Vertical distribution and feeding of larval blue whiting in turbulent waters above Porcupine Bank

In April 1995 a patch of blue whiting larvae Micromesistius poutassou was found at low illumination levels below 20 m depth near Porcupine Bank, west of Ireland, together with high densities of copepod nauplii and reduced turbulence rates, suggesting that larval blue whiting vertical distribution was determined by prey concentration, illumination and turbulence. Most (83·8%) larvae (2·0–7·5 mm L _s) had food in their guts. Feeding incidence and feeding intensities increased with increasing larval length. Only larvae >5·5 mm reduced numerical in favour of weight-based feeding intensity, indicating a shift in dietary composition. Maxima of the diel rhythms of feeding incidence and intensities occurred at 1800 and 2100 hours and minima at dawn (0600 hours). Proportionately, more nauplii were eaten by day but more copepod eggs and tintinnids at night. The distinct diel pattern in larval blue whiting feeding suggests that any analysis of factors mediating feeding must take into account diel feeding cycles. Larval feeding was significantly affected by wind speed. The larvae ate more and larger items at low than at higher turbulence levels. The data suggest that the maximum level of turbulence was not beneficial for larval blue whiting, but more moderate wind speeds could have had an enhancing effect on larval feeding success.