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Workshops on Lake Trout age assignment in annual stock assessment: Accuracy and Precision of Methods Specific to Age Ranges and Length Bins
Ji X. He1
1 Michigan Department of Natural Resources, Alpena Fisheries Research Station, 160 East Fletcher Street, Alpena, Michigan 49707
Two workshops were organized to review age assignment procedures around the Great Lakes and in other regions beyond of the Great Lakes. The workshops emphasized the needs for lake trout stock assessments, and some topics also covered other cold water species such as lake whitefish. A recently developed method using maxillae to estimate ages of lake trout has been applied as a primary method by many agencies, for the easy collection of large samples, the simple and inexpensive lab procedure of maxilla section, and the straightforward interpretation of annuli. The images of maxilla sections are reliable to be used for multiple reads and reference collection, and thus it is also relatively easy to implement a quality control procedure with the maxilla method. Many studies have found that using the method of maxilla section provided more consistent and repeatable estimates of lake trout ages than using otolith section, although with more lab times and more specific training the otolith method is recognized as the standard method. More studies are needed to explicitly compare the maxilla and otolith methods. From available comparisons between the two structures collected from a same set of fish samples, the maxilla ages appeared to be younger than the otolith ages; however, the MDNA lab at Alpena demonstrated a routine quality of maxilla sections and indicated applicability of the method to lake trout near or more than 30 years of old. Like the use of all other calcified structures for fish age assignment, two typical issues with the maxilla method are related to the interpretations of the edge and center of a maxilla section (image). On the image of a maxilla section, the edge outside of a dark zone, even without a complete light band, should be counted as the last age for a lake trout caught in the spring, but the same type of edge should not be counted for a lake trout caught in the fall. This decision rule implied that as maxillae grow over seasons, how an annulus is developed is not as clear as with other calcified structures. In the center of a maxilla section (image), potential complexity mostly stems from the process of hatchery production and stocking. Identifying the presence or absence of the stocking signore on a maxilla section is suggested as the key to objectively decide where the first annulus is in the center. To ensure the decision rules are clear and are consistently applied, it is recommended to use reference collection in a quality control procedure to closely match new age assignments to previous age assignments .