Freshwater shrimp, or prawns, occur throughout the world and many of the largest species belong to the genus Macrobrachium. Although four native species of Macrobrachium are found in rivers and streams in the U.S., aquaculture production of freshwater prawns in this country, and most others, is based on Macrobrachium rosenbergii, a species that is native to the Indo-Pacific region. Culture techniques for this species were originally developed in Malaysia, Hawaii and Israel, but production methods have been modified to fit local conditions in many tropical and sub-tropical countries throughout the world. The tropical nature of M. rosenbergii has resulted in limited farming success in cooler climates, including much of the U.S. The terms “prawn” and “freshwater shrimp” used here will refer to M. rosenbergii.
Unlike marine shrimp, freshwater prawns are much more reptant. They prefer to walk around the pond bottom and climb on structures. As a result, stocking densities in culture ponds are limited to some degree by the surfaces available for them to crawl or climb on.
Cold temperature intolerance, and limited availability of post-larvae (small prawns that have passed through the larval stages of the life cycle) for nursery culture or juveniles for grow-out are the two major technical constraints for freshwater prawn production in the U.S. Growth is limited at water temperatures below 70o F, and prawns will become stressed at 65o F. Death begins to occur at 60o F, with few animals surviving temperatures below 58o F for any period of time. Accordingly, recommended production practices for U.S. growers have evolved to accommodate these constraints.
Production schedules are typically based on stocking juveniles (30 to 60 days old) in the spring once morning water temperatures are consistently above 68o F, followed by a grow-out period of 3.5 to 6 months, then total harvest. Some producers purchase post-larvae one to two months earlier and raise them to juveniles in tanks, either indoors or in greenhouses. If a producer chooses to produce their own post-larvae, broodstock must be set aside the previous autumn and maintained throughout the entire winter. This results in significant labor, energy and feed costs, as well as the need for additional specialized facilities.
Pond stocking rates of 15,000 to 20,000 juveniles per acre have given satisfactory results in growth studies throughout the Southeast, but lower densities may be required if aeration is not available, or in more northerly regions in order to attain marketable animals during a limited growing season. Prawns cannot eat floating feed, so sinking feed must be offered. Daily feeding rates vary, but are usually somewhere around 10 lb. per acre for the first month and 20 lb. per acre thereafter.
Ponds used for prawn production should have characteristics similar to those of fish culture ponds, although they can usually be operated with lower water depths. A depth of 2.5 to 3 ft. allows for good water quality, predator evasion and more rapid warming during the summer months. Shallower ponds also improve oxygen dynamics for prawn production. Vertical substrates, such as orange plastic construction fence or lightweight bird netting, are often hung like curtains throughout a pond to provide additional areas for prawns to climb and spread out, which can increase yields.
Unless they are stocked at low densities, ponds used for prawn production must be aerated from time to time, and an aeration capacity of 1 hp. per acre is considered the minimal requirement. Oxygen levels must be measured on or near the pond bottom, since prawns spend most of their time there when not climbing vertical substrates. If dissolved oxygen decreases to 2.0 near the pond bottom, aeration must be applied immediately to prevent mortalities.
The fastest growing prawns in a pond can sometimes be selectively harvested using a 1-inch bar mesh seine, occasionally up to a month before the full harvest takes place. However, the time and labor involved with this type of partial harvesting may not be justified, especially if vertical substrates are deployed in the pond. For efficient harvesting, ponds should have catch basins. Large ponds, old ponds with soft bottoms and ponds that cannot be easily drained should all be avoided because they are difficult to harvest, especially rapidly and on short notice as is often required when cold fronts are approaching in the fall.
Once a pond is ready for harvest, water level should be lowered gradually to encourage prawns to congregate in the catch basin, where they can be seined or netted. Invariably, some will become stranded in low spots throughout the pond and these animals must be harvested with dip nets or by hand. Final harvest is accomplished by complete draining. The key to success is to harvest the pond and get the product clean and chilled as quickly as possible.
The USDA’s 2018 Census of Aquaculture indicated 13 farms were producing freshwater prawns in the U.S. at that time, down from 40 operations reported in the 2013 census. Of these, 11 were producing food size prawns while 3 were selling larvae and juveniles. Hawaii and Ohio each reported 4 farms, followed by 2 each in Texas and Kentucky and 1 in Missouri. Total reported sales were $1,084,000, down from $1,376,000 in 2013.
Handling of prawns during and after harvest will have an impact on their marketability, especially in terms of off-flavor and texture. Prawns should be rinsed in clean water for up five minutes to remove all mud and vegetation, then placed in ice water or an ice slush. Failure to thoroughly wash off any mud on the surface or in the gill chambers will contribute to off-flavors when the prawns are cooked. Additionally, failure to immerse prawns in ice water or slush (a small amount of salt can be added to further lower the slush temperature) within five minutes of harvest may lead to product discoloration and softness of the tail meat.
However, once their temperature has been sufficiently reduced (after approximately 20 min) prawns should not be left immersed in water, but drained and held with crushed ice that can drip off as it melts. De-heading prawns once they have been chilled will greatly extend the shelf-life of the tail meat, but at the expense of the visual appeal of a whole, intact animal.
Marketing problems for domestic prawn producers have largely been a direct result of the cold intolerance of this species. Producers in the U.S. are forced to grow their prawns for as long as possible to maximize the average size at harvest, but all animals must be removed from outdoor ponds or tanks before temperatures drop to lethal levels. Few marketing outlets are available for this type of one-time sale, especially for a species most buyers are not familiar with. An additional problem with this approach to batch harvesting is excessive size variability, with many animals still too small to market once temperatures begin to drop.
Live markets for freshwater prawns are potentially lucrative, but there are a number of difficulties and expenses involved in holding and transport. Mortality from handling damage and aggressive interactions can reduce potential profits from live prawn sales. Cooler temperatures are desirable to reduce activity during holding and transport, but water temperatures below 70o F will stress animals. Aeration and abundant substrate for climbing (preferably 1 ft.2 per animal) are essential during holding and transport.
Pond-bank sales at the time of harvest are common among U.S. prawn operations. Although they are somewhat variable depending on the location the growing season and harvest period are locked in by local climate patterns. As a result, harvest sales have become somewhat of an annual ritual for some established producers. The advantages of this approach are that prawn producers can be close to large urban markets, and therefore offer a live or fresh product that is not possible with imported products. And although U.S. growers cannot compete on price directly with foreign, imported products (or even domestic wild-caught shrimp), they have the advantage of much lower costs associated with transport and cold storage.
An analysis conducted in 2004 suggested break-even prices for freshwater prawns ranging from $4.50 to $12.60 per lb. with 60% survival (the most optimistic projected survival rate in commercial ponds under good management). However, prices for both fixed and variable costs associated with prawn production have increased significantly in the years since that study was conducted. Current management recommendations often result in higher yields than were typically realized at the time in 2004, but profitability still varies greatly from one operation to another.
Retail, restaurant and direct to consumer sales often result in higher prices, but these sales are less predictable and based on smaller volumes than what would typically be seen in wholesale markets. Most prawn producers can only be profitable when focusing on these smaller, direct markets. If prawns will be processed in any manner, including simple de-heading, local and federal regulations will apply. Any facility that processes freshwater prawns must be approved, even when selling direct to consumers. A HACCP plan will be required if selling to distributors or restaurants. Whole- or live sales do not require a HACCP plan, but any sales – whole, live or de-headed, across state lines do.
When farming freshwater prawns, production schedules will determine marketing activities. One aspect of prawn production that allows for competition with traditional shrimp supplies involves a size-based marketing advantage. Production must be tweaked to produce the largest number of large animals possible. The availability, size variation and costs of juveniles in the spring will impact the types of markets a producer can utilize at harvest in the fall.
When planning a freshwater prawn farming operation it will be necessary to calculate the carrying capacity of the system in order to estimate capital costs, projected production, revenues and input requirements. At this point, an evaluation can begin in order to determine if the operation will be large enough to generate profits and if so, a market analysis can be undertaken. Total yields of 0 to 1200 lb. per acre are seen in prawn production in the U.S., with typical yields more often in the 300 to 700 lb. per acre range. Survival rates of 30% to 60% are typical, but lower and higher survival rates are not uncommon. Average sizes of 10 – 20 prawns per lb. are generally required to compete with traditional shrimp supplies in most markets. Input suppliers (especially for feed and juveniles) must be reliable and trustworthy, even when larger customers are competing with you. Consider your feed requirements – who will supply it, at what cost, and in what quantities will it be delivered?
Since harvests occur only once per year, identifying buyers prior to harvest is crucial for prawn producers. Shelf life of harvested prawns is limited and can only be extended by de-heading, which can require a number of permits or licenses.
Prepared by C. Greg Lutz, Louisiana State University Agricultural Center