REDUCED OXYGEN CONCENTRATIONS IN HERRING WINTERING AREAS

Since 1987 the major part of the adult Norwegian spring-spawning herring slock has been wintering in the Ofolfjord and the Tysfjord, two tributaries to the Vestljord in northern Norway. The herring enter the fjords in August-September, and leave again in January to start their spawning migration. Since 1975 the Institute of Marine Research has made regular surveys in the area in November of each year, in which the herring population has been measured acoustically. Since 1976 environmental parameters have also been recorded, including temperature, salinity, and oxygen and nitrate concentrations. This time series shows a gradual decrease in oxygen concentration in the Ofotfjord since 1987, when the herring started wintering in the ijord. High concentrations of herring are found in areas with oxygen concentrations as low as 1-2 ml. The paper presents and discusses the available data. A simple oxygen budget demonstrates that the reduction in oxygen content may have been caused by the wintering herring. © 1994 International Council for the Exploration of the Sea.