Nutrient dynamics in integrated aquaculture–hydroponics systems

  Feed, Nutrients

Nutrient dynamics in integrated aquaculture–hydroponics systems

https://doi.org/10.1016/S0044-8486(97)00168-3Get rights and content

Abstract

Changes in concentrations of dissolved Ca, Cu, Fe, K, Mg, Mn, NO3-N, Na, P, and Zn, as a function of increasing biomass of Nile tilapia (Oreochromis niloticus) fed a commercial diet in integrated aquaculture–hydroponic systems growing romaine lettuce (Lactuca sativa longifolia cv. Jericho), were monitored over three, 28-day experimental trials. The integrated systems, 320–330 l in volume, were filled initially with a complete nutrient hydroponic solution, and were operated at a 100% recirculation rate. Treatment levels were 0 (control) 151, 377, 902, and 1804 g of fish/system and treatments were conducted in duplicate. Nutrient concentrations and mutual ratios of nutrients quickly departed from initial conditions because the relative proportion of dissolved nutrients excreted by fish and subsequently absorbed by plants differed. The removal of nutrients by lettuce, fish, and solids collection was determined as a function of treatment size. The objective of this study was to quantify both the flow of nutrients through representative integrated aquaculture–hydroponics systems and the effects of different quantities of feed nutrient input on changes in nutrient-specific concentrations.

Introduction

The integration of aquaculture and the hydroponic cultivation of plants has been examined repeatedly over the past three decades with a wide variety of system designs, plant and aquatic animal species, and experimental protocols (Rakocy and Hargreaves, 1993). Closed, recirculating systems appear to be the most appropriate aquaculture systems for integration with hydroponics (hereafter referred to as `integrated systems’) because nutrients can be maintained at concentrations sufficient for hydroponic plant culture (Nair et al., 1985). Although recirculating systems appear to be the most appropriate system type, the prevalence of nutritional deficiencies and yield reduction in plants in prior studies, caused by deficient nutrient solutions and excessive salt accumulation, indicated that optimal hydroponic concentrations cannot be maintained over prolonged periods of time if commercially available diets are used (Van Toever and MacKay, 1981; Sutton and Lewis, 1982; Burgoon and Baum, 1984; Wren, 1984; Fitzsimmons, 1985a, Fitzsimmons, 1985b, Fitzsimmons, 1985c; Nair et al., 1985; Zweig, 1986; Rakocy and Nair, 1987; Rakocy, 1989a, Rakocy, 1989b, Rakocy et al., 1989; Parker et al., 1990; Clarkson and Lane, 1991; Rakocy et al., 1993). As a result, nutrient concentrations must be continuously monitored and nutrient supplementation and water replacement must be used to correct for nutrient deficiencies and salt accumulation, respectively.

For a given integrated system operating at steady state with no additional nutrient supplementation, nutrient concentrations will increase, decrease, or remain constant over time if nutrient production by fish is greater than, less than, or equal to nutrient assimilation by plants and nutrient losses, respectively. The rate of change in nutrient concentration can be influenced by varying the ratios of plants to fish (Rakocy et al., 1989, Rakocy et al., 1993), but since the relative proportions of soluble nutrients made available to the hydroponic plants by fish excretion do not mirror the proportions of nutrients assimilated by normally growing plants, the rates of change in concentration for individual nutrients differ. The disparity in accumulation (or reduction) rates of different nutrients quickly results in suboptimal concentrations and ratios of nutrients, thereby reducing the nutritional adequacy of the solution for plants. Apparently, there does not exist an optimal ratio of plants to fish capable of sustaining nutritionally adequate nutrient solutions if standard fish diets are used.

Manipulating the mineral contents of diets used in integrated systems has been suggested as a means of influencing the rates of accumulation of nutrients and reducing or obviating the need to supplement nutrients artificially (Seawright, 1993). Theoretically, the nutrient content of a diet can be manipulated to make the relative proportions of nutrients excreted by fish more similar to the relative proportions of nutrients assimilated by plants. With such a diet, there would exist an optimal ratio of fish to plants and optimal nutrient concentrations could be maintained over prolonged periods without nutrient supplementation. However, the commercial promise of such a diet would require that the fish fed the diet grow at commercially acceptable rates.

This study was conducted to examine both the flow of specific nutrients through representative integrated systems and the effects of different quantities of dietary nutrient on change in nutrient concentration, so that parameters for a model to predict optimal dietary nutrient inclusion rates could be developed.

Section snippets

System description and treatment determination

The control and integrated systems utilized are depicted in Fig. 1. Integrated systems consisted of both recirculating fish culture and hydroponic subunits and had operational volumes about 325 l. Each system had four hydroponic troughs, each with a different age group of romaine lettuce (Lactuca sativa longifolia cv. Jericho). Control systems consisted of four, interconnected hydroponic troughs with no fish culture subunit. Control system volumes were 242 l. For a more detailed description of

Fish

Mean daily fish growth rates (MDGR) did not differ significantly between treatment levels (Table 2); when average MDGR values were compared by trial rather than by treatment, a significant difference was detected between trials 2 and 3 (Table 2). MDGR values averaged 4.77, 5.13, and 4.43% for trials 1, 2, and 3, respectively. Feed efficiencies (weight gain/weight diet fed×100%) were high and did not differ between treatments or between trials. Average trial feed efficiencies were 100, 112, and,

Acknowledgements

We thank JoAnne Hudson and Erich J. Gauglitz, Jr. of the Northwest Fisheries Science Center (Seattle, WA) for mineral analysis and Douglas Ewing of the botany department for providing greenhouse space, technical advice, and project assistance.

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