Age determination through shape analysis and validation of otolith annular increments in red snapper, Lutjanus campechanus

Abstract

The periodicity and timing of otolith growth rings in red snapper was examined through a mark-and-recapture study and with known-age, hatchery-reared fish. In 2005 through 2008, red snapper (n = 265) were caught hook-and-line, injected with oxytetracycline, and released 15 to 20 km south of Dauphin Island, Alabama. Fish were recaptured approximately one year after release (n = 6). From recaptured fish, sagittal otoliths were sectioned and the number of opaque growth rings past the OTC mark was compared to time at liberty of the fish. Otoliths from hatchery reared, age-2 red snapper were sectioned and the number of growth rings visible on the otolith compared to known age. An annual periodicity of growth ring formation was shown in both adult and young red snapper. However, if the reading transect was slightly altered from a radius immediately next to the sulcus, false increments were detected and showed two rings formed during one year at liberty. The timing of annulus formation was in the summer to early fall months which correlated with the summer spawning season of red snapper. This was in contrast to previous literature that indicated late winter mark formation. All adult recaptured fish were less than six years of age and questions remain as to the validity of annual ring formation in older red snapper. In younger (age-0, 1 and 2) hatchery-reared red snapper, a new method of age determination was attempted independent of counting otolith growth rings. Otolith shape analysis was applied to whole sagittal otoliths and morphological shape indices were able to distinguish among age-0, age-1, and age-2 otoliths. Significant differences in the aspect ratio, box x/y, and radius ratio showed juvenile red snapper otoliths grew faster along the anterior-posterior axis compared to the dorsal-ventral axis. A discriminant function analysis and cross-validation showed an age classification success of 70% based on shape variables alone. The addition of otolith weight to the discriminant function increased classification success to 93%. This method is especially significant considering that age determination in young red snapper by ring counts is problematic, especially with identification of the first opaque ring.