The first annulus of otoliths: a tool for studying intra-annual growth of walleye pollock (Theragra chalcogramma)

We quantified the growing season of yearling walleye pollock (Theragra chalcogramma) and related it to annual cycles of water temperature and day length. The study was restricted to members of the 2000 year class and thereby controlled for inter-annual variability. Juveniles were sampled from the year class during 10 cruises in the western Gulf of Alaska (GOA). Fifty percent of juveniles exhibited an annulus on 16 March 2001 (+/- 11 days 95% confidence interval). No regional difference was detected in the timing of annulus formation or in post-annulus growth trajectories. A model, derived from growth trajectories, estimated that the growing season lasted 204 days (22 March to 13 October 2001) and that growth rate peaked at 0.59 mm day(-1) on 2 July 2001. Growth rate increased with day length and water temperature during spring and decreased in late summer possibly due to thermal stress. Secondarily, we explored the utility of otolith size at the first annulus as a natural tag to identify nursery area, but this potential was curtailed by overlap in length among regions. Our results indicate that the first annulus can be used to advance our understanding of climate forcing on marine fish growth by providing fine temporal resolution of the growing season.