HARVESTING, PROCESSING, MARKETING AND ECONOMICS

Harvesting

Ponds where prawns are reared may either be harvested once, at the end of the growing season dictated by environmental conditions or management practice, or regularly, as in the continuous stocking/culling management system, or some combination of the two. Harvesting may be by drain-down, either by gravity or pumping, or by seining.

In ‘pure’ batch culture, the prawns are harvested by a combination of seining, hand collection and, where there is one, collection from a harvesting sump. This is carried out when sufficient of the prawns have reached marketable size or when the cessation of farming activities is necessitated by lack of water, temperature fall, etc. Where continuous culture is practical, ponds are rarely drained. Marketsized animals are removed by seining with an appropriate mesh size net on a regular basis. This may be once every 2–4 weeks; sometimes only part of the pond is seined on any particular occasion to avoid total disturbance of the habitat. Re-stocking may take place in the continuous system one to six times per year.

In Thailand, some of the larger farms use the continuous management technique (New and Singholka, 1985) while some of the others use the batch technique, mainly because of seasonal shortages of water. Most Thai farmers use a combination of the two methods, cull harvesting from 5 months after stocking onwards until the pond is completely harvested after about 8 months.

In Hawaii, the seine net used for harvesting (Malecha, 1983b) retains mostly prawns over 30 g in weight; other smaller prawns may also be trapped unintentionally. Malecha(1983b), and New and Singholka (1985) describe harvesting techniques, both seining, draining and pumping, in detail. Aids to improved management of prawn ponds have been suggested. Some Hawaiian farms use mechanical harvesting. A simple inexpensive prawn grader to separate prawns into the three Hawaiian marketable sizes (Arndt et al., 1984) and a mechanized seine for cull-harvesting (Williamson and Wang, 1982; Losordoet al., 1986) are two improvements suggested. Lam and Wang (1986) have reviewed engineering research for improving prawn yield in Hawaii. Canal type ponds (i.e. long narrow ponds not more than 30 m wide) assist seining operations. Seining operations are typically slow, labour intensive and inefficient and often exacerbate the losses of animals during harvesting.

Comparing ordinary manual seining with modified seining (where a seine stretched across the pond is pulled at each end by tractors)and drain harvesting (by gravity or pumping) followed by pumping the harvest into a dewatering box with a fish pump, Peterson (1982) found that either type of seining had a low efficiency in catching marketable prawns. Although the drain harvesting system was 100 per cent efficient, much labour was necessary for post-harvest hand sorting. Peterson (1982) concluded that manual seining would continue to be the methods of choice on small farms, being slightly more efficient than the modified seining technique. In terms of equipment costs per kg of prawns harvested over a five-year period, drain harvesting was the most expensive at farm sizes from 4–96 ha; the manual method was the cheapest. The efficiency of the manual technique in harvesting marketable prawns could be improved through other pond management practices including minimizing muddy and uneven pond bottoms, removing obstacles such as sticks or stones and controlling below-water-level vegetation.

The composition and appearance of harvestable shrimp and prawns can be modified by dietary modification. For example, Sandifer and Joseph (1976) showed that shrimp fed a diet augmented with 3 per cent shrimp head oil had fifteen times more carotenoid content than the control. The levels of n-3 fatty acids in the shrimp mirror those in the diets utilized. Those who compare the flavour of prawn flesh to that from marine shrimp sometimes find it bland, although taste panels comparing both marine shrimp and freshwater prawns with lobster have reported Macrobrachium rosenbergii to be of similar quality to lobster flesh. Free amino acids and chloride ions have a generally flavour-enhancing effect on aquatic foods. Elevating the rearing salinity several days before harvest may enhance the flavour of prawns (Fox et al., 1985). Taking prawns off feed three days before harvesting is said to obviate the need for de-veining (Lee, 1981).

Seine-harvesting nears its goal.

The goal: a good harvest about to be size-graded.

Harvesting techniques will have to be modified for prawn-fish polyculture. Pavel et al.,(1985) postulated that channel catfish ponds, normally rarely drained, would need to be drained if prawns were also stocked. Methods for sorting mixed harvests of fish and prawn would also need development.

Processing

Processing yields of prawns are size and sex dependent (Smith, et al., 1980). Tail weight percentage decreases with increasing prawn size. Female prawns give consistently greater tail yields than males, mostly due to differences in the size and weight of their claws. In general, processing yields of deheaded prawns are less than those of penaeid shrimp but greater than those obtained from crayfish and crabs. On average, 50 per cent of the total prawn weight is lost when tails only are sold (Huner, 1980).

Many freshwater prawns are sold close to the farm site in Thailand (New and Singholka, 1985) either on ice or alive. Fresh prawns only have a shelf life of up to three days. After this they become mushy and begin to decompose. Mushiness also occurs if prawns are cooked for more than a few minutes (Lee, 1981). The quality deterioration of raw prawns under iced or refrigerated storage, characterized by softening (mushiness), may be caused by the activity of a collagenolytic enzyme released postmortem from the hepatopancreas. After 4 days storage, proteolytic and collagenolytic microflora also contribute more extensively to mushiness (Pre-maratne et al., 1986). Glude (1984), commenting on the short shelf-life of fresh (chilled) prawns, postulated that this might be extended by storage under CO₂ or that bacterial decomposition might be controlled by O₃. Some farms have their own on-site restaurants. Prawns can be shipped in aerated water containers to other restaurants and to hotels if the extra value obtainable for live prawns warrants it. Freshwater prawns are particularly susceptible to enzymatic damage after harvesting and death, and some farms ‘kill-chill’ by dipping the prawns in iced water. This is followed by blanching them at 65°C for 15–20 seconds. The prawns are then iced and transported to market. In experiments on processing prawns in Thailand (Anonymous, 1980a), corrugated fibreboard boxes were suitable for transporting prawns for up to 14 hours with a dry ice/prawn ratio of 1:2.5. One alternative, the use of styrofoam boxes, was extremely expensive.

In Thailand, most prawns are sold whole (head and shell-on). The original quality of the prawns in terms of microbial count and hypoxanthine value is important (Passy et al., 1983). The initial microbial load can be reduced significantly by heading and intensive washing. Packing in ice was found by Passy et al., (1983) to prevent desiccation and to lengthen the microbial growth lag phase from 1–2 days to 5. Flushing with CO₂ and a temperature of 4°C extended shelf life by 2–3 days (to 8–9 days) without affecting flavour. Cooking tends to remove off-flavours and odours and toughens up the texture of prawns. Some information on the frozen storage stability of prawns has been published by Hale and Waters (1981). Chemical, physical, microbiological and sensory evaluations were made over a 9 month period of -20°C glazed storage on whole (raw or precooked) and deheaded (raw) prawns. Prawns can be frozen for up to six months with almost no deterioration in flavour (Anonymous, 1981).

Marketing

Values of cultured freshwater prawns are recorded in Table 4. Farm-gate prices are time, size and site specific. Many of the high prices quoted in the USA appear to be based on the value of very large prawns rather than on average value for all the marketable animals. Farm-gate values appear to have fallen severely in Thailand between 1980 and 1986 due to a ten-fold increase in farmed production. However, while farmers complain about the prices obtainable there, this is undoubtedly because profits are not so great. That prawn farming in Thailand remains economically viable at the reduced farm-gate prices obtainable is demonstrated by its continuing expansion.

Marketing problems do not exist in the home market in Thailand where prawns are long established as a desirable product. Liao and Smith (1981) reported that most South Carolina consumers evaluated freshwater prawns as similar to marine shrimp but, unlike those in Thailand, preferred them heads-off. Retailers preferred a 78–110 tails/kg size category. Direct sales to consumers, as well as through seafood retailers and restaurants, were the most attractive marketing approaches there (Liao and Smith, 1982). An advertizing campaign in Hawaii in 1978 stressed heads-on prawns as the product, rather than tails, despite the fact that some residents and visitors found this unattractive; it was thought that ‘heads-off’ marketing might lead to “hepatopancreas contamination” (Aquaculture Digest, 6.11.51.). Lee (1981) reports that Boston chefs find fresh freshwater prawns to have more flavour than saltwater shrimp and that serving them shell-on helps retain the flavour. In north-east USA, freshwater prawns are said to be requested in preference to marine shrimp because the latter have an ‘iodine taste’ and an objectionable cooking odour (Aquaculture Digest, 13.3.13.). In a South Carolina marketing test, restaurateurs found good acceptability of dishes prepared either with head-on or head-off prawns. Microwaved tails stuffed with crab was the most preferred dish (Liao et al., 1981). A consumer survey in the same state indicated that the typical purchaser of freshwater prawns would be “a 44 year old white-collar worker with an average yearly income of US$ 23,182 and 12 years of formal education” (Liao and Smith, 1983). This seems a little over specific and would be greeted with some amusement in Thailand.

In Thailand, different types of marketed prawns are distinguished (Sastradiwirja, 1986) and valued (Table 4). A detailed analysis of prawns population structure in various marketing categories and its relation to economic yield from a typical Thai prawn farm has been made by Lin and Boonyaratpalin (1987). About one-third of the 15,000 mt of prawns estimated to have been produced in Thailand in 1987 were exported to Europe (B. Saisithi, personal communication, 1988). The possibility exists in Hawaii (Lee, 1979) for developing specialized markets for egg bearing and soft shelled forms. Soft shelled and old (terminal growth) prawns are usually separated from those to be sold. Lee (1979) found that 63.5 per cent of Hawaiian cultured prawns were ultimately used by direct consumers and the rest by restaurants. The basic farm-gate outlets for farmers were wholesalers (17.8 per cent), retailers (58.6 per cent), restaurants (18.7 per cent), and consumers (4.9 per cent). In 1982 only 15 per cent of the prawns farmed in Hawaii were being marketed off-island, 85 per cent being sold live in Hawaii. The rest went to mainland U.S., chilled on ice (Lee Ure, reported in Aquaculture Digest, 8.8.2.). The major consumer in Hawaii was the Filipino community. It would be necessary to ‘educate Caucasians’ to consume this product. The Hawaiian market for prawns was reported to be stable, at 117 tons/year (Lee, 1984). Only about 21 tons/year was being shipped to continental U.S. Demand, both there and Hawaii, was said to exist but no firm estimates of potential had been made. One of the problems experienced by the expanding Hawaiian prawn farming industry was being economically able to supply the quantities needed on a regular basis to continental U.S. restaurateurs interested in featuring them. One marketing agent in New York in 1985 appealed for suppliers of prawns; 45 kg was required on a regular weekly basis (Shnider, 1985).

Table 4. Farm-gate prices for whole (head-on) freshwater prawns, Macrobrachium rosenbergii

¹ If reported
² Larger prawns have higher value
³ Retail market price
⁴ Price sold to restaurants

In early 1982 the Amfac Food Group was telling its stockholders that it predicted that the U.S. ‘heads-on’ market for prawns was over 4,500 tons/yr with a value of US$ 50 million (Anonymous, 1982); their Hawaiian cultured prawns were being test marketed in a frozen, whole, size-graded form in Los Angeles and Chicago. However, later in the same year, Amfac were reported to be abandoning their two-year pilot project in Hawaii and returning the land to sugar production. This was because test marketing showed “weak consumer support for the product at cost-effective prices” (Amfac news release dated 10 September 1982, quoted by Aquaculture Digest 7.11.8.). General Mills, Inc., which owned the Red Lobster Inn seafood restaurant chain began a pilot prawn farm in Honduras in 1972 to supply at least part of its need for more than 4,500 tons/year of shrimp tails in 1981 (Wulff, 1982b). General Mills sold their Honduran farm in 1983 to a local company in order to concentrate on its retailing activities and to develop a technology transfer business in prawn culture (Aquaculture Digest, 8.12.11.).

The Weyerhauser Company, starting its own pilot prawn farm in Brazil in 1981, intended to test market them on the East Coast of the United States and in Europe and were expecting them to end up in “the white table cloth class of restaurants” (Kadera, 1981). In 1986, Israeli-produced prawns were reported as being exported head-on to Europe and the U.S.A. (Aquaculture Digest, 11.2.53.). The Israeli export potential for prawns was, in 1978–1979, thought to be for 30 g head-on animals (Arieli et al., 1981). Initial quantities were culled from monoculture and were 35–40 g in average weight. When the major supply became available in October from polyculture, they were marketed at 45–90 g. Again regularity of supply, if marketed fresh on ice in styrofoam boxes with a covering of dry ice as they were, would be a problem. Prawns originating from Jamaican aquaculture were being marketed in the U.S. head-off in small quantities but the Israeli company concerned believed they should be marketed head-on (Anonymous, 1986b). Jamaica and Puerto Rico are now said to be exporting large quantities of freshwater prawn to the US (Aquaculture Digest, 13.3.13.).

In 1980, restaurants and processing firms in Japan were reported to be buying prawns being cultured in rice paddies as a response to rice surpluses (Anonymous, 1980). In 1986 freshwater prawns, of which 20 tons/year were exported to Japan in 1983, were said to be in high local demand in Taiwan (Aquaculture Digest, 12.9.5.). Restaurant and fish buyers come to the farm-gate to buy the prawns.

Economics

Few economic studies of freshwater prawn farming have been published and many of these were feasibility studies rather than case studies of existing farms. By 1986, US$ 3 million had been invested in a Puerto Rican prawns farm (Anonymous, 1986a). A study of the feasibility of farming prawns in Jamaica examined the projected economics of a 4-acre farm (8 ponds ×½ acre) buying in its postlarvae and marketing its product fresh on ice in Jamaica after 9 months batch culture (ADCP, 1983a). The annual return, after interest and depreciation was about US$ 2,580. A similar, unpublished, economic appraisal of the potential of freshwater prawns farming in the Dominican Republic (ADCP, 1983b) calculated that the annual return on a 4-acre farm would be US$ 15,000. A feasibility report on freshwater prawn farming in Dominica in 1977 concluded that 2,5 or 10-acre farms would be economically viable as an alternative land use for existing private or government land owners, assuming certain site characteristics (New et al., 1978). Based on a mixture of projected farm sizes and types with a total annual national output of 50 tons of prawns per year a positive cash flow would have appeared in year 5 and cumulative cash flow would have turned positive in year 9. A study on the feasibility of a 100 acre prawns farm with hatchery in Surinam in 1976 estimated capital costs at US$ 852,000 and operating costs at US$ 427,645 per year (Pacific Aquaculture Corporation, dba Fish Farms Hawaii, P.O. Box 899, Kihei, Hawaii 96753, Unpublished MS).

A study of the economic status of prawn farming in Thailand in 1980 (Anonymous, 1980a) found that both hatcheries and grow-out units of all sizes were highly profitable. In most cases the initial investment could be paid off within the second year of operation. The initial investment for hatcheries ranged from US$ 1,000 for ‘backyard’ operations, to more than US$ 50,000 for large units. Annual hatchery operating costs were 5.6 times the initial investment for small hatcheries but only 1.8 times for medium and large hatcheries. (In the study, the small hatcheries were defined as having an annual production of < 2 million postlarvae and most were inland.) Feed and water costs accounted for 42–67 per cent of operating costs. Annual hatchery profits ranged from US$ 2,200 for small hatcheries to about US$ 37,200 for large ones. The rates of return on investment were 236 per cent for ‘backyard’ hatcheries, 154 per cent for medium and 62 per cent for large hatcheries. Costs of production were US$ 11.5, US$ 9.0 and US$ 10.0 for 1,000 postlarvae for small, medium and large hatcheries respectively. Hatcheries integrated with growout units produced postlarvae at lower cost (US$ 6.0/1,000) and coastal hatcheries produced at lower costs than inland hatcheries trucking sea water or brine. Labour is said to constitute 52 per cent (Shang, 1974) to 77 per cent (Fujimura and Okamoto, 1972) of the costs of postlarval prawn production.

Investment in Thai grow-out farms ranged from US$ 3,125–6,250/ha (Anonymous 1980a). Land costs were US$ 3,125–5,000/ha. Annual operating costs were US$ 4,396, US$ 3,319 and US$ 4,225 for small, medium and large farms respectively. (In the study the small grow-out farms were defined as < 1.6 ha, medium were 2.4–6.4 ha and large were > 8 ha.). All of the farms studied were profitable. The annual profit averaged US$ 2,754, US$ 5,431 and US$ 3,275/ha for small, medium and large farms respectively. Corresponding rates of return on investment were 51 per cent, 108 per cent and 58 per cent. For medium size farms the initial investment could be paid off within one year: for others it took two years. Generally speaking, at the average market value of prawns in 1980 in Thailand (US$ 7.50/kg) the report gave a break-even annual productivity of 588 kg/ha, 444 kg/ha and 563 kg/ha (small, medium and large farms). If 1986 prices had prevailed then the breakeven annual productivities would have been about 1,100,831 and 1,056 kg/ha respectively. It is interesting to note that prawn farming in Thailand was still rapidly expanding in 1987, despite the drop in the market value of the crop. A recent study by Shang and Lin (1988) confirmed that all sizes of grow-out farms in Thailand were profitable but that those in the medium range were the most successful.

The author of the economic study of grow-out production of prawns in Thailand included in Anonymous (1980a) later compared this with the situation in Hawaii (Shang, 1982). In this study the investment required to start a farm in Hawaii was shown to be much higher than in Thailand, mainly due to the 2.5–8.9 times higher cost of pond construction (US$ 20,500–43,000/ha). In Hawaii, pond construction costs decreased with increasing farm size due to economics of scale, whereas the converse was true in Thailand where the larger farms spent more on construction and equipment per unit area. Hawaiian labour costs per unit area averaged 5–6 times those in Thailand. On average, the cost of land, interest, feed and depreciation were much higher in Hawaii. Average annual productivity at 2,273 kg/ha was also higher than in Thailand (1,068 kg/ha). The lower productivity in Thailand was attributed to lower stocking rates and the shortage of water in certain seasons. The cost of labour and feed per unit of prawns produced was 6.8 and 2.1 times higher, respectively, in Hawaii than Thailand but the cost of postlarvae per unit of marketable weight was lower due to the lower cost of government hatchery supplied postlarvae at that time in Hawaii and to better survival rates. Total production costs per kg of marketable prawn were US$ 8.61–10.50 in Hawaii, compared to US$ 3.00–4.50 in Thailand. Farm-gate prices for prawns in Hawaii averaged a little higher (US$ 8.80/kg) than in Thailand (US$ 7.30/kg) but the low production costs in Thailand generated profits. While small (0.4 ha), medium (4 ha) and large (8 ha) Thai farms made average profits of US$ 1,113, US$ 2,297 and US$ 1,139 in 1980, corresponding farms in Hawaii made an average loss of US$ 1,543 for small farms, and US$ 1,344 for medium farms, and only just broke even at a profit of US$ 170 for large farms. Clearly, in Hawaii prawn farms would need to be considerably larger than 8 ha to be profitable, whereas profitability in Thailand was achievable at all farm sizes studied. 4 ha farms were the most profitable size in Thailand. Shang (1982) saw clear competitive advantages, should prawns become a global commodity for production locations with low production and marketing costs. By 1987, accelerating Thai exports to Europe were demonstrating the truth of this forecast.

Samples and Leung (1985), during conceptual modelling of Hawaiian prawn production, showed that at a wholesale price of US$ 11/kg there is a 50 per cent possibility that a representative 8 ha farm utilizing ponds greater than 0.8 ha in surface area will incur a financial loss in any given year. The probability falls to 30 per cent if ponds smaller than 0.4 ha are utilized.

Taiwanese freshwater prawn farmers estimate that total production cost is US$4.16/kg (Aquaculture Digest, 12.9.5.), while revenue is US$ 8.00–9.60/kg. However, farms are small and, like those in marine shrimp farming there, utilize family labour. The production cost does not include rent or pond construction and maintenance. Rent for a freshwater pond in Taiwan, at US$ 3,200/ha/yr, is one quarter that of ponds for marine shrimp culture. Ponds are, after several culling harvests, totally harvested after a growing season of about 5½–6 months. Production is high, varying from 2.5–7 mt/ha per crop, reflecting the use of paddlewheel aeration, intensive feeding, heavy stocking rates (in excess of 20/m²) and careful water quality management. The unit feed cost is about US$ 640/mt and the FCR ranges from 1.4:1 to 2.2:1, averaging 2:1. Compounded feed costs therefore represent about 31 per cent of total costs. However, Taiwanese farmers also use yellow bean cake, wheat and trash fish as dietary supplements when available; they also use fertilization.

Many other reports on the economic feasibility of the hatchery and grow-out phases of prawn farming in various locations exist but are not reviewed here. These include studies in Martinique (Elizabeth-Mesnager, 1981), Samoa (Popper, 1982), Malaysia (Tiensongrusmee, 1983), the Indian sub-continent (Karim, 1981), Suriname (Pacific Aquaculture Corporation, dba Fish Farms Hawaii, P.O. Box 899, Kihei, Hawaii 96753, Unpublished MS), a geothermal farm in Oregon, U.S.A. (Oregon Institute of Technology, 1981), and in South Carolina, U.S.A. (Smith et al., 1983). Bauer et al., (1983), investigating the costs and returns for prawn farming as a supplemental enterprise in South Carolina, reported that potential profitability was particularly likely if existing pond facilities, already discounted into the value of the land or having been constructed during a period of lower investment costs, were available. Pashen (1984), examining the feasibility of prawn farming in Queensland, Australia, estimated that an 8-ha prawn farm would require an initial investment of US$ 210,000. After two or three years' operation the break-even price of prawns would be US$ 8.13/kg. Prawns would not have completed favourably with marine shrimp in Australia at this price. In Israel, six month seasonal prawn monoculture was said (Aquaculture Digest, 10.11.9.) to raise a gross revenue of US$ 14,250/ha. After postlarval costs of US$ 5,500/ha and feed costs of US$ 1,250, US$ 7,500/ha was left to cover other costs and profit.

Not everybody makes money growing prawns. Some of the hazards that attend all aquaculture ventures have befallen prawn farming. Wulff (1982b) recorded that all ponds were flooded and the prawns escaped from a farm in Honduras, following hurricane Fifi. Cook (1980) noted that a farmer in Louisiana, who stocked 252,000 young prawns costing US$ 10,000, harvested only 28,000 harvest-sized prawns which were sold to Antoine's Restaurant in New Orleans for US$ 5,100! This brave farmer tried again, only to lose his second crop in an unexpected cold front.