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Starting a business in temperate climate regions of Europe or Northern America requires a larger investment since the systems have to be kept frost-free requiring more electrical energy for plant lighting when operated throughout the year. In Europe, there are two strong horticultural production powerhouses, one in Westland/NL and the other in Almeria, southern Spain. The market concentration is high and contribution margins are slim. As a result, some aquaponic producers presumed that in aquaponics the contribution margin from aquaculture is more interesting than that of horticulture, which is probably why some of the few commercial operators chose to oversize the aquaculture part of the setup. This can lead to technical issues because a larger quantity of nutrients than required by the plant side is being produced in the aquaculture side. The excess process water has to be discarded (Excursion Graber 2016 and Interview Echternacht 2018), putting the sustainability claims of aquaponics in question. Christian Echternacht from ECF reports that the contribution margin of the aquaculture has been overestimated in early calculations, rendering the oversizing of the aquaculture part of the farm counterproductive for the overall profitability of the farm.
Numerous different fish species have been reported to be produced in commercial aquaponics in Europe. Popular species for aquaponics production are tilapia, African catfish, largemouth bass, jade perch, carp and trout. There is no known commercial aquaponics farm currently rearing European catfish, but researchers at the University of Applied Sciences of South Westphalia (Morgenstern et al. 2017) found this species to be suitable for aquaponics production. The selection of fish species is influenced by a large number of different project-specific parameters. Most importantly of course are the market needs, price and distribution options. Within Europe, coastal regions have a traditionally strong market for marine fish with a diverse set of species and products. This creates a marketing challenge for freshwater aquaculture production. Ivo Haenen from Uit je Eigen Stad, Rotterdam, and Ragnheidur Thorarinsdottir from Samraekt Laugarmyri, Iceland, talked about this effect in their interviews. Rotterdam customers are used to a rich and diverse supply of marine fish products, making it difficult to market freshwater Tilapia and African catfish. The marine wild catch tradition is so ingrained into Icelandic culture that the aquaculture aspect of aquaponics is probably not going to be actively promoted in future aquaponics projects.
Tilapia, one of the most commonly used fish species in aquaponics in the USA (Love et al. 2015), is a fish species that is not commonly known in Europe. As the experiences from NerBreen in Spain show, European Tilapia aquaponics producers face a double marketing challenge: Their marketing attention needs to be put not only towards building customer awareness on the benefits of aquaponics production but also on the benefits of this relatively unknown fish species.
The suitability of the selected species for elevated water temperatures is another important factor. Fish are poikilothermic; thus their growth and consequently their production yields speed up with higher water temperatures. But elevated water temperatures require more energy, which, depending on the selected energy source for heating the process water, is connected with higher operational costs. Therefore, the positive effect of higher yields has to be balanced with the elevated costs for heating the water. From this perspective, it is desirable to tap into the potential of residual heat usage from adjacently located power plants or industries. These locations, however attractive and sensible they may be from an economical and ecological point of view, might pose a challenge for the overall marketing of the farm and its products. Industry sites are usually not idyllic and emotionally attractive, and worse still in case of anaerobic sewage plants or similar industries, they might even appear to be repulsive. Consequently, the available locations, and the context the farm can reasonably be placed in, is one factor for species selection.
The influence the different fish species have on plant yield and quality has not yet been completely researched. Knaus and Palm (2017) conducted experiments comparing plant yield in two identical aquaponics systems with identical operating parameters rearing Tilapia and carp and found that plant performance with Tilapia was better than with carp. These results show that there is indeed a difference in fish-plant interaction, but these have not been researched for a wider range of different species. In addition, the potential for fish polyculture, where two or more different fish species are reared in the same aquaculture cycle, has not yet been systematically researched.
One of the important operational factors for the fish selection is juvenile availability. Most of the commercial aquaponics producers buy juveniles from hatcheries. One notable exception is the company Aqua4C in Belgium that produces jade perch juveniles and uses these fish in their aquaponics system. A common recommendation is to select a species with at least two known suppliers with significantly larger capacity than the projected demand for the aquaponics farm. The rationale behind this recommendation is risk mitigation. If the supplier of juveniles experiences production issues and cannot deliver, the whole aquaponics production is in jeopardy.
As with the horticulture side of aquaponics, similarly the aquaculture part of aquaponics faces high technical risks, such as the death of the fish due to electricity outages, as reported by both Ponika from Slovenia and NerBreen in Spain. Ivo Haenen, former operator of the aquaponics system of the Urban Farm "Uit je Eigen Stad" in Rotterdam, reports that the heating system of the initial system setup was not dimensioned appropriately. An unexpected period of cold weather led to lower than tolerable process water temperatures resulting in losses in the aquaculture part of the operation. These kinds of instances have to be attributed to the pioneering character of early commercial aquaponics operations in Europe. The presented cases illustrate why Lohrberg and his team classified aquaponics in the "experimental" category of the seven identified business models of urban agriculture (Lohrberg et al. 2016).