Introduction
Human food production contributes nearly 20% to global greenhouse gas emissions. Diet choices can significantly influence these greenhouse gas emissions. The environmental footprint of aquaculture largely depends on the production method and species cultured. In Europe, rainbow trout Oncorhynchus mykiss is one of the main species farmed while land-based flow-throug systems are the predominant production form. By using the streams natural hydraulic gradient such systems are considered to be relatively energy efficient compared to RAS.
In the recent study the ecological footprint of trout production in the temperate climate zone with respect to global warming potential, eutrophication potential, acidification potential, and ozone layer depletion was investigated. Data was collected under the typical farm framework and a cradle-to-gate life cycle assessment was conducted to find the processes contributing the most to the emissions of a typical trout farm. Two further scenarios were than analyzed, with either differing components in the fish feed or a differing amount of photovoltaic systems used on farm.
Material and methods
To estimate the environmental impact of 1 kg live weight (LW) rainbow trout in a traditional flow-through system four impact categories were analyzed: Global warming potential (GWP), Ozone layer depletion (ODP), Acidification (AP) and Eutrophication potential (EP). The impact categories were chosen due to their direct effect not only on the environment but also on human health. Data, which originated from a typical trout farm in South Germany, was considered with respect to energy and material flows. The model farm rears rainbow trout Oncorhynchus mykiss from egg to market size in three different river systems. ~500tonnes of trout are produced per year.
The chosen impact categories were analyzed with the software program SimaPro 9.1.1.1, using the CML_IA baseline/EU 25 method. Furthermore, the databases Ecoinvent v. 3.6, Agri_footprint v. 5.0, and Agribalyse v. 3.0 were used. To test for uncertainties, a Monte-Carlo simulation was run in SimaPro with 5000 runs to the 95th confidence interval
The model farm uses feed with 35% fishmeal and ~8% of the production site is covered with photovoltaic systems. The FCR of the fed feed is 0.89 (farmer’s calculation)
For the analyses two further models were created. One scenario considering either no fishmeal (No FM) or 61.8% fishmeal (61.8% FM) in the feed, compared to the current model farm (35% FM). Different FCRs were considered for the differing formulations. The other scenario considering either no (No PV) or 100% (100% PV) coverage of the production site with solar panels compared to the current model farm (~8% PV). Here, different amounts of excess energy were considered as well.
Results
The production of 1 kg LW trout is most environmental friendly with a share of 61.8% fishmeal in the feed and a 100% coverage of the production site with solar panels (Table 1). The current model farm (35% FM, ~8% PV) emits around 1.18 kg CO2eq. For the fish feed production itself, the no fishmeal variant emits 1.75 kg CO2eq, while the 61.8% fishmeal variant emits 1.13 kg CO2eq. Main shares of the emissions of feed production come from wheat, soy and fishmeal.
Discussion
The 1.18 kg CO2eq of the recent study are in the lower range of values found in literature of farmed rainbow trout in flow-through system. Compared to terrestrial farming traditional trout production has a comparably low impact on climate. Nonetheless, there is room for improvement. Especially through the use of renewable energy sources and by improvement of feed formulations. Photovoltaic systems can offer the possibility to drastically reduce emissions by the production of excess energy. Additionally, the installation of roofs for PV-systems provides further benefits, such as temperature reduction by shading or the protection against predators. Regarding feed composition fishmeal seems to be the most environmental friendly solution, although this has to be considered carefully. The fish in: fish out ratio decreased within the last years, as fish meal and fish oil are more and more substituted by mainly plant based ingredients. Nevertheless, the increase in aquacultural production lead to a rise in fishmeal use, which can affect wild populations. According to the FAO, more than 50% of marine stocks are fully exploited. Vice versa, the use of some plants as fishmeal replacement can lead to high GHG emissions, mainly related to cultivation, deforestation and land use. Furthermore, some of the mainly used plants in fish feed need to by highly processed to be well digestible for fish.
In summary, it can be stated, that production of Oncorhynchus mykiss in Europe could be furthermore reduced by increasing the use of renewable energy sources such as photovoltaic and by reducing the impact of feed as the main single factor for most impact categories. Here, the use of modern technologies such as genetic manipulation of plants or sustainable insect meal could have the potential to decrease the environmental footprint of fish feed production.