SEARCHING FOR SOLUTIONS IN AQUACULTURE: AQUAPONICS

Aquaponic production combines intensive production with waste recycling and water conservation. Aquaponic join recirculating aquaculture with hydroponics to use nutrient waste from aquaculture as an input to plant growth. Traditional aquaculture systems treat or dispose nutrient-rich wastewater. In aquaponics, the waste products from the fi sh are converted by a bio-fi lter into soluble nutrients which are absorbed by the plants, and allow “clean” water to be returned back to the fi sh. Th us, it produces valuable fi sh protein with a minimal pollution of fresh water resources, while at the same time producing horticultural crops. Fish in aquaponic production systems can be raised in ponds, tanks, or other containers. Plants are grown separately in hydroponic tanks, submerged in water but suspended in gravel, sand, perlite, or porous plastic films, as well as on floating raft s. Systems vary greatly in design and construction, but most perform the following key functions: finfish and plant production, removal of suspended solids, and bacterial nitrification. Th is review discusses applications, eff ects and perspective of aquaponics.


INTRODUCTION
Aquaponics has been considered as a sustainable agriculture system that amalgamates aquaculture and hydroponics in an enclosed symbiotic environment (Nelson, 2008). Th e word 'aquaponics' is derived from a combination of 'aquaculture' and 'hydroponics' , and refers to the integration of hydroponic plant/vegetable production with aquaculture. It is a bio-integrated system linking recirculating aquaculture with hydroponic production of plants such as vegetables, culinary or medicinal herbs. Aquaponics may provide an eff ective and effi cient means to provide both animal protein (fi sh) and mineral and vitamin sources (fresh vegetables) to populations where water/and or fertilizer resources are limited with a minimum of environmental pollution.
Th e basic principals of aquaponics is that fi sh are fed "waste plant and animal material", which they convert into protein. Th e waste material from the fi sh is then used by plants as the nutrient source, and the water is then recirculated back to the fi sh tank. An essential component of this is a biofi lter (between the fi sh and the plants) which essentially comprises bacteria which converts the waste products from the fi sh into soluble nutrients for the plants. An absolutely critical component of this is the conversion of urea (excreted by the fi sh) into nitrite, and then nitrate because high levels of urea in the water are toxic to fi sh. Th e solid waste (fi sh faeces and unconsumed food) is usually fi ltered off and converted into soluble nutrients in a separate bypass.
Design Aquaponic systems are usually designed as an enclosed recirculating system, but a few systems can be open, depending upon environmental factors. Fish or other aquatic organisms are reared in tanks and excrete nutrientrich waste or effl uents into the water. Metabolic byproducts excreted by fi sh, unionized ammonia NH3-N, ionized ammonia NH4+-N, or combined equal Total Ammonia Nitrogen (TAN) are oxidized and broken down into nitrite (NO2 --N) by nitrifying bacteria of the genera Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio. Genera that oxidize nitrite to nitrate (NO3--N) include Nitrobacter, Nitrococcus, Nitrospira, and Nitrospina (Timmons and Ebeling, 2007). Th ese nitrifying bacteria are also known to be light sensitive (Yoshioka and Saijo, 1984). Mineralization also occurs, releasing essential inorganic nutrients into the water for plant uptake (Timmons and Ebeling 2007). Th ese dissolved nutrients accumulate and reach concentrations equal to hydroponic nutrient solutions (Timmons and Ebeling 2007). Th e water is continuously circulated to hydroponically grown crops that absorb nontoxic nutrients from the water to fulfi ll growth requirements. Th e water is then circulated back to the rearing tanks where the process starts again.
Th ere are multiple aquaponic system designs that have been analyzed and utilized for crop production. Depending upon the system scale there are fi ve main components to an aquaponic system: rearing tank, solids removal, biofi lter, hydroponic subsystem, and sump (Rakocy and Hargreaves, 1993). Some systems are able to eliminate one or two of the components, and scale and primary production focus are the key factors determining the system design. Some aquaponic systems are able to effi ciently operate with the use of hydroponic subsystems acting as a biofi lter. Th is is possible with the aid of media such as hydroton, pea gravel, and expanded shale (Rakocy 1984;McMurtry et al. 1990). Floating raft hydroponics also known as DWC, which utilize polystyrene sheets and net pots for plant support, may also provide adequate biofi ltration provided the hydroponic subsystem is large enough (Rakocy 1995). When utilizing media within hydroponic subsystems, care must be taken to prevent an overload of suspended solids; therefore, media fi lled subsystems are not ideal for commercial scale production (Timmons and Ebeling 2007).
One of the most important components of an aquaponic system is the hydroponic subsystem: media fi lled, NFT, and DWC (Lennard and Leonard 2006). A media fi lled hydroponic subsystem contains a grow bed fi lled with a soilless medium for plant support.
Popular soilless media include hydroton (expanded clay pebbles), gravel, sand, and perlite. Th e NFT system consists of troughs that expose suspended plant roots (net cup) to a thin fi lm of water. DWC is similar to the media fi lled subsystem but instead of using media in the hydroponic bed, a fl oating raft (polystyrene sheets and net cup) supports the plants.
Currently there are two main irrigation methods for hydroponic subsystems, fl ood and drain or continuous fl ow. Flood and drain system uses a siphon to periodically drain water when it reaches a specifi ed level. A continuous fl ow system allows water to constantly run throughout the system (Rakocy et al. 2006). Lennard and Leonard (2006) found that hydroponic subsystem design and water fl ow have a signifi cant eff ect on Green oak lettuce (Lactuca sativa) yield where media>DWC>NFT; NFT systems were 20% less effi cient in nitrate removal. Lastly, producers should realize that diff ering aquaponic or hydroponic methods (system designs) do not alter the genotypic characteristics of plants. Production will not surpass genetic limitations regardless of growing techniques, and plants will reach peak production when optimum requirements are met (nutrient assimilation, light, temperature, etc.).

Fish
Th ere is no real limitation on the types of fi sh which can be used. Today it is common to grow Nile tilapia (Oreochromis niloticus), channel catfi sh (Ictalurus punctatus), rainbow trout (Oncorhynchus mykiss), and various carp species (Cyprinus sp.), in aquaponic systems. Tilapia appear to be one of the most popular species of fi sh reared in aquaponic systems, because the warm temperatures for optimal growth of tilapia are also needed for the growth of plants (Rakocy and McGinty, 1989).

Plants
Common plants that do well in aquaponic systems include various lettuce (Lactuca spp.), tomato (Solanum spp.), spinach, and herb species including sweet basil (Ocimum basilicum), mint, watercress, chives, and most common house plants. Species of plants that have higher nutritional demands and will do well only in heavily stocked, well established aquaponic systems include tomatoes, peppers, cucumbers, beans, peas, and squash, among others (Rakocy, 1999). Many of the fruit vegetables (tomato, pepper, cucumber, melon, etc) appear to require higher levels of nutrients in hydroponics, than the leafy vegetables. Nutrient wastes from tanks are used to fertilize production beds via the water. Th e roots of plants and associated rhizosphere bacteria remove nutrients from the water. Th ese nutrients, generated from the feces of fi sh, algae and decomposing feed, are contaminants that could otherwise increase to toxic levels in the tanks. Instead they act as liquid fertilizer for hydroponically grown plants. In turn, the hydroponic beds function as biofi lters, and the water can be recirculated to the tanks. Bacteria in the gravel and associated with the roots of the plants have a critical role to play in the cycling of nutrients; without these organisms, the system would stop functioning (Diver, 2006).
Aquaponic plants are subject to many of the same pests and diseases that aff ect fi eld crops, although they seem to be less susceptible to attack from soilborne pests and diseases. Because plants may absorb and concentrate therapeutic agents used to treat parasites and infectious diseases of fi sh, these products cannot be used in aquaponic systems.Even the common practice of adding salt to treat parasitic diseases of fi sh or to reduce nitrate toxicity would be deadly to plants. Instead, non-chemical methods are used, ie., biological control (resistant cultivars, predators, antagonistic organisms), barriers, traps, manipulation of he environment, etc.). It also seems that plants in aquaponic systems may be more resistant to diseases that aff ect those in hydroponic systems. Th is resistance may be due to the presence of some organic matter in the water, creating a stable, ecologically balanced growing environment with a wide diversity of microorganisms, some of which are antagonistic to pathogens that aff ect the roots of plants (Rakocy, 1999).

CONCLUSIONS
Aquaponic system is advantageous compared to other agriculture production systems, and has become very popular today (Rakocy et al. 2006). Since aquaponic systems are designed as enclosed recirculating systems, their agricultural waste and environmental footprints decrease, compared to conventional agriculture practices. Furthermore, utilization of plants as a secondary crop reduces the pollution load (waste concentration) through nutrient uptake and assimilation (Timmons and Ebeling 2007). Nitrate accumulation has been shown to be reduced by 97% within aquaponic systems compared to regular recirculating aquaculture systems (RAS) (Lennard 2006). Since water within systems is recirculated, the quantity of water needed to run the system is minute compared to most fi sh and crop production systems. On average, 98% of the water in aquaponic systems is recycled for the duration of operation (Al-Hafedh et al. 2008). Th e periodical input of water is only necessary when too much water has evaporated from the system. Aquaponic systems decrease the amount of space needed to produce two crops at once. Th is allows plants and fi sh to be raised together within a relatively small environment.
Aquaponics can range from an in-home counter top system to large scale commercial systems. Additionally aquaponics on average utilizes less than 1% of land compared to conventional agriculture systems. Along with space, aquaponic systems use fewer resources than average crop and fi sh production systems due to symbiotic relationships (Treadwell et al. 2010). For example, aquaponics utilizes 90-99% less water than conventional agriculture systems. Also, carbon dioxide (CO 2 ) from fi sh rearing tanks can also be used to increase crop production within an indoor facility (Timmons and Ebeling, 2007). Furthermore, aquaponic systems can be deployed in various environments allowing for year round crop production, and potentially a closer farmer-toconsumer interaction. Lastly, successful aquaponic systems utilize secondary crops that are of economic importance or benefi cial to the aquatic organisms being produced (Timmons and Ebeling, 2007).
As with all food production systems, there are a few disadvantages with aquaponic systems. First, the ratio of hydroponic growing area compared to fi sh rearing surface area is relatively large. Ratios have been used ranging from 1:1 to 10:1, which are dependent upon the scale of the system, primary species of focus, and space. Another disadvantage includes the labor involved with plant management. Th e majority of aquaculturists do not have horticulture experience or knowledge, so additional personnel is oft en needed. Furthermore, due to the close relationship between fi sh and plants within an aquaponic system, poor management practices can easily aff ect the sensitive system. Pesticides cannot be utilized within systems and thus, biological control or natural methods must be used to eliminate plant pests (Timmons and Ebeling 2007). When entering into a competitive market, aquaponic producers should evaluate competitors and their species of production. It has been stated that hydroponics can produce heads of lettuce cheaper than what aquaponic systems can produce (Ako and Baker 2009). Lastly, materials utilized for aqua-ponic production (hydroton, fi sh feed, etc) are not considered sustainable. For example, hydroton (clay) is mined from the earth, and fi sh feed may come from wild caught fi sheries or commodity crops. Th ese materials utilize nonrenewable resources for production and may also contribute to environmental pollutants.

ACKNOWLEDGEMENT
Th is study was fi nancially supported by a grant from the Ministry of Education and Science of the Republic of Serbia, under the project TR31075, TR-31011.