The role of passive transport and the influence of vertical migration on the pre-settlement distribution of a temperate, demersal fish: numerical model predictions compared with field sampling

We compared spatial variation in the abundance of King George whiting Silla ginodes punctata post-larvae in the southern part of Port Phillip Bay, Aust ralia, with predictions of hydrodynamic and dispersal numerical models that included passive transport, and vertical migratory behaviour previously ob served in the field. Post-larvae were sampled at 20 sites on cruises in the spring of 1994 and 1995. Modelling included a 'passive' case with particle s mixed through the water column, 'active' behaviour mirroring diel and tid al variation in the field, and 'day/night' behaviour where behaviour in the field was averaged across tides. Correlations between model simulations an d post-larval distribution were highest in 1995 and were similar amongst th e 3 simulations (58 % of variation explained for both the passive and day/n ight cases). In 1994, variation in the spatial distribution explained was h ighest for the passive case, intermediate for the active case and lowest fo r the day/night case. Prevailing winds were quite different between years, with westerlies prevailing in 1994 but significant periods of easterly wind s in 1995. These differences were reflected by particle distributions from simulations including behaviour, but were not reflected in post-larval dist ributions. A negative correlation was found between post-larval abundances and distance from shore. When model predictions and distance from shore wer e combined in a multiple regression, approximately 70 % of the spatial vari ation in post-larval abundance was explained in 1995. The results imply tha t although the passive transport model was an excellent predictor of post-l arval abundance in both years, observed vertical migration was not influenc ing transport, and post-larvae were closer to shore than expected, possibly due to behaviours other than vertical migration. The close association of S, punctata post-larvae with the coastline provided a mechanism for transpo rt further into the bay, against the prevailing wind field.