North Sea fish stocks, United Kingdom, Spawning stock biomass (thousand tonnes)
Trends in spawning stock biomass vary from species to species and stocks can fluctuate substantially over relatively short periods. Most stocks are over-exploited and some stocks are at near historically low levels. For example, the spawning stock biomass of North Sea cod fell from 157,000 tonnes in 1963 to 38,000 tonnes in 2001, a decrease of 76 per cent, although it has since increased slightly to 46,000 tonnes in 2004.
The North Sea herring population was seriously affected by over-fishing in the 1970s. The closure of the North Sea fishery between 1978 and 1982 allowed stocks to recover. From the late 1980s there was another decline in stocks of North Sea herring. This recovered again from the mid-1990s and in 2004 the stock was at the highest level recorded for 40 years.
Biomass estimates are also used to evaluate whether the spawning population of each stock is sustainable. In 2003 the percentage of fish stocks around the UK which were categorised as being at full reproductive capacity and harvested sustainably was 30 per cent. This means that for 70 per cent of stocks spawning levels were insufficient to guarantee stock replenishment.
Sources: The Centre for Environment, Fisheries and Aquaculture Science (CEFAS), International Council for the Exploration of the Sea (ICES), Department for Environment, Food and Rural Affairs (Defra)
Notes: The spawning stock biomass estimates are those produced by the ICES based on mathematical models that use time series information on international catches and fishing activity and estimates of relative abundance from research vessel surveys. The spawning stock biomass is the mass of the stock of mature fish with reproductive potential.