Emission factor and balance of mercury in fish farms in an artificial reservoir in NE Brazil

Abstract

This paper estimated the mercury (Hg), emission factor, and mass balance from caged fish farming in the Castanhão Reservoir, NE Brazil, based on monitoring of a typical farm of Nile tilapia (Oreochromis niloticus). The total Hg input to the farm reached 1.45 gHg ha⁻¹ year⁻¹, from which 0.21 gHg ha⁻¹ year⁻¹ was exported out as fish biomass, ultimately resulting in an emission factor of 1.24 gHg ha⁻¹ year⁻¹ for the reservoir or approximately 8.27 mgHg ton fish⁻¹ year⁻¹ produced. Most of the input came from aquafeeds with concentrations varying from 1.4 to 31.1 ng g⁻¹, depending on the type of aquafeed. The Hg concentrations in fish were very low and varied from 1.0 to 2.9 ng g⁻¹. These values are two orders of magnitude lower than the legal limit for human consumption. The estimated total annual discharge of Hg from farming into the reservoir is 174 g for 18,000 tons of fish produced and may reach 387 g when the reservoir reaches its total capacity (40,000 tons), which is expected to occur in 2020. The mass balance, considering the deposition and accumulation rates, showed that approximately 40 % of the total Hg input accumulate in farm sediments (0.72 g ha⁻¹ year⁻¹), which is approximately 60 % of the deposition rate estimated through the sediment traps and suggests that 0.54 gHg ha⁻¹ year⁻¹ could eventually be transported out of the farm to the reservoir. Notwithstanding these facts, the total annual input of Hg from fish farming to the Castanhão Reservoir is less than 1.0 % of the total input from anthropogenic sources.