Comparison of scales and sagittal otoliths to back-calculate lengths-at-age of crappies collected from midwestern waters
Back-calculation is commonly used in age and growth studies to increase the sample size when few fish are captured from a population; however, which structure to use for back-calculation has been often disputed. We compared back-calculated lengths of crappie (Pomoxis spp.) scales and sagittal otoliths from seven Midwestern lakes using linear regression. To examine this relationship on a finer scale, a residual sum of squares (RSS) analysis was also run to determine whether the two structures suggested different mean lengths at age for one population (Cedar Lake, Illinois). A total of 150 fish, age-1 to age-10 (mean 2.8 years), was collected from all lakes; a total of 296 back-calculated lengths was obtained for each structure. Thirteen fish were discarded because of lack of age agreement between structures. Overall for all lakes, the otolith versus scale back-calculated lengths regression yielded an intercept of-22.19 mm and a slope of 1.00 (R2=0.77, df=1, 296, P<0.0001). The slope was not different from 1.00 (95% CI of 0.94–1.07), but the intercept was slightly different from zero (95% CI of-34.66 to −9.72). Furthermore, the RSS analysis suggested that the growth curves for Cedar Lake did not differ between structures (F3,2=0.29, p>0.75), indicating that the pattern seen across all lakes held true for one population. Either structure could be used for back-calculation studies. However, perfect agreement of ages between structures (i.e., scales and otoliths) did not occur. Crappie length at age should be back-calculated using otoliths when more precise age estimates are needed and with scales when fish are too few to be killed.
Wahl, Nicholas, Quinton E. Phelps, James E. Garvey, Sean T. Lynott, and Wells E. Adams. "Comparison of scales and sagittal otoliths to back-calculate lengths-at-age of Crappies collected from Midwestern Waters." Journal of Freshwater Ecology 24, no. 3 (2009): 469-475.
Journal of Freshwater Ecology