Management Guide 2: Trout Logic
Troutlodge founder, Ed McLeary, is one of the very few individuals who revolutionized fish culture. Trout farming world-wide is different today from...
Trout Logic Guide:
The Importance of Good Record Keeping
Part 2
The world of trout breeding
Introducing Part 2
Introduction
After 75 years, Troutlodge continues to be an innovator in trout breeding and genetics. Our global reach enables us to partner with trout producers and researchers from all over the world. We are happy to bring you Series 2 of Trout Logic where we examine the importance of record keeping; arguable the most important aspect of managing a profitable, successful farm. Without accurate record keeping and data collection and tracking, it is impossible to make informed business decisions. You will also get a feel for the fun stuff in the other chapters of this edition of Trout Logic.
This management guide series covers the following topics:
Record Keeping
Discover the types of records that are useful for tracking and improving a farm’s efficiency and performance.
Record Keeping
Begin with Key Metrics
-
SFR = Specific Feed Rate
-
How many kilograms of feed to offer per kg of biomass per day?
-
FCR = Feed Conversion Rate
-
How well does the feed I give turn into biomass?
-
SGR = Specific Growth Rate
-
What percent increase in body weight do my fish show each day?
This section covers the types of records that are useful for tracking and improving a farm’s efficiency/performance. While every farm has a unique mix of challenges and resources, the data recording system described here should provide a strong foundation to any farm’s data set. Types of data points can be separated into several categories:
- Inputs
- Transfers
- Outputs
- Performance
- Environmental
Inputs
Inputs are characterized as the resources one invests in the production process. In the aquaculture context, the primary inputs are stock (the number of eggs/animals, as well their average weight and associated biomass), feed and any chemicals used.
Inputs - Stock
When eggs (or fish) first arrive on the farm, an input code should be generated that will be associated with the stock throughout the production cycle. An input code should contain as much information as possible about the origin of the eggs or fish, taking a form such as “supplier – date received – incubator/rearing unit”. Additional information, such as the strain or variety of the species being raised should also be collected and either attached to the input code or noted alongside the input for reference later. Assigning an input code to every batch of eggs or fry that enter your facility will allow for proper traceability should any issues arise in production where you must isolate a group of fish that may have encountered a virulent pathogen, been exposed to a feed having manufacturing or storage defects, or received a particular medicinal treatment.
Inputs - Feed
When feed first arrives on a farm from the manufacturer, the following information should be recorded:
-
Feed type (it is best to use a product code supplied by the manufacturer, if available)
-
Arrival date
-
Amount of feed
-
Any associated lot or batch number (there can be multiple lot or batch numbers associated with a single shipment)
As feed is provided to a group the amount, type and batch number of the feed used should be recorded each day per rearing unit. Recording this information enables the calculation of daily feeding rates (as a % of fish body weight) and feed conversion ratios (FCR) as well as proper inventory management and feed traceability.
Economic FCR (EFCR) is a very practical metric, and is calculated as follows:
EFCR = (Total Feed Used During Period) ÷ (Final Biomass – Initial Biomass) or (Total Feed Used During Period) ÷ (Change in Biomass from Start to Finish)
Goals for an EFCR depend on many factors:
- Species
- Feed Type
- Life Stage
- Survival and Timing of Mortality Events
If any health issues arise in the stock that are suspected to be feed-related, having a record of what exact batch of feed was being used for the stock allows for speedier identification of issues in either the manufacturer’s production process, transport conditions or the feed storeroom on site. A feed records sheet can be generated for either a single day’s feedings or for a series of days. Using a format that allows for multiple days of feed records has the advantage of allowing a reviewer to see any trends in appetite (up or down) as well as reducing the total amount of paper records generated.
Keeping track of feed types and amounts used on a farm will help with feed ordering in the future, analysis of performance indicators such as FCR, and inventory management.
Inputs – Chemicals
There are often a variety of chemicals used in the production of trout with purposes ranging from hormone enhancement to parasite removal. Whenever applying or using a chemical, the following information should be recorded (as well as any other information required by the governing body/agency in charge of monitoring the use of the chemical):
- Name of the chemical used
- Amount of the chemical used
- Purpose of use
- Date of use
- Ultimate concentration and duration of use
- Stock (input code, lot number or individual ID tag) to which the substance is applied and what rearing unit the stock occupies at the time
Transfers
Transfer records relate to the movement of stock, feed, or other production inputs from one location to another. Transfers of feed, medicinal products or any other consumptive input can be recorded simply by generating a sheet where one lists the source location, destination, and amount transferred. Transferring stock from one unit to another is a bit more complex, and requires that the following information be recorded:
Outputs
Outputs of a farm are mostly classified in terms of biomass exiting the population in the form of mortality, culling, sale, or harvest. When recording mortality, a simple table of how many individuals expired on a given date per rearing unit is appropriate. Any culling should be recorded with an average weight, number and total biomass of fish culled from the population. Astute farmers will also include in their culling summary a breakdown by percentage of why these fish were culled (type of deformity, poor condition factor, etc.).
Sales and harvests should be well accounted, as they represent the cash inflow to the farm and will be used to extrapolate important metrics that guide production decisions in the future. In addition to the standard data points of average weight, number of fish and biomass sold/harvested, one should record the price per piece or weight (in unit of preference, pounds, or kilograms). Keeping record of biomass, average weight and population harvested or sold will allow calculations at a later date of several important production figures: FCR, SGR, survival, average price, and ultimately the cost to produce a given piece or pound/kilogram of harvestable product. Any harvest that does not remove all the stock in a given unit should result in an adjustment of number of fish and biomass in the remaining population, meaning less feed will be provided to this unit.
| Temp (°C) |
Weight (grams)
0.12
|
Weight (grams)
0.5
|
Weight (grams)
1
|
Weight (grams)
2.5
|
Weight (grams)
5
|
Weight (grams)
10
|
Weight (grams)
15
|
Weight (grams)
20
|
Weight (grams)
30
|
Weight (grams)
40
|
Weight (grams)
50
|
Weight (grams)
60
|
Weight (grams)
80
|
Weight (grams)
100
|
|
7°C
|
2.70 |
2.45 |
2.10 |
2.24 |
2.16 |
2.21 |
1.98 |
1.86 |
1.62 |
1.47 |
1.32 |
1.30 |
1.30 |
1.30 |
| 8°C |
3.12 |
2.82 |
2.40 |
2.54 |
2.40 |
2.52 |
2.34 |
2.19 |
1.89 |
1.68 |
1.47 |
1.40 |
1.40 |
1.40 |
| 9°C |
3.48 |
3.16 |
2.70 |
2.86 |
2.72 |
2.84 |
2.61 |
2.43 |
2.07 |
1.86 |
1.65 |
1.60 |
1.60 |
1.60 |
| 10°C |
3.90 |
3.53 |
3.00 |
3.19 |
3.04 |
3.15 |
2.88 |
2.70 |
2.34 |
2.10 |
1.86 |
1.80 |
1.80 |
1.80 |
| 11°C |
4.32 |
3.90 |
3.30 |
3.51 |
3.36 |
3.47 |
3.15 |
2.97 |
2.61 |
2.34 |
2.07 |
2.00 |
2.00 |
2.00 |
| 12°C |
4.68 |
4.21 |
3.54 |
3.79 |
3.68 |
3.78 |
3.42 |
3.21 |
2.79 |
2.52 |
2.25 |
2.20 |
2.20 |
2.20 |
| 13°C |
5.10 |
4.61 |
3.90 |
4.13 |
3.92 |
4.10 |
3.78 |
3.54 |
3.06 |
2.73 |
2.40 |
2.30 |
2.30 |
2.30 |
| 14°C |
5.46 |
4.92 |
4.14 |
4.41 |
4.24 |
4.41 |
4.05 |
3.78 |
3.24 |
2.91 |
2.58 |
2.50 |
2.50 |
2.50 |
| 15°C |
5.88 |
5.31 |
4.50 |
4.78 |
4.56 |
4.73 |
4.32 |
4.05 |
3.51 |
3.15 |
2.79 |
2.70 |
2.70 |
2.70 |
| 16°C |
6.18 |
5.59 |
4.74 |
5.06 |
4.88 |
5.04 |
4.59 |
4.32 |
3.78 |
3.39 |
3.00 |
2.90 |
2.90 |
2.90 |
| 17°C |
6.60 |
5.98 |
5.10 |
5.43 |
5.20 |
5.36 |
4.86 |
4.56 |
3.96 |
3.57 |
3.18 |
3.10 |
3.10 |
3.10 |
| 18°C |
6.60 |
5.98 |
5.10 |
5.43 |
5.20 |
5.36 |
4.86 |
4.56 |
3.96 |
3.57 |
3.18 |
3.10 |
3.10 |
3.10 |
Table 1. Examples of SFR table. (Remember: these values are percentage.)
A practical scenario
For a tank with 13 degree water, 20,000 fish in it averaging 30 grams - how much feed should you give per day?
- Consulting the SFR chart, the SFR is 3.06%.
- Find the biomass: 20,000 fish x 30 grams = 600,000 grams / 1,000 grams per kilogram = 600 kg.
- The SFR is 3.06%, so daily feed is (3.06/100) x 600 kg = 18.36 kg feed.
If the rate of feed you calculate exceeds safe levels, you need to either reduce the biomass in the tank or lower the feed rate (and lower your growth expectations).
EFCR: A practical scenario
- Say you have a tank with 100 kg of biomass in it.
- Over the next month, you feed out 100 kg of feed.
- Your final biomass is 185 kg. What is the EFCR?
- EFCR = 100 ÷ (185-100) = 1.18
This means for every kg of biomass produced, it took 1.18 kg of feed to generate it.
Performance
While each farm may have indicators that they watch more closely than others, there are a few that are common to all farmers and should be accounted for monthly and yearly. Feed conversion ratio (FCR) is a metric that describes how much feed is required to produce a given pound, kilogram or other unit of weight/mass of product. There are two main types of FCR calculation: Economic FCR and Biological FCR. Economic FCR divides total feed use by current biomass of a group, while Biological FCR divides total feed use by the current biomass and all biomass lost due to mortality, predation, escape and culling. Specific growth rate (SGR) relates the weight gain as a percentage of the average individual in a population over a specified time period. Typically, this percentage gain is evaluated on a per day basis and the SGR can be evaluated over days, weeks, months, or any other unit of time one prefers. It is important to track SGR as it can be compared to benchmark performance either on one’s own farm or against other farms’ performances. Making this comparison allows the producer to determine if stock is underperforming and, if so, what changes could be made in their production process to bring stock up to expectation.
Specific Growth Rate (SGR)
- The Specific Growth Rate is a metric that tells us what percent of an individual’s body weight they gain during a given time period (usually one day).
This contrasts with the Absolute Growth Rate, which says the actual weight the individual gains in one day (2g per day, 10g per day, etc)
- An animal weighs 100g at the start of the day, and the next morning it weighs 102g.
Specific Growth Rate = (102 – 100) ÷ 100 = .02, or 2.0%
Absolute Growth Rate = (102 – 100) ÷ 1 day = 2g per day
Specific Growth Rate: Multiple Days
SGR gets more complicated when you consider multiple days, and the formula becomes:
SGR = ln(Final Weight ÷ Initial Weight) x (100 ÷ # days of growth)
“ln” stands for Natural Logarithm, which you can research if you need help understanding. It is a common function that appears on phone calculators as well as in Excel.
SGR: a practical scenario
- Let’s say a tank of fish grew from a 100g average weight to 150g over 30 days. What is the SGR?
- SGR = ln(150÷100) x (100÷30) = (0.4055) x (3.333) = 1.35
- The ability to calculate this value let’s you determine how quickly your stock grows relative to your expectation.
- Your expectations should be developed from doing this process over time by experimenting with different feeding practices
Interrelationship between SGR, SFR and FCR
SGR = SFR÷FCR
It’s that simple. In practice, this means I can increase my growth rate by either feeding more or by feeding more efficiently
- Watch out! Just because a strain eats more does not mean they are more efficient.
- When assessing what strain or supplier to use, consider both SFR and FCR. If someone shows you data regarding how much more their strain will eat than others, make sure they show you information regarding FCR as well.
- SGR is a great metric because it incorporates both SFR and FCR.
Survival as a metric indicates the percentage of individuals surviving from the original input stock. A producer should track survival throughout the grow-out of a population to determine where the largest challenges to survival occur (typically in the earliest stages of production) so that when changes are made to the production process, they can determine if this change positively or negatively affects survivability.
Environmental
The environment in which stock is grown has a tremendous impact on stock performance. While each site is different and will need to watch some parameters more closely than others, there are several environmental metrics that are common and germane to all aquaculture production. Temperature and dissolved oxygen levels are at the forefront of necessity in environmental monitoring as they have the largest effect on the success of a given population. Records should be kept at least daily for each of these figures in a simple table to be able to track changes over time and adapt production processes when environmental conditions become unfavorable. Several other parameters to monitor include:
- Dissolved gas levels
- Temperature
- PH
- Ammonia
- Nitrites and Nitrates
- Salinity
Figure 1: Relationship between the ammonia/ammonium ratio and pH
Figure 2. Reaction of ammonia excreted into water by fish
Conclusion
Though record keeping is often one of the more mundane activities to perform on a farm, it is highly important to the short- and long-term success of a production operation. Proper record keeping enables the understanding of the enterprise’s productivity, offering insight into how operations can be improved or amended to achieve desirable results. Diligent record-keeping not only maintains the economic health of an operation but also the general health of the animals for which it is responsible. Keeping and monitoring records can help diagnose production issues before they arise to keep everything, and everyone, healthy, happy and thriving.
Grow-out Systems
Find out the categories of grow-out systems that help employ deriving from the resources available to the grower and the regulatory environment of the location.
Grow-Out Systems
There are numerous options for grow-out system design, with decisions on which to employ deriving from the resources available to the grower and the regulatory environment of the location. In general, grow-out systems can be grouped into the following categories:
- Flow-through systems
- Recirculating aquaculture systems (RAS)
- Closed pond systems
- Cage systems
Flow-through systems
Flow-through systems are characterized by the capture of water either through pumping from an aquifer or collection of naturally flowing surface waters and the direction of these waters through rearing units in a single pass. As water quality naturally diminishes during the passage of water through a production system, methods such as falls, settling basins and in some cases mechanical filtration are used to improve the critical parameters of dissolved gas and fish metabolite concentrations. Flow-through systems require less intensive water quality improvement measures by relying on an abundance of fresh water constantly flowing into the system, leaving water re-purification unnecessary. Flow-through systems are often the easiest to manage due to their relative simplicity, requiring only that the infrastructure be maintained, and any screens or inlets be kept free of debris that would hinder flow.
Figure 1. Water source supplying the farm using gravity vs pumping. (Lekang, 2007, pg296)
Historically flow-through systems began as earthen structures, gradually moving toward rearing units of more regular shape (raceways, circular tanks, etc) to allow for easier management and harvest. The style of rearing unit may depend on factors such as local topography, water resources available, and capital on hand to use in new construction.
Rectangular concrete raceways have been a popular design choice in the US and elsewhere due to their relative ease of management. In this design water from a head flume is distributed equally among a series of first-use raceways designated for the stock of smallest average size. As the water moves through the raceway dissolved oxygen levels decrease and metabolite/manure concentrations increase, decreasing water quality and causing most of the stock to occupy the upper two-thirds of the raceway. In many cases the tail screen is set well ahead of the actual end of the raceway to create a quiescent zone for the settling and ultimate removal of any solids generated in the raceway. As water moves from one series to another, some measure of water quality improvement is implemented to regain productive capacity for the subsequent series. A common and simple water quality improvement method is to allow the water to plunge or cascade into the next series of raceways. A direct plunge can entrain air bubbles deeper in the receiving raceway, allowing for oxygen infusion at greater pressure, while installation of a splash plate or other device to diffuse the water over a larger area before entering the receiving raceway can increase the surface area of the water droplets with respect to their volume – allowing for increased diffusion of detrimental gases to the atmosphere and infusion of oxygen into the receiving water. Waters can be used and recharged in this manner through subsequent series of raceways until their productive capacity can no longer be adequately increased, at which point they undergo treatment (settling of biosolids, re-oxygenation, etc.) before being discharged to a receiving water.
The primary advantages of raceways are the ability to view, manage and harvest stock with ease. Unlike in sea cages or earthen impoundments fish can be surveyed for signs of disease, behavior abnormalities, and level of appetite. Should clinical signs of disease be present, an attentive member of the production team can quickly detect them and begin the process of recuperating the stock.
Although they are more popular in the Recirculating Aquaculture Systems of today, circular tanks can also be incorporated into flow-through aquaculture if local circumstances permit or require. Circular tanks have many of the same management advantages as raceways (ease of observing stock, applying therapeutics and harvest) while bringing the benefits of self-cleaning characteristics and increased uniformity of water quality through the unit. Influent water is typically introduced parallel to the tank wall, helping to create a spinning or swirling effect as water moves through the tank and concentrating solids further toward the center drain over time. As water swirls in the tank it will mix with influent water, preventing the development of a water quality gradient and letting the stock spread more evenly throughout the unit.
Recirculating Aquaculture Systems
Recirculating aquaculture systems are managed with the intention of re-using as much water as possible in the production process to avoid discharging any effluent to the environment and to reduce the cost of pumping water continuously. To accomplish this the water used in the facility must be filtered and treated in between uses in rearing units to remove biosolids, destroy pathogens, strip carbon dioxide, add dissolved oxygen and purify any toxic dissolved metabolites. The next few sections will discuss what technologies are applied toward water treatment in RAS.
Figure 2. A centralized re-use system serving several fish tanks
Particulates generated in aquaculture production systems range in size, thus different methods are used for separating out these particulates. Coarse, dense particulates can be effectively removed from a system through physical manipulation of the water (radial flow separation, swirl separation, settling basins) or mechanical filtration (drum filter, parabolic screens, sand filtration). Selection of which system is most appropriate should be undertaken with site-specific considerations in mind.
When coarse waste removal is accomplished, ultraviolet (UV) sterilization can occur. UV sterilizers use intense, constant supply of ultraviolet light directed into water as it passes through a plumbing system to kill any pathogens present in the water. It is important this technology is used only after coarse solids have been removed from the water as any turbidity in the water will limit the penetration of the UV waves through the entire volume of water, wasting the effort of UV sterilization.
Following UV sterilization, dissolved metabolites such as ammonia (NH3/NH4) and nitrite (NO2) need to be converted via nitrification to the far less toxic form of nitrogen in water – nitrate (NO3). Biofiltration is a common technique used to perform this transition, wherein nitrifying bacteria are cultivated in an aerobic environment through which water is constantly passed. The amount of water requiring treatment as well as its nitrogen load will determine the appropriate size of the biofilter and its required footprint in the facility.
Once toxic metabolites have been processed from the effluent water, any remaining harmful dissolved gases such as carbon dioxide and dissolved nitrogen that may remain in the system must be removed, while oxygen must be added. Techniques for stripping water of dissolved gases typically involve pumping to the top of a column packed with media through which the water falls to allow diffusion. The same diffusion effect can be generated through any number of means that cause agitation of the water and thus an increase in the area of the water-air interface, but the packed column remains a simple, passive method that requires a small footprint in the RAS facility.
Closed-pond systems
Closed-pond systems are those that have no constant influent and effluent, and no water treatment systems in place. Closed-pond systems are almost always earthen in nature and typically possess a larger water surface area than other production systems. These systems rely on natural productivity of the water body in many ways, from providing early feeding opportunities to fry/juveniles to allowing phytoplankton to oxygenate the water during the day. Closed-pond systems need less intensive management than recirculating systems, but still require diligence from the grower to keep conditions in balance (most of all during night-time hours).
A fundamental necessity in establishing a productive closed-pond system is the cultivation of a steady, managed plankton population (zooplankton during early rearing, and phytoplankton during all stages). By establishing a well-managed phytoplankton population, the water will maintain strong dissolved oxygen levels during sunshine hours while not overly consuming this oxygen during the night. Closed-pond system growers rely on various aeration technologies during the low-oxygen night, most commonly one of the various paddlewheel designs and others such as compressed air diffusion.
Due to the typically high algal populations in these systems, observation of stock is extremely limited. Disease events are typically only recognized when feeding ceases, mortalities appear on the surface, or fish behavior changes dramatically and drives them to the surface. This inability to observe stock effectively poses a major challenge to fish health management, and due to the typically large surface area of these systems applying therapeutants is in many cases an uneconomical recourse even if a disease is identified early.
Despite these challenges, closed-pond systems offer several advantages to the producer. In well-balanced ponds, much of the water quality management and zooplankton forage production is taken care of by natural processes, meaning a grower will only need to monitor environmental conditions and allow nature to do the work. The generally larger size of these units offers growers the ability to stock them with a number of fingerlings equivalent to the number they plan to harvest, considering mortality losses in the interim. Whereas growers in raceways, tanks, and other systems are often forced to split and transfer their stock to different rearing units as they grow to economize space, closed-pond growers avoid this effort by stocking for harvest, lowering their manpower effort per kilogram harvested.
Cage Systems
Cage system aquaculture, popular for many of the largest and most advanced growers of finfish in the world, is characterized by the raising of stock in net pen cages located in open or near shore waters favorable to the cultural requirements of the stock. Cages offer the opportunity to use the natural flow of an open water body to replace water in the net pen, and as long as site selection is done well growers may benefit from consistently good water quality without the effort of treating or constantly monitoring water quality that RAS and closed-pond system growers face. Provided sites are properly selected, cage system producers can benefit from an ability to use very large cages for production – increasing the production capacity of a farm without having to perform expensive earthwork or other infrastructure installation.
Proper site selection is paramount in the productivity and success of a cage production operation. Growers often look for sites that are deep, maintain predictable and gentle water current patterns, and offer easy access from shore to allow for transport of feed and personnel to the site as well as harvested biomass from the site. Depth of the water body is important to cages in that feces will be deposited far below the stock being grown, and if appreciable current occurs below the cages feces will be deposited over a wide area of the bottom – preventing the accumulation of feces in a concentrated area, which can lead to the development of an anoxic mound of organic material beneath the cages. An appreciable level of predictable current through the site, often as a result of tidal movement in marine environments and wind patterns in freshwater bodies, allows for a steady replenishment of water within the cage.
Stocking Density
Commonly called just ‘Density’, this metric can indicate to a producer relative risk levels to a stock’s production efficiency as low density generally carries lower risk while high density represents a risky situation (relative to that producer’s context).
Stocking Density
Stocking density is a metric often used by producers to relate the proportion of biomass in a production unit relative to the total tank, pond or cage volume. Commonly called just ‘Density’, this metric can indicate to a producer relative risk levels to a stock’s production efficiency as low density generally carries lower risk while high density represents a risky situation (relative to that producer’s context). Regarding stocking density in this way allows the producer to stock units appropriately to avoid mortality, reduced growth rates, and other obstacles to good production performance over time.
A common way to express stocking density is in kilograms of biomass per cubic meter of rearing space. For instance, if a 10 cubic meter unit holds 100 kilograms of biomass then the stocking density would be 100 kilograms per 10 cubic meters = 10 kg/m3. Each type of production environment will have different limits for maximum safe density in each rearing unit, depending on several factors.
Average weight of the stock being raised, temperature and dissolved oxygen, and the water exchange rate will all affect the maximum permissible density. As fish grow, the oxygen required for optimal growth per kilogram of body weight decreases. In other words, one kilogram of trout with an average weight of 2 grams will consume more oxygen during a given period of time than one kilogram of trout weighing 500 grams on average. This is a natural biological effect, and as nothing can be done to remediate this condition the producer needs to take this effect into account when planning densities in production units.
Temperature and dissolved oxygen will in most cases work hand in hand toward limiting or increasing maximum permissible stocking density in any rearing unit. Since trout prefer cooler waters between 10-15 degrees Celsius, they will not survive at similar densities when reared in water that is 20 degrees
Celsius. Not only does increasing temperature increase metabolism, thereby increasing oxygen demand, but the saturation point of dissolved oxygen decreases as temperature rises, limiting the total amount of oxygen available to the fish. Production facilities that experience wide temperature fluctuations through the year will need to take this into account and adjust stocking densities according to the season to avoid exceeding maximum permissible density during the warmer seasons.
Water exchange rate is a factor of total influent flow into a rearing unit and the volume of the unit. Dividing the volume of the unit by the influent flow rate (keeping the volume units the same – if flow is measured in liters, the volume of the unit should be expressed in liters as well, etc.). A higher exchange rate means that low oxygen water is replaced by fresh influent water more frequently, meaning more oxygen is available for respiration and a higher stocking density is possible. For farms that experience variable seasonal flow, care should be taken to monitor stocking density relative to water exchange rate and standards should be established relating maximum stocking density to exchange rate throughout the year.
In addition to concerns related to diminishing water quality, producers may encounter other issues when stocking densities are high. Increasing the number of animals in a given rearing unit increases the ability of pathogens to multiply quickly, should they enter the farm. High stocking densities may also lead to aggressive interactions between animals, such as tail-nipping or other signs of violent behavior. With these considerations in mind, producers should seek to establish maximum stocking densities in all of their rearing units based on experience and knowledge of the changing environment on their farms.
Harvest Techniques
Methods of harvesting generally depend on the culture system, the facility, and the form in which the product is to be marketed.
Harvest Techniques
Methods of harvesting generally depend on the culture system, the facility, and the form in which the product is to be marketed.
The length of the grow-out stage depends on input size, harvest size and specific growth rate (SGR) in relation to fish size. For that reason, when a pond, tank or cage is to be harvested, it is a great advantage if the fish have been size graded. If not, the slaughterhouse must deal with several sizes of fish which must go to the customers in different weight classes. It is also more difficult to estimate precisely the amount of fish in each weight class and therefore to achieve a good price.
Harvesting from cage farms can be a little more complex, depending on the location and size of the cages. One solution is to size grade during the harvest and send fish that are too small back to another production unit. For this, it is advisable to carry out a representative sampling prior to harvesting, to determine the actual size range of the fish in the cage to be harvested. The production manager will then know if it is advisable to harvest that cage or choose another that has less size variance and that better fits the customers’ specifications.
Ponds
Extensive rearing of trout can be done in small lakes or ponds. Before use, ponds should be cleared of natural predators. Ponds can be separated into those for fry production and those for on-growing production; the difference is normally the size of the ponds. Commonly, relatively small ponds are used for brood stock, fry, and juvenile fish. In nursery ponds, when fry have reached the required size they can be removed with small-mesh seines, even ones made of mosquito nets, and the smaller pond dimensions enable easy fishing. As the ponds become larger, controlling the water level and managing the stock becomes more difficult.
Full production ponds are also possible, where spawning, fry production and on-growing all occur, although harvesting can be quite difficult. It is important that the levee is sufficiently wide to carry traffic, for instance for feeding, maintenance or harvesting. Drainable ponds offer the advantages of control over the water level and a more effective harvesting process. To carry out the harvesting the pond must normally be emptied and/or drained several times, otherwise the fish density will be too high during harvesting when the water level is lowered. A seine net may also be used for harvesting.
Pond depth is usually between 0.5 and 2.4m, depending on what the pond is used for. For on-growing fish it is normal to choose a depth sufficient to prevent any light reaching the bottom of the pond. In this way growth of vegetation at the bottom is prevented and harvesting is easier. It is important to have a slope towards the outlet on the bottom to make drainage possible and harvesting easier: this can be in the range 1/1000 to 1/100, with the largest slope in the smallest pond. The length–width ratio of ponds is normally about 2:1, but of course is adapted to the site conditions. If ponds are too wide, harvesting will be more difficult. The shape of watershed ponds depends on the terrain. Harvesting with a seine net is easier if the pond is rectangular. Properly designed fish ponds have special provisions for draining and easy harvesting. Special material must be used at the end of the outlet pipe to prevent erosion. Concrete is usually used here. Concrete can also be used to construct the collecting basin for fish so that fry can be harvested or collected.
There are 4 general classifications for harvesting pond facilities.
- Harvesting without emptying the ponds:
If there is insufficient water to refill the pond and you only need to harvest part of the population, a seine net can be used to capture the fish without emptying the pond. The seine net will be placed initially on the deeper side and will culminate on the shallow side for harvesting.
- Harvesting partially emptied ponds:
If water is a little scarce a siphon can be used to lower the water level in the pond by half. A seine net is used to capture the fish, starting from the deepest area. The smaller fish can be returned to the pond for on-growing.
- Harvesting completely emptied ponds:
If there is good water flow, it is better to empty the pond completely. This ensures that all of the fish will be caught. When the pond is almost empty, use hand nets to catch fish that may be in small pools of water.
- Harvesting fish with a monk (both within and outside the pond)
If a monk is built into the pond, fish can be harvested either in front of the monk inside the pond or outside the pond once they have passed through the monk and the drainpipe.
Figure 1. Concrete monk outlet
Net pens
For harvesting fish in net pens operation is usefull to have a pump near to the cage to direct the fish to tanks located in the land or transport the fish to the coast and from a dock pump the fish to a harvesting processing area.
Vacuum–pressure pump: A vacuum–pressure pump consists of a tank to which inlet and outlet tubes are connected via valves ; a small pump is also attached. This pump can either pressurize the larger tank or withdraw the air from it, causing a partial vacuum. The function of the pump is first to evacuate the tank; then the valve to the inlet pipe is opened and water and fish are sucked into the tank; after this the inlet valve is closed and the tank is pressurized; lastly the outlet valve is opened and the fish are forced out through the outlet tube. The operation is repeated, and a new batch is pumped through. The pump does not deliver water and fish continuously, because it operates in two phases: vacuum and pressure. However, two pumps can be used alternately to obtain more equal delivery of fish and water. A vacuum head of more than 5m H2O is normally avoided to prevent injuries to the fish; use of less than 40% water relative to fish should also be avoided for the same reason. Here the manufacturer’s recommendations must be followed (Lekang, 2007).
Ejector pump: In an ejector pump, a high velocity, high pressure part flow creates a region of low pressure (suck) in the larger main stream. The fish travel with the water in the main stream. When the water flows past the ejector it will go from low to higher pressure. The pump can therefore deliver fish in a continuous flow of water. The pump has no moveable parts that can injure the fish. A suction head that is too high must be avoided; it is better to take a larger part of the lifting head on the pressure side (Lekang, 2007).
Stress in the harvesting
Special care must taken to avoid undue stress to the fish during harvest, since meat quality is known to be affected by stress. This stress response may increase the consumption of glycogen stored in the muscle resulting in an earlier occurrence and shorter duration of rigor mortis after slaughter, reducing flesh quality. Sedatives used before or during harvest (ie, "harvest at rest") can minimize these effects. The use of chemical sedatives is regulated by the Food and Drug Administration of the United States and, sadly, none are approved for harvest at rest. Currently, electro-sedation technology is not subject to the same regulatory restrictions as chemo-sedation, but its effectiveness in the context of harvesting at rest has not been adequately proven. Some scientists have used eugenol treatment at a dose of 10 mg / L, as well as electro-sedation protocols which seem to improve product quality and are perceived as a more humane means of slaughter.
References
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