Abstract

Cobia, Rachycentron canadum, are migratory, coastal pelagic fish that are distributed worldwide in tropical and subtropical seas, except for the eastern Pacific, and are abundantin the Gulf of Mexico. Previous research established that cobia from the northem Gulf of Mexico could be aged using sectioned sagittal otoliths, however the collection of sagittae from cobia typically requires removal of the entire head for subsequent otolith extraction and sectioning in the laboratory, generally a difficult and time-consuming process. In this study, first dorsal spines and whole sagittal otoliths were assessed as ageing structures for cobia collected from the northcentral Gulf recreational fishery. The accuracy of age estimates, based on counts of opaque bands on whole sagitta and translucent bands on dorsal spine sections, was determined by comparison with age data from sectioned sagittae from the same fish. Spine sections exhibited concentric translucent bands for most fish, but outer bands were often closely spaced and difficult to interpret. Vascular erosion of the central core of spines from some large fish obscured the inner (early) growth bands which required their "replacement". Vague bands were typical of many whole otoliths which led to numerous discrepancies in band counts among readers. Agreement amongthree readers for the number of bands on sectioned otoliths, whole otoliths and sectioned spines was 100%, 86.1% and 97.7%, respectively. Age agreement between sectionOed otoliths and sectioned spines (61.9%) and sectioned otoliths and whole otoliths (61.3%) was generally not satisfactory for any age category, and some whole otoliths and spines over-and under-estimated ages by as much as two years. Under-and over-estimations of age obtained from whole sagittae and sectioned spines have obvious implications in estimating mortalities for use in fisheries management. Although dorsal spines were far more practical in terms of ease of collection, they do not represent a viable altemative for deriving age estimates for cobia. This study demonstrated that it is essential to section sagittal otoliths from cobia in order to obtain an accurate age.

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