Prawns

Prawn Culture in British Columbia
Prawn or shrimp culture is practised extensively worldwide. In terms of yield in tonnage and market value, shrimp farm production constitutes one of the most significant aquaculture activities. Almost all of it, however, is concentrated in tropical or sub-tropical areas where extensive shrimp ponds allow for high volume production. Culture of cold water species has only recently been attempted and operations are still in the research and development stage.

Species
In British Columbia, recent efforts have been exerted on developing a culture system for a high value species, the Spot Prawn (Pandalus platyceros). Presently, the spot prawn is fished commercially at an annual rate of approximately 1500 tonnes. Management of the wild fishery is based upon maintaining a minimum threshhold number of “spawners” which will theoretically ensure a sustainable population level for future harvesting. The Spot Prawn is the largest of the prawn species of the west coast with females growing to a size as large as 23 cm in total body length. The natural range of this species is the North Pacific ocean, from Alaska down to San Diego, California in the east Pacific and the Sea of Japan through the Korea Strait in the west Pacific.

Markets
There is no over-supply of Spot Prawn in the marketplace. Even though the availability of the product is dependant on seasonal limits of the capture fishery, which can be as long as six months or as short as 10 weeks, it is entirely absorbed by high demand. The major markets are Japan and other Asian countries which buy either fresh or frozen at sea product for as high as $28cdn/kg. This bouyant market in combination with uneven annual supply suggests that cultured prawns can occupy an important niche in the global seafood market.

Development of Production Systems
For several years, the Cultured Crustacean Company has the impetus behind the efforts to develop a culture system for the Spot Prawn. The company has been working with researchers at Island Scallops to develop hatchery and juvenile rearing techniques which will create a consistent supply of prawns for grow-out operations. Additional work has been done to develop feeds and proper containment conditions for on growing prawns to market size.

Hatchery Technology
Spot prawns are potandrous hermaphrodites, switching from being males to females. They exist as males for the first two to three years of life, during the latter part of which they are sexually active. At this time the males will turn into females and will mate with the males in late summer or fall. The fertilised eggs in the female will be extruded and attached to its pleiopods where it will incubate them. The 2000 or so eggs will be cleaned and irrigated by the female for 5-6 months. Under natural conditions, the eggs will hatch in march or april and the larvae will spend 2-3 months as free-swimming nauplii and late larval and post-larval stages on the bottom in relatively shallow water. Prawns are omnivorous, with the young feeding on smaller crustaceans such as mysids and amphipods, and larger ones more opportunistic feeders. The initial stage of developing hatchery techniques has been to capture gravid females and utilize the emerging larvae as seed stock for culture. However, research into controlled hatchery production of juveniles has been advanced to improve yield and survival up to 50% through the use of probiotics and other techniques.

The relatively large size (8mm total length) of the hatchlings makes them quite amenable to hatchery rearing in containment using readily available feeds. The planktonic larvae initially feed on zooplankton and commercially available Artemia works well for this stage. Other solid food such as fresh or frozen seafood byproducts can be introduced a few days later. The four larval stages will be completed in 15-19 days. From this point on the challenge has been to develop a feed source onto which they can be readily weaned and which will be amenable and economical for grow-out at marine culture sites.

Nursery and Grow-out Rearing
Post-hatchery rearing at nursery and grow-out sites is dependent on the development of a satisfactory pelleted feed and determining the optimum bio-physical conditions for the animals. Although they are frequently captured in the wild at depths of 200m, it appears that they are not averse to living nearer the surface so that containment can be suspended from floating structures.

Temperature tolerance ranges from 5-18oC. Certain sites may be unsuitable due to high summer water temperatures. Salinity tolearnces range from 26-31 ppt again suggesting that certain sites influenced by fresh water intrusions and especially shallow depths may not be suitable for culture.

Containment systems for grow-out will likely consist of some kind of suspended net cages although experiments have been conducted using shallow raceways.

The most significant issue for grow-out culture, however, is feed. The animals require regular feeding from weaning to market. The fact that these animals are omnivores makes this a somewhat easier task. In 1999, the researchers developed a nutritionally sound proprietary feed formulation that can be pelletized and will likely be manufactured by commercial animal feed processors in the future. Feed conversion rates for this pelleted material appear to be around 1.5:1. Initial investigations suggest that the grow-out period at marine sites may take two years before stock reaches market size.

A significant bonus may be achievable by integrating prawn culture with the culture of finfish or shellfish. Because they are omnivores, the prawn will be able to consume a significant amount of waste and/or fall-down from fish farms or deep water shellfish culture operations. For example, biofoiling that covers equipment and stock in deep water shellfish aquaculture operations can be removed at harvest and fed to prawns held in containment below the harvest area. Such polyculture systems may be developed at less cost than mono-culture systems.

Current Status and Prospects
The project to develop a viable prawn culture industry in British Columbia is now at a stage where further capital is required to develop hatchery and grow-out facilities and take the next step after successful first-stage research and development. The plan is develop a prawn culture sector which will grow to fill a quarter of the market in 5-7 years. This will require the develoment of two hatchery/nursery facilities as well as 25 grow-out sites at both new and existing aqua farms. This sector alone is expected to produce 150-300 jobs by the completion of phase one in 5-7 years, with both prawn fishers and aquaculturists actively working in the sector.

Printed sources to consult about Spot Prawn:

  • Campbell, Chris. 1999 “Cultivated Spot Prawns Offer Promise in British Columbia”. Shellfish World Vol.1 No.2, pp.14-15.
  • “Spot prawn makes gains at BC hatchery”. Northern Aquaculture, Vol. 4 No. 4, April 1998.

Web sites to consult:

  • Fisheries and Oceans Canada:Species and Habitat of Shellfish.
  • Fisheries and Oceans Canada:Diseases and Parasites of Shellfish

Thanks to Chris Campbell for information and photographs.