Mackerel Scomber scombrus

Distribution

Mackerel (Figure 9.23) are found in warmish water on both sides of the North Atlantic. Their range extends from the south coast of Norway and northern North Sea, along the west coasts of the British Isles and into the English Channel, and as far south as the Canaries. They also occur in the Mediterranean and on the western side of the Atlantic from south Labrador to North Carolina.

Life history

The life history and migrations of mackerel have been well documented by Lockwood (1976, 1978, 1989). Two stocks of mackerel are found in north-west European waters: a western stock which spawns mainly along the shelf edge west of Britain, in the Celtic Sea and Bay of Biscay; and a North Sea stock which spawns in the central North Sea and off southern Norway.

During their spawning period, mackerel congregate on the spawning grounds in huge shoals. The western stock spawns between February and July, the most intensive spawning occurring during April in a spawning area some 50-80 miles west of the Scilly Isles (Figure 9.24b), mainly along the line of the continental edge extending southwards into the Bay of Biscay. As the season proceeds, spawning

Figure 9.23 The mackerel, Scomber scombrus. Identification features: 1 = zebra stripes along the back; 2 = Finlets in front of tail; 3 = Dorsal fins widely spaced.

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

Publications Ltd.)

Figure 9.23 The mackerel, Scomber scombrus. Identification features: 1 = zebra stripes along the back; 2 = Finlets in front of tail; 3 = Dorsal fins widely spaced.

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

Publications Ltd.)

fish are found in progressively shallower water further to the east, until by July they are spawning in St. George's Channel, the Bristol Channel and the western part of the English Channel. The North Sea stock spawns from May to July, peaking in June.

The females produce about half a million pelagic eggs. These are not all shed together but are produced in successive batches over a period. The eggs are about 1.2 mm in diameter and are laid directly into the upper part of the water column. At first they float to the surface, but after 2 days they lose buoyancy slightly and sink to mid-depth water. They hatch after about 7 days (depending on temperature) into a larva about 2.5 mm long. The yolk-sac is resorbed in about 9 days. Young fish spawned in the Scillies area are presumably carried eastwards by the drift of the water towards the English, Irish and Welsh coasts. In some years, sizable shoals of young mackerel of about 13-17 cm in length are found along the English south-west coast during the summer. Mackerel post-larvae are also abundant in the northern part of the North Sea. In autumn and winter small mackerel have sometimes been brought up from the bottom in fine-mesh trawls, suggesting a pattern of behaviour similar to that of the adult shoals (see below).

Growth is rapid, reaching approximately 23 cm during the first year. Mackerel mature when two years of age at approximately 29 cm, and grow to about 36 cm by their sixth year, but after this period there is no satisfactory method of determining their age (see Section 9.4.4).

Shoals in surface waters feed mainly on copepods, but small fish such as sand eels and pilchards are also taken. Fishermen associate good catches of mackerel with an appearance of the sea which they call 'yellow water', and it has been shown that these areas are particularly well filled with the copepods Pseudo-calanus and Calanus. On the other hand, leaden-coloured water with a slightly unpleasant smell, known to fishermen as 'stinking water' and associated with the poorest catches, was found to be loaded with phytoplankton. Feeding intensity is greatest in summer after spawning and as the shoals approach the coast they tend to disperse, the fish ranging to and fro along the coastline seeking food which

20* 15* 10* 5* 0* 5* 10* 20* 15* 10* 5* 0* 5* 10' 15* 10* 5* 0* 5*

20* 15* 10* 5* 0* 5* 10* 20* 15* 10* 5* 0* 5* 10' 15* 10* 5* 0* 5*

Figure 9.24 (A) The main mackerel overwintering grounds and the annual migration paths to and from the spawning and feeding grounds.

Figure 9.24 (A) The main mackerel overwintering grounds and the annual migration paths to and from the spawning and feeding grounds.

(B) The mackerel spawning grounds. Spawning occurs to some extent in all shelf waters around the British isles, but the main areas are in the central-northern North Sea and the western Celtic Sea. (C) The main mackerel fisheries. The Norwegian fisheries are concentrated in the northern North Sea while the UK fisheries are in the Minch, west of Scotland, and off Cornwall. (From Lockwood, S.J. (1978)).

at this time may include various inshore crustacea such as mysids, shrimps and prawns as well as fish and copepods.

Migrations

At the end of the spawning season, part of the western stock of mackerel migrates up the west coast of Britain to the northern North Sea and Norwegian Sea to overwintering grounds (Figure 9.24). The return movement of the western stock down the west coast towards the spawning grounds has become progressively later in recent years and now occurs mainly from January to March (Walsh and Martin, 1986). It is postulated that this may be associated with a change in flow of the North Atlantic Drift at the continental shelf edge. There is also some migration through the English Channel from the west into the southern North Sea and here too the return migration seems to be later in the season than previously. Other major overwintering shoals are thought to extend well down the continental slope to the west of the Scillies and there are also other concentrations along the south coast of Cornwall.

Mackerel migrations off the south-west of the British Isles were investigated by Steven in the late 1940s (Steven 1948, 1949). He found that overwintering fish were demersal congregating on the bottom and feeding on any available food such as small crustacea, polychaetes and small fish. Cascade currents (see Section 5.6) may carry food down from the surface in some areas.

Towards the end of winter, fish begin to leave their overwintering concentrations. At first they still keep to the bottom, but soon begin to perform diurnal vertical movements, ascending during darkness and eventually forming surface shoals. During the early months of the year the surface plankton is sparse and the majority of the fish are still without food, but they readily feed when suitable food is found, for example, on shoals of small fish such as pearlsides (Maurolicus muelleri).

During the late winter and spring the surface shoals begin to move towards the spawning areas (Figure 9.24). Those which overwintered in the English Channel and southern North Sea move westwards, those from the Irish Sea move south-westwards, while those which wintered on the continental slope move towards the east, all tending to converge towards the west of the Scilly Isles. From the northern North Sea fish move towards the Norwegian coast, or southwards into the central North Sea.

After spawning the shoals move away from the spawning grounds, some over considerable distances. Many that spawn in the Celtic Sea move eastwards towards the English and French coasts. Some pass up the English Channel into the North Sea and others go northwards west of Ireland or through the Irish Sea to the Hebrides, or even beyond to the Shetlands where there is some mingling of the western and Norwegian stocks. Fish that spawned in the central North Sea return mainly northwards towards the Norwegian coast.

Towards the end of the autumn the fish disappear from coastal regions and return to their overwintering concentrations on the sea-bed. It is thought that the fish tend to return to the same overwintering sites as they left the previous spring.

Fishery and over-exploitation

Mackerel are caught by trawling in the winter, when they are demersal, and through most of the year by various pelagic fishing methods, mainly purse seines and pelagic trawls. Some small boats still use handlines in winter in areas such as along the Cornish coast. The trolled lines carry a number of feathered hooks. The North Sea fishery is exploited mainly by Norwegian purse seiners.

During the mid-1960s the North Sea fishery underwent enormous expansion from normal levels of 100 000 tonnes or less to nearly a million tonnes in 1967. This was partly as a result of the purse seine fleet transferring most of its attention to mackerel after the herring fishery collapsed. The catch was used mainly for fishmeal and oil. During the early 1970s the landings declined rapidly as the stock diminished. The Norwegian government instituted controls to limit mackerel fishing. However, the size of the spawning stock in the North Sea is presently less than 50 000 tonnes and biologists believe the North Sea mackerel will probably become effectively extinct, apart from influxes from Atlantic waters in the north.

The south-western fishery has traditionally been of interest mainly to British, French and Spanish ships, and landings were always less than from the North Sea and remained fairly stable at around 30 000 tonnes per year. The importance of this fishery increased with the decline in herring landings, and in 1968 Soviet ships began to fish the south-western stock. By 1975 annual landings from this fishery had increased to about 500 000 tonnes, with ships from the USSR taking over half this total. In 1977 Soviet ships were excluded from EEC waters. The western spawning stock was still relatively large in 1989, approximately 2 million tonnes, and it is mainly this stock that the international fishery now exploits.

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