European hake Merluccius merluccius

Distribution

Hake (Figure 9.15) extend along the eastern side of the Atlantic from Norway and Iceland (where it is only seasonally common) to Morocco in North Africa. Hake are also found in the Mediterranean. It is a fish of deep water, occurring mainly at depths below 200 m, but throughout adult life it performs seasonal migrations into shallower water for spawning during spring and summer, returning to deep water in the autumn. Further south, along African and American coasts, extending down to South Africa and Patagonia, there are other closely related species of hake.

Around the British Isles, the fish are mainly found off the west coast but may enter the northern part of the North Sea during the summer months when a migration occurs from beyond the Orkneys and Shetlands. The commercial fishery mainly operates over shelf regions, but the fish can be trawled from depths well below the continental edge.

Figure 9.15 The hake, Merluccius merluccius. Identification features: 1 = Shallow dip in second dorsal, and anal fins: 2 = Large jaws and teeth; 3 = Straight lateral line.

(Illustrated by Robert Irving.)

Figure 9.15 The hake, Merluccius merluccius. Identification features: 1 = Shallow dip in second dorsal, and anal fins: 2 = Large jaws and teeth; 3 = Straight lateral line.

(Illustrated by Robert Irving.)

Life history

Hake spawn in numerous areas off the west coast of the British Isles between April and October (Figure 9.16). During the early part of the season, spawning mainly occurs close to the continental edge, but as the season proceeds the spawning fish are found in progressively shallower water. The fish also spawn in the Mediterranean, off the Atlantic coast of Morocco and in various areas in the western part of the North Atlantic.

The female probably deposits between a half million to 2 million eggs on or near the bottom. The eggs are small, approximately 1 mm in diameter, and buoyant. They float up to the surface and hatch after about seven days, the newly hatched larva being about 3 mm long. From spawning areas to the west of the British Isles, the eggs and developing larvae are carried mainly eastwards by the surface drift towards shallower coastal water.

Little is known of the biology of the early stages of hake. It seems that the fish remain pelagic for their first two years of life, and then descend to the sea-bed. They grow to about 10 cm by the end of their first year, 20 cm by the end of the second year and after that probably add about 7-8 cm per year, eventually reaching over 1 m in length.

There is a difference between the sexes in the age of onset of maturity. The majority of male hake mature in groups III to VI (see Section 9.4.4) when between 28 and 50 cm long, but females not until they are Group VI to VIII fish at about 65-75 cm.

Throughout life, hake feed almost entirely on pelagic prey. The young fish feed at first on copepods, later on larger planktonic crustacea such as euphausids, and also on small fish and small cephalopods. The diet of older fish consists almost entirely of fish and cephalopods. The hake is cannibalistic, and sometimes small hake comprise as much as 20 per cent of the food of the larger adults. In addition a large range of shoaling fish is taken. Bottom-feeding fish are seldom eaten and hake appear to feed mainly at night, making diurnal feeding migrations from the

Merluccius Merluccius
Figure 9.16 Main area and times of hake spawning around British Isles. Arrows indicate summer drift of currents transporting pelagic eggs and larvae.

bottom to mid-depth water during darkness. Trawling for hake must therefore be performed during daylight while the fish are on the sea-bed.

9.2.4 Plaice (Pleuronectes platessa)

Distribution

Plaice are commercially important flatfish (Figure 9.17) occurring in the north-east Atlantic area. They range from the Barents Sea and Iceland, south to south Spain and the western Mediterranean but are most common throughout the North Sea, English Channel and Irish Sea. Plaice are most commonly found on sandy bottoms on the shallow continental shelf in depths of less than 80 m. They

Underwater World

Figure 9.17 The plaice, Pleuronectes platessa. Identification features: 1 = Bright orange or red spots; 2 = Row of bony knobs on head; 3 = Right-eyed.

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

Figure 9.17 The plaice, Pleuronectes platessa. Identification features: 1 = Bright orange or red spots; 2 = Row of bony knobs on head; 3 = Right-eyed.

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

occasionally extend down to 120 m or more and can be found on mud and gravel as well as sand. Plaice are caught mainly by trawl or Danish seine, and make up about 7 per cent of the landings of British fishing vessels. They are taken in shallow water all around the British coasts but a large part of the commercial catch comes from the southern North Sea. They are fished for all year round although the quality varies and is rather poor around spawning time.

Life history

Although not great long-distance swimmers, adult plaice from the main Irish Sea, English Channel and North Sea populations, make regular migrations between known feeding grounds and spawning areas. Spawning (Simpson, 1959) occurs all round the British Isles but the main well-defined areas are in the southern North Sea and the Irish Sea and here large numbers congregate (Figure 9.18). The largest area is in the Flemish Bight between the Thames estuary and the Dutch and Belgian coasts. A large part of the southern North Sea plaice population migrates southward during the early winter months towards the tongue of slightly warmer and more saline water which flows into the North Sea through the Strait of Dover. Here, spawning occurs between January and March. After spawning, the spent fish return northwards to their main feeding grounds in the central North Sea. Other spawning concentrations in the North Sea occur to the east of the Dogger Bank, off Flamborough Head and further north in the Moray Firth and around the Shetland Isles.

Spawning also occurs in the Irish Sea, the English Channel and the Baltic, mainly during February and March. In more northerly latitudes, plaice spawn a little later in the year, for example, in the Barents Sea in April, and around Iceland

Plaice Nursery Norway

Figure 9.18 Main spawning areas of plaice around the British Isles as determined by egg surveys. The darker the shaded areas, the more eggs per square metre (darkest = >27 m2; lightest = 1 m2).

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

Figure 9.18 Main spawning areas of plaice around the British Isles as determined by egg surveys. The darker the shaded areas, the more eggs per square metre (darkest = >27 m2; lightest = 1 m2).

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

between March and June. There is no special uniformity of depth, temperature or salinity in the spawning areas, but they are never far from the coast, and always in areas where water movements will carry eggs and larvae towards sandy coasts for nursery grounds.

Sexual maturity depends on size rather than age. In the North Sea most females mature and first lay eggs when between 30 and 40 cm in length and 4-5 years of age, and males mature at 20-30 cm when most are four years old. A female plaice spawns between 10 000 and 600 000 eggs in a season. The eggs average about 1.9 mm in diameter. They are spawned close to the bottom but are buoyant, and are mostly fertilized in mid-water as they float up to the surface, especially at night. The eggs increase slightly in density as the embryos develop, so that by the time the larvae hatch they are drifting in near-surface waters. They hatch after about 2 to 3 weeks into a symmetrical larva, pigmented canary yellow,

6.0-7.5 mm in length with a ventral yolk-sac which is resorbed within about eight days. When active feeding commences, the larvae take chiefly flagellates and small diatoms. As the larvae grow, larger diatoms, molluscan larvae, early stages of copepods and the larvaceans Fritillaria and Oikopleura are taken, the larvaceans being a particularly important component of the diet of plaice larvae, often forming virtually the entire food.

Normal symmetry and a planktonic mode of life continue until about 4 to 6 weeks after hatching, sometimes longer. At this stage, when the plaice larva is about 10 mm long and 2 mm in height, its metamorphosis begins, and within the next 17 days the larva becomes gradually converted into a 'flatfish'. It becomes laterally flattened and acquires a new swimming position with its left side downwards. The skull is progressively transformed by the movement of the left eye to a new position dorsal and slightly anterior to the right eye on what now becomes the uppermost side of the body. The swimbladder which is present in the planktonic larva is gradually lost. Colour disappears from the new underside, and the upper parts develop the characteristic pigmentation and spots of the plaice. During this period of metamorphosis, the fish becomes demersal and grows to about 14 mm in length and 7 mm in 'height', i.e. the maximum distance between the bases of the dorsal and ventral fins.

The chief nursery areas for young plaice are along sandy stretches of coastline. Eggs and larvae spawned in the Flemish Bight are carried by the drift of the surface waters in a north-easterly direction towards the Dutch, German and Danish coasts, usually travelling some 1.5-3 miles per day. By the time their metamorphosis is complete, the young fish have drifted close to the coast where they settle upon the sea-bed. The extensive sandy expanses along these shores, particularly the west coast of Denmark and the Waddensee, form a major nursery area for young plaice. Larvae spawned in various other places near the British Isles also drift towards regions of shallow sandy bottom, and the Wash and the greater part of the Lancashire and Cheshire coasts are other plaice nursery areas. Considerable fluctuations occur from year to year in the numbers of young plaice which successfully complete their metamorphosis. Of several factors which may influence survival at this stage, an important one may be the strength and direction of the wind, upon which depend the speed and direction of drift of the surface water. Poor survival may occur in years when a large proportion of the larvae fail to reach, or are carried beyond, the sandy regions required by the young fish.

Once the fish have become demersal, they feed at first on a variety of small benthic organisms including annelids, harpacticoid copepods, amphipods, small decapods, small molluscs, etc. While very small, much of their food is obtained by biting the siphons off molluscs such as Tellina or the palps off spionid worms, which then regenerate. As they grow larger they begin to take whole polychaetes and small crustacea, and molluscs form an increasingly important part of the food. The diet varies from place to place according to the nature of the food available but molluscs often account for about 25 per cent of the total. Venus, Cardium, Spisula, Solen, Mactra and Tellina are important plaice foods, and they also take the tectibranch Philine and numerous annelids, crustacea, echinoderms, actinians and sand-eels.

Tagging experiments (see Section 9.4.2) have shown that during their first year of life, the majority of plaice are to be found close inshore, mainly in sandy areas in water of less than 5 m depth. With increasing size they migrate into deeper water. This movement usually occurs during the latter part of each summer. At the end of their first summer, the fish move into the 5-10 m zone and are thought to spend much of their first winter buried in the deposit. They are now 6-8 cm in length. In the following spring, they return to shallower water and, during the summer, range to and fro along the coast in search of food. As autumn approaches, they again migrate into deeper water, this time to about the 20 m line. Thus, until their third summer, by which time they are about 20 cm in length, most plaice are found near the coast in water of less than 20 m depth. Thereafter, as they increase in age and size, they inhabit increasingly deep water further from the shore. The size reached by a particular age depends partly on the food supply. North Sea plaice reach a size of between 35 and 45 cm by about their sixth year of life. In most areas the females are slightly larger than males of the same age.

Plaice occasionally hybridize with both flounder and dab. Various races of plaice can be distinguished to some extent by differences in numbers of fin rays or vertebrae.

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