A Standardized Technique to Back-Calculate Length at Age from Unsectioned Walleye Spines

Abstract

The primary assumption of using back-calculated length-at-age (BCL) methods to estimate fish growth is that the body length and hard-part radius relationship is linear. Therefore, standardized techniques are required for BCL methods to be comparable over time. Recent studies have found that unsectioned dorsal spines of Sander spp. improve efficiency and provide accurate age estimates. We evaluated a simple, standardized method of using unsectioned Walleye S. vitreus (N = 162) dorsal spines to estimate age and BCL (direct-proportion and Fraser–Lee methods) by comparing these estimates with those of a sectioned spine technique. We validated the technique by comparing observed growth (measured via a mark–recapture study) with BCL growth estimated via sectioned and unsectioned techniques. The time required to process, prepare, view and perform BCL for each of the two techniques was also compared. Both reader age estimates and exact reader agreement rates were significantly better using the sectioned spine technique. Age frequencies varied significantly among readers and techniques as differences ranged up to 8 years. No difference was detected among the slopes of the back-calculation methods among techniques, but there were significant differences in the mean BCL estimates for ages < 10 years old. The relationships between observed growth and growth estimated via BCL methods were significant; however, BCL methods significantly overestimated Walleye growth. Total processing time for the unsectioned technique was significantly less, but based on our poor ability to replicate ages, we do not suggest the use of our standardized unsectioned spine technique to estimate the age or BCL older Walleye. In addition, no BCL method accurately estimated annual growth regardless of technique or reader experience. We conclude that more research is necessary to validate dorsal spine-based BCL estimates on Walleye populations. Once validated, further research evaluating new or modifying our technique for unsectioned Walleye spines needs to be explored.

Received February 14, 2013; accepted June 2, 2013

ACKNOWLEDGMENTS

The Sport Fish Restoration Act provided funding for the project. We would like to thank the crew at the Spirit Lake Fish Hatchery and management and research Iowa Department of Natural Resources staff that assisted with fish and data collection. We would also like to thank all of the reviewers who contributed comments to this manuscript.