Microzooplankton, vertical mixing and advection in a larval fish patch

Abstract

A large (˜30 × 75 km) patch of larval walleye pollock, Theragra chalcogramma, was located south of the Alaska Peninsula during May 1986. A drifter deployed in this patch followed an anticyclonic path consistent with dynamic topography. Changes in community composition and vertical distribution of microzooplankton >40 μm were sampled for 4 days alongside this drifter to examine feeding conditions for larvae. Biological and physical changes during the first 2 calm days revealed substantial small-scale variability within the larger circulation pattern. Changes during the last 2 days were dominated by vertical mixing due to strong winds. Despite mixing, prey concentrations remained adequate for feeding by larval pollock as determined by laboratory studies. A satellite-tracked drifter replaced the first drifter and was still located within the patch 6 days later. Overall distributions of larvae and movements of the drifters show a net translation of 7.8 km day⁻¹ south-westward, but details of the study reveal complex interactions between coastal waters and a coastal current. During the 10-day period there was an increase in standard length of the larval fish population of 0.13 mm day⁻¹ and a decline in abundance of ˜7.6% day⁻¹. Both calculated rates must be underestimates due to continuing recruitment of small larvae from hatching eggs.