**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION. The title, authors, and abstract for this completion report are provided below.  For a copy of the full completion report, please contact the author via e-mail at hej@michigan.gov or telephone at 989-356-3232 ext. 2573. Questions? Contact the GLFC via email at stp@glfc.org or via telephone at 734-662-3209.**

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

December 2017

ABSTRACT:

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 .

RESEARCH HIGHLIGHTS:

1. When age structured stock assessment integrates multiple data sources, each of those data sources typically represent different selectivity in harvesting or sampling the fish population. Thus, one challenge in conducting age structured stock assessment is to collect a large sample of calcified structures for reliable age assignments and maintain consistent age assignments over years, with a given data source and among multiple data sources.
2. Using maxilla sections to estimate ages of lake trout is a new method recently developed at MDNR Alpena Fisheries Research Station, and has been applied by many agencies as a primary method in their annul stock assessment. The advantage of this new method includes the easy collection of a large number of samples from all needed data sources, the simple and inexpensive lab procedures, the straightforward interpretation of annuli, and reliable use of images in the procedures of quality control.
3. Attention to details in lab procedure can substantially improve the quality of maxilla sections, and reliable age assignments are demonstrated for lake trout over 30 years of old. Continued efforts are needed to compare the new method against the standard otolith method and further demonstrate applicability of the new method for age assignment to old lake trout. 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 .