Mortality of marine cultured rainbow trout, Oncorhynchus my kiss (Walbaum), during the summer in the Netherlands

. In northern Europe, rearing of rainbow trout, Oncorhynchus mykiss (Walbaum), in sea cages is a substantial part of fish culture. One of the major constraints for the development of this system in other parts of Europe is the unexplained mortality of fish during the summer. To investigate environmental aspects of this mortality, rainbow trout, cultured in cages in sea water, were monitored from May to October for three successive years. No evidence of bacterial involvement could be found, while water temperature and salinity strongly influence the mortality. This supports the suggestion that the mortality has been caused by patho-physiological changes due to higher metabolic requirements for osmoregulation at elevated water temperature in the saline environment which might result in a dissipation of O2 by CO2 on the hemoglobin.