In recent decades several factors have changed the practices of aquaculture drastically to change it from small scale homestead activities to large scale commercial farming. The decrease in consumption of meat and egg and increase in consumption of fish and legume per capita from one side and increased regulatory pressure focusing on the discharge to natural water bodies are forcing aquaculture industry for more innovative approaches. One of the growing fields in aquaculture both in terms of research and commercial activities is Recirculatory Aquaculture System (RAS). But there are big challenges that RAS is facing like, limitations in quality and quantity of water, availability and cost of land and limitations on water discharge and natural environmental impacts. Besides these another major challenge for RAS is increasing costs of energy for heating and pumping.
This paper will present an overview of some basic research areas like manipulation of species adapted to cold water like arctic char, mutualism of which with phytoplankton like Tilapia and Chlorella and the challenges that RAS is facing in internal treatments of contaminants. RAS is using the knowledge of ecosystems in internal treatments of contaminants by manipulating nitrifying bacteria to make RAS a sustainable aquaculture practice by increasing in production but with lesser costs and more environmental friendlier practices.
Table of Contents
1. Recirculatory Aquaculture System (RAS)
2. Arctic char
3. Manipulating mutualism between West African Tilapia and Chlorella in warm water RAS
4. Biological filters
5. References
1. Recirculatory Aquaculture System (RAS):
RAS is a set of approaching in fish farming which use a closed-loop system that retain, treat and reuse the water. The main objective is to preserve the water the water quality and minimize the nutrient pollutants. RAS is mainly composed of a growing tank, Tank of particulate removal device, biofilter, oxygen injection with U-tube aeration and water circulating pump. The water start flowing from growing tank and after passing treatment components return back to the growing tank and due to this circulation of water this closed-loop, it is called Recirculatory Aquaculture System. To get more from this system some RAS, grow herbs and vegetables in nutrient rich water. Vegetables uptake of nutrients from water not only helps in their rapid growth but also in filtration of water. This practice is called aquaponics as contrasting to hydroponics.
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2. Arctic char:
The natural home of Arctic char is Scandinavian countries and it is fished in Siberia, Iceland, Greenland, Norway and northern Canada. In northern countries listed above the wild char is fished and there are also extensive aquacultures of char in these countries. Arctic char is well known for its adaptation of cold water and it is the only fish species in the Lake Hazen that is commonly thought of as the Northern most Lake of Canada, though smaller lakes are present further north. The Arctic Char live both in fresh water and sea water. Based on living habitats, Arctic Char is of two basic types. First type lives in fresh water in winter and migrates to sea in summer to feed. The second types is live totally in fresh water and do not migrate. Arctic Char store food during summer feeding and use these stored fat by feeding very less during winter.
The farming of Arctic Char is dominated the fresh water production of fish in Nordic countries. There are several factors that make char a preferred fish in cold water regions. Char is considered as the most cold adapted species within Salmonid family (Johnson, 1980). Char have excellent fillet yield, tolerate high density culture conditions, amenable to niche marketing and are suitable for production within super-intensive Recirculatory system (Jobling, 1987, Johnston, 2002, Summerfelt et al., 2004 a, b.)
Despite the mentioned characteristics of char, the seasonal changes in growth rates of char are a real concern in farming of char. In autumn the growth rate of wild char drops (It is known as autumn depression), so it is a problem for those char farmers who catch wild char and farm them to marketable sizes. In contrast to wild char, hatchery-produced char didn’t show seasonal changes in their growth rates.
To test this observation Sten and his colleagues in Nofima (The Norwegian Institute of Food, Fisheries and Aquaculture Research) has compared the growth performance of wild and hatchery-produced char in a commercial coldwater recirculation system (Villmarksfisk, Bardu, 688N, 198E, Norway). They reared both hatchery-produced char and wild char for 240 days by measuring length and weight of sampled fishes at intervals of 0, 40, 90, 150, 200 and 240 days. The hatchery-produced fishes were taken from Sjoblink Blokken hatchery facility and Wild char is caught from Lake Altevan in Bardu. The initial body mass of char fishes were 115 gram. Two weeks before start of experiments 240 char comprising of 120 wild char and 120 hatchery-produced char were anesthetized and tagged. Similarly, Wild char is weaned to dry feed during two weeks prior to experiments by adding 10% cod roe at day first and gradually reducing to zero at the day 14. The tagged fishes were then randomly distributed among four 2000 L fiberglass tanks, two tanks per group each holding 60 tagged and 240 untagged fish. All the fish were reared under identical condition at a mean temperature of 9.2 8C and on continuous light (150 lx).
The results indicated that there was no much difference till day 40 between wild and hatchery produced char but at day of 90, there was sharp difference between wild and hatchery produced char. The difference between growth rates continues till end of experiments at day 240, in which hatchery produced char gained a weight of 451 as compared to 231 g of wild char. Besides the growth differences the mortality rates are also showed significant differences. The mortality rate in hatchery produced char was 4% as compared to 42% of that of wild char. The result of the this study by Sten clearly show that the superiority of hatchery produced char as compared to wild char both in terms of weight gains and less mortality. Hatchery-produced char would certainly a better choice for cold water RAS.
3. Manipulating mutualism between West African Tilapia and Chlorella in warm water RAS;
Following carp, Tilapia is the second as most important farmed fish in the world. Tilapia is an omnivore, with high growth rate, high resistant to diseases, tolerant to low oxygen levels, live in diverse range of salinities and could be cultured in different density levels. These characteristics along its palatability make it a fish of choice for fish farming. Tilapia is a common name used for many cichlid species of three genera, Oreochromis, Sarotherodon, and Tilapia (Watanabe et al. 2002). Tilapia live mostly in fresh water but due to its tolerance for diverse salinity it shows a diverse range of habitats like, stream, rivers, ponds, lakes and estuaries. Two disadvantage of Tilapia are warm water habitat and self-reproducing populations. As Tilapia can’t survive in cold waters below 16 degree Celsius so it is a limiting factor for its farming in temperate areas. Tilapia’s juveniles matures at six months but spawning occurs year-around. Males grow faster and larger in sizes as females use energy for reproduction and mature earlier. Due year-round spawning and high tolerance they become invasive species in warm waters.
Although Tilapia is an omnivore fish and can adapt to any food available but it mainly feed on the phytoplankton and benthic alga. In closed fish farming like RAS the removal of ammonia excreted by fish is a challenge. On other hand availability of CO2 in Chlorella algae is limiting factor for it. Phytoplankton based RAS is one of the current eco-technological solution. In such a RAS, Tilapia fish could be benefited by feeding on Chlorella as well as by removal of ammonia and oxygenation of water by their photosynthesis. Similarly, Chlorella could be benefited from Co2 produced by Tilapia’s respiration as well as cleaning its environments by their filter feeding habits.
To analyze the practical advantage this mutualism in warm water RAS, Sylvain Gilles and his colleagues at IRD, Senegal, conducted an experiment. The experiment duration was 24 hours, in which 36 tubs filled with filtered water (to remove rotifers) from a phytoplankton pond is used. The tubs were arranged in two rows in a greenhouse to avoid dilution and contamination by rainwater and other falling particles.
Fishless Unfed Fishes Fed Fishes
A graphical presentation of the arrangements of tubs and treatments of the tubs with fish and Chlorella
A 3-level trilapia treatment (C, fishless control: F, Unfed fish and Ff, Fed fish) with four-level Chlorella treatment (With 10%, 50%, 80% and 100% of routine algal density in the prototype “phytoplankton RAS or recycle pond”) was conducted. To achieve initial Chlorella density, prototype water (Salinity 15 g/L, 100% algal density = 33 x 10 6 cells mL -1 ) was mixed with a water from a well dug 30 m from ocean shoreline (Same salinity but no phytoplankton).
The 24 female tilapia which were starved for 24 hours, prior to start of experiment were distributed in 24 of 36 tubs. Only fed fish got a single 2 gram ration in pellets of 1.5 mm. The phytoplankton were sampled successively just in half hour of start of experiment (8:30), in 2 hours (10:00), in 6 hours (14:00), in 12 hours (20:00) and in 24 hours (8:00, next morning). The samples collected in first half an hour of were experiment was taken as a mean density of chlorella. Samples were collected at mid depth with 30-mL container and phytoplankton immediately fixed by adding a 3 drops of 4% of formaldehyde.
Chlorella concentrations were determined by colorimetry with a Hanna C203 photometer and counts were performed using a Burker cell under an OLYMPUS CX41 stereomicroscope (40X magnification).
The result of experiment showed that the density of Chlorella decreased in fishless tubs at lowest and highest initial Chlorella densities respectively. Compared to fishless tubs, the tubs with unfed fish shown an increase in Chlorella density. Similarly, fed fish tubs showed more increase in density than unfed fish tubs. Beside an increase in algal density from fishless to unfed fish and fed fish tubs, the initial density of Chlorella is also affected the resulting Chlorella. Higher initial densities resulted in higher Algal densities. Researchers in this experiment concluded that this mutualism between Tilapia not only can be manipulated in mass production in photosynthetic suspended-growth systems but can also be used for treatment of wastewater.
4. Biological filters:
The wastes in effluent are a major problem in aquaculture. Wu for example estimated that 85% of phosphorus, 80-88% carbon and 52 -95% of nitrogen of mass feed will end up as particulate matter, dissolved chemicals and gases (Wu, 1995). On other hand RAS technology can reduce the effluent waste stream by a factor of 500-1000 (Chen et al., 1997: Timmons et al., 2001). RAS technologies use biofilters to treat internally the dissolved ammonia and other dissolved organics and greatly reduces the amount of water use as well as discharge from aquaculture operations.
Biofilters are mediums for bacteria to colonize and remove unwanted dissolved gases and organic content. Biofilters are for different purposes like improve air quality, improving water quality and waste water managements. In aquaculture biofilters are used to convert ammonia to nitrites and nitrite into nitrate and oxidize organic matter. For this purpose the bacteria medium requires oxygen to continue nitrification process. In RAS, there are two types of biofilters are used. Fixed biofilms, in which bacteria is provided a fixed substrate to colonize and grow. In contrast, there are also suspensions biofilters in which bacteria is maintained suspended.
Although RAS is an environmental friendly technology but its operational costs are higher than pond or flow through systems so currently it is more feasible for higher priced fishes like fingerlings and bloodstock. Increasing efficiency of biofilters is key in making them more cost competitive. Maria Teresa Gutierrez-Wing and Ronald F. Malone have summarized a few suggestions in their paper that is based partially on ideas come of the discussions that took place at Workshop Design and Selection of Biological Filters for Freshwater and Marine Applications held in Honolulu, Hawaii on 8-11 November 2004, hosted by Oceanic Institute’s Aquaculture Interchange Program under leadership of Dr. Cheng-Sheng Lee. Notable suggestions by authors are listed as,
Though suspended growth systems is less favored to fixed film systems due to poor water quality however to meet economic expectations force the aquaculture community to revalidate the suspended biofilters especially for hardy species or fishes with high levels of tolerance for contaminations and low oxygen levels. Waste water managements have developed more efficient biofilters like MBBR (Moving bed biofilm reactors) and BAF (Biological aerated filters). These biofilters have started to be introduced into RAS and it is needed that the introductions of these new biofilters should be compared and evaluated with currently used biofilters and in RAS. Two factors of marine species make them attractive for RAS. The first is the high demand for reliable supply of marine species as sea bass, sea bream, flatfish and cobia (Schwarz et al., 2004) and their higher market prices make them an attractive niche for RAS. The second is the demand of marine larval systems for ultraoligotrophic category of water (maximum oligotrophic category for fresh water and fingerlings are set 0.3 mg N/L and marine larval systems requires TAN concentration below 0.1 mg/L) and sensitivity to feed sizes in different stages of developments. For example, larvae of the shrimp have three distinctive stages of growth, nauplius, zoea and mysis and each of them requiring different feeding regimes and very high quality water, as they are very sensitive to suspended solids, and bacterial infections.
References
1- Atanabe, W. O., T. M. Losordo, K. Fitzsimmons, and F. Hanley. 2002. Tilapia production systems in the Americas: Technological advances, trends, and challenges. Reviews in Fisheries Science 10:465-498.
2- Alon Singer, Shmuel Parnes, Amit Gross, Amir Sagi, Asher Brenner, A novel approach to denitrification processes in a zero-discharge recirculating system for small-scale urban aquaculture, Pages 72-77
3- FAO. © 2003-2009. Fisheries Topics: Technology. Fisheries technology. In: FAO Fisheries and Aquaculture Department [online]. Rome. Updated 2006 15 09. [Cited 20 October 2009]. http://www.fao.org/fishery/sofia/en
4- Hamlin, HJ; MichaelS, JT; Beaulaton, CM; Graham, WF; Dutt, W; Steinbach, P; Losordo, TM; Schrader, KK; Main, KL. Comparing denitrification rates and carbon sources in commercial scale upflow denitrification biological filters in aquaculture. Aquacultural Engineering. 2008; 38(2): 79-92.
5- Maria Teresa Gutierrez-Wing, Ronald F. Malone, Biological filters in aquaculture: Trends and research directions for freshwater and marine applications, Pages 163-171,
6- Nafsika Karakatsouli, Sofronios E. Papoutsoglou, Georgios Panopoulos, Eustratios S. Papoutsoglou, Stella Chadio, Dimitris Kalogiannis, Effects of light spectrum on growth and stress response of rainbow trout Oncorhynchus mykiss reared under recirculating system conditions, Pages 36-42
7- Steinar Skybakmoen, Sten Ivar Siikavuopio, Bjørn-Steinar Sæther, Coldwater RAS in an Arctic charr farm in Northern Norway, Pages 114-121,
8- Sten Ivar Siikavuopio, Steinar Skybakmoen, Bjørn-Steinar Sæther, Comparative growth study of wild- and hatchery-produced Arctic charr (Salvelinus alpinus L.) in a coldwater recirculation system, Pages 122-126,
9- Steven T. Summerfelt, Mark J. Sharrer, Scott M. Tsukuda, Michael Gearheart, Process requirements for achieving full-flow disinfection of recirculating water using ozonation and UV irradiation, Pages 17-27
10- Sylvain Gilles, Gérard Lacroix, Daniel Corbin, Ngansoumana Bâ, Carla Ibañez Luna, Jacob Nandjui, Allassane Ouattara, Ousséni Ouédraogo, Xavier Lazzaro, Mutualism between euryhaline tilapia Sarotherodon melanotheron heudelotii and Chlorella sp.—Implications for nano-algal production in warmwater phytoplankton-based recirculating systems, Pages 113-121,
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