Buoyancy and vertical distribution of eggs and larvae of blue whiting ( Micromesistius poutassou): observations and modelling

Abstract

A mathematical model of the vertical distribution of fish eggs and larvae in response to changes in their buoyancy was developed and applied to eggs and larvae of blue whiting ( Micromesistius poutassou). The model balances buoyancy and vertical mixing and allows for changes in buoyancy during development, as well as for variations in the physical properties of the water column with depth. Input data for the buoyancy of eggs and larvae were derived from experiments using a density-gradient column; the neutral buoyancy of blue whiting eggs was observed to increase over the duration of embryonic development from a salinity of below 34 to near 40 while larvae immediately after hatching were neutrally buoyant in a salinity of around 36.6. Input data for the vertical distribution of blue whiting eggs at hatching were derived from the results of plankton sampling to the west of the British Isles. These showed early stage eggs to be distributed in most abundance in the depth range 460–580 m, with relatively little observed change through the remainder of embryonic development, and for larvae to became progressively concentrated in the upper 50 m of the water column. For both eggs and larvae, the model results were in close agreement with field observations. The lack of any marked displacement of the egg distributions was shown to be a reflection of their change from being initially buoyant, to sinking in the water column during the later phases of development, this behaviour being an adaptation to their bathypelagic habit. The model results also indicated that the ascent of the larvae from the spawning depths to the near-surface layers could be due entirely to buoyancy-induced passive rising of the larvae.