Gadus morhua (Linnaeus 1758), Cod (Gadidae)

Author Ronald Fricke, Germany

1. Description of the habitat/autecology of the species

The cod, Gadus morhua, is a benthopelagic species, distributed in a variety of habitats, from the shoreline down to the continental shelf and beyond to depths of 600 m. The species forms large schools during the day. In the western Baltic Sea, populations spawn from January to April as spring spawners; for the eastern stock in the Bornholm basin, the main spawning season is from June to September as summer spawners (Bleil & Oeberst, 2005), in deeper water. The eggs and larvae are planktonic. Maximum total length 200 cm; maximum total weight 96 kg; maximum individual age 25 years (Fricke, 1987: 71; Froese & Pauly, 2005).

2. Distribution (past and present)

Distributed from Kattegat to Gulf of Finland. Three separate populations: One in Kattegat, the second from Kiel Bay to Arkona Sea, and the third from Bornholm Sea northeastward. Between the first and second populations there is possibly a hybrid zone in the Belt sea area (Nielsen et al., 2003). Outside the HELCOM area, cod is found across the North Atlantic, in the east from North Sea and Barents Sea to Greenland and the east coast of North America

3. Importance (sub-regional, Baltic Sea-wide, global)

In the Baltic Sea, Gadus morhua is considered as of global importance according to the HELCOM (2007) definition, as it is one of the largest populations of the species and as it is genetically and biologically separated from other major cod population units. Baltic cod is the most important species for commercial fishery in the Baltic Sea with total international yearly landings of 70 000 tonnes.

4. Status of threat/decline

Cod is a commercially overexploited species that has at present declined in many parts of the Baltic Sea. Due to the special hydrographic conditions of the Baltic Sea, recruitment is impaired in most years as it is dependent on inflows of oxygenated ocean water, i.e. the recruitment success is strongly dependent on salinity and oxygen conditions in the spawning area. An important feature of the eastern Baltic cod stock is the size distribution; big female cod produce larger eggs which have a higher bouyancy, which in turn result in a better survival rate (Cardinale and Arrhenius 2000). In the Skagerrak and Kattegat, as well as in the North Sea, cod stocks have severely declined in recent years (Svedäng 2003, Svedäng and Bardon 2003, Cardinale and Svedäng 2004). From Canadian and European research, we know that cod stocks do not recover after a massive decline, even under a long-term moratorium preventing fisheries on this species (Anonymous, 2005, Fisheries and Oceans Canada, Svedäng and Svenson 2006).

The species is threatened throughout its range in the HELCOM area and also in other European Atlantic areas and in Atlantic coastal waters of eastern North America. Its populations in the Baltic Sea are considered as probably declining, though highly fluctuating due to environmental conditions, especially salinity. In most other parts of its distribution range outside the HELCOM area, populations have severely declined. In the HELCOM area, cod is classified as endangered (EN) according to IUCN criteria; when applying OSPAR/HELCOM prioritisation criteria, it is a high priority species (HELCOM, 2007). The species is listed as endangered by Germany and Sweden (Gärdenfors, 2005). It is also included as a prioritary species in regions II and III on the OSPAR list (Anonymous, 2004); according to the OSPAR list explanation, protected stocks include the North Sea and Skagerrak cod stock, Kattegat cod stock, Cod west of Scotland, Cod in the Irish Sea, Cod in the Irish Channel and Celtic Sea. In the IUCN red list cod is defined as vulnerable (VU).

5. Threat/decline factors

Threatened by fisheries as a target species and as by-catch in other pelagic and demersal fisheries, eutrophication and habitat loss.  This species may need a minimum spawning school size and thus may collapse if a local population gets too small. The populations in the Baltic proper are also affected by salinity fluctuations (the recruitment gets impaired if the salinity is too low); they depend on high salinity water inflow from the North Sea. Cod is not considered rare at the moment, at least not in the southern Baltic Sea, but it is sensitive prolonged over-exploitation and eutrophication. In the Kattegat the decline may have resulted in reduced productivity as the stock structure is hampered. It is a keystone species.

The decline of cod in the Baltic Sea is conservatively classified as probable, though stocks were definitively much higher 25 years ago. However, in Swedish waters cod is declining in all areas, and the spawning stock biomass has declined by at least 60 % between 1990 and 2004 (ICES, 2006).

6. Options for improvement

Baltic cod TAC (Total Allowable Catch) is fixed and quotas have been allocated by IBSFC recommendations to the Baltic Sea countries. After abolition of IBSFC (2005), cod fisheries is managed by EU regulations within agreement of Russian Federation. Regulative measures include TACs, and quotas in yearly basis, effort limitation rules and other measures including summer ban and fisheries technical rules among others minimum mesh sizes.

The Baltic cod is managed in two management areas: the eastern stock in ICES subdivisions 25-32 and the western stock (ICES 22-24). The eastern stock has in recent years been outside of the precautionary parameters and the fishing effort has probably been too high. Recent scientific advices from ICES have indicated that the eastern cod stock has declined to levels where it is suffering from reduced reproductive capacity and that the stock is being harvested unsustainably. The western cod stock of the Baltic Sea is over-exploited and has reached levels where it is at risk of reduced reproductive capacity. There is also some evidence of illegal and unreported fisheries of cod in the Baltic Sea.

The EU Council has 12 June 2007 accepted a multi-annual plan of recovery measures for cod stocks. The objective of the plan is to ensure that the Baltic cod stocks can be exploited under sustainable economic, environmental and social conditions. The plan means gradual reduction of fishing effort in fisheries catching cod to levels that are consistent with the objective, and by fixing the total allowable catches (TACs) for the cod stocks at levels consistent with the fishing effort. The new plan will also mean a continuation of other technical measures including tighter rules for control and surveillance for fishing of Baltic cod.

7. Additional notes

In the Baltic Sea, cod is considered to be a keystone species, because it is an important predator on fish but also other species like sea birds (e.g. cormorants) depend on it as the main food item. It is also of global importance (see above).

8. References

Anonymous 2004. 2004 Initial OSPAR List of Threatened and/or Declining Species and Habitats (References number 2004-06). OSPAR Convention on the Protection of the Marine Environment of the North-east Atlantic, Meeting of the OSPAR Commission (OSPAR), Reykjavik, 28 June-1 July 2004.

Anonymous 2005.  Underwater world. Groundfish. Atlantic Cod.  Fisheries and Oceans Canada, Ottawa, Ontario. Homepage, 16 June 2005.Available in: http://www.dfo-mpo.gc.ca/zone/underwater_sous-marin/atlantic/acod_e.htm

Bleil M. & Oeberst R. 2005.  Die Reproduktion von Dorschen (Gadus morhua L. und Gadus morhua callarias L.) in der Ostsee unter besonderer Berücksichtigung der Arkonasee. Teil I. Allgemeiner Verlauf des jährlichen Reifeprozesses und der laichaktivitäten in den verschiedenen Gebieten. Informationen zur Fischereiforschung, 52: 74-82.

Cardinale, M., Arrhenius, F., 2000. The influence of stock structure and environmental conditions on the recruitment process of Baltic cod estimated using generalized additive model. Can. J. Fish. Aquat. Sci. 57, 2402–2409.

Cardinale M, Svedäng, H. 2004. Recruitment and abundance of Atlantic cod, Gadus morhua, in the Kattegat-Eastern Skagerrak (North Sea): evidence of severe depletion due to a prolonged period of high fishing pressure. Fish Res 69: 263-282

Fricke R. 1987. Deutsche Meeresfische. Bestimmungsbuch. Hamburg (DJN), 219 pp.

Froese R. & Pauly, D. (eds) 2005. FishBase. Available in: www.fishbase.org, version (11/2005).

Gärdenfors U (ed). 2005. The 2005 red list of Swedish species. Uppsala (ArtDatabanken).

HELCOM 2007. HELCOM Red list of threatened and declining species of lampreys and fish of the Baltic Sea. Baltic Sea Environmental Proceedings, No. 109, 40 pp.  Available in: http://www.helcom.fi/stc/files/Publications/Proceedings/bsep109.pdf

ICES, 2006. Report of the Baltic Fisheries Assessment Working Group. ICES CM 2006/ACFM.

Nielsen E.E., Hansen M.M., Ruzzante D.E., Meldrup D. & Grønkjær P. 2003.  Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis.  Molecular Ecology, 12: 1497-1508.

Svedäng, H. 2003. The inshore demersal fish community on the Swedish Skagerrak coast: regulation by recruitment from offshore sources. ICES J Mar Res 60: 23-31

Svedäng, H.& Bardon, G. 2003. Spatial and temporal aspects of the decline in cod (Gadus morhua L.) abundance in the Kattegat and eastern Skagerrak. ICES J. Mar. Sci. 60, 32–37.

Svedäng, H. & Svenson, A. 2006. Cod (Gadus morhua L) populations as behavioural units: inference from time series on juvenile cod abundance in the Skagerrak. J Fish Biol 69 (supplement C): 1-14