Effects of feeding frequency and photoperiod on water quality and crop production in a tilapia–water spinach raft aquaponics system

Highlights

• Water spinach was effective in fish waste water treatment.

• Only 3.3% water used in 4 weeks.

• Water consumption for fish production was 20.0 L kg⁻¹.

• Increase photoperiod and feeding frequency favored water quality.

• Accumulation of nitrogen and phosphorus nutrients slowed down in fish production.

Abstract

A factorial arrangement of 6 treatments, 2 photoperiods for water spinach Ipomoea aquatica (12-h or 24-h light per day) X 3 feeding frequencies for red tilapia Oreochromis sp. (an equal daily ration evenly fed 6, 4 or 2 times at 4-h, 6-h or 12-h interval, respectively) were assigned to 12 tanks. Each tank was an aquaponics system containing fish and raft-supporting plant. Water loss in 4 weeks was 3.3%, due to leaf transpiration mainly and evaporation. Water quality remained safe and stable. No fish died. Overall average weight gain was 43.9% for fish and 169.0% for plant. 24-h light resulted in 2.4% higher fish growth, 12% higher plant growth and lower accumulation of all nitrogen and phosphate species in water than 12-h light. Increased feeding frequency favored stable and good water quality and fastened fish growth and plant growth by as much as 4.9% and 11%, respectively.

Introduction

Aquaculture is the culture of aquatic organisms, commonly referred as animals, in a designated water body. The water needs to be treated whenever toxicants in it have built up beyond animal's safe level. Toxicants such as ammonia and nitrite are derived from decomposition of unconsumed feed and metabolites or waste of the animals. Hydroponics is the culture of aquatic plants in soilless water where nutrients for plant's growth come entirely from a formulated fertilizer. Aquaponics (a portmanteau of the terms aquaculture and hydroponics) integrates aquaculture and hydroponics into a common closed-loop eco-culture where a symbiotic relationship is created in which water and nutrients are recirculated and reused, concomitantly fully utilized and conserved. In aquaponics system, waste organic matters from aquaculture system, which can become toxic to animals, are converted by microbes into soluble nutrients for the plants and simultaneously, hydroponics system has already treated the water and recirculates back to aquaculture system with cleansed and safe water for the animals. Besides its ecological merits, aquaponics system can obtain extra economic advantages: saving cost (input) on water treatment for aquaculture system, saving another cost on formulated fertilizer for hydroponics system and benefit from double outputs, harvest of animal and plant, by a single input, fish feed.

Tilapia is the most commonly used fish in aquaponics systems (Rakocy et al., 2006) for their high availability, fast growing, stress and diseases resistant and easy adaptation to indoor environment (Hussain, 2004). Water spinach or swamp cabbage Ipomoea aquatica is a semiaquatic tropical macrophyte and commonly grown as a leaf vegetable in East and Southeast Asia. It has hollow stems, rooting at the nodes and flourishes naturally in waterways or moist soil. It requires little care to grow and hence low cost and popular in Taiwan. It has been found effective in treating aquaculture waste water (Li and Li, 2009) and eutrophic water with undesirable levels of nitrogen and/or phosphorus (Hu et al., 2008). Nutrients dynamics are quite complex in aquaponics system (Seawright et al., 1998). In such system, feed is the primary source of nutrients which are eventually tied up as the biomass of animal, plant and microbes or stayed free in water. When no discharge, no nutrients are output until the animal and plant are harvested as economic crops. Through microbial decomposition, the insoluble fish metabolite and unconsumed feed are converted into soluble nutrients which then can be absorbed by plant. Therefore, plant growth and production are indirectly related to feeding strategies, fish metabolic condition and microbial activity. While plant removes the soluble nutrients, water is filtered. Consequently, water quality or safe guard of fish growth and production depends highly on the disposal capacity of the plant.

Besides the above factors which affect the nutrient availability for plant and fish, system designs, plant and fish species and other physical factors such as temperature, light sources and photoperiod all add up the managing complexity for a steady state of nutrient flow, which can be essential for the stable and predictable production of fish and plant in aquaponics system. Since photoperiod affects photosynthesis and plant growth, the increase of photoperiod may also increase the removal capacity of nutrients in aquatic macrophyte. Some studies have showed that the growth and productivity of floating aquatic macrophytes are directly related to the intensity and amount of light, so are the absorption rates for nutrients (Gopal, 1987; Urbanc-Bercic and Gaberscik, 1989). Petrucio and Esteves (2000) found that 2 h longer photoperiod a day enabled two aquatic macrophytes to reduce more nutrients from the water. Feeding frequency can affect feed intake of fish, quantity of uneaten feed, feed utilization efficiency and consequently, metabolite and excreta of fish and water quality. In an intensive culture of fingerling walleye Stizostedion vitreum, Phillips et al. (1998) found that higher frequency feeding resulted in higher daily dissolved oxygen (DO) and lower total ammonia nitrogen. Postlarval Ayu Plecoglossus altivelis with higher feeding frequency at lower feeding rate had higher survival and growth (Cho et al., 2003). When fed at 10% body weight daily, newly weaned Australian snapper Pagrus auratus fed 8 times a day had higher growth and lower size heterogeneity than fed 4 and 2 times a day (Tucker et al., 2006). Therefore, in the present study we investigate under a constant nutrient input, namely, the feeding rate, if increasing photoperiod can increase plant production, concomitantly, plant's filtering capacity and nutrient concentration in water and also if increasing feeding frequency can even out through time, fish metabolite and excrete, concomitantly, stabilize water quality and increase fish production.