The Age & Growth Workshop
BY KATHERINE JARDINE, LA CROSSE FWCO
This winter Biological Science Technicians Trevor Cyphers and Katharine Jardine with La Crosse Fish and Wildlife Conservation Office (FWCO) attended two short courses offered by the Minnesota Chapter of the American Fisheries Society. These courses were held on the St. Paul Campus of the University of Minnesota. Together the courses provided twenty hours of training in data collection and analysis.
Dr. Daniel Isermann of the Wisconsin Cooperative Fishery Research Unit at University of Wisconsin- Stevens Point taught the first class: Estimating Age and Growth of Fish. Primary discussion topics included: purposes for estimating the age and growth rates fish; sample size; selection, removal, and processing of calcified structures by both lethal and nonlethal means; and interpretation of annuli.
Fish scales, otoliths (ear bones), fin rays and cleithra (a bone at the rear margin of the gill cavity) are commonly used for estimating age and growth rates. Similar to the annular growth rings found in trees, layers of calcium are deposited in the bony structures of fish at different rates throughout the year forming visible annuli (growth rings) that can reveal the approximate age of fish. Methods for removing and processing a bony structure, as well as the reliability of a given structure to reveal the age of fish, can vary among different species with some structures considered more dependable than others.
For example, a nonlethal method includes collecting a scale(s) or fin ray from the exterior of a fish. In contrast, otolith or cleithra removal is lethal and requires knowledge of size, location, and the precise removal technique to avoid damaging the structure. Only in certain circumstances is the lethal method an option.
Stations were set up for instructor demonstrations and for the class to actively participate in locating, removing, processing and aging various bony structures. I really enjoyed the hands on activities, as it gave me the chance to learn proper technique and the opportunity to ask Dr. Isermann specific questions.
Dr. Derek Ogle, Professor of Mathematics and Natural Resources at Northland College in Ashland, Wisconsin taught the second class: Analyzing Age Data with R. Here we learned how to analyze and interpret age and growth data by writing scripts and functions using the R software programming language.
Dr. Ogle focused on back-calculating past fish length, deriving and applying an age-length key, computing measures of precision, and producing growth models using R software. The class was able to follow along with Dr. Ogle as he explained each topic and complete provided exercises using our personal laptops.
As a Northland College alumnus and a former student of Dr. Ogle, it felt as if I were back in college once again! It was great to see him again and have the opportunity to build on my past education as well as apply that knowledge to my current profession.