A field dose-response approach as a tool for environmental Hg contamination assessment. Case study: bioaccumulation of Hg by Tucunaré (Cichla spp.) from Brazilian Amazon ecosystems

Z. C. Castilhos* & E. D. Bidone

*Center for Mineral Technology, Ilha do Fundão, Brazil; zcastilhos@CETEM.gov.br Department of Geochemistry, Fluminense Federal University, Brazil, geobida@vm.uff.br

ABSTRACT

Many factors control the bioaccumulation and/or biomagnification of Hg by fish and its tissue levels have been normalized by several ways for spatial and/or temporal comparisons. We suggest using fish length in order to infer the time of exposure to Hg and as an indirect dose-normalizer from field data. The fish specie chosen is the Tucunaré (Cichla spp.), a carnivorous icthyophagous specie, considered as good bioindicator of Hg accumulation in fish. It is sedentary, territorial, commercially important, and does not exhibit seasonality in Hg accumulation. The estimated fish growth rate for Cichla spp., was used to correlate potential ages to different percentages of the asymptotic length, and assuming this to be the potential time of exposure to Hg; the linear relationship, the dose-response relationship and the exposure-response relationship were established by using Hg tissue levels from a background and a contaminated area impacted by goldmining. All approaches corroborated the presence of significant higher uptake rates by a factor of 2 of the Hg dose in contaminated area in contrast to background area.

INTRODUCTION

A lot of factors have been considered important in the bioaccumulation and/or biomagnification of Hg in fish. Among them, the Hg load-dependent factors in the aquatic environment, specially in those related to Hg in sediments and environmental conditions, like bioproduction (Häkanson, 1980); as well as local biota’s physiological-dependent factors, like size, length, age and metabolic rate (Phillips, 1980; WHO, 1990); and also food-chain characteristics (Cabana et al, 1994).

Hg levels in fish for spacial and/or temporal comparisons have been normalized by mean of Hg content in 1-Kg fish (as pike) (Johnels et al., 1967), or by using only fish of one year of age (Post et al, 1996) or by using a specific length (Scruton, et al., 1994). We suggest using the length to infer upon the time of exposure of fish to Hg, which could be considered as indirect dose-normalizer. This assumes that the length increases along the time (hormonally controlled) (Zaret, 1980).

The Tucunaré (Cichla spp.) specie was chosen for many reasons. This specie may be considered good bioindicator of Hg accumulation in the Amazonian ecosystem, specially because of its time-integration capacity and its carnivorous icthyophagous feeding habit. Their fine taste and abundance in native habitat have made it an important commercial specie.

The objectives of this work are: (i) to estimate the daily Hg uptake rate by Cichla spp. during growth up to the attainance of its maximum length; (ii) to establish and compare the dose-response relationship for Hg accumulation by Cichla spp. for a contaminated and a background area, and (iii) estimate the potential time of exposure necessary for Hg accumulation to reach 500mg.Kg^-1, the concentration limit for human consumption adopted by many countries.

MATERIALS AND METHODS

We sampled and analyzed 69 specimens of Cichla spp.; 41 specimens from the contaminated site and 28 specimens from the background site. The contaminated area is located in the Tapajós river between the cities of Jacareacanga and Itaituba (04°15’23”S-55°54’33”W), where the gold mining sites are distributed alongside the tributaries of the Tapajós river; and the background site is located in a fluvial lacustrine system near Santarém city (02°25’11”S-54°42’16”W), 250 Km downstream from the contaminated site (Castilhos & Bidone, 1998). Each specimen was weighed, and its length was measured at the time of collection. Hg was analyzed in the fish muscle through Atomic Absorption Spectrophotometer (A-G/VARIAN MODEL) using a Vapor Generation Acessory-VGA (CVAAS).

The growth of fish is considered as an interaction between the specimen and the environment. Growth has many aspects. Growth in relation to age can be described by entirely empirical mathematical equations. The relationship between length and age of fish can be expressed by the von Bertalanffy (1957) mathematical equation.

RESULTS AND DISCUSSION

The elimination of MeHg by fish is very slow relativelly to the uptake and the accumulation rates. Positive correlation between concentrations in muscle and both size, length and/or age for a given specie have been well documented (WHO, 1990). In this study significant linear correlation (Pearson’s correlation) between Hg levels in muscles and length (0.67; p<0.0001; n=28) and weight (0,9; p<0.006; n=10) was found for Cichla spp. in the background area. The correlations for Cichla spp. from the contaminated area were not significant.

The estimated fish growth rate for Cichla spp., can be used to correlate potential ages to different intervals of length. The estimated ages were used in order to infer the equivalent potential time of exposure.

Toxic substance accumulation rates vary between individuals in populations. The dose-response relationship has the competence to absorb such individual variabilities. A certain percentage of test organisms will show the respond within some stated conditions. The quantal test is designed to estimate the concentration of a test material that affects 50% of the test organisms, the median effective dose (D 50% or D₅₀) (Ross & Gilman, 1985). The D₅₀ for accumulation of Hg by Cichla spp. (DA₅₀) indicates the time of exposure necessary to attain those tissue concentration levels by half of the exposed individuals.

The results showed that half of Cichla spp. individuals exposed to a potential DA₅₀ in the background area would accumulate at least 100mg.Kg^-1, whereas in the contaminated area, the expected response would be 300mg.Kg^-1.The determination of DA₅₀ for each area estimates that close to 1.2 year of exposure, half of Cichla spp. exposed to a potential dose in the contaminated area could accumulate at least 300mg.Kg^-1, and in the background area at least 100mg.Kg^-1 after 1.4 year of exposure. Regardless of the large uncertainties always present in estimates of fish age from its length, the specie-pollutant pairing is unique in terms of pollutant kinetics or flux.

If we accept that exposure and response can be interrelated as: t _exposure * C = constant (adapted from Dämgen & Grünhage, 1998); in which a certain response (constant) can be achieved from a time of exposure t _exposure and a concentration of aquatic environment C. In this study we found: (i)for contaminated area: 420 (days) * C = 300mg.Kg^-1 ; C @ 0.7mg.Kg^-1.day^-1 and, (ii) for background area: 510 (days) * C = 100mg.Kg^-1 ; C @ 0.2mg.Kg^-1.day^-1. It must be assumed that these Hg doses are from both sources, by water and food. Considering that the latter for Cichla spp. from contaminated area takes 1.9 years to achieve 500mg.Kg^-1 and 6.8 years for the background area. We believe that the difference between daily doses (~3,5 times), is significant and could be attributed to Hg load differences between the studied areas, and also, could be a consequence of a potetial pollution source, the goldminig activity. If the fish contamination by Hg can be considered a risk, the risk-benefit analysis of goldmining activity will take into account the increase of the Hg dose to fish in the contaminated area relativelly to the background area.

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