These findings provide valuable information that tilapia farming in net cages in reservoirs may be associated with increased methane emissions, but the impact is restricted to a small area when compared to the total area of ??the reservoir. This is an important fact to be considered and indicates that fish farming has the potential to be one of the animal production activities with the lowest carbon footprint, says Paula Packer, general manager of Embrapa Meio Ambiente, a unit located in Jaguariúna (SP), and one of the authors of the study.
Approximately 75% of the ebullitive (bubble) methane emissions in fish production were recorded in locations with net cages, compared to other surfaces of the reservoir. However, the bubbles were isolated events and did not occur throughout the entire area of ??the net cages. The emission of GHG (greenhouse gases) in reservoirs occurs due to physical-chemical processes resulting from the degradation of organic matter and oxidation of chemical compounds in the water column and sediment. Methane is 34 times more powerful than carbon dioxide in influencing global warming over a one-hundred-year time horizon.
According to Marcelo Gomes da Silva, from CNPq (National Council for Scientific and Technological Development Scholarship Program), little is known about the contribution of fish farming to the processes of GHG emission or removal in tropical reservoirs.
One of the main concerns associated with fish farms is related to the release into the water of considerable amounts of organic matter, originating from unconsumed food and fish excrement, which can be converted into GHGs.
GHGs produced in water bodies are transported to the surface mainly by two mechanisms. In the diffusive flux, gases formed in the sediment and water are transported through the water column to the atmosphere. The ebullitive flux, originating from bubbles formed in the substrate, is released when the water is agitated or when the bubble overcomes the hydrostatic pressure, crossing the water column and being released into the atmosphere.
For a reliable result, it was necessary to study both transports. The diffusive transport, despite being common and occurring throughout the entire reservoir, tends to have lower methane emissions when compared to the emission by bubbles.
The study in the Ilha Solteira reservoir showed that average methane emissions by diffusion were 5 milligrams per square meter per day. On the other hand, the emission of ebullitive methane, which is less frequent, reached bubbles of 5,000 to 10,000 milligrams per square meter per day. Studies abroad hardly evaluate emission by bubbles because this is a characteristic of tropical reservoirs, in areas with warmer waters, with a lot of available organic matter.
To include data in national inventories, researchers from tropical areas suggest considering bubbling emissions. “When we perform methane analysis, using modeling to determine the gas flow that leaves the water column for the atmosphere, or using equipment that only measures the atmospheric air above the reservoir, these methods are unable to capture these bubbles,” says Gomes da Silva.
The great importance of this work is to show that, in addition to increasing greenhouse gas emissions in the form of methane, net cages also greatly increase the ebullition flux, that is, the amount of bubbles and the impact of methane emissions. Hence, it is important to understand the two modes of transport of gases into the atmosphere. According to Silva, it was observed that the increase in methane emissions was associated with the tilapia production area. The increase was approximately six times for diffusive methane and approximately 2,000 times for ebullition emissions, compared to the areas outside the net cages. (With Embrapa)st widely farmed fish in Brazil. For the first time, Brazilian researchers have carried out a very important study for national fish farming. They measured methane emissions in production of this fish in net cages for tropical conditions.
Some of these results were published in the journal Environmental Challenges and announced this week by Embrapa (Brazilian Agricultural Research Corporation). Intensive aquaculture has increased worldwide in recent decades. In Brazil, fish farming in net cages in reservoirs has expanded in recent decades, driven by the presence of dams used in the production of electricity, which provide high-quality water. Brazil is among the countries with the largest aquaculture production, with 612 thousand tons in 2019, according to IBGE (Brazilian Institute of Geography and Statistics).
The study is important because international calculations usually consider temperate climate conditions in which so-called ebullitive emissions (by bubbles) from fish are less frequent. The studies were conducted in three secondary tributaries that flow into the Ilha Solteira reservoir (SP): Formoso, Cancan and Ponte Pensa, suitable for producing 800 tons of tilapia per year, the first, and 3,000 tons per year, the other two tributaries.
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