Differential growth of larval sprat Sprattus sprattus across a tidal front in the eastern North Sea

Spatial variations in abundance and growth of larval sprat Sprattus sprattus L. were examined across a tidal front in the eastern North Sea, off the west coast of Denmark. The aim of the study was to evaluate the potential advantage for sprat larvae of residing in the vicinity of a tidal front. The hydrographic front in the area followed the 20 to 30 m depth contour and separated a mixed water regime to the east and a stratified regime to the west.

Primary production and phytoplankton abundance were highest in the mixed water (>2000 Mg C m-2 d-1 and 150 mg chlorophyll m-2 respectively) and declined offshore towards stratified water. Zooplankton biomass peaked in the central parts of the investigated area in the vicinity of the front (up to 43 mg dry weight m-3).

Peak densities of sprat larvae (> 2 m-2) were found in the front, in a water column stability range from 5 to 30 J m-3. The length distributions of larval sprat were dominated by modes of 12 and 16 mm, and growth rates of these 2 size classes were estimated from a Laird-Gompertz curve of larval size at age.

Size at age of larvae was estimated by analysis of sagittal otoliths. Growth rate estimates were highest in the mixed water (absolute growth of 12 mm larva: 0.47 mm d-1; weight-specific growth: 0.16 d-1). At the mixed side of the front, where larval abundance peaked, growth was slightly lower, whereas growth rate declined significantly further offshore (to a minimum for 16 mm larva of 0.13 mm d-1, weight-specific growth: 0.03 d-1).

Growth rate of larvae was positively related to chlorophyll content of water (when <110 mg m-2) and to the available prey biomass. In conclusion, both abundance and growth of larvae were connected to characteristics of the tidal front.