Reduced oxygen concentrations in herring wintering areas

Abstract

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.

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 fjord. High concentrations of herring are found in areas with oxygen concentrations as low as 1-2 ml l ^-1 .

The paper presents and discusses the available data. A simple oxygen budget demonstrates that the reduction content may have been caused by the wintering herring.