General results from modeling studies suggest that wind mixing of the upper water column can both be beneficial or detrimental to larval survival depending on the intensity of the turbulence [ Davis et al., 1991]. Many processes exist that connect pollock larval survival to mixing [ Bailey and Macklin, 1994]. These authors determined a time series of abundance of larvae hatched on a given day that survived through the early feeding stage, and established a mixing index using the cube of the wind speed. Comparing these series revealed two patterns: strong wind events during the first-feeding period coincided with lower than expected survival, and periods of higher than expected larval survival were associated with calm periods of wind often bracketed by strong mixing. During a spring with moderate winds and a shallow mixed layer, concentrations of food, growth at age and mortality rates were more conducive to larval survival than during a spring when strong winds were accompanied by a deep mixed layer [ Bailey et al., 1995b].
Bailey and Macklin suggest how larval survival may be related to an integration of wind-mixing, stratification within an eddy, and larval behavior. The eddy observed in 1989 had enhanced prey and feeding conditions and a low-salinity core relative to surrounding waters. Pollock larvae in the laboratory avoid turbulence [ Olla and Davis, 1990] by moving deeper in the water column. Reduced light intensity with increasing depth has detrimental effects on the ability of larvae to search for and capture prey [ Heath, 1989]. The vertical stratification of the eddy required more wind-induced turbulence than adjacent waters to mix to comparable depths. Thus, under similar winds, larvae within the eddy could remain higher in the water column in better feeding conditions than larvae outside the eddy. Hence, first-feeding larvae are more likely to survive in the eddy than in the surrounding waters giving similar prey fields.