North Sea - Key Species
Cod
Cod Gadus Morhua are top predators although during their first six months, cod are pelagic and feed mainly on copepods. At a size of approximately 7 cm they adopt a demersal way of life. Food is initially comprised of crustaceans, but as they grow, bigger fish become more and more important as prey. The average trophic levels of juvenile and adult cod are 4.43 and 4.83, respectively. Most cod become mature at a length of approximately 40 cm and age of 3 years.
In the North Sea, cod are targeted mainly by otter trawl and gillnet vessels. The fishery is year-round, although some fleets exhibit seasonal fishing patterns. Trawlers usually catch cod as part of a mixed fishery with haddock and whiting. Gillnet vessels are better able to target their effort towards cod alone. Cod also form a by-catch in the beam trawl fisheries targeting plaice and sole and in the otter trawl fisheries targeting Nephrops.
ICES classifies the stock as being harvested unsustainably and suffering reduced reproductive capacity. Spawner stock biomass (SSB) is well below the Blim of 70 000 t. Fishing mortality has shown a decline since 2000 and is currently estimated to be around Flim. The 2001-2004 year classes are all estimated to have been well below average; the 2005 year class is estimated from surveys to be more abundant, but still below average.
Whiting
North Sea whiting Merlangius merlangus are similar to cod in that they are predominantly piscivorous, but select mainly smaller prey such as sandeels, sprats and Norway pout (due to the smaller size of whiting compared to cod); benthos is also an important part of the diet especially for younger/smaller fish. Average trophic levels of juvenile and adult whiting are 4.27 and 4.41, respectively. Whiting reach maturity at approximately 2 years of age (20+ cm in length).
Whiting are caught throughout the North Sea especially in the northwestern North Sea, off the northeast coast of England and off the coast of Denmark, Germany and the Netherlands. The majority of landings are from the mixed demersal fishery in the northern North Sea mostly by Scottish light trawlers and seiners and off the northeast coast of England. By-catches are taken by trawlers targeting Nephrops, beam-trawlers targeting flatfish and by vessels fishing for industrial species. In the eastern English Channel, whiting are caught mainly by bottom trawlers as a by-catch in a mixed demersal fishery, but a seasonal fishery targets whiting using midwater trawls. A high proportion of the whiting catch is discarded at sea. The stock is estimated at or near the lowest observed level. Landings and fishing mortality remain at a low level.
Haddock
North Sea haddock Melanogrammus aeglefinus are caught in the mixed demersal fishery, mainly by Scottish light trawlers, seiners and pair trawlers. Smaller quantities are taken by Nephrops trawlers. By-catches of haddock are also taken in the small mesh industrial fisheries. Haddock are a by-catch in the mixed demersal fishery and the small mesh industrial fishery, in the Skagerrak. They are caught throughout the year, especially in the north-western North Sea but they are largely absent from the southern North Sea (ICES Division IVc). A high proportion of the haddock catch is discarded at sea.
Haddock mature at approximately 2 and years and older (20 cm+). Juvenile haddock is dominantly benthivorous, but also feeds on small fish such as sandeels and dab; adult haddock feeds on various fish and benthic species. Average trophic levels of juvenile and adult haddock are 4.06 and 4.28, respectively.
Based on the most recent estimate of spawner stock biomass and fishing mortality, ICES classifies the stock as having full reproductive capacity and being harvested sustainably.
Saithe
Saithe are mainly targeted by French, German and Norwegian otter-trawlers but some UK vessels also participate in the fishery. Catches occur year-round in the northern North Sea, to the west and north of Shetland and on the southern and western limits of the Norwegian Trench (Rhinne) usually in depths greater than 150 m.
Juvenile saithe Pollachius virens reach adult maturity at age 4 years and older (approx. 40 cm). Juveniles dominantly feed on Norway pout, sprat and euphasiids, and zooplankton, and adult saithe is mainly piscivorous (Table). Average trophic levels are 4.03 for juvenile - and 4.36 for adult saithe.
Based on the most recent estimates of spawner stock biomass and fishing mortality, ICES classifies the stock as having full reproductive capacity and being harvested sustainably. Fishing mortality has declined since 1986, and since 1997 has been below Fpa (Precautionary Approach reference point for fishing mortality).
Norway pout
Distributed in the northern North Sea, the Norway pout Trisopterus esmarkii has a bentho-pelagic to pelagic mode of life, living in shoals in the open sea, mostly in waters between 100 and 200 m depth. Norway pout is a target species for the industrial fishery. Because of large by-catches of juvenile haddock, an area in the northern North Sea has been closed to the fishery (Norway pout box). Norway pout are zooplanktivorous with an average trophic level 3.59 (tables). Norway pout is one of the dominant prey fish species in the North Sea.
Gurnards
Grey Gurnard Eutrigla gurnardus is an abundant demersal species. In winter they form dense aggregations locally to the northwest of the Dogger Bank, whereas in summer they are more widespread.
Grey gurnard are important for the fisheries as a by-catch species in demersal fisheries for which only a small market exists. Catches are largely discarded.
Gurnards are key predators in the North Sea and have become more import in recent years (Floeter et al. 2005). Juveniles feed on a variety of crustaceans. The diet of older specimens consists mainly of juvenile fish. Their average trophic level is 4.52.
Herring
North Sea herring stocks have shown enormous fluctuation in the past. There was a rapid stock decline in the late 1970s due to overfishing and recruitment failure. This was followed by a four-year closure of the fishery and then another decline in the mid 1990s, mainly due to high by-catch of juveniles in the industrial fishery. This led to the implementation of a recovery plan in 1997, which was successful. Two fisheries exploit the autumn spawning herring: the directed herring fisheries with purse seiners and trawlers in the North Sea and ICES Division IIIa (Skagerrak and Kattegat) and the one where herring is taken as a by-catch in the industrial small-mesh fisheries that operate in the same areas.
Herring is zooplanktivorous with trophic level 3.44 for juveniles and 3.45 for adults. Some herring are mature at age two but most are three or four years old before they spawn for the first time.
Based on the most recent estimates of spawner stock biomass (SSB) and fishing mortality, ICES classifies the stock as having full reproductive capacity but at risk of being harvested unsustainably. SSB in 2005 was estimated at 1.7 million t, and is expected to decrease to Bpa (Precautionary Approach reference point for spawning Biomass) (1.3 million t) in 2006. Both the 1998 and the 2000 year classes were strong. However, all year classes since 2001 are estimated to be among the weakest since the late 1970s. Due to the current circumstances of four poor recruiting year classes of North Sea herring, it is particularly important that the decline of future spawning stock biomass be addressed with sufficient caution to ensure the safety of the spawning stock in the next few years.
Mackerel
There are two components to the mackerel stocks in the North Sea, a resident population called North Sea mackerel and a migratory population called Western mackerel (ICES 1997). Historically the resident population has been very large (c.a. 2.5 million tonnes, Hamre (1978)), but since the 1970s it has decreased (36 - 110 thousand tonnes SSB, ICES (2002a)). It is not possible to distinguish between the two stock components in the catches, however due to the differing time of residency, ICES working groups treat the two stock components differently. Mackerel are mainly exploited in a directed fishery for human consumption, which tends to target bigger fish, potentially causing smaller, less marketable fish to be discarded (high-grading). Based on the most recent estimates of fishing mortality,
ICES classifies the stock as being harvested unsustainably.
Mackerel feeds on both pelagic and benthic organisms: fish (sprats, sandeel), euphasiids, copepods, as well as polychaetes, mysides and other benthos. Trophic level is, on average, 3.9.
Sandeels
Sandeels (genus Ammodytes) are caught in large quantities by specialized industrial fisheries. Feeding of sandeels (Ammodytes) reflects their bentho-pelagic mode of life: diet is dominated by zooplanktoon, but polychaetes, meiofauna and other small benthos have also important share. They have an average trophic level of 3.35. Sandeels are the dominant prey fish in the North Sea.
Plaice
The main fishery for plaice in Europe is in the North Sea. English vessels have traditionally caught plaice in a directed beam-trawl fishery using 120 mm mesh north of 56°N, and in a mixed fishery with sole, using 80 mm beam trawls in the southern North Sea. Plaice are also a by-catch in inshore and offshore otter-trawl fisheries. Historically the main English ports were Lowestoft and Grimsby, but with the decline of the English beam trawl fleet, plaice is mainly landed into Grimsby and Hartlepool and abroad. In 2005, international landings amounted to 55 700 t compared with a peak of 170 000 t in 1989. About 40% of the total international landings were reported by Dutch vessels, the UK accounted for 23%, Danish landings for 20%, while Belgium, Germany, France and other countries landed the remaining 17%. Based on the most recent estimate of spawner stock biomass and fishing mortality, ICES classifies the stock as being at risk of reduced reproductive capacity and as being harvested sustainably.
Plaice is predominantly benthivorous, although larger specimens also feed on small demersal fish, with an average trophic level of 3.99.
Dab
Dab Limanda limanda is one of the most frequent and abundant species in the North Sea at present. It has a growing importance as a by-catch and obviously an important role in the food web as both consumer and prey, mainly because of its great abundance. Dab is a dominantly benthivorous fish species, although larger specimens occasionally feed on small demersal fish, with and average trophic level of 4.01
Sole
Sole is mainly caught by Dutch beam-trawlers in a mixed fishery with plaice in the southern North Sea using 80 mm mesh south of 56° N. There is also a directed gillnet fishery around the Danish coast, mainly in the second quarter of the year. In the English fishery, the high value of sole makes it one of the most important species targeted by inshore vessels using trawls and fixed nets. The main fishery is from March to October. Sole is also taken as a target and by-catch species by offshore beam- and otter-trawlers and gillnetters. In 2005 the Netherlands reported about 67% of the total landings of sole in ICES Division IV, Belgium accounted for 8% and France, Germany, the UK and Denmark made up the remaining 18%. Landings during the period 1990-1995 were dominated by two strong year-classes, 1987 and 1991, and averaged about 32 000 t. Since then, reported landings decreased to 16 355 t in 2005. Based on the most recent estimate of SSB and fishing mortality, ICES classifies the stock as being at risk of reduced reproductive capacity and as being at risk of being harvested unsustainably. The main preys of sole are polychaetes and meiofauna, but large specimens also eat small fish such as gobies and dragonets. Their average trophic level is 4.