Research Article: Effect of stocking density of fish on water quality and growth performance of European Carp and leafy vegetables in a low-tech aquaponic system

Date Published: May 30, 2019

Publisher: Public Library of Science

Author(s): Carmelo Maucieri, Carlo Nicoletto, Giampaolo Zanin, Marco Birolo, Angela Trocino, Paolo Sambo, Maurizio Borin, Gerolamo Xiccato, Jay Richard Stauffer.


Aquaponics (AP) is a semi-closed system of food production that combines aquaculture and hydroponics and represents a new agricultural system integrating producers and consumers. The aim of this study was to test the effect of stocking densities (APL, 2.5 kg m-3; APH, 4.6 kg m-3) on water quality, growth performance of the European Carp (Cyprinus carpio L.), and yield of leafy vegetables (catalogna, lettuce, and Swiss Chard) in a low-technology AP pilot system compared to a hydroponic cultivation. The AP daily consumption of water due to evapotranspiration was not different among treatments with an average value of 8.2 L d-1, equal to 1.37% of the total water content of the system. Dissolved oxygen was significantly (p < 0.05) different among treatments with the lowest median value recorded with the highest stocking density of fish (5.6 mg L-1) and the highest median value in the hydroponic control (8.7 mg L-1). Marketable yield of the vegetables was significantly different among treatments with the highest production in the hydroponic control for catalogna (1.2 kg m-2) and in the APL treatment for Swiss Chard (5.3 kg m-2). The yield of lettuce did not differ significantly between hydroponic control and APL system (4.0 kg m-2 on average). The lowest production of vegetables was obtained in the APH system. The final weight (515 g vs. 413 g for APL and APH, respectively), specific growth rate (0.79% d-1 vs. 0.68% d-1), and feed conversion (1.55 vs. 1.86) of European Carp decreased when stocking density increased, whereas total yield of biomass was higher in the APH system (4.45 kg m-3 vs. 6.88 kg m-3). A low mortality (3% on average) was observed in both AP treatments. Overall, the results showed that a low initial stocking density at 2.5 kg m-3 improved the production of European Carp and of leafy vegetables by maintaining a better water quality in the tested AP system.

Partial Text

Aquaponics (AP), the integrated multi-trophic production of fish and plants in a semi-closed synergetic recirculating system, is one of the newest sustainable systems of food production [1–4]. In AP systems, the biological wastes excreted by fish (e.g., ammonia, salts) and those generated from the microbial breakdown of feed for fish (i.e., nitrite and nitrate) are absorbed by plants as nutrients for growth. Thus, this method allows the removal of undesirable nutrient wastes from the water by plants and the reuse of the water for fish [5]. Indeed, in AP systems, the majority (> 50%) of the nutrients sustaining the optimal growth of plants is derived from the waste originating from feeding the aquatic organisms [6]. Owing to its integrative character and multiple application (ranging from high- to low-technology systems) scenarios, AP is an atypical and complex production system for food [7]. As reviewed by Goddek et al. [2], AP can be considered a sustainable system of agricultural production, i.e., agricultural practices that do not undermine our future capacity to engage in agriculture and those that reduce the inefficiencies of the production process by designing systems that close cycles of nutrients, which is one of the main aspects of an AP [2]. This technique can play a crucial role in the environmental and socio-economic sustainability of smart cities of the future [8]. Finally, AP has been classified as one of the “ten technologies which could change our lives” by the European Union (EU) Parliament [9].

The experimental system was located inside a plastic greenhouse that was 50% shaded at the experimental farm of the University of Padova, North-East Italy (45°20′N, 11°57′E, 6 m a.s.l.). It consisted of nine independent units (Fig 1) divided as follows: three hydroponic units (HP), three AP units with low stocking density of fish (APL), and three AP units with high stocking density (APH). The AP units were designed as “low-technology systems” because they were characterized by: 1) the simplest hydroponic section with the capacity to act as biofilter; 2) the absence of energy to regulate water temperature; 3) the absence of probes for the continuous evaluation of water quality; 4) the absence of probes and systems for remote management; and 5) the absence of devices for water sanitation (UV, ozone).

The stocking density of European Carp influenced the yield of the tested AP system with better results, in terms of water quality and production of vegetables, achieved with an initial stocking density of 2.5 kg m-3. Moreover, growth and feed conversion of fish were negatively influenced by stocking density, but total biomass yield increased with increasing density of fish. Considering the species-specific response of fish to different stocking densities, our findings are reliable for European Carp, and further investigations are needed to establish the more suitable stocking densities for other species raised in AP. Good production of vegetables, performance and optimum health of fish suggest that the proposed low-tech system could be successfully implemented in the field on a larger scale at low construction costs.