sustainability Article Evaluation of Pacific Whiteleg Shrimp and Three Halophytic Plants in Marine Aquaponic Systems under Three Salinities Yu-Ting Chu and Paul B. Brown *   Citation: Chu, Y.-T.; Brown, P.B. Evaluation of Pacific Whiteleg Shrimp and Three Halophytic Plants in Marine Aquaponic Systems under Three Salinities. Sustainability 2021, 13, 269. https://doi.org/10.3390/ su13010269 Received: 30 October 2020 Accepted: 28 December 2020 Published: 30 December 2020 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2020 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907, USA; chu141@purdue.edu * Correspondence: pb@purdue.edu; Tel.: +1-765-494-4968 Abstract: The effect of salinity on the growth performance of whiteleg shrimp (Litopenaeus vannamei) and three halophyte plants, red orache (Atriplex hortensis), okahijiki (Salsola komarovii), and minutina (Plantago coronopus), in a marine aquaponic system with biofloc was evaluated in this study. The experiment was conducted for 4 weeks, and the three treatments were 10, 15, or 20 ppt (parts per thousand). The growth performance of the shrimp and the three halophytes were affected by the salinity. Compared to the shrimp reared in 10 ppt, those reared in 15 and 20 ppt had higher final weight, weight gain rate (WGR), and specific growth rate (SGR), and lower feed conversion ratio (FCR). The results from shrimp raised in 15 ppt were 2.0 ± 0.1 g, 89.9 ± 2.2%, 2.3 ± 0.0%, and 1.5 ± 0.0, respectively, and those in 20 ppt were 2.0 ± 0.1 g, 93.9 ± 5.4%, 2.4 ± 0.1%, and 1.4 ± 0.1, respectively. On the other hand, the growth performance and nutrient content in halophyte plants decreased with the increasing salinity. In general, the three halophyte plants had better results in the 10 and 15 ppt treatments than those in 20 ppt. Therefore, the salinity of 15 ppt was suggested as the optimal condition for the integrated cultivation of whiteleg shrimp and the three halophytes in marine aquaponics. Additionally, they are compatible species for the development of marine aquaponics. Keywords: marine aquaponics; Litopenaeus vannamei; halophyte; Atriplex hortensis; Salsola komarovii; Plantago coronopus; biofloc; probiotics; water-pump-less system design; sustainable food production 1. Introduction Aquaponics is a new, rapidly emerging, eco-friendly food production system (FPS) that links recirculating aquaculture systems (RASs) with hydroponics (plant production in water, without soil) [1]. This system combines the merits of both aquaculture and hydroponics, such as reduction in water usage, increased food production per unit area, reuse of wastewater and eliminating environmental pollution [2–4]. Given these merits, aquaponics has been viewed as a promising approach for sustainable food production in the future. However, high electricity demand is a drawback of aquaponic systems [4]. To overcome this shortcoming, airlift, a technology that raises liquids by the pressure of air, is a more electrically efficient and alternative approach to promoting water flow and reducing electricity consumption from water pumps [4]. Moreover, this technology can be beneficial to areas with less power supply. Therefore, the technology was applied to the system design of the present study. On the other hand, most aquaponics systems are freshwater, which is an increasingly limited resource while one of the primary elements for food production. Approximately 70% of the global supply of freshwater is in use in current food production systems [5,6]. Another option for aquaponic FPS would be marine or saline systems. There are thousands of potential fish and invertebrates that could be raised in marine aquaponic systems and numerous high-value plant crops are tolerant of low salinity environments [7]. Currently, marine aquaponics is a relatively new concept compared to freshwater aquaponics and is still in the early stages of development. To date, relatively few species Sustainability 2021, 13, 269. https://doi.org/10.3390/su13010269 https://www.mdpi.com/journal/sustainability