Cod Gadus morhua

No other single species has been of such importance as cod for human consumption. Many millions are still taken each year although there is now growing concern over the state of many stocks. For example the Canadian Grand

Banks cod fishery in the NW Atlantic finally collapsed by 1992. Cod are mainly caught by trawl, and some are also captured by long line.

Distribution

The cod (Figure 9.11) has an extensive range over the continental shelf and slope to a depth of about 600 m throughout the Arctic and the northern part of the north Atlantic. Although the isotherms do not set firm limits to its distribution, cod is most abundant in seas within the temperature range 0-10°C. It is found around Greenland (mainly on the west coast) and Iceland, in the Barents Sea and around Nova Zemlya. It occurs around the Faroes, along the Norwegian coast, in the North Sea and Baltic, the Irish Sea, and the English Channel. On the eastern side of the Altantic, the Bay of Biscay is as far south as cod extend in any numbers. It is also found along the coasts of Labrador, Newfoundland and south along the North American coast as far as Virginia. In the northern part of the Pacific, a closely related form, G. macrocephalus, occurs over a wide area.

Although tagging experiments (see Section 9.4.2) have revealed extensive migrations by individual cod throughout the North Atlantic and into the North Sea and Barents Sea, there does not appear to be any large-scale movement of populations between different areas apart from the tendency to congregate for spawning. Results from tagging experiments in the North Sea showed a maximum distance travelled from the release point of about 200 miles. Occasionally much longer migrations have been reported. For example, a tagging experiment in the central North Sea resulted in two fish being recaptured off the Faroe Islands and one from Newfoundland (Macer and Easey, 1988).

The cod population consequently comprises a number of fairly distinct stocks; notably those of the Arcto-Norwegian (Cushing, 1966), North Sea, Faroe,

Does Cod Fish Have Scales And Fins

Figure 9.11 The cod, Gadus morhua. Identification features: 1 = Three dorsal and two anal fins; 2 = Light coloured lateral line; 3 = Overhanging upper jaw; 4 = Long chin barbel; 5 = mottled colour.

(Modified from British Sea Fishes, F. Dipper (1987), by kind permission of Robert Irving and Underwater World Publications Ltd.)

Figure 9.11 The cod, Gadus morhua. Identification features: 1 = Three dorsal and two anal fins; 2 = Light coloured lateral line; 3 = Overhanging upper jaw; 4 = Long chin barbel; 5 = mottled colour.

(Modified from British Sea Fishes, F. Dipper (1987), by kind permission of Robert Irving and Underwater World Publications Ltd.)

Iceland, East Greenland, West Greenland, Newfoundland and Labrador regions. There appears to be little interchange of stocks by movement of cod across intervening areas.

Life history

During winter, mature cod move towards particular areas for spawning in late winter and spring. In the northern part of their range there is a general tendency to move southwards in winter prior to spawning, returning northwards for feeding in summer.

The following information relates chiefly to cod in the North Sea (Macer and Easey, 1988). Spawning occurs between January and April, the peak spawning period in the North Sea usually being March to April in water temperatures of 4-6°C. The fish collect in shoals close to the sea-bed, and the female sheds between

3 and 7 million pelagic eggs, diameter about 1.4 mm. The male cod sheds its milt (sperm) into the water and fertilization is external. The fertilized eggs are buoyant and gradually float to the surface.

The major spawning areas around the British Isles are shown in Figure 9.12. There are probably many other subsidiary spawning areas. Further afield, cod are known to spawn around Iceland, Faroes and the Lofotens, also on the west coast of Greenland, on the Newfoundland Banks and along the Atlantic coast of North America.

After spawning the shoals disperse. Time to hatching depends on the water temperature but is typically 2 to 3 weeks. At the time of hatching the larva is about

4 mm long, the mouth has not yet formed and the animal is at first entirely dependent for food on the ventrally attached yolk-sac beneath which it floats upside down. About a week later, the yolk-sac has become completely resorbed, and the mouth has perforated and the young fish begin to feed for themselves in the surface waters. At this early stage the nauplius larvae of copepods are a major part of their food.

The planktonic phase lasts for about ten weeks, by the end of which time the young cod has grown to about 2 cm in length and increased in weight about forty times. Throughout this period copepods remain the chief food. In the North Sea, Calanus, Paracalanus, Pseudocalanus and Temora are important foods for the cod fry, which in their turn are preyed upon by carnivorous zooplankton, particularly ctenophores and chaetognaths.

At the end of the planktonic phase the cod fry disappear from the surface layers and go down to the sea-bed. In the North Sea there are nursery areas to the south-east of the Dogger Bank and around the Fisher Banks. Maps showing the distribution of cod at various stages in their life history in the North Sea are given in Macer and Easey (1988). The fish are not easy to find at this stage because they are quite small and occur mainly in areas where the sea bottom is rocky, making

North Sea And British Isles Fish Stocks

Figure 9.12 Main areas of cod spawning around British Isles as identified from egg surveys. Numbers in sea areas refer to the average international catch in tonnes during the period 1973-77.

(From Lee, A.J. and Ramster, J.W. (eds). (1981). Atlas of the Seas around the British Isles. MAFF, London. © Crown Copyright, 1981.)

Figure 9.12 Main areas of cod spawning around British Isles as identified from egg surveys. Numbers in sea areas refer to the average international catch in tonnes during the period 1973-77.

(From Lee, A.J. and Ramster, J.W. (eds). (1981). Atlas of the Seas around the British Isles. MAFF, London. © Crown Copyright, 1981.)

it difficult to operate nets. Young fish sometimes occur in rock pools on the shore. At this stage they are often an orange-brown with a distinctly chequered pattern in contrast to the more usual adult sandy brown.

The change from planktonic to demersal life involves a change of diet. The young demersal cod feed at first on small benthic crustacea such as amphipods, isopods and small crabs. As the fish increase in size, they take larger and faster-moving prey. Shoals of adult cod actively hunt, chasing pelagic prey through the middle depths. They feed mainly on other fish such as sand eels, whiting and

Figure 9.13 The haddock, Melanogrammus aeglefinus. Identification features: 1 = three dorsal and two anal fins; 2 = Pointed triangular first dorsal fin; 3 = Black thumbprint mark on sides.

(Modified from British Sea Fishes, F. Dipper (1987), by kind permission of Robert Irving and Underwater World Publications Ltd.)

Figure 9.13 The haddock, Melanogrammus aeglefinus. Identification features: 1 = three dorsal and two anal fins; 2 = Pointed triangular first dorsal fin; 3 = Black thumbprint mark on sides.

(Modified from British Sea Fishes, F. Dipper (1987), by kind permission of Robert Irving and Underwater World Publications Ltd.)

haddock, and also squid. A variety of benthic annelids, crustacea and molluscs are also included in the diet, when the fish are feeding on the bottom.

The growth rate of cod varies in different areas. In the North Sea they have reached approximately 8 cm in length at the end of their first six months, 14-18 cm by the end of the first year, and 25-35 cm by the end of the second year. Further north the growth rate is slower. Off the Norwegian coast, cod attain only about 8 cm during their first year, reaching 30-35 cm by the end of the third year. The fish begin to be taken in trawl nets once they exceed about 25 cm in length.

In the North Sea, cod reach maturity when about 50 cm long at 3-4 years of age. Given the chance, they grow to considerable size, sometimes reaching about 1.5 m in length and weighing 30 kg or more.

9.2.2 Haddock (Melanogrammus aeglefinus)

Distribution

The distribution of haddock is similar to that of cod, but with slightly narrower temperature limits, not extending quite so far as cod to the north or south. It is most abundant in the northern part of the North Sea, but is also widely distributed around Britain and Ireland including the Orkneys, Shetlands and Rockall. It also occurs around the Faroes, Iceland, the west coast of Greenland, around Newfoundland and on the Atlantic coast of North America.

Life history

The spawning period is February to June, the peak spawning in the North Sea being between March and April. The female haddock deposits up to three million eggs very like those of cod. The spawning shoals usually congregate in rather

Atlantic Cod North Sea Spawning Ground

Figure 9.14 Main spawning areas of haddock around British Isles as determined by egg surveys. Dark shading indicates spawning areas; all shaded areas indicate fishing ground. Numbers in sea areas refer to the average international catch in tonnes during the period 1973-77.

(From Lee, A.J. and Ramster, J.W. (eds). (1981). Atlas of the Seas around the British Isles. MAFF, London. © Crown Copyright, 1981.)

Figure 9.14 Main spawning areas of haddock around British Isles as determined by egg surveys. Dark shading indicates spawning areas; all shaded areas indicate fishing ground. Numbers in sea areas refer to the average international catch in tonnes during the period 1973-77.

(From Lee, A.J. and Ramster, J.W. (eds). (1981). Atlas of the Seas around the British Isles. MAFF, London. © Crown Copyright, 1981.)

deeper water than cod, mainly 80-120 m depth. This preference for deeper water restricts the haddock spawning areas of the North Sea to a more northerly distribution than those of cod, the main area extending from the east coast of Shetland to off the coast of Norway (Figure 9.14). When preparing to spawn, the North Sea haddock migrate northwards towards the deeper water to the north of the Fisher Banks.

After spawning the fish return to shallower water and form feeding shoals in the central part of the North Sea. This movement of the fish has its effect on the

North Sea haddock fishery, which generally shows two peaks of landings (a) during February to May, when the best catches are made in the spawning areas in the northern part of the North Sea; (b) in September to October, when the main fishery occurs amongst the feeding shoals in the central part of the North Sea.

The buoyant eggs rise to the surface layers, and in the northern North Sea the eggs take some 14-20 days to hatch, a 4 mm larva with attached yolk-sac emerging. The miniature adult form is reached when about 2-2.5 cm in length. Haddock tend to remain in the surface and mid-depth waters rather longer than cod, sometimes until 5 cm or more in length. At night Group 0 fish (less than 1 year old) can be caught in surface and mid-water trawls, as well as on the bottom, indicating a tendency to make diurnal vertical migrations. As they grow larger the fish become demersal and show a greater tendency than cod to congregate in shoals. Young haddock prefer rather deeper water than young cod, which sometimes occur close inshore.

During their pelagic phase, haddock feed mainly on copepods. Once they have become demersal, haddock obtain much of their food by searching about in the deposit for crustacea, molluscs, annelids and echinoderms. Although they feed to some extent on fish, they do so far less than cod, spending more of their time foraging on the bottom, and there is thought to be no strong competition for food between these two closely related species.

Growth rates vary considerably in different areas, the southern part of the North Sea being a rapid growth area where haddock average about 45 cm in length at 4-5 years of age. In the northern part of the North Sea, haddock of the same age average only about 30 cm in length. They can grow to about 90 cm. Probably most haddock reach maturity during their third year.

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