TILAPIA AND VEGETABLE PRODUCTION

UPDATE ON TILAPIA AND VEGETABLE PRODUCTION IN THE UVI AQUAPONIC SYSTEM

James E. Rakocy, Donald S. Bailey, R. Charlie Shultz and Eric S. Thoman

University of the Virgin Islands Agricultural Experiment Station RR 2, Box 10,000 Kingshill, VI 00850, USA

Abtract

The UVI commercial-scale aquaponic system has produced Nile and red tilapia continuously for 4 years. During that time, two trials have been conducted to evaluate the production of basil and okra. Tilapia were harvested every 6 weeks from one of four 7.8-m3 rearing tanks. Nile and red tilapia were stocked at 77 and 154 fish/rrr', respectively. During the last 20 harvests, production of Nile and red tilapia averaged 61.5 and 70.7 kg/rrr', respectively. Mean harvest weight was 813.8 g for Nile tilapia and 512.5 g for red tilapia. Nile tilapia attained a higher survival rate (98.3%) and a lower red conversion ratio (1.7) than red tilapia (89.9% and 1.8, respectively). Projected annual production is 4.16 mt for Nile tilapia and 4.78 mt for red tilapia. Batch and staggered production of basil in the aquaponic system was compared to field production of basil using a staggered production technique. Annual projected yield of basil is 25.0, 23.4 and 7.7 kg/rrr' for batch, staggered and field production, respectively. Annual projected yield of basil for the aquaponic system is 5.34 mt for batch production and 5.01 mt for staggered production. However, batch production was not sustainable with the current fish output because nutrient deficiencies occurred. The okra trial compared the production from three varieties (Clemson, Annie Oakley and North South) and two planting densities (2.7 and 4.0 plants/nr') in the aquaponic system. One variety (Clemson) was cultivated in a field plot at the low planting density. The highest production (3.04 kg/rrr') was attained by the variety 'North South' at the high density.

Projected annual production of 'North South' is 13.37 kg/rrr' and 2.86 mt per system. Field okra grew slowly and produced only 0.15 kg/rrr'. The aquaponic system performed well over a sustained period of time. Aquaponic production of basil and okra was dramatically higher than field production.

Introduction

Aquaponics is the combined culture of fish and plants in recirculating systems. Nutrients, which are excreted directly by the fish or generated by the microbial breakdown of organic wastes, are absorbed by plants cultured hydroponically (without soil). Fish feed provides most of the nutrients required for plant growth. As the aquaculture effluent flows

through the hydroponic component of the recirculating system, fish waste metabolites are removed by nitrification and direct uptake by the plants, thereby treating the water, which flows back to the fish-rearing component for reuse.

Aquaponics has several advantages over other recirculating aquaculture systems and hydroponic systems that use inorganic nutrient solutions. The hydroponic component serves as a biofilter, and therefore a separate biofilter is not needed as in other recirculating systems. Aquaponic systems have the only biofilter that generates income, which is obtained from the sale of hydroponic produce such as vegetables, herbs and flowers. In the UVI system, which employs raft hydroponics, only calcium, potassium and iron are supplemented. The nutrients provided by the fish would normally be discharged and could contribute to pollution. Removal of nutrients by plants prolongs water use and minimizes discharge. Aquaponic systems require less water quality monitoring than individual recirculating systems for fish or hydroponic plant production. Aquaponics increases profit potential due to free nutrients for plants, lower water requirements, elimination of a separate biofilter, less water quality monitoring and shared costs for operation and infrastructure.

A commercial-scale aquaponic system was developed at the University of the Virgin Islands in St. Croix. The status of the system was reported in the proceedings ofISTA 4 and 5 (Rakocy et al. 1997, Rakocy et al. 2000). The development of the system initially required many design changes. There have been no major changes in system design since ISTA 5. The system has produced tilapia continuously since that time. During the continuous production of tilapia, two short-term trials were conducted to determine the production capacity of basil and okra. This paper will report the highlights of this work.

Methods

The design of the UVI aquaponic system is shown in Figure 1. The water pump, which is located on the left side of the sump, pumps water a short distance to the fish rearing tanks. The flow to individual tanks is regulated by ball valves. As the carrying capacity of a fish- rearing tank is reached, a greater portion of the flow (378 L'min.) is diverted to that tank. Water flows from the fish-rearing tanks through the rest of the system by gravity and returns to the sump, which is the lowest point in the system.

Each fish-rearing tank has 22 air diffusers (14 L'min), which are cleaned weekly. There are four air diffusers in the degassing tank and one in the base addition tank. Each hydroponic tank has 24 air diffusers (10 L'min), which are positioned every 1.2 m in the center of the tank. ........... Continue....