During the month of January, Fishery Biologists Scott Koproski and Adam Kowalski finished aging samples collected during the 2008 fishery independent lake whitefish and fall lake trout spawning surveys. 

As a signatory of the 2000 Consent Decree, the Service is responsible for working with State and Tribal agencies to establish safe harvest limits of lake whitefish and lake trout in 1836 Treaty waters. 

Alpena NFWCO fulfills the Service’s responsibilities in northern Lake Huron by assessing lake whitefish and lake trout populations in two management units:  WFH-04 and WFH-05.  The study sampling design was established by the Modeling Subcommittee (MSC) of the Technical Fisheries Committee (TFC).  The MSC uses data collected from each management unit to establish safe harvest limits using catch-at-age models.     

In 2008, Alpena NFWCO fished 40 gangs of gill nets in WFH-04 and WFH-05.  Over 1400 fish were collected during field operations, and aging structures (i.e., scales, otoliths, and fin rays) were removed from almost 400 of these fish.  Scales and otoliths were collected from all lake trout and lake whitefish, scales were collected from all round whitefish, and scales and dorsal fin rays were collected from all percids encountered.  Ages were assigned to each structure by Koproski and Kowalski. 

Scales were examined using a stereo-microscope and a transmitted light.  While examining a scale sample, the seasonal patterns of circuli formation can be identified and counted.  An annulus within a scale sample is assigned when circuli spacing is compressed and circuli cutting over is observed.  This cutting over is typically associated with the change in growth patterns present on the scale between slower winter and faster summer growth.

Fin rays were also examined using a stereo-microscope and a transmitted light.  The fin rays are cross sectioned and then a drop of vinegar is placed on each sample.  While examining a cross-sectioned fin ray, broad summer and narrow winter growth bands can be identified.  The winter growth bands are then counted and an age is assigned for that sample.

Similar to fin rays, patterns in summer and winter growth can be identified on an otolith sample.  However, preparation is different and a crack and burn technique is used along with a reflected light.  Individual otoliths are cracked transversely and placed in an alcohol flame for a brief period of time. 

Burning the otolith allows researchers to differentiate between the two distinct growth patterns within the structure: broad summer and compressed winter growth patterns. 

While viewing the sample a drop of mineral oil is placed on the structure to provide a clearer image.  By counting the bands of winter growth, age estimates can then be obtained from the otolith.

Age data, along with other biological parameters, are used by the MSC in the catch at age models to develop the safe harvest limits of 1836 Treaty Waters.