DISTIBUTION: 

Eastern Atlantic: Bay of Biscay to Spitsbergen, Barents Sea, Iceland and eastern Greenland. Western Atlantic: southwestern Greenland and Labrador in Canada to Virginia in USA (Ref. 7251). 

DIAGNOSIS: 

Dorsal spines (total): 0-0; Dorsal softrays (total): 98-110; Anal spines: 0-0; Anal soft-rays: 73-85. Uniformly dark brown or black; young marbled or spotted with paler marks (Ref. 4705). 

BIOLOGY:

Benthic but occasionally caught pelagically (Ref. 4705). Feeds mainly on other fishes (cod, haddock, pogge, sand-eels, herring, capelin), but also takes cephalopods, large crustaceans and other bottom-living animals. Utilized fresh/ dried/salted, smoked and frozen; can be steamed, fried, broiled, boiled, microwaved and baked (Ref. 9988). Growth rate varies according to density, competition and availability of food. Slow growth rate and late onset of sexual maturity, halibut populations can be seriously affected by overfishing (Ref. 35388). 

MAXIMUM SIZE: 

240.0 cm TL; 300.0 cm TL (female); max.weight: 320 kg; max. reported age: 50 years 
Environment: demersal; marine ; depth range 50 - 2000 m 

CLIMATE:

Temperate; 74°n - 37°n 
Importance: fisheries: commercial; aquaculture: experimental; gamefish: yes; aquarium: show aquarium 

BIOLOGY, CAGE CULTURE AND MARKETING:

H. hippoglossus can reach an age in excess of 50 years and such fish may grow to a length of up to 4 m, weighing over 300 Kg. More commonly, fish from the wild average 5-10 Kg.

NATURAL LIFE-CYCLE: 

H. hippoglossus has a complex life cycle - outlined at right. Fortuitously, Shetland's geographical location is close to the natural habitat of halibut and it provides excellent environmental conditions to enable successful rearing and on-growing of halibut. Constant, 'cool' temperatures and low levels of intense sunlight are perfect for year round halibut growth

NURSERY STAGE:

A nursery stage has been recognised as an important 'stop-gap' between the hatchery and sea-site stage of the production cycle. The fish are grown from a few grams to 30 - 40 g over approx. 10 months in land based, flow-through tanks, feeding on a proprietary pellet diet 

CAGE CULTURE:

Post-nursery halibut adapt well to cage culture and on growing in cages appears to be very effective with low mortalities. 
Adapted salmon cages with flat bottom inserts constructed from tarpaulin or knotless mesh has been found to be successful. The bottom of the cage is usually 3 - 4 m deep and is kept taut with a sinker tube, to give the halibut a substrate to rest upon. The cages, being quite shallow, can cause some problems since halibut are quite susceptible to sunburn especially if their skin pigmentation is underdeveloped (see earlier). To avoid this, it is usual to shade the fish with a suitable cage cover. Increasing the productivity of a site by increasing the surface (bottom) area available to the stock may be an important consideration for the future. 
Strong water currents necessary for salmonids, for example, do not appear to be particularly beneficial for halibut culture. This means that some salmon sites may not be applicable for halibut and additional sites, not necessarily suitable for salmon, are potentially viable. Halibut cage sites should be quite sheltered to reduce cage bottom movement. 
Halibut do not have the same ravenous appetite of salmon, apparently preferring to eat few but large meals. Some on-growers have fed their halibut 2 - 3 times per week whilst others feed every day. Because the fish are relatively docile, there is a very good feed conversion ratio (e.g. 1.1:1), meaning food is efficiently converted to body mass. Trash white fish mixed with vitamins have been used but this has the related problem of disease introduction. Commercial dry pellets have been used successfully, however experience with rearing broodstock has shown that the fish much prefer a moist pellet/sausage. 
It is commonly found that (post-nursery) halibut can reach a weight of 1.5 - 7 Kg within 2 years of cage growth (the current market prefers fish of between 5 and 10 Kg although 3 - 5 Kg weight is also thought to be marketable). The composition of commercial halibut diets has not been optimised but it is anticipated that growth rates will increase when the necessary components are understood, as seen in the salmon industry. 
Only 1 - 3 % of the fish's body mass is lost during gutting and yields of around 49 % for fillets is normal. There is still much to learn for on-growing halibut - stocking densities; feed constituency and rates; harvesting procedure(s), etc., but it appears that cages give good growth prospects and produce farmed fish akin, or better than, wild fish. 

DISEASE: 

Halibut are susceptible to a range of diseases but are relatively robust with a well developed immune system. A real concern for hatchery operations, in particular, is the susceptibility of halibut to IPN; this is a fatal infection for the larvae and fry. For the on-grower, halibut are susceptible to a number of parasites and diseases, sometimes the infestation being so heavy that the fish ceases to eat and may die as a result. Atlantic halibut are susceptible to vibriosis but vaccines are available. It is thought that halibut are not affected by ISA. It is anticipated that veterinary costs will be relatively low for halibut but it is difficult to speculate as disease incidence is bound to increase as farming intensifies. 

SALES AND MARKETING:

 The European nominal landings for Atlantic halibut are between 3000 and 4000 tonnes (FAO Fishery Statistics Vol. 74). It is predicted that around 1,000 tonnes of farmed halibut will be harvested in the UK alone by the turn of this century. In comparison to the European farmed salmon volumes of over 500,000 tonnes including 30,000 tonnes from Shetland, it appears there is considerable potential for profitable growth. With the various technical and environmental limiting factors for farming halibut it is unlikely that major volumes are likely to be produced in the foreseeable future such that the market will be saturated and price adversely impacted upon. Current thinking is that a sustainable level of farmed halibut price at existing market demands is likely to be £5 - £6 per Kg for a 3 - 4 Kg fish. 

REFERENCES:

Nielsen, J.G., 1986. Pleuronectidae. p. 1299-1307. In P.J.P. Whitehead, M.-L. Bauchot, J.-C. Hureau, J. Nielsen and E. Tortonese (eds.) Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO, Paris. 3: 1301-1302.

http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Hippoglossus&speciesname=hippoglossus

http://www.nafc.ac.uk/publish/info2/info2.htm

compiled by Zhengzhong Zang