The problem in temperate climates is that production of freshwater prawns is limited to a single season, with growout taking place in approximately 100 days. Under these constraints of temperate and time, pond production rates must be maximized to achieve commercial viability. Another problem is how to achieve greater yields without increasing the number or size of the ponds. The larger the pond means using more water, a precious natural resource.
Prawns are primarily bottom feeders and as such are constrained to a two-dimensional area rather than the whole area of the pond like finfish. This is further aggravated by the fact that they are territorial and cannibalistic. Several studies have documented the benefits of adding artificial substrate to production units (e.g., tanks) to increase available surface areas besides the bottom for prawns.
This added surface area has the ability to create a high-density pond. This means using the same pond and same quantity of water; yet able to add more prawns to produce a higher yield. This could be a cost/benefit issue.
This grant will determine if adding the substrate will generate a higher yield compared to the pond with no substrate. Will the higher yield (income) offset the added cost of the substrate or pay for itself?
By research, we will prove by adding substrate to a pond, we can use the same amount of pond space and water, a valuable resource, yet increase the size and yield of the prawns. This grant will also see if the higher volume and increased revenue will offset the cost of the substrate.
To create this added surface area at the bottom of the pond, a plastic bird netting will be staked into the pond. Bird netting was used because it has a lower cost than the orange construction fencing.
As part of the research, we utilize FFA students to help us with the harvest. The students do most of the hard, physical work but are also very curious about producing and harvesting fresh water prawns and usually ask lots of questions. For several, it is a first time experience working in the aquaculture industry. We also have a few students who have worked previous harvests. It is interesting how these students become the leaders showing and teaching the others various aspects of the harvest. It is an incredible educational opportunity for everyone –students and adults.
During the harvest, we will take total weight (pounds) from each pond and compare the yield.
We will then take random samples from each pond, one without substrate and one with, during the harvest. From the random samples we will count total weight and pieces. Next, sort by size, weight and average count of pieces per pound. (Count = prawns/lb)
Originally, we decided to measure the pawns by length using rulers. We quickly discovered that was not going to work because the prawns are not perfectly straight. We switched to a scale measuring in ounces. We used ounces instead of metric because shrimp are sold in the store by weight and by count.
For instance, if you buy 16 count shrimp at the store it means there are 16 shrimp per pound, which are fairly large shrimp. After weighing them, we sorted them into the three sizes and then sold according to size and weight.
Large- (1 ounce and over)
Medium- (1/2 ounce to just under 1 ounce)
Small- (less than ½ ounce)
No substrate (East pond) – 352.4 lbs
With substrate (West pond) – 406.5 lbs (13% higher yield)
Pond with substrate – additional 54.1 pounds,
average selling price of $11.00 pound, yields an
additional income of $595.10.
Initial Sample by Count and Weight
8 lbs-112 prawns Count-14 prawns/lb
Large-34 prawns, 31%
Med- 54 prawns, 48%
Small-24 prawns, 21%
6.5 lbs-70 prawns Count-11 prawns/lb
Large-25 prawns, 36%
Med- 35 prawns, 50%
Small-10 prawns, 14%
The size of the prawns from both ponds were almost identical because we evenly distributed the feed throughout the pond based on our test of increased yield and size from 2014 SARE’s Grant (FNC14-962 Feeding Freshwater Prawns through Mechanical Means to Increase Yields and Size).
The data from the samples (Count) showed the pond with the substrate had slightly larger prawns in it. To make a pound, it took an average of 11 prawns compared to Count on the other pond of 14 prawns.
As we harvested the ponds, the yield continued to show larger-sized prawns. We also noticed more blue-clawed males being harvested from the second pond. The blue-clawed males are the largest and most territorial. The substrate must have been providing more area for them to grow so large.
The pond with the substrate- 86% were medium or large prawns.
The pond with no substrate- 79% were medium or large prawns.
Cost of Subtrate-
Number of substrate runs-8
The length of the run-100 feet.
The bird netting comes in 100×7 foot lengths.
The height of the netting was cut down from 7 feet to 3.5 feet
effectually doubling each roll.
The cost for the rolls-$14.98 (Lowest).
The posts (lightweight fiberglass electric fence posts from Tractor Supply)- $1.99 each x 70 posts.
Bird netting 4 x $14.98= $59.92
Fiberglass posts 70 X $1.99 = $139.30
Total cost =$199.22
This added surface area from the substrate increased our profit.
Increased profit = $395.88 ($595.10-$199.22)
The unknown factor in this equation is how many seasons will the bird netting last before replacement. The fiberglass posts should have an indefinite life cycle.
At current costs replacing the bird netting annually (conservative replacement), the annual increases in profit could be $535.18.
Impact of Results/Outcomes
May 15 through June 1- Preparations for the delivery of the post larvae prawns. This included installing fence posts in the pond and attaching the substrate to the posts.
June 6, 2015 – 30,000 post larvae prawns were delivered. The count was done and 15,000 were distributed into each pond.
September 11 and 12, 2015- Harvested both ponds.
East Pond (No substrate) -352.4 lbs. West Pond (With substrate)-406.5 lbs.
September 12, 2015 – We had our public harvest selling locally-grown prawns. We had enough for all our customers who came. This is the first year we didn’t sell out early. We were also able to fill our own freezer with several pounds.
September, 2015-This was the second year we sold prawns commercially. Don’s Prawns had a contract with a local seafood distributor for some of the large prawns.
September, 2015 -We had a booth at the Urbana Fish and Shrimp Festival and sold out within four hours after the gates opened even though it rained the entire time.
Educational & Outreach Activities
Before utilizing local FFA students from Fairfield Union High School to help with the harvest; we gave a PowerPoint presentation to the FFA classes. The FFA teachers were kind enough to grant us access for the entire day. The presentation was given to all the classes, approximately 300 students. It was about aquaculture and raising and harvesting freshwater prawns. We were also generating an interest in working the harvest.
Also prior to the harvest, we presented a Question & Answer session hosted by our local Ohio State Extension office at our farm. Questions ranged from “How can I start raising prawns?” “How do you increase yields?” or simply “You’re raising what in Ohio?” We had between 20-40 participants.
After the harvest and the research is complied, it will be presented in multiple venues. One venue will be our website www.donsprawns.com. We will also update information on our Facebook account at facebook.com/donsprawns.
Furthermore, I will ask to present at the Ohio Aquaculture Association (OAA) annual meeting held in January/February. I do not anticipate any issues with this request, since the association is always looking for innovative and sustainable speakers within the industry.
This will be in conjunction with a published overview of the study in the OAA newsletter. We will ask our local Ohio State Extension Agent to post the study on their website
First and foremost the contribution of this grant was the education and first time experience most of our FFA students had with the aquaculture industry.
Our student participants showed interest in the process and our operation. They also asked lots of questions about the harvest. Many asked if it would be possible to return next year to work.
If we could increase yield, we could develop best practices that would benefit our farm and the US industry as a whole. The success of this project could result in a value-added revenue from a sustainable product with a small cost investment. This would lessen the environmental impact of overfishing the oceans by producing a locally grown, aquaculture product. There would also be less disruption of the natural food chain.
Overall, by adding substrate, the result is increased yields and profits for the producers of locally grown fresh water prawns.
Based on the research from this grant, the recommendations would be to place substrate in ponds to produce higher yields and income for growing fresh water prawns. The substrate had a relatively low cost, yet increased the yield by 13% and profits by almost $400.00.
Other recommendations would be to mechanically feed the prawns for larger and more evenly sized fresh water prawns. (2014 SARE Grant FNC14-962 Feeding Freshwater Prawns through Mechanical Means to Increase Yields and Size).
Growing fresh water prawns can be profitable. Using good practices, we are producing a locally grown, sustainable product which can take some pressure off overfishing the oceans.