Environ Sci Pollut Res (2015) 22:679–688 DOI 10.1007/s11356-014-3382-x

RESEARCH ARTICLE

Mercury toxicity and the protective role of selenium in eel, Anguilla anguilla Lucyna Polak-Juszczak & Stanisław Robak

Received: 15 January 2014 / Accepted: 23 July 2014 / Published online: 8 August 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The aim of the study was to determine the impact trace metals, mainly toxic ones, on the condition of eel (Anguilla anguilla) inhabiting four regions of Poland. Metal concentrations in eel muscle tissues were studied as functions of size, region, and season 2011–2012. The levels of metals were also used for risk assessment on consumer health. Copper and zinc occurred at concentrations that could only have positive impacts on eel condition. Low levels of cadmium and lead did not impair the condition of the fish. However, mercury occurred at high levels and increased with fish length and season. The mercury levels in eels were compared with the threshold of toxicity (500–1,200 μg kg−1), which can cause changes in biochemical processes and impair fish reproduction. The concentration of mercury was 1,010 μg kg−1 in one specimen of the 120 samples examined, and in 16 specimens, it exceeded 500 μg kg−1. The toxic effects of the mercury could have been attenuated by the selenium in the muscles of the eel, especially in the muscles of smaller specimens in which the Se/Hg molar ratio was higher than 1 with a positive correlation between these two elements. In larger specimens measuring in excess of 70 cm, this coefficient was below 1, and the mercury to selenium correlation was negative, which meant that the protective effects of selenium were weaker. The mercury in the muscles of large specimens at levels exceeding 500 μg kg−1 could have weakened eel condition and also posed a threat to consumer health. The cadmium and lead in the muscles of the eel did not affect the condition of the fish. Responsible editor: Henner Hollert L. Polak-Juszczak (*) Department of Food and Environmental Chemistry, National Marine Fisheries Research Institute, ul. Kołłataja 1, Gdynia 81-332, Poland e-mail: [email protected] S. Robak Department of Ichthyology, Inland Fisheries Institute in Olsztyn, ul. Oczapowskiego 10, Olsztyn 10-719, Poland

Mercury weakened the condition of large eel, A. anguilla. Selenium protected small- and medium-sized eel against the toxic effects of mercury. Keywords Trace metals . Anguilla anguilla . Risk assessment . Muscle tissue

Introduction Over the past years, populations of European eel (Anguilla anguilla) have been on the decline in Europe, and this species is currently threatened with extinction. In Poland, the Eel Management Plan was developed based on Council Regulation (EC) no. 1100/2007 of 11 September 2007 establishing measures for the recovery of the stock of European eel. The reasons for the collapse of eel stocks might be in the characteristics of the species. Eel is a migratory, catadromous fish that inhabits fresh waters and undertakes spawning migrations to the Sargasso Sea, which means it travels distances of approximately 8,000 km in periods ranging from 180 to 250 days. Migrating eel encounter many obstacles on their migration routes including various types of weirs, dams, and other constructions (Bartel et al. 1998). Other causes of reduced eel abundance include recreational catches, cormorant predation, the lack of suitable food resources, and water pollution. At the outset of migration, eel should be in excellent physical condition and at the peak of biological development. To be able to survive migration, the eel should have sufficient reserves of high-calorie fats that provide energy for the long journey and ensure proper reproduction cycles. The unsatisfactory health of eel (A. anguilla) emigrating from European waters to ocean spawning grounds could be one of the main causes of decreased stock abundance. One of the factors that could lead to the deteriorating health of eel is the high levels of contaminants accumulated in their bodies. Belpaire and

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Goemans (2007) suggest that, depending on the degree of pollution in their habitat, the levels of certain contaminants in yellow eels can be high and might affect their potential for reproduction. Contamination can affect the metabolism of lipids and lead to decreased reserves of energy. Large loads of contaminants and low reserves of energy in eel could be the reason behind failed migrations and/or reproduction. Toxic compounds can cause disruptions in the nervous and immune systems and hormonal balance, but they can also negatively impact the ability of individuals to reproduce, and, if reproduction does occur, the development of progeny. Studies of toxic compounds in European eel (A. anguilla) have been conducted in Belgium (Maes et al. 2008), Germany (Stachel et al. 2007; Karl et al. 2010), France (Ribeiro et al. 2005), Italy (Ferrante et al. 2010), Sweden (Ankarberg et al. 2004), Norway (Knutzen et al. 2003), and Portugal (Cid et al. 2001; Neto et al. 2011). Toxic metals and organic compounds are the most frequently studied, and their levels fluctuate widely depending on the habitat of the eels. Some researchers suggest that this species could be used in biomonitoring environmental pollution (Neto et al. 2011; Belpaire and Goemans 2007; Ribeiro et al. 2005). To date, studies of eel contamination in Poland have only been conducted on a rather limited basis and have focused on organic compounds (Szlinder-Richert et al. 2010) but have not included metals. Fish accumulate metals and some, such as mercury, in high concentrations (Sormo et al. 2011; Polak-Juszczak 2012). This element requires a special focus because of its toxic properties. All forms of mercury are toxic to both fish and those who consume them, especially with regard to the organic form of methylmercury. The primary source of mercury exposure in humans is from fish consumption (Rice et al. 2000), and levels of methylmercury in some fish are high enough to cause toxic effects in the fish themselves and in top-level predators, including humans who consume the fish (WHO 1989). Eel is a fish with very desirable organoleptic properties, which is why its meat is consumed despite the high mercury content that can pose health risks (Belpaire and Goemans 2007). The aim of the current study was to determine the concentrations of trace metals, mainly those that are toxic, which could potentially impact eel condition and pose a threat to the consumers of these fish. The main focus was on mercury because of its high levels and toxicity and selenium, which is thought to protect against mercury exposure.

Materials and methods Characteristics of tested samples The study was performed on the muscle tissues of eel (A. anguilla) caught in 2011 and 2012 in four regions of Poland: the southern Baltic Sea in Puck Bay and the vicinity

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of Świnoujścia and Mielno; the inland lakes of Jamno, Bukowo, Święcajty, Nidzkie, Śniardwy, and Mamry in northeast Poland; in the Vistula and Szczecin lagoons; and in the Vistula, Poland's largest river (Fig. 1). A total of 126 samples were tested. Samples comprised one specimen when the eel weighed over 900 g, while samples of smaller eel comprised from two to five specimens. Forty samples were prepared from eel caught in the lagoons, 38 from the Baltic Sea, 34 from lakes, and 14 from the Vistula River. Analytical methods Sample digestion The samples tested were handled to the stage of measuring metal concentrations in clean rooms (Class 10,000). Dorsal muscles were excised from the fish and homogenized. Approximately 2 g of wet weight sample, 6 ml of nitric acid (65 %), and 2 ml of (30 %) hydrogen peroxide were placed into PTF vessels. The sample in the vessel was then subjected to a Microwave Digestion System (MDS 2100 and MARS 5). Mineralized samples were transferred to 25-ml volumetric flasks. Concentrations of copper, zinc, cadmium, and lead were measured directly in the solutions. Before measuring selenium concentrations, the mineralized samples were heated for 2 h at 70 °C. Then 3 ml of the sample solution was added to 3 ml of 25 % hydrochloric acid, and the selenium concentration was measured. Sub-samples of homogenized tissue were preserved intact until mercury analysis. Concentration determinations Metal concentrations were determined with atomic absorption spectrophotometry using a Perkin-Elmer 4100 atomic absorption spectrometer equipped with a graphite furnace to determine concentrations of cupper, cadmium, and lead. Concentrations of zinc were determined with the acetylene–air flame method using a Perkin-Elmer 1100 atomic absorption spectrometer. Concentrations of selenium were measured with the atomic absorption hydride generation technique using a Perkin-Elmer spectrometer coupled with a Fias 200. After removing the nitric acid by heating the sample, selenium hydride was generated with sodium borohydride (0.2 %). The hydride was transported by argon into a heated absorption cell (900 °C) where the hydride decomposed, and atomic Se was determined at a wavelength of 196.0 nm. Mercury concentrations were determined with the cold vapor atomic absorption technique in a mercury analyzer (AMA 254). We calculated Se/Hg molar ratio by dividing the concentration micrograms per kilogram by the molecular weight. For each individual fish, we divided the selenium concentration (micrograms per kilogram) by 78.96, and for each individual fish, the mercury concentration (micrograms per kilogram) by

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Fig. 1 Eel, A. anguilla, sampling locations

200.59, and calculated the ratio Se/Hg. Mean Se/Hg molar ratio were calculated from the ratio for each individual fish. The ratios reported here were calculated from total selenium and total mercury.

stations and seasons. The correlations between metal concentration in eel and length were assessed with the nonparametric Spearman’s test. The level of significance was designated as p

Mercury toxicity and the protective role of selenium in eel, Anguilla anguilla.

The aim of the study was to determine the impact trace metals, mainly toxic ones, on the condition of eel (Anguilla anguilla) inhabiting four regions ...
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