Dale Hanson, fishery biologist at the Green Bay National Fish and Wildlife Conservation Office, has initiated work on a reference collection of known age lake trout otoliths, or ear bones. 

Otoliths contain a history of fish growth.  Annuli, or yearly rings, form as a result of seasonal growth patterns; the opaque dark bands represent periods of rapid fish growth, followed by translucent white bands that form when fish growth is slow (Photo 1).  Using a specialized saw, a 20 micron cross-section is sliced from the center of the otolith to expose the annuli.  The sections are then mounted on a microscope slide for a permanent record that can be viewed under a microscope. 

To date the reference collection contains 50 known age specimens, but it will eventually grow to 200 or more representing a range of ages, collection years and locations in Lake Michigan.

Management of fish stocks relies on fish age determination to evaluate factors that may affect year-class strength, fecundity of a population, fish growth, and age-specific mortality rates.  Ageing workshops held last year demonstrated that only thin-sectioned otoliths provided relatively unbiased age estimates for Lake Michigan lake trout over 12 years of age. 

Historically most agencies have used fish scales to determine age, however scale age determinations severely underestimate age in mature lake trout and therefore more agencies are currently switching to otoliths for age determinations.  The reference collection that is currently being constructed will provide an invaluable training tool for new otolith readers, and may be mixed in with fishery samples to enable monitoring of reader accuracy and precision.