Effect of small-scale turbulence on feeding rates of larval cod and haddock in stratified water on Georges Bank

Abstract

A set of vertically stratified MOCNESS tows made on the southern flank of Georges Bank in spring 1981 and 1983 was analyzed to examine the relationship between larval cod and haddock feeding success and turbulent dissipation in a stratified water column. Observed feeding ratios (mean no. prey larval gut−1) for three size classes of larvae were compared with estimated ingestion rates using the Rothschild and Osborn (Journal of Plankton Research, 10, 1988, 465–474) predator-prey encounter rate model. Simulation of contact rates requires parameter estimates of larval fish and their prey cruising speeds, density of prey, and turbulent velocity of the water column. Turbulent dissipation was estimated from a formulation by James (Estuarine and Coastal Marine Science, 5, 1977, 339–353) incorporating both a wind a tidal component. Larval ingestion rates were based on swallowing probabilities derived from calm-water laboratory observations.Model-predicted turbulence profiles generally showed that dissipation rates were low to moderate (10−11−10−7 W kg−1). Turbulence was minimal at or below the pycnocline (≈ 25 m) with higher values(1–2 orders of magnitude) near the surface due to wind mixing and at depth due to shear in the tidal current near bottom. In a stratified water column during the day, first-feeding larvae (5–6 mm) were located mostly within or above the pycnocline coincident with their copepod prey (nauplii and copepodites). The 7–8 mm larvae were most abundant within the pycnocline, whereas the 9–10 mm larvae were found within and below the pycnocline. Feeding ratios were relatively low in early morning following darkness when the wind speed was low, but increased by a factor of 2–13 by noon and evening when the wind speed doubled. Comparison of depth-specific feeding ratios with estimated ingestion rates, derived from turbulence-affected contact rates, generally were reasonable after allowing for an average gut evacuation time (4 h), and in many cases the observed and estimated values had similar profiles. However, differences in vertical profiles may be attributed to differential digestion time, pursuit behavior affected by high turbulence, vertical migration of the larger larvae, an optimum light level for feeding, smaller-scale prey patchiness, and the gross estimates of turbulence.Response-surface estimation of averaged feeding ratios as a function of averaged prey density (0–50 m) with a minimum water-column turbulence value predicted that 5–6 mm larvae have a maximum feeding response at the highest prey densities (> 30 prey 1−1) and lower turbulence estimates (<10−10 W kg−1). The 7–8 mm and 9–10 mm larvae also have a maximum feeding response at high prey densities and low turbulence, but it extends to lower prey densities (> 10 prey 1−1) as turbulence increases to intermidiate levels, clearly showing an interaction effect. In general, maximum feeding ratios occur at low to intermediate levels of turbulence where average prey density is greater than 10–20 prey 1−1.