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.

Limnology of the largest multi-use artificial reservoir in NE Brazil: The Castanhão Reservoir, Ceará State

“…After, however, differences between surface and bottom water temperatures were well below 1.0°C. At the Castanhão reservoir, the monitoring of current velocities and directions obtained through the deployment of an ADCP Oliveira et al 2015) showed the highest current velocities at the surface and a decrease towards the reservoir bed. This supports the relatively small remobilization of bottom sediment and the permanence of the water column stratification.…”

“…During the 2010-2014 period, total tilapia production from the Castanhão reservoir averaged 18,000 tons per year, but considering the total potentially available area on the reservoir legally able to harbor fish cages, production figures could reach 40,000 tons per year. A major environmental aspect of intensive fish aquaculture practiced in the Castanhão reservoir is the intensive use of aquafeed to sustain high production figures of about 150 t.ha -1 .yr -1 ; to reach such a production, with a conversion rate of 1.7, about 258 t.ha -1 .yr -1 of aquafeeds are necessary (Oliveira et al 2015). Excess aquafeeds and fish excreta result in this activity displaying the largest emission factors for nitrogen and mercury and the second largest for phosphorus, among all anthropogenic sources emitting these substances to the Castanhão reservoir (Table III).…”

“…Reduction of the reservoir volume results in nutrient accumulation and concentration, leading to increasing algal density and the frequency of cyanobacteria blooms, thus rendering these systems much more vulnerable to eutrophication (Molisani et al 2010). The recent development of intensive fish cage aquaculture in many reservoirs in the Brazilian semiarid and the expansion of irrigated agriculture, additionally contribute to a further deterioration of water quality, in particular during extended drought periods (Oliveira et al 2015). To a certain extent, however, once normal rainfall reestablishes, lakes and reservoirs may return to their typical trophic state, depending on the extension of the interval between two consecutive drought periods.…”

“…Cage fish farming have become the main activity of this reservoir with projected yield of 50,000 ton/year when the legal 1% limit of reservoir area will be used for aquaculture (Bezerra et al 2014). Annual yield reached its maximum in 2011 (19,000 t) but has decreased to less than 10,000 t due to decreasing water quality following reduction of the reservoir volume (Oliveira et al 2015).…”

Hydrochemistry and trophic state change in a large reservoir in the Brazilian northeast region under intense drought conditions

“…Molisani et al (2015) reported, based on stable C and N distribution, current velocities and deposition rates of fecal pellets, that effluents from cage aquaculture (feed and feces) were not extensively reaching the water column and the bottom sediments of the cage area, on the contrary, effluents from farms are carried out to the deeper, central region of the reservoir. Hydrodynamic modeling of the reservoir (Molisani et al, 2015;Oliveira et al, 2015) showed surface wind-drive currents to move westwards from the center deeper part of the reservoir to shore, whereas bottom currents flush-out fish farm areas and incoming runoff eastwards to the deeper regions (Group 1 region), where stratification exists during high volume periods (e.g., in station 6 in November, 2011 -Supplementary Tab. 1), but is broken when the reservoir volume decreases.…”