Biodiversity and exploitation of the main fish stocks in the Norwegian - Barents Sea ecosystem

Abstract

Juvenile herring and capelin are the main stocks of plankton feeders in the Barents Sea, the cod is the dominant predator. Warm climate favours recruitment of herring and cod, but large stocks of juvenile herring hamper survival of the capelin fry. Since the early 1970s, the herring stock has been grossly overexploited, which could have led to an imbalance in the state of the predatorprey relationships in the Barents Sea. In the 1970s and early 80s, however, cod could feed on capelin which had excellent growth and recruitment conditions when the herring was absent. The consequences of the reduced herring stock were triggered in the mid 1980s, when excellent recruitment conditions for herring and cod occurred. Three abundant year classes of cod were recruited, but the herring stock was not sufficiently large to take full advantage of the favourable recruitment conditions. Given the lack of juvenile herring and a reduced capelin stock, the rapidly growing cod stock grazed down all other available prey species in the area, including its own progeny, and starved cod, seabirds and seals have in later years appeared on the north Norwegian coast. The capelin fishery collapsed, and the traditional coastal cod fisheries have been struck by the most serious crisis on record.

Competition among fishermen and fish causes the collapse of Barents Sea capelin

“…3, 7, 11, and 12), and the capelin harvest was undoubtedly at a moderate level (1.1⅐10 6 tons) when the stock collapsed again in the 1990s. An alternative hypothesis is that the collapses are mainly caused by young herring feeding on capelin larvae (7,13). The herring of the Norwegian spring-spawning stock, living mainly in the Norwegian Sea, spend the first 2-3 years of their life in the Barents Sea.…”

Estimated food consumption of minke whales <i>Balaenoptera acutorostrata</i> in Northeast Atlantic waters in 1992-1995

“…The 1992 diet data from area ES were excluded from the analyses due to the collapse in the capelin stock between 1992 and 1993 (see Hamre 1994, Gj0sreter 1995. All diet composition data were based on reconstructed prey biomass in minke whale forestomachs (see Haug et al 1995a, Haug et al 1996a, and the prey organisms were grouped into the following 8 taxa: krill (Thysanoessa spp.…”

“…However, only 3% of the total consumption of herring took place in area EC, while 96% occurred in area EB, where immature fish were eaten. The southern Barents Sea has served as the main nursery area for immature herring (O-group and recruits up to 3-4 years old) since 19 88 (R0ttingen 1990, Hamre 1994, Gj0sceter 1995 (Haug et al 1997b) : herring was a less prominent component of the di et of Northeast Atlantic minke whales in 1996 than in the years 1992-1995.…”

“…Capelin appeared to be important as prey for the minke whales in the northernmost area in 1992, whereas, following the collapse of the Barents Sea capelin stock between 1992 and 1993 (Hamre 1994, Gj0sceter 1995, capelin was replaced by krill (Haug et al 1996b, Haug et al 1996a. These dramatic changes led us to exclude data for 1992 from the present analyses of food consumption by minke whales in area ES.…”

“…There is some evidence that krill is a less preferred prey than herring and capelin (Haug et al 1996a, so it is to be expected that the impo11ance of cape lin will increase (from its present consumption level of 142,000 tonnes) as the capelin stock recovers. In 1995 , the Barents Sea capelin stock was at an all-time low level, with a total estimate of 195 ,000 tonnes (of which 120,000 tonnes were maturing) and with very poor year classes being produced in , 1994(Anonymous 1996b. The Barents Sea capelin has been protected from fisheries since autumn 1993.…”