Research Document 2016/115

Comparison of Scales, Pectoral Fin Rays, and Otoliths for Estimating Age, Growth, and Mortality of Lake Whitefish, Coregonus clupeaformis, in Great Slave Lake

By Zhu, X., Wastle, R., Leonard, D., Howland, K., Carmichael, T.J., and Tallman, R.F.

Abstract

It has been well documented that otoliths are the preferred hard structure for estimating the age of Coregonids. Additionally, the slower growth due to short growing seasons experienced in populations from the sub-arctic may alter the utility of alternative, non-lethal structures for estimating ages. In this study, scales, pectoral fin rays and otoliths from Great Slave Lake (GSL) Lake Whitefish Coregonus clupeaformis (Mitchill, 1818) were compared for differences in processing time efficiency, annular assignments, precision biases and reader uncertainty.

Among the three ageing structures, pectoral fin rays took longest to process followed by scales and then otoliths. Readers’ confidence was highest for otoliths, followed by pectoral fin rays and then scales. Annuli in ground and baked otoliths appeared as dark, narrow lines with regular spacing; however, annuli in pectoral fin rays and scales were more variable in appearance. Readings of pectoral fin rays had the highest and largest uncertainty while readings of otoliths had the lowest uncertainty. Within-reader precision was highest for age estimates from otoliths, followed by scales and pectoral fin ray sections. Reader confidence, within reader precision and age estimates themselves, were all affected by age structure and age class.

Pairwise comparisons between age estimates from otoliths and scales, and otoliths and pectoral fin rays found no significant differences when fish were younger than 10 and 12 years, respectively, suggesting that these non-lethal structures could be conservatively used to reliably estimate ages of younger (<10 years) and smaller (≤300 mm) Lake Whitefish.

Derived estimates of growth and total mortality for Lake Whitefish in GSL varied significantly among ageing structures. Mean length-at-age based on otoliths ages was significantly lower than that based on scale ages. Of particular significance is that divergence between scale and otolith age estimates is delayed by 5–6 years relative to more southern populations, and in contrast to southern examples, fin rays do not offer a suitable non-lethal alternative for estimating ages of older Lake Whitefish (>11 years).

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