The "Crack and Burn" aging techique used on a lake whitefish otolith. - FWS photo

Fishery Biologist Scott Koproski began working on otolith samples collected for age and growth analysis during the 2006 fishery independent lake whitefish survey. 

Otoliths were collected from all lake whitefish, lake trout, and burbot sampled during this survey. 

The otolith is the first calcified structure that begins to develop during the egg stage.  It grows towards the anterior end of a fish and by viewing a cross-section of the otolith you can begin to see zones of summer and winter growth. The pattern resembles the rings of a tree. 

Annual growth can be identified by counting the number of zones of compressed winter growth.  In addition to counting the zones of winter growth you must pay special attention to the edge of the structure.  Annulus formation varies geographically with northern regions seeing annulus formation later in the summer compared to southern regions.  The growth past the last annuli must be classified as either new growth or growth from the previous year.  If it is growth from the pervious year, the annulus has not formed yet and the edge must be counted when assigning an age to the structure.

Koproski uses the “crack and burn” technique to differentiate the zones of summer and winter growth.  By cracking the otolith laterally and placing the cracked portion into an alcohol flame, the cracked portion begins to darken to a golden brown color.  Once the otolith is burned, it can be viewed using a stereo-microscope.  To help view the structure a drop of mineral oil is placed on the cracked section.  The mineral oil smoothes out the structure while viewing it under the stereo-microscope and allows the age interpreter to see the image more clearly.