Factors Influencing Timing of Annulus Formation in Walleye and Smallmouth Bass Otoliths

Walleye and smallmouth bass were sampled monthly (May-October) from Lake Sharpe, South Dakota during 2006 and 2007 to estimate the timing of otolith annulus formation and to evaluate the influence of intrinsic and extrinsic factors on the timing and detection of annulus formation.  Timing of annulus formation was evaluated using marginal increment analysis.  Walleye samples were stratified by age, sex, and location (i.e., upper and lower Lake Sharpe) and smallmouth bass samples were stratified by age and sex.  Monthly mean marginal increment measurements for both species generally increased from May to June, declined in July, and slowly increased from August to October.  Although not significant (p > 0.05), July consistently had the lowest mean marginal increment across species and strata, suggesting that annulus formation in walleye and smallmouth bass in Lake Sharpe likely occurs in July.  The lack of differences in timing of annulus formation across species-specific strata was surprising given the well-known influences of age, sex, and water temperature on somatic growth.  Nonetheless, results will aid managers in improving the accuracy of age estimates and in the design and implementation of better-informed management strategies.