Age-specific mortality and transport of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

Abstract

A field study was conducted in May 1988 to estimate mortality rates of individual cohorts of larval walleye pollock Theragra chalcogramma in Shelikof Strait, Gulf of Alaska. Two surveys of the area of larval distribution were done 12 d apart and daily mortality rates estimated from changes in abundance of each cohort. This study was designed to minimize potential sources of bias to mortality estimates, such as larval extrusion or escapement from sampling gear, advective or diffusive gain or loss of larvae into or out of the sampling area, the uncertainty incurred by the use of length as a proxy for age, and the often erroneous assumption of constant production and mortality rates. The age- specific mortality rates from this study were low compared to rates previously estimated for larval wall- eye pollock. Four significant mortality estimates, ranging from 0.041 to 0.0f6 d-', came from among the youngest cohorts. The low larval mortality rates in 1988 coinaded with very calm ocean conditions. Larval drift rates estimated in this study were coherent with the movement of a satellite-tracked drift buoy and averaged 3.1 to 3.9 an s-', which is comparable to rates reported from other studies.