| Summary: | Since 1987 the major part of the adult Norwegian spring spawning herring stock has been wintering in the Ofotfjord and the Tysfjord, two tributaries to the Vestfjord in northern Norway. The herring enter the fjords in August-September, and leave again in January, to start their spawning migration. The Institute of Marine Research has had regular surveys in the area in November each year since 1975, in which the herring population present 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 fjord. High concentrations of herring are found in areas with oxygen concentrations as low as 1-2 ml1^-1. This paper presents and discusses the available data.
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