Evaluation of Methods for Estimating Age and Growth of Lake Sturgeon

Abstract

Estimating age and growth is a crucial component to the management of fish populations as age data are used to estimate mortality, survival, age at maturity, etc. Pectoral fin rays are currently the preferred structure used to estimate the age of lake sturgeon (Acipenser fulvescens). Previous research has shown that ages estimated via fin rays underestimate true age of lake sturgeon > age 14. Age estimates from otoliths were previously reported as valid for lake sturgeon up to age 52, but difficulties related to otolith collection, removal, processing, and readability have resulted in little to no comprehensive assessment. Additionally, mark-recapture based growth modeling has been used to estimate growth parameters when age estimates from calcified structures are inaccurate, imprecise, or structures are difficult to collect. I evaluated multiple structures and techniques for estimating age and growth of lake sturgeon in the Lake Winnebago System (LWS) including: sectioned pectoral fin rays, otoliths prepared via alternative sectioning methods, correcting pectoral fin rays using a correction factor and agecorrection matrices, and estimating growth using the Fabens mark-recapture growth model. My study objectives were to determine if: 1) among-reader precision and age estimates were similar between ages estimated from pectoral fin rays and otoliths; 2) pectoral fin rays and otoliths provided accurate ages for lake sturgeon on the LWS and 3) raw or corrected pectoral fin ray and otolith ages provided growth estimates that were similar to estimates from Fabens mark-recapture models. All calcified structures were collected from lake sturgeon legally speared during the annual winter harvest season on the LWS. Otoliths from 814 lake sturgeon were sectioned, but only 13% of the sections were considered readable and precision of ages estimated by three readers from readable otolith sections was lower (CV = 32%) than precision of ages estimated using pectoral fin rays (CV = 17%). Analysis of known-age fish indicated accuracy of age estimates between structures was reader dependent. However, conclusions on structure accuracy were limited given the small known-age sample (pectoral fin rays N = 43; otoliths N = 36) consisting of relatively young fish (ages 4-17). Comparisons of von Bertalanffy growth parameters indicated that readable otolith sections produced growth trajectories that generally aligned with predicted growth from Fabens growth models which rely on mark-recapture observations of individual fish. Additionally, when compared to raw fin ray age estimates, correcting fin ray ages using correction matrices provided growth trajectories that were more similar to trajectories estimated from mark-recapture data. My assessment demonstrates that using otoliths to estimate the age of lake sturgeon will likely be problematic given the low number of readable structures and the fact that sacrificing fish is not possible for most populations. Furthermore, fin rays did not provide estimates of growth that were similar to mark-recapture, but age correction could provide a solution for using fin rays to describe lake sturgeon population metrics. My evaluation also suggests that using mark-recapture methods like the Fabens model might offer the easiest, most practical, and likely the most accurate method for estimating the growth of lake sturgeon when compared to using ages estimated from calcified structures. However, the accuracy of mark-recapture based estimates of growth will likely be affected by measurement error and the ability to recapture sufficient numbers of fish across a wide range of lengths. My work also suggests that the Wisconsin Department of Natural Resources should continue to collect calcified structures from known-age fish harvested in the winter spear fishery on the LWS to further assess the accuracy of both pectoral fin rays and otoliths for fish older than age 17 and to refine age correction methods for fin rays.