It requires specific bottom substrate for spawning, from coarse gravel to small stones, where the eggs are attached. It reaches maturity at the age of three to four years. The number of eggs produced by the female varies from 10 000 to 60 000, depending on the length of the fish. The main spawning grounds are in the Shetland-Orkney area and off the Scottish east coast, where spawning takes place from July to September. The larvae are transported by currents into their nursery areas in the North Sea and Skagerrak/Kattegat where they stay during the first one or two years of their life. Herring is mainly a plankton feeder (calanoid copepods and krill) but will also take small fish like sprat and sand eel. Larvae and immature herring are important food for other species.
Fishery
Herring is caught in a directed fishery by purse-seiners and trawlers and as by-catch in the industrial fishery. The Norwegian fishery is mainly by purse-seiners. The fishery is most intense in the second half of the year, aiming at the mature and spawning stock component.
Over the last 45 years, the annual landings have varied between 11 000 and 1.2 million tonnes. In 2005, the total reported landings were 664 000 tonnes; 633 000 tonnes in the directed fishery and 31 000 tonnes as reported by-catches (Denmark).
Regulations and by-catches
The North Sea herring is managed according to the EU-Norway Management Agreement from 1997, revised in 2004. Quotas are set for the directed fishery and for by-catches in other fisheries with the aim of minimising the by-catch of immature fish. The agreement sets limits for maximum fishery mortalities (0,25 for adults and 0,12 for juveniles) when the spawning stock is above 1,3 mill tonnes (Bpa). The revised agreement includes a TAC (total allowable catch) change limit of +/-15 %.
In the present situation, where four consecutive weak year-classes are recruiting to the population, the 15 % rule is considered as inconsistent with the Precautionary Approach. This rule might be consistent with the precautionary approach in situations where the manager departs from the rule early enough to secure the development in the stock.
The EU has a quota for by-catch of herring in the industrial fisheries while by-catches of herring taken in Norwegian fisheries are allocated against the quota. Norway has 29 % of the annual TAC. Part of the international catch control system has not been fully efficient, and unreported catches in excess of quotas have been revealed for the years 2002-2004. International efforts are made to solve these problems.
Stock status and setting of quotas
The stock of North Sea herring is classified as having full reproduction capacity and as being harvested sustainably. The biomass of the spawning stock in 2005 was estimated to 1.7 mill tonnes and at risk to decrease to 1, 3 mill tonnes in 2006 (Bpa). The year-classes 1998 and 2000 were strong, while the four year-classes 2002-2005 are estimated to be among the lowest recorded.
Having three poor consecutive year-classes is rare, and it is particularly important that the management ensure the safety of the spawning stock in the next few years. It is thus recommended to reduce the fishing mortality on the adult and the juveniles in order to bring the spawning stock biomass back to the Bpa in 2008.