Validation of Annuli and Identification of Discontinuities in Sagittal Otoliths of Juvenile Alligator Gar

Abstract

AbstractAnnual increment periodicity has been validated in sagittal otoliths of Alligator Gar Atractosteus spatula; however, discontinuities (often referred to as false annuli or checks) are common in early years, potentially affecting both accuracy and precision. We used up to four injections of oxytetracycline (OTC) to induce time stamps associated with annuli (opaque zones) in the otoliths of juvenile Alligator Gar (ages 2–6; n = 68). These OTC marks allowed us to test a reader's ability to distinguish discontinuities (hereafter, “checks”) from annuli by using established criteria. In addition, the OTC marks were used to infer the timing of translucent zone and opaque zone formation. Although the reader's error rate was low (6%), some checks were essentially indistinguishable from annuli. Checks were common, occurring in over half of the otoliths examined and within all annual increments in these juvenile fish. The association of all OTC marks with the opaque zones of the annual increment further validates annulus formation in juvenile Alligator Gar and indicates that these narrow zones formed approximately between November and June. Subsequently, it can be inferred that the much wider translucent zones formed from about June to November. Although opaque zone formation was complete by June, new annuli were not distinguishable from the ventral edge of the otolith until translucent material was deposited beyond the annuli. In our samples, only 22.2% of the otoliths from fish sacrificed on June 30 (n = 9) had fully visible annuli along the edge of the otolith. Based on multiple studies, it is apparent that annual increments form in the sagittal otoliths of Alligator Gar throughout their lives; however, the interpretation of these increments can be very difficult. In our experience, the OTC marking of fish can provide a valuable tool for training and verification programs needed for this species and for other long‐lived fishes that pose difficulties in age estimation.