Modelling the vertical distribution of eggs of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus)

Abstract

Understanding and modelling the vertical distribution of fish eggs in the water column is a major challenge for future use of underway‐continuous egg samplers as estimators of the total egg abundance. This study presents modelling of field data of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) egg vertical distribution obtained from LHPR sampling in the Bay of Biscay. Starting from Sundby's model [Deep‐Sea Res.30 (1983) 645], improvements were achieved through successive modifications concerning egg buoyancy and vertical propagation of wind‐induced turbulence. In addition, measurements of egg settling velocity and buoyancy by stages were included as inputs for the model. The best model fitting was achieved through the adoption of a gradual turbulence vertical decay model (proportional to the inverse of the water density profile), a Gaussian variability of egg density and adaptability of the egg density to the surrounding water by means of permeability of the chorion. This led to improvements over the Sundby original model. The coefficient of determination (R2) of the modelled egg abundance profiles compared with the observed ones was around 80% for both sardine and anchovy. The model described successfully the vertical distribution of eggs for waters of high surface salinity (R2 of almost 90%), but less so for waters of low surface salinity (R2 of about 70%).