Feeding conditions of Arcto-norwegian cod larvae compared with the Rothschild–Osborn theory on small-scale turbulence and plankton contact rates

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Abstract
Data on first-feeding Arcto-norwegian cod larvae in Lofoten, northern Norway, sampled during the period 1976–84, are examined to verify the theory on the influence of small-scale turbulence on the contact rate between predator and prey [Rothschild and Osborn (1988) J. Plankton Res., 10, 465–474]. The number of prey per cod larval gut are compared with the concentration of prey, Calanus finmarchicus nauplii, and with data on wind speed and static stability of the upper layer. The data indicate that the contact rate increases by a factor of 2.8, when the average wind speed increases from 2 m s−1 to 6 m s−1. Independent data on cod larval cruising speed, the velocity and concentration of prey organisms, inserted into the model of Rothschild and Osborn (1988), show a comparable average increase in contact rate of 2.2 times for the same increase in wind speed. The data indicate that the turbulent velocity has a greater contribution to contact rate than the larval swimming speed, when wind speed exceeds 4 m s−1. The larval period of Arcto-norwegian cod is an important period for the formation of the year class. Larval stages are confined to the mixed layer, and therefore the variable contact rate induced by wind mixing must be an important regulatory mechanism for the formation of year class strength.