Midwest Fish & Wildlife Conference 2018

AUTHORS. Daniel J. Dembkowski, Wisconsin Cooperative Fishery Research Unit; Daniel A. Isermann, USGS, Wisconsin Cooperative Fishery Research Unit; Ryan P. Koenigs, Wisconsin Department of Natural Resources

ABSTRACT. Within most state or provincial resource management agencies, Walleye Sander vitreus ages are usually assigned by many different individuals. Under the assumption that among-reader precision is high, these age estimates are often combined to estimate age-based population metrics or develop state or regional growth standards. To test this assumption, we distributed images of sectioned otoliths and dorsal spines from a random sample of 50 Walleye to seven readers who routinely estimate Walleye ages for the Wisconsin Department of Natural Resources. Precision among the seven readers was low for both sectioned otoliths (mean CV = 37%) and dorsal spines (mean CV = 35%), and there were no instances where all readers agreed on the age of an individual fish, regardless of structure. To determine if levels of among-reader precision can be improved, we designed and implemented a group age estimation workshop focusing on standardization of structures that readers identify as annuli and interpretation of images from known-age fish. We distributed the same 50 images of sectioned otoliths and dorsal spines, along with images of structures from 25 known-age Walleye, to 22 readers grouped into beginner, intermediate, and advanced experience levels. Across all readers, among-reader precision was substantially higher after the workshop (otolith mean CV = 16%; dorsal spine mean CV = 15%) than before the workshop (otolith mean CV = 27%; dorsal spine mean CV = 26%), and improvements in precision were observed for all experience levels. Furthermore, agreement of age estimates from both structures with known ages (i.e., accuracy) improved after the workshop. Our results suggest that, while among-reader precision at a statewide level may be lower than previously thought, periodic re-trainings may lead to improvements in precision and accuracy among readers of all experience levels.