Introduction to Cage Aquaculture/Cage Fish Farming:
Today, let us get into details of Fish Cage Culture and Cage Aquaculture Technology.
Fish can be cultured in one of four culture methods ponds, raceways, recirculating systems, cages or in-water closed containment. A cage or net pen is a method that confines the fish in a mesh enclosure. This fact sheet will use the word “cage” to minimize any confusion. Cage culture uses existing surface water resources (ponds, rivers, estuaries, open ocean, etc.), but confines the fish inside some kind of mesh enclosure. The mesh retains the fish, making it easier to feed, observe & harvest them. The mesh allows the water to pass freely between the fish and surrounding water resource, thus maintaining good water quality.
Today cage culture is receiving more attention by both researchers & commercial producers. Factors rising consumption of fish, some declining wild fish stocks, & a poor farm economy have produced a strong interest in fish production in cages. Many small or limited source farmers are looking for alternatives to traditional agricultural crops. Aquaculture appears to be a rapidly expanding industry & one that may offer opportunities even on a small scale. Cage culture offers the farmer a chance to utilize existing water resources which in most cases have only limited use for other purposes.
Cage Aquaculture involves growing fishes in existing water resources while being together with this in a net cage which allows free flow of water. It is an aquaculture making method made of a floating frame, net materials & mooring system (with rope, buoy, anchor, etc.) with a round or square shape floating net to hold & culture a large number of fishes and can be installed in a reservoir, river, lake or sea. Economically speaking, cage culture is a low impact farming practice with high returns & least carbon emission activity. Farming fish in an existing water body removes one of the biggest constraints of fish farming on land, the need for a constant flow of clean, oxygenated water. Cage farms are positioned to utilize natural currents, which provide the fish with oxygen & other appropriate natural conditions while also removing waste.
Origin of Cage Fish Culture:
The origins of cage culture are a small vague. It can be assumed that at the beginning, fishermen may have used the cages as holding arrangement to store the captured fish until they are sent to the market. The first cages which were used for producing fish were developed in Southeast Asia approximately the end of the 19th century. Wood or bamboos were used to construct these ancient cages & the fish were fed by trash fish and food scraps. In 1950s modern cage culture began with the initiation of manufacture of synthetic materials for cage construction. Fish production in cages became highly popular among the small resource farmers who are looking for alternatives to traditional agricultural crops.
Advantages of Cage Aquaculture/Cage Fish Farming:
- Fast expansion of the stock compared to that in ponds.
- Very effective use of fish feeds and Very less manpower requirement.
- 10 to 12 times higher yields than pond culture for comparable inputs and area.
- Monitoring (feeding, sampling, observations, etc.) is through simple.
- Treatment of disease is more simple than that of pond culture.
- It requires less investment because it uses existing water bodies and simple technology & swift return of investment
- There is no need for water replacement
- Issues of pond excavation & dependence on soil characteristics are not there
- Avoids the proximity of agricultural areas reduces the hazards of pesticide contamination
Cage Aquaculture/Cage Fish Farming is important because of:
- Social benefits.
- Economic benefits.
- Environmental benefits or impact.
- Cage Aquaculture takes place in much exploited coastal environments.
- Cage Aquaculture will be an increasing source of fish in the future.
- There is a growing concern for food safety (use of antibiotics, heavy metals).
- Aquaculture is still a minor basis of pollution, but it can be a problem in inshore coastal waters.
At the same time, there is a growing awakening for the environment of our oceans & aquatic areas against pollution.
That pollution comes from:
– Sewage 25%, – Air pollutants 25%,
– Agriculture 20%, – Industrial wastewater 10%,
– Ships 10%, – Oil 5%,
– And litter 5%
What is Cage Fish Culture?
Cage culture is an aquaculture production method where fish are held in floating net pens. Cage Aquaculture of fish utilizes obtainable water resources but encloses the fish in a cage which allows water to pass freely between the fish and the pond permitting water exchange & waste removal into the surrounding water. Cages are used to culture several types of shellfish & finfish species in fresh, brackish and marine waters. Cages in freshwaters are used for food fish culture & for the fry to fingerling rearing.
Potential challenges in Cage Culture:
The feed is critical: Feed must be nutritionally complete & kept fresh. Caged fish will get no natural food & so depend on the manufactured diet for all essential nutrients. Feed must give all necessary proteins, carbohydrates, fats, vitamins, and minerals for maximum growth. Nutrients start to deteriorate quickly when exposed to heat & moisture.
Therefore, food must be stored properly & fed quickly. Fortunately, there are a number of fish feed suppliers in Canada with high-quality feed resulting from years of collaborative research between farmers, feed suppliers, & academic researchers.
Water quality problems: Localized water quality problems, particularly low dissolved oxygen, are a probable outcome of cage culture if cage systems are not properly sited. Much has been learned about sitting cages such that water quality issues are minimized.
Vandalism and poaching: Caged fish are can be a simple target people bent on theft or vandalism. Cages must be placed where access can be controlled & poaching risks reduced. Increasingly, operators are employing electronic security technique as additional protection.
Predation: Predation can be a problem if cages are not constructed properly. Turtles, snakes, otters, raccoons & fish-eating birds will take fish or damage cages unless precautions are taken. In general, though, predation is less of a problem in freshwater versus the marine environment.
Design and Construction of Cage Aquaculture/Cage Fish Farming:
Cage range in size from one to several hundred cubic meters & can be any shape, but rectangular, square or cylindrical shapes are typical. Cage shape does not appear to affect the manufacture with most freshwater species. Cage size depends on the size of the pond, the availability of aeration, & the method of harvesting. Small cages are more easily managed than large cages and generally provide a higher economic return per unit volume.
Cage frame: Cage frame can be constructed from wood, iron, and steel. Different cage materials used for cage frames. Materials normally used are High-Density Poly Ethylene (HDPE), Galvanized iron (GI) pipes and PVC pipes etc. HDPE frames are costly but long-lasting. Cost efficient epoxy coated Galvanized Iron (GI) frames are recommended for Small groups & fishermen. GI frames have a very less life span compared to HDPE frames.
Flotation: Floating cages require a flotation machine to stay at the surface. Flotation can be provided by metal or plastic drums, sealed PVC pipe, and Styrofoam. Floats should be placed around the cage so that it floats regularly with the lid about 30 cm out of the water.
Mesh or netting: Cage mesh made from wire mesh or nylon netting. Plastic netting is durable, semi-rigid, lightweight & less expensive than wire mesh. Nylon mesh is inexpensive, moderately durable, lightweight & easy to handle. Nylon is susceptible to damage from predators such as turtles & crabs. An additional cage of larger mesh & stronger twine may be needed around nylon cages. Mesh size has a significant crash on production. Mesh sizes of Tilapia cages are at least 1.5 cm, but 2.0 cm is preferred. These mesh sizes give adequate open space for good water circulation through the cage to renew the oxygen supply and remove waste. The use of large mesh size requires a larger fingerling size to stop the escape
Cage cover: Cages should be equipped with covers to stop fish losses from jumping or bird predation. Covers are often removed on large nylon cages if the top edges of the cage walls are supporting 30 to 60 cm above the water surface.
Cage Size: It is a fact that costs per unit volume decrease with increasing cage size, within the limits of the materials & construction methods used. CMFRI has developed open sea cages of 6-meter diameter and 15-meter diameter for growing out fish culture & 2-meter diameter HDPE cages for seed rearing. Ideal size for growing out cage is 6 m due to its easy maneuvering & reduced labor. For fingerling, two-meter cages can be used.
Feeding rings: The feeding rings are generally used in smaller cages to retain floating feed and prevent wastage. The rings consist of small-mesh (2 mm or less) screens balanced to a depth of 45cm or more. Feeding rings should enclose only a portion of the surface area since rings surrounding the entire cage perimeter may reduce water movement through the cage. However, feeding rings that are too small will permit the more aggressive fish to control access to the feed. If sinking feed is used, small cages may need a feed tray to minimize loss. These rectangular trays can be prepared of galvanized sheet metal or mesh (2 mm; galvanized or plastic) and are suspended from the cover to a depth of 15 to 45 cm.
Different criteria must be addressed before site choice for cage culture. The first is mainly concerned with the physicochemical conditions like temperature, salinity, oxygen, currents, pollution, algal blooms, water exchange, etc. that verify whether a species can thrive in an environment. Other criteria that must be considered for site selection are weather conditions, shelter, depth, and substrate, etc. Finally legal aspects, access, proximity to hatcheries or fishing harbor, security, economic, social & market considerations etc. are to be taken care.
Species Selection for Cage Culture:
Species selection depends on local interest & market value. For fingerling production, the species selected were primarily Indian major carps and, to some extent, common carp & grass carp. During winter, the fish had contact with natural food sufficient for further growth after stocking in the reservoir. For growing table fish, the species must have high market value, such as freshwater prawns, air-breathing species, and sea bass and, to some extent, carps and Pegasus sp. A small indigenous fish species (SIFS) with high market demand are also suitable.
Cage Maintenance in Aquaculture System:
Regular monitoring of some water quality parameters, including dissolved oxygen, pH & free ammonia, inside cages is necessary. Normally in Indian reservoirs, with the purpose of raising fingerlings in cage culture, water quality is not conducive to good fish health & on very rare occasions with a dense algal bloom, some parameters cause stress for the fish. So, cages should be cleaned with soft coir brush fortnightly to remove biofouling organisms like algae, sponges & debris. Routine checking for loose twine, torn meshes from predators, anchors & sinkers is also necessary.
Although stocking densities should be found by species requirements & operational considerations, the influence of stocking densities on growth and production has been determined empirically. The stocking density depends on the carrying capacity of the cages & the feeding habits of the cultured species. Optimal stocking density varies with species & size of fish.
Feeds and Feed Management in Cage Aquaculture/Cage Fish Farming:
Fresh or frozen trash fish, moist pellet (MP) & floating dry pellets are the common feed for growing fish in cages. Feeding in cages is somewhat easy compared to that in ponds. The ration can be divided into equal portions & supplied at regular intervals. Feeding can be done either by broadcasting and using feeding trays. Feeds must be complete and provide all the necessary proteins, carbohydrates, fats, vitamins and minerals needed for growth & health. Feeds cannot be allowed to deteriorate through storage.
Harvest of fish in cages is less labor intensive compared to the ponds. Cages can be towed to a convenient place & harvest can be carried out. Moreover, based on demand, partial or full harvest can be done. Marketing of fishes in live conditions as a value addition can be done.
Environmental Impacts of Cage Aquaculture:
The success of Cage Aquaculture depends on maintaining good water quality around the fish cages & so it is in the farmer’s best interests to minimize environmental impacts. Some of the environmental issues connected to cage culture are:
- Nutrient enrichment of waters that may lead to increased algal growth & downstream impacts.
- The interactions with wild fish populations.
- Accumulation of uneaten feed & fish excreta under the cage can become an environmental problem, but this can be avoided by selecting a site with excellent water exchange to install the cage.
Normally, the environmental impacts of cage culture can be minimized through:
- The proper site selection.
- Suitable anchoring or mooring systems.
- Using an extruded feed or steady pellet diets avoids leaching of nutrients.
- Keeping feed wastage in low level & higher Food Conversion Ratios.
Dissolved oxygen: Dissolved Oxygen (DO) level and its availability are critical to the health & survival of caged fish. In general, warm water species such as catfish & tilapia need a dissolved oxygen level of 4 mg/l DO (or ppm) or greater to maintain good health & feed conversion. Dissolved Oxygen stage below 3 mg/l can stress fish. If this level goes below two mg/l can increase the mortality of fish.
Temperature: The main physical factor controlling the life of a cold-blooded animal like fish is temperature. It is critical in growth, reproduction & sometimes survival. Each species of fish has an optimum temperature range for growth, as well as upper & lower lethal temperatures.
pH: Uptake and release of CO2 during photosynthesis and respiration effect on pH in a pond & due to this, it fluctuates daily. The lowest level of pH appears at or by dawn whilst the highest is at mid-afternoon. The desirable range of early morning pH for fish production is from 6.5 – 9. Acid death point of the fish is about pH 4 and the alkaline death point is approximately pH 11. Slowed growth of fish, reduced reproduction, & susceptibility to disease increasing can be caused if the pH is not at the optimum range.
Other-Turbidity, Nitrate and Phosphate levels, Alkalinity, & Salinity are also affecting fish culture in cages.
Cage culture management results in optimizing production at minimum cost. The management should be so efficient that the cultured fish should develop at the expected rate with respect to feeding rate and stocking density, minimize losses due to disease and predators, monitor environmental parameters & maintain the efficiency of the technical facilities. Physical protection of cage structures is also of vital importance. The raft and net-cages must be routinely inspected. Necessary repairs & adjustments to anchor rope and net-cages should be carried out without any delay.
Fouling of cage net:
Fouling of cage nets and other structures has been observed on several instances of cage farming. Nets get enclosed with biofoulers. Fouling by mollusks, particularly edible oyster sand barnacles have to be checked before its growth advancement. Algal mats & other periphytons can be removed by the introduction of omnivorous grazers in cages. A fouled net will be heavier, thereby increasing drag and this result in loss of nets & fish.
To avoid or reduce fouling, nets should be changed as and when required, which may vary from two to four weeks depending on the intensity of fouling. During oyster fouling, the net exchange has to be done instantly after the seasonal spat fall. Herbivorous fish such as rabbitfish, pearl spots & scat can be used to control biofoulers, but their application on a large scale needs to be assessed.
Monitoring of fish stock health is essential & early indications can often be observed for changes in behavior, especially during feeding.
Disadvantages of Cage Aquaculture/Cage Fish Farming:
- Feed must be nutritionally complete & kept fresh. Fish simply affected by the external water quality problems example, low oxygen levels.
- Diseases are very common problems in cage culture. The crowding in cages promotes stress & allows disease organisms to spread rapidly. Moreover, wild fish around the cage can transmit diseases to the caged fish.
- Caged fish are unable to get the normal food of their choice, whereas it is readily available to the free fish.
- In public waters, cage culture faces many competing interests & its legal status is not well defined.
- It is very difficult to overwinter warm-water fish in cages. There is usually a high mortality rate because of bacterial & fungal diseases.