Hatching period, growth and survival of young walleye pollock Theragra chalcogramma as determined from otolith analysis

Abstract

Daily increments in otoliths were used to estimate growth rates and hatch dates of young walleye pollock Theragra chalcogramma from the western Gulf of Alaska, USA. Growth rate increased through the first 70 d after hatching and was described by a nonlinear, Laird-Gompertz model. There were no interannual differences in growth rates of young larvae from the limited size range collected in 1983 and 1985 to 1987. Growth rates of older larvae and pelagic juveniles increased from southwest to northeast along the Alaskan Peninsula. Hatch dates were calculated from age of fish at time of collection; spawning dates were estimated after accounting for temperature-dependent egg stage duration. From 1983 to 1987, spawning and hatch dates seem to have shifted to later in the season. This can indicate an overall change in time of spawning or increased removal, through mortality and dispersal, of eggs and larvae spawned early in the season. Hatch dates were earlier in the southwest near Unimak Island and later in the season to the northeast around Kodiak Island. Mortality was estimated from the difference in abundance of each daily cohort between the May and June-July surveys in 1987 and was age dependent. Mean daily instantaneous mortality rate was 0.11 for young larvae and decreased to 0.02 for older larvae and pelagic juveniles. From these rates, it is estimated that about 97% of the larvae will die or be removed from the Shelikof Strait area within 35 d of hatching.