A Comparison of Ageing Methods in Red Snapper Lutjanus campechanus

Abstract

Sectioning otoliths and counting annuli is an accepted method for ageing of red snapper (Lutjanus campechanus). To reduce effort, time, and costs, whole otolith increment counts were compared to sectioned otolith increment counts to investigate the accuracy of whole otolith age estimations. Increments on whole otoliths were counted for all fish (n = 1743), and subsets were randomly selected across all whole increment counts for sectioning and counting. A linear regression of whole on sectioned otolith increment counts had a slope of 0.85 (n = 694, R2 = 0.91, p < 0.01). Mean differences between whole and sectioned otoliths showed that estimated ages of older, larger fish were underestimated by whole otolith counts. Therefore, an adjusted count was derived by removing large and unreadable otoliths, which increased the regression slope to 0.89, but the slope was still significantly different from 1 (n = 534, R2 = 0.92, p < 0.01). Mortalities and von Bertalanffy growth curves were compared between methods to determine if the adjusted count method was accurate relative to the sectioned count method. Similar mortality estimates and growth curves were produced. Furthermore, shape analysis performed on age-0, age-1, and age-2 fish supported the whole count method with an 89% agreement rate between classified ages from the discriminate function analysis and whole increment counts. Marginal increment analysis was performed on the sectioned otoliths in conjunction with an oxytetracycline study to determine increment formation. The marginal increment analysis indicated opaque formation from May through September, which is most likely due to reproductive stress. Most (69%) OTC-marked fish also supported a Summer-Fall formation of opaque increments. This study provides a new method for otolith ageing in red snapper that could increase efficiency, reduce costs, and remove much of the inherent reader based bias typical of traditional otolith counting.