive training and is a subjective classification. Therefore, CPW initiated a research project to determine if otolith mass can be used as a reliable indicator of age. Such a method would require less training, would be faster and would be relatively objective.

During the kokanee salmon spawning runs, researchers collected kokanee salmon from four reservoirs in Colorado. Researchers noted kokanee salmon length and whether the fish was male or female. Otoliths were also extracted from each fish and weighed. A small subset (~30) of otolith sections was used to determine kokanee salmon age and "calibrate" a computer model.  All t​he length and gender data from each fish (hundreds) was then input into the model to estimate ages for all the fish collected. With a relatively low error rate, researchers determined that using otolith mass is a promising addition to traditional age-classification methods.

Results from this study provide managers with a faster and more reliable manner in which to determine fish age in Colorado's lakes and reservoirs. This information will inform future management decisions, streamline methods required to determine fish age and will be applied to the valuable kokanee salmon fishery in Blue Mesa Reservoir. ​

Associated Publications

Lepak, J.M., Cathcart, C.N., and Hooten, M.B. 2012. Otolith weight as a predictor of age in kokanee salmon (Oncorhynchus nerka) from four Colorado reservoirs. Canadian Journal of Fisheries and Aquatic Sciences. 69(10):1569-1575.