Validation of daily otolith increments in glass-phase American eels Anguilla rostrata (Lesueur) during estuarine residency

Prior to making inferences from otoliths about the residence time and growt h rate of glass-phase anguillid eels Anguilla in estuaries, it is necessary to validate the deposition rate of microincrements in the otoliths. Glass- phase American eels Anguilla rostrata (Lesueur), which had been captured ne ar the mouth of an estuary in Maine, USA, prior to freshwater exposure, dep osited increments at a daily rate at ambient temperature and salinity in a field and laboratory study. The regression for glass eels not possessing a transition ring was: I = 0.976(D - 1) + 0.434, where I is the number of oto lith increments distal to a fluorescent mark placed on the otolith at the b eginning of the experiment, and D is the number of days in the experiment, which ranged from 7 to 49. The slope was not significantly different than 1 . Unexpectedly, many glass eels deposited the transition ring during the ex periment, although this ring had previously been thought to mark entry into fresh water. The regression for these glass eels was: I = 0.961(D - 1) - 3 .880, and the slope was not significantly different than 1. The negative in tercept suggests that approximately il days were lost from the otolith reco rd during deposition of the ring. This study demonstrated daily deposition of increments prior to freshwater exposure and demonstrated that deposition of the transition ring is not linked to freshwater entry. (C) 2001 Elsevie r Science BN. All rights reserved.