Recruitment depressions in North Sea herring

Two periods of sustained low recruitment have been observed in North Sea herring in recent history; one from 1971 to 1979 and one that started in 2002 and continued up until the most recent year for which information is available (2010). This paper compares both periods of recruitment depression and considers the possible causes for each of them. The first depression (1971-1979) has been commonly ascribed to insufficient egg production by the depleted parent stock. However, recruitment to the central and northern populations was probably also affected by an environmental factor. There are indications of a reduced Atlantic inflow into the northwestern North Sea at the time of the depression. This could have affected survival of the larvae by slowing down their transport to the nursery areas in the southeastern North Sea. For the second period (2002-2010), the low recruitment has been ascribed to an increase in temperature and/or a regime shift in the ecosystem. An alternative explanation could be predation by the large stock of adult herring on its own larvae.