An Evaluation Of Ageing Methodologies For Southern Lake Michigan Alewife
Steven R. LaBay, Aquatic Biology and Fisheries Center, Department of Biology, Ball State University, Muncie, IN 47306; (765) 285-8825; FAX (765) 285-8804; srlabay@bsu.edu
Thomas E. Lauer, Aquatic Biology and Fisheries Center, Department of Biology, Ball State University, Muncie, IN 47306
The alewife (Alosa pseudoharengus) has been recognized as a primary prey source for sport fishes in Lake Michigan. Therefore, it is imperative that growth rate, recruitment, and mortality calculations are derived from the most accurate aging structure possible as these dynamics have important stocking rate and harvest implications for sport fish. Currently, the alewife population exists at the highest density recorded in Southern Lake Michigan since 1984, and is presumably dominated by a strong 1998 year class. However, ageing methodologies of the alewife have not been fully evaluated. The objective of this study was to evaluate the precision of four commonly used bony parts for aging including: scales, opercles, vertebrae, and whole otoliths. Three readers of different skill levels aged 135 alewives from Southern Lake Michigan. One age determination was made on each of the four structures from a single fish. The process was replicated three times and a coefficient of variation was calculated to examine precision. Age bias plots were constructed to examine both reader and structure biases. Otoliths were most precise and significantly different from all other structures, followed by scales, vertebrae, and opercles, respectively. Otoliths also consistently produced lower coefficients of variation both within readers and among structures. Because true age is often difficult to validate, we used length frequency histograms as a corroborative method to assess the accuracy of the ages given by otoliths. Age bias plots confirmed that scales underestimated the true ages of the fish. Numerous authors have also concluded that otoliths were the most precise aging structure in other species of clupeids. Our study has implications for the use of otoliths as an aging structure for other taxonomically similar species in North America.