Biomass transport to and from an upper estuarine area by migration of juvenile Atlantic herring Clupea harengus

A model describing fish abundance resulting from fish migration and mortali ty in an upper estuary is presented. This model can be used to estimate bio mass transports by fish migrations between estuary and sea and to assess pr oduction assimilated in the estuary. It was applied to herring Clupea haren gus L., a marine species with 0+ juveniles that migrate during winter to te mperate European estuaries. It was shown that different mortality regimes g reatly affected the number of fish that eventually emigrated and, hence, th e biomass that was exported during the seaward migration. The difference be tween imported and exported biomass was assessed under different growth con ditions and varied from positive to negative as mortality rates increased. The discrepancy between export of biomass and in situ produced biomass show ed that fish production was not necessarily transported to the sea when emi grating. It was tentatively concluded that export of biomass out of the est uary only occurs if populations move seaward before winter.