Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture Diurnal stratification of oxygen in shallow aquaculture ponds in central Europe and recommendations for optimal aeration Martin Oberle a,1 , Sebastian Salomon b , Bruno Ehrmaier b , Peter Richter c , Michael Lebert c , Sebastian M. Strauch c, ⁎ ,1 a Bavarian State Research Center for Agriculture, Institute for Fisheries, Department for Carp Farming, 91315 Hoechstadt a. d. Aisch, Germany b University of Applied Science Weihenstephan-Triesdorf, Department of Environmental Engineering, 91746 Weidenbach/Triesdorf, Germany c Friedrich-Alexander-University Erlangen-Nürnberg, Department of Biology, Cell Biology Division, Gravitational Biology Group, Staudtstr. 5, 91058 Erlangen, Germany ARTICLE INFO Keywords: Aeration Cyprinus carpio Hypoxia Thermal stratification ABSTRACT The common carp (Cyprinus carpio) is an important livestock in regions of Central Europe, where it is bred in shallow, turbid ponds (mean depth 1 m). A frequent problem observed in this form of aquaculture is hypoxia, which confines the fish close to the surface, preventing them from optimally feeding at the bottom. According to information propagated in textbooks and good practice guidebooks and internet documents, the best solution to that is aeration in the early morning hours, when oxygen concentration is supposed to be minimal. While we wanted to test the feasibility of photovoltaic power for aeration of ponds, we detected that on days with bright sunshine the lowest oxygen concentration occurs in the afternoon at the bottom layer of a shallow pond, and that oxygen distribution is highly influenced by thermal stratification. This formation of layers of different tem- peratures within the water body inhibits the diffusion of oxygen. A breaking of the stratification effectively increases the oxygen concentration, elevating the overall amount of oxygen in the pond. Consequently, the drop in oxygen concentration at night is less dramatic, and critically low levels of oxygen in deeper zones of the pond are avoided. A comparison between the oxygen distribution and daytime or nighttime aeration showed that on days with bright sunshine it is more effective to force mixing of the entire water column during the day, making the use of a photovoltaic power system feasible. This offers a simple and elegant solution for ponds which are not connected to the power grid. 1. Introduction Fish and fishery products are of enormous importance as protein source for a fast growing human population. The total demand of fishery products in 2016 was about 175 million tons, which is 4 million tons more than in 2015 (FAO, 2016). Due to the overexploitation of natural resources over decades, marine and fresh water aquaculture play an increasing role in meeting market demands: almost half of fishery products on the market originate from aquaculture. From the 171 million tons of fishery products consumed in 2015, 93.5 million tons were provided by capture fisheries and 77.5 million tons by aquaculture (FAO, 2016). Various types of marine and fresh water aquaculture exist, for example sea cages for cultivation of Atlantic salmon (Salmo salar), seawater ponds for production of prawn (e.g. Penaeus monodon, P. merguiensis, and P. japonicus), closed loop circu- lation systems for intensive cultivation of trout (Salmo truta), and shallow fresh water ponds. The latter plays an important role in pro- duction of fish and other commercially relevant species all over the world, such as the giant river prawn (Macrobrachium rosenbergii) or the noble crayfish (Astacus astacus). In Bavaria (Germany), four regions are traditionally commercially important for cultivation of the common carp (Cyprinus carpio), the “Aischgrund” in the region around Höchstadt (Franconia), and the regions Ansbach/Dinkelsbühl, Tirschenreuth, and Schwandorf (Bätzing, 2013). Typical carp ponds of this area are shallow (average depth about 1–1.2 m) and often lack a natural water influx, which means the only replenishment of lost water comes from rain and the surrounding area, thus making an elaborate water management necessary. While the most important fish in these ponds is the common carp, other species are sometimes co-cultivated, such as catfish (Silurus glanis), pike (Esox lucius), tench (Tinca tinca) or pikeperch (Sander lu- cioperca). Besides the production of fish, these areas play an enormous role as habitats for many rare and threatened plants and animals, and https://doi.org/10.1016/j.aquaculture.2018.12.005 Received 29 May 2018; Received in revised form 2 December 2018; Accepted 3 December 2018 ⁎ Corresponding author. E-mail address: sebastian.m.strauch@fau.de (S.M. Strauch). 1 These authors contributed equally to the work. Aquaculture 501 (2019) 482–487 Available online 04 December 2018 0044-8486/ © 2018 Elsevier B.V. All rights reserved. T