Article written by: Jason Danaher Ph.D.
A large portion of the fish farmer’s operational cost is allocated to the price of the fish diet. The fish diet is fed to increase growth rates and maintain fish health in high-density aquaculture environments. The farmer needs to make sure he or she is optimizing feed use, reducing costs, and minimizing the environmental impact by not overfeeding. In aquaculture, a low Feed Conversion Ratio (FCR) is particularly desirable to reduce the volume of feed required and thus, reduce production costs and improve farm profitability.
The FCR is a practical method to directly measure the fishes’ efficiency in converting fish diet into fish biomass. The formula for the FCR is:
The FCR is calculated by dividing the total cumulative weight of fish diet provided from stocking to the time of harvest by the total cumulative fish weight gained from time of stocking up to the time of harvest. The numerator and denominator in the formula can be reported as either imperial (pounds) or metric (kilograms) units. Let’s look at a couple of scenarios and go through the calculations.
Example Calculation #1:
This calculation assumes 100% survival over the entire production period. A farmer stocked a tank with 500 fish. The farmer fed 1,000 pounds of fish diet during a 6-month production period. At time of stocking the individual fish each weighed 0.125 pounds for a total initial biomass of 62.5 pounds. At harvest the individual fish weighed 1.6 pounds for a total final biomass of 800 pounds.
The above example reveals 1.36 pounds of fish diet was needed to produce 1.0 pound of fish biomass during the 6-month production period. The ideal FCR will vary depending on the fish species, age of the fish, and production system (i.e. ponds, raceways, recirculating systems); however, a good FCR is generally between 1.0 to 1.75.
It is recommended to check the FCR more frequently (i.e. monthly) when possible; therefore, a monthly FCR is calculated by dividing the total weight of fish diet provided over the entire month and dividing that weight by the difference in fish biomass at the start of the month and end of the month.
Example Calculation #2:
This calculation assumes 100% survival over the entire month. A farmer stocked a tank with 750 fish. The farmer fed 175 pounds of fish diet during the month of November. At the start of November the individual fish each weighed 0.8 pounds for a total initial biomass of 600 pounds. At the end of November the individual fish weighed 0.96 pounds for a total final biomass of 720 pounds.
The above example reveals 1.46 pounds of fish diet was needed to produce 1.0 pound of fish biomass during the month of November. With a proper water quality monitoring program and record keeping program this monthly FCR can be compared to previous FCR values to reveal seasonal variations or to better understand effects of recent management adjustments in feeding strategies.
An FCR can also be used as a planning tool for anticipated business expenses. In the following example a farmer is planning to operate a recirculating aquaculture system and is trying to budget for the annual amount of fish diet. After doing some industry research the farmer anticipates the fish he or she will be growing and the production system technology to grow the fish has yielded an FCR of 1.28 at other farms growing the same fish species, similar process flow, and life support technologies. The farmer wishes to achieve an annual production of 125 tonnes. How much feed can the farmer expect to use and what is the expected cost of the feed?
Example Calculation #3:
How many tons of fish diet should the farmer expect to purchase, if the anticipated FCR is 1.28 and the cost of fish feed is $55 per 50 pound bag?
Tons of fish diet to purchase = 125 tons of annual fish production x 1.28 = 160 tons
The cost of the fish diet is $55 per 50 pound bag. What does the farmer need to budget for fish diet over the anticipated production period?
There are several management strategies to improve feed conversion ratios. The first is to maintain feed quality. The farmer should properly store the feed. Bagged feed should be stored in a cool, humidity free facility void of direct sunlight and rodents. Bagged feed should be used in the order of date purchased and is known as the “first in, first out” protocol. Bulk feed should be stored in bins preventing rain and moisture from contacting the feed pellets; thus, averting mold. Bins should not become overly heated either and should be free of pests. Feed in outside bins should be used within a two-week time period. Bulk feed should also be used in the order it was manufactured and delivered to the facility.
The second strategy a farm manager needs to evaluate and implement is efficient feed presentation. This can be performed either by manually feeding the fish or by mechanically feeding the fish. Manual feeding or hand feeding is fairly simple where farm staff directly feeds the fish by the handful or bucket full. Manual feeding can be accomplished by feeding fish to “apparent satiation” within a specific time period or applying feed based on percent body weight of the total fish biomass. Feeding based on percent body weight requires frequent sampling over the production period. Referencing feed tables, often supplied by feed manufacturers, that give a recommended feed rate expressed as percent bodyweight per day for different size fish is important. Mechanical feeding is accomplished using automatic feeders or demand feeders mounted over tanks or feed blowers mounted to farm vehicles driving around ponds. Mechanical feeding is often based on an assumed percent body weight. Both manual feeding or mechanical feeding require periodic adjustments to the daily feed amount.
Finally, proper feeding requires the appropriate diet formulation is provided for the present age of fish, the proper feed size is given to the fish, feed is disturbed evenly across the surface of the grow unit, and the total feed amount is divided into equal rations and fed at appropriate times during the day. The latter can be precarious in outdoor production systems often affected by environmental conditions such as temperature and diurnal oxygen concentrations.
Many factors can influence growth rates of fish (i.e., population density, water chemistry, water temperature and daily management procedures). Knowing how to calculate an FCR will help the farmer determine if any of those factors are having a significant impact on the efficiency of production and can help the farmer determine if new management strategies or technologies may be needed to keep the farm operating in an economically viable manner since feed price is a major variable cost to farming operations. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com about questions you may have with your feeding protocols.