This note gives information on the intrinsic quality of blue whiting, its shelf life in unchilled, chilled and frozen storage, its suitability for grading, filleting or mincing by machine, and the likely yield. Ways of utilizing it in products for human consumption are also discussed. The information on handling is preceded by a brief description of the species and its fishery.
The preferred scientific name for the blue whiting found in the north Atlantic is Micromesistius poutassou. The name Gadus poutassou is also sometimes used. A closely related species caught in the southern hemisphere and sometimes imported has the scientific name Micromesistius australis. This note refers exclusively to Micromesistius poutassou from northern waters.
The now generally accepted common name is blue whiting. The names poutassou and Couch's whiting have been used in the past but have gone out of favour.
Danish: sortmund, blåhvilling
Dutch: blauwe wijting
German: Blauer Wittling
Norwegian: kolmule, blågunnar
Swedish: kolmule, blå vitling
The body is long and narrow, greyish blue on the back and silvery underneath. Unlike the whiting, the lower jaw is slightly longer than the upper one; both jaws have large teeth. The third of the three back fins is widely separated from the other two, and the first of the two anal fins, which is very long, starts further forward than the first of the back fins. There is no barbel. The eyes are large.
The blue whiting occurs throughout the north east Atlantic from Spain to Iceland and Spitzbergen, but it is most abundant during the spring spawning period in deep water to the west of Scotland and Ireland and along the Faroe-Shetland channel.
Knowledge of the behaviour and movements of blue whiting is far from complete, but what follows is a summary of the known life history of the stock that spawns to the west of the United Kingdom.
Adult blue whiting spawn at a depth of 300-500 m over deep water and along the edge of the continental shelf and banks. Peak spawning time ranges from February in the Celtic Sea to May at south Faroe, that is it moves northwards with the advance of spring. Eggs and larvae move slowly towards the surface as they develop, and drift away from the spawning grounds. Many of the young fish, up to 2 years old, remain in the upper 100 m of the open ocean in the north east Atlantic and Norwegian Sea until they mature and join the breeding population. Some of the adult spent fish migrate northwards as far as Bear Island and Spitzbergen to feed before returning to the spawning area in the autumn via Jan Mayen and the east of Iceland, but the full extent of the feeding migrations is not known: feeding blue whiting are distributed so widely that location is difficult until dense shoals begin to form on the return journey. Small blue whiting feed on plankton; they change to krill and then to myctophid fish as they grow to full size.
Some blue whiting reach maturity in their third year, but recruitment to the breeding stock is not complete until most fish are 7-8 years old. Blue whiting 2 years old are about 17 cm long; 3-year-olds are about 20 cm, and 7 to 8-year-olds about 30 cm. Males mature earlier and reach their maximum length faster than females, but the females eventually grow slightly bigger than the males.
A conservative estimate of the spawning stock to the west of the UK is 5-15 million tonnes, which could give an annual yield of at least 1 million tonnes. Shoals large enough to give a catching rate of 5-10 tonnes an hour or higher are found at the southern end of the Faroe-Shetland channel from mid February and extend over a wide area to the west of Scotland and Ireland until the fish move northward and disperse again by the end of May.
Blue whiting are caught mainly by large midwater trawl, lowed at a depth of 200-500 m, either in very deep water or close to the sea bed when the fish layer is near to the edge of the shelf. Trawlers over 40 m long and at least 1200 hp are required to give the necessary manoeuvrability and power, and a headline transducer firing upwards and downwards is essential for accurate positioning of the net in relation to the shoals and the sea bed. Fishing is best during daylight; the shoals tend to disperse during darkness. Further details of the fishery are given in Laboratory Leaflet 45 'Blue whiting', MAFF Lowestoft 1979.
Over 200 000 tonnes of blue whiting were taken by European countries in 1977, mostly for reduction to animal feeding stuffs; the 1978 catch was about 500 000 tonnes; the UK caught 4500 tonnes in 1977 and 6300 tonnes in 1978. Norway, USSR, Faroe and Iceland are the principal catching countries at the present time, and catches may continue to increase rapidly over the next few years.
Blue whiting occasionally attain a length of about 40 cm, but 95 per cent of those taken in waters to the north west of the UK in spring are between 25 and 36 cm long, with an average length of about 31 cm. In some catches there may be a preponderance of larger or smaller fish.
A blue whiting 31 cm long caught in spring just before spawning typically weighs about 180 g. The range is from about 135 g for a 27 cm fish to about 280 g for a 35 cm fish. In contrast, a 31 cm fish just after spawning weighs about 140 g. Rested, well fed fish caught north of Norway in the autumn weigh from about 135 g for a 29 cm fish to 340 g for a 37 cm fish. The figures are all for ungutted fish.
The chemical composition of the flesh of blue whiting is generally similar to that of other members of the cod family, namely about 80 per cent water, 19-20 per cent protein and less than 1 per cent fat, but spent fish taken in April have an average water content of 83 per cent, with some fish as high as 91 per cent, with some consequent depletion in protein.
Condition can range from good to poor among fish taken during the spring spawning fishery; prespawning fish are likely to be relatively thick and reasonably firm, with large livers, whereas spent fish are thin and soft with small livers. The variation in condition is more marked than in other species of the cod family.
Blue whiting, in common with other species such as whiting and herring, are heavily infested with Anisakis, a parasitic round worm, which is found in the larval form in great numbers in the gut cavity and in the flesh, particularly in the belly flaps.
Whole ungutted blue whiting held unchilled while awaiting stowage or processing spoil rapidly; with a surrounding air temperature of about 10°C they should not be kept more than 6 hours. After 8 hours they are perceptibly softer, after 12 hours they are soft, and after 24 hours the guts are digested, off odours are present and the flesh is extremely soft and discoloured.
Blue whiting packed ungutted in plenty of ice in boxes show little adverse change for 45 days apart from slight softening, and are still of good quality after 5-6 days; they become unacceptably soft and spoiled after about 8 days in ice. Gutted and nobbed fish keep a little longer in ice, and are unacceptable after 12 days.
Blue whiting stowed ungutted in chilled or refrigerated sea water show little adverse change for 4-5 days, but beyond this time they spoil rather more rapidly than those in ice. Blue whiting, whether ungutted or gutted and nobbed, become unacceptable after about 8 days in chilled sea water.
Blue whiting are intrinsically too soft and small to withstand deep bulk stowage in ice; shallow boxes or closely spaced shelves are necessary. Chilled sea water has a number of advantages over ice for short term stowage up to 5 days; the fish are of better appearance, suffer less damage, and are firmer, which makes them easier to handle by machine.
The flesh of blue whiting is intrinsically not as while as that of cod or haddock. Skinned fillets from fresh fish display a pinkish hue when packed in bulk, due to the presence of blood concentrations in the dark muscle along the skin side; these areas gradually darken to muddy brown during storage in ice. The flesh of ungutted fish stowed in ice is somewhat darker than that of gutted fish because they remain unbled; bruising during handling can increase the discoloration.
Considerable advances have been made in the development of filleting machines that can accept almost the entire size range of blue whiting from the spring spawning stock. Uncertainties in the potential market, however, make it unlikely that machines designed specifically to handle blue whiting will be available; the need will be met by machines developed for a range of small species of white fish.
Size grading may be necessary to meet demands for a particular size of fillet; for example if block fillets of 100 g and upwards are required for say the frying trade these can be obtained only from fish longer than about 33 cm; machine grading would he necessary to segregate the 15 per cent or less of large fish from a batch of mixed sizes to enable selected fillets to be marketed at a premium. Because of the marked difference in thickness between prespawning and spent fish of the same length, a tapered gap roller grader cannot size blue whiting to fine limits; should grading prove essential, grading by length and weight may prove more accurate.
The small size of blue whiting makes hand filleting labour intensive and thus uneconomic. Two makes of filleting machine can be considered. The Baader 121, which is still under development, could take when on trial about 100 fish a minute in the size range 24-37 cm, that is an input of about 1 tonne an hour. The Arenco filleting machine takes fish from which the head, guts, belly flaps and bellywall lining have already been removed on a modified Arenco CIS/CIF machine; the SFA 4 system is still under development and no details of performance are available. Both machines can produce single skinless fillets. For comparison a hand filleter can cut 30-40 kg blue whiting an hour to produce single or block fillets with the skin on; hand skinning is impracticable, but the fillets can subsequently be skinned by machine. The soft flesh and fragile skin cause difficulties on some skinning machines, but the Trio model is satisfactory.
Trials of a prototype Baader 121 on small quantities of thawed fish have given some guide to fillet yield. The values given below are based on these trials. The proportion of fish rejected as unsuitable for filleting, because they were too thin, had broken backbones or were otherwise damaged, increased as condition became poorer. About 1 per cent of firm prespawning fish was rejected, whereas about 8-9 per cent were rejected from spent fish caught in April. Yield of skinless fillets from thawed whole ungutted blue whiting fed through a Baader 121 ranges from about 25 per cent from spent fish to about 34 per cent for firm prespawning fish, after inspection and trimming; the poorer the condition the lower the yield. Poor handling reduces yield even further. For example a delay before freezing of 12 hours at surrounding air temperature can reduce subsequent filleting yield to below 20 per cent. Blue whiting fillets tend to pick up considerable amounts of water during processing, and the apparent yield can be considerably affected by the amount of draining permitted before weighing. Given good practice at sea and on shore, with ample time for excess water to drain away, the likely yield from thawed blue whiting in good condition is about 27-29 per cent. Yield of block fillets, using a Baader 121 and a Trio skinning machine, is about 27 per cent.
Yield of skin-on fillets from hand filleting is 40-41 per cent; assuming a yield of about 73 per cent of skinless fillets from skin-on fillets fed through a skinning machine, the yield of handcut skinless fillets is about 29 per cent. Hand filleting yield is largely unaffected by change in condition of the fish.
Whole ungutted blue whiting can be frozen soon after capture and stored for subsequent processing. The fish should not be held unchilled more than 6 hours after capture before freezing them. It is better to freeze them at sea than ashore. Blue whiting should be handled carefully prior to freezing; being small and intrinsically soft they are susceptible to physical damage and bruising, and damage at this stage can result in a high proportion of rejected fish when the thawed fish are subsequently presented to a filleting machine.
The proportion of damaged fish is most affected by initial softness and subsequent handling practice, for example delays between operations. The thickness of a frozen block of whole blue whiting, and the manner of making it in a vertical plate freezer, whether packed dry or with added water in an outer wrapper, make little difference to damage. Nevertheless blocks 50 mm rather than 100 mm thick and packed wet rather than dry, are likely to give a better subsequent filleting yield. Blocks of frozen whole blue whiting remain suitable for filleting and further processing when kept in cold store at - 30°C for up to 12 months.
Blue whiting are unlikely to find a ready market in chilled form, either as whole fish or as fillets; their small size, discoloration due to autolysis and bruising and the presence of parasites all weigh against them in competition with other well established white fish species.
One likely outlet is as skinless fillets from chilled or frozen whole fish for the manufacture of frozen laminated blocks for finger or portion production. Laminated blocks of fillets from fresh chilled whole fish have a pleasant pinkish white appearance when newly prepared; they become gradually darker and browner during cold storage, and laminated blocks made from fillets of thawed frozen whole fish are darker in colour from the outset. Dark muscle and blood discoloration from bruising show up noticeably in random packed blocks; careful block preparation, with the outer layers of fillets laid skin side innermost, look more attractive, and washing of fillets in 1 per cent polyphosphate solution also helps to improve the general appearance of a block. Laminated blocks of blue whiting fillets are less attractive than those made from cod, haddock or whiting, but they compare favourably with blocks made from saithe, hake or mixtures of fillets and mince. Fish fingers cut from blue whiting blocks are not as acceptable as those made from cod, but are more acceptable than some 'economy' fish fingers on the market. Acceptability is enhanced by the addition of salt and glutamate. Blocks of fillets cut from a Baader 121 still contain small fine pinbones. Trials of fish fingers made from such blocks show that the bones go largely unnoticed by the consumer. Incidence of detection compared favourably with commercial fish fingers against which they were tested.
A variety of portion shapes of controlled weight can be made directly from skinless fillets of blue whiting by using a commercial automatic food forming machine. The shapes are fragile and must be conveyed immediately into a continuous freezer to retain their shape. Small surface irregularities and discoloration are not noticeable after coating with batter and crumb. Long thin shapes, such as fish fingers, are prone to deformation between moulding machine and freezer Fillet shaped portions of 90 g and upwards can be made by compressing three or four blue whiling fillets laid lengthwise together in a mould; the composite fillet portion can be frozen, and subsequently battered and fried by fish fryers in the same manner as conventional frozen fillets.
Another likely outlet for blue whiting is as mince, prepared from skinless fillets, using commercial bone separators such as the Baader 694. The freshly prepared mince from fresh fish has a pleasant pinkish white appearance, which darkens gradually in cold storage to a pale brown. Fish fingers made from blocks of frozen mince compare well with existing commercial economy fingers, and there is little to choose between fingers of blue whiting made from laminated blocks of fillets and blocks of minced fillets. Blue whiting mince is also a suitable raw material for the manufacture of fish cakes, fish pies and cook freeze dishes.
Mince can be prepared more economically from headed and gutted fish fed directly to a bone separator; the product from a Baader 694 is less discoloured if a drum with 3 mm holes is used, lubricated with water, and with belt tension adjusted to retain the maximum amount of skin. Little is yet known about the properties of this material in products such as fish cakes and fish fingers.
One possible export outlet for blue whiting is to Japan as surimi, an intermediate product in mince form used there for the manufacture of kamaboko, a speciality product of high value. Several trials have been made to produce batches of surimi to the Japanese specification; the main difficulty is in obtaining the perfect white colour demanded, particularly from frozen raw material; experiments are continuing to overcome this problem. Good quality surimi can be made from fresh chilled fish.
Blue whiting could possibly be marketed in smoked form, either as split whole fish, cold smoked like kippers, or as hot smoked snacks after removal of the head, guts and belly flaps. Both products when prepared experimentally have been excellent.
If you have any queries, write, phone or call at either of the addresses below:
The Officer in Charge
Torry Research Station
PO Box 31
135 Abbey Road
Tel: 0224 877071
Tel: 0482 27879
61 Gaping of fillets, by R. M. LOVE.Earlier notes in the series, most of which are still available, are summarized in:
62 The freezing time of fish, by F. J. NICHOLSON.
63 Fishing ports in the UK, by J. J. WATERMAN.
64 Fish silage, by I. TATTERSON and M. L. WINDSOR.
65 Fishworking machinery, by S. MAIR.
66 Handling and processing mackerel, by J. N. KEAY.
67 The haddock, by J. J. WATERMAN.
68 Icemaking plant, by J. GRAHAM.
69 Cook-freeze fish products, by J. N. KEAY.
70 Advice for the fish industry; who does what, by J. J. WATERMAN.
71 Processing cod; the influence of season and fishing ground, by R. M. LOVE.
72 Reducing odour in fish meal production.
73 Stowage of fish in chilled sea water, by J. H. KELMAN.
74 Handling and processing rainbow trout, by A. MILLS.
75 Freezing small pelagic fish, by I. MCDONALD.
76 Dark colour in white fish flesh, by R. M. LOVE.
77 Squid, by G. D. STROUD.
78 Health hazards of handling industrial fish, by A. WARD.
79 Minced fish, by J. N. KEAY.
80 Round worms in fish, by R. WOOTTEN and D. C. CANN.
60 Key to Advisory Notes 1-59, by J. J. WATERMAN.
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