Marble goby (Oxyeleotris marmorata Bleeker) is a freshwater food fish that commands a high price in Southeast Asia, ranging from a wholesale price of 20–30 US dollars/kg (Lam et al., 2008, Chew et al., 2009, Loo et al., 2013). The fish is conventionally cultured in lakes, rivers, and ponds in countries such as Malaysia, Thailand, and Vietnam (Luong et al., 2005, Chew et al., 2009, Loo et al., 2013). Some attempts to culture the fish in earthen ponds and cages have failed because of the disease problem caused by Aeromonas hydrophila and Lernaea cyprinacea (anchor worm), the slow growth of the fish during juvenile stage, and the high fish mortality rate (Ang Kok, 1980, Cheah et al., 1994, Tng et al., 2008, Nhi et al., 2010, Idris and Amba, 2011). It is thought that these problems are derived from the poor water quality and the lack of appropriate control present in the conventional culture of marble goby. Therefore, it is important to find an alternative culture technique that could rectify these deficiencies in order to ensure better production of the fish.

Recirculating aquaponic system (RAS), a water recirculating system that is designed to produce both fish and plants, can potentially be a good culture technique for the fish as it ensures a good controlled culture condition by providing better control of water quality, reduced water usage, improved waste management and nutrient recycling (Hamlin et al., 2008, Endut et al., 2009, Martins et al., 2010, Lam et al., 2014). In RAS, the biological nutrient wastes excreted by fish (e.g. ammonia) and those generated from the microbial breakdown of fish feed (nitrite, nitrate) are absorbed by plants as nutrients for growth, and thus this method allows the removal of undesirable nutrient wastes from the water by plants and the water can then be reused for fish culture. These could potentially lead to faster growth and higher production of both the fish and plants. The use of RAS techniques has been reported to be highly efficient as it utilizes the generated fish waste as nutrients for the plants and thus providing a symbiotic environment for producing fish and plants in a closed system (Martins et al., 2010). However, the need for viable methods for protection of plants grown in RAS is currently under development as there has not been a fish-safe insecticide or fungicide developed for use in aquaponics (Pilinszky et al., 2015). There is also a need to design the RAS to maintain the balance of nutrient production and uptake in order to ensure effective nutrient removal (Buzby and Lin, 2014).

Owing to the tremendous market demand for marble goby and the limitations and poor fish yield shown by conventional culture methods, it was thought useful to investigate the development of a RAS for efficient control of water quality and improved fish production from the culture of the fish. The distinct advantages shown by RAS may overcome the poor growth and disease problem shown by conventional cage and pond culture systems and lead to the potential for the greater production of the fish. So far, limited information is available on the characteristics of the culture of marble goby. Studies on the application of RAS have been reported in the culture of other species, such as seabass (Franco-Nava et al., 2004), African catfish (Endut et al., 2010) and carp (Martins et al., 2009), however, no similar studies have been reported on marble goby.

In this study, marble goby was cultured in a fish rearing tank treated by a RAS containing a hydroponic tank grown with leaf vegetable (i.e. water spinach, Ipomoea aquatica). The influence of the dimension ratio between the hydroponic tank and the fish rearing tank (termed “component ratio”, i.e. hydroponic tank volume to rearing tank volume) on the fish growth, vegetable yield, and nutrient removal was investigated; ammonia-N (TAN), nitrite-N (NO₂–N), nitrate-N (NO₃–N), total-N (TN), 5-day biochemical oxygen demand (BOD₅), total suspended solid (TSS), and total phosphorus (TP) were determined and assessed for this study. The component ratio is an important process parameter to study as it could have influence on the rate and extent of nutrient accumulation and removal in the fish rearing tank, and also the nutrient supplementation to the plants in the hydroponic tank; these two factors could have effects on the production of both the fish and the plants in the RAS. These evaluations are important to assess the technical feasibility of using RAS as an alternative method for the culture of the fish. So far, there has been little research reported on the influence of the component ratio used in RAS, and existing literature is limited to mostly studies performed on the use of RAS for the culture of tilapia, catfish, tomato, and pak choi (Brassica campestris L. subsp. chinensis) (Palm et al., 2014, Hu et al., 2015).