Methylmercury in fish from the southern Baltic Sea and coastal lagoons as a function of species,size,and region

Abstract

Background:

Methylmercury (MeHg) is a highly toxic compound that traverses the blood–brain barrier with deleterious effects to the central nervous system. Exposure is generally through the ingestion of contaminated fish. Fish are a main source of MeHg.

Goals and methods:

The aim of this study was to determine the dependence of MeHg concentrations on fish species and age, the percentage of MeHg in total mercury (THg) and risk assessment depending on the size of fish. Assays of THg and MeHg were performed on the muscle tissues of 18 species of fish.

Results:

The investigations indicated there were differences in the mercury concentrations depending on fish size. THg and MeHg concentrations in the muscles of fish species that have a wide length distribution were strongly, positively correlated with fish length. However, concentrations of MeHg were strongly, positively correlated with those of THg in all the fish species investigated. Variation in the percentage share of MeHg in THg in the muscles of fish of large sizes was also noted within species, but this correlation was not noted in small-sized fish. The dose of MeHg in small-sized fish species was estimated and the risk posed to consumer health was assessed using mean MeHg concentrations determined for different fish species.

Conclusions:

For species of fish that occur within a wide length distribution, the dose of MeHg should be assessed separately in different length classes. Fish consumption of small-sized species poses no health risk.

Keywords

Fish methylmercury risk assessment total mercury

Introduction

The Baltic Sea, where most of the fish were caught, is surrounded by industrialized countries, which means that this region is exposed to pollution with toxic substances. Among the compounds contaminating the marine environment, mercury is especially dangerous because of its high toxicity. One of the critical consequences of mercury cycling is its bioaccumulation and biomagnification through the aquatic food chain, especially among consumers at higher trophic levels, including fish (Chen et al., 2008; Kehrig et al., 2010). Mercury concentrations in the muscles of fish increase as the fish age, with the fish position on the trophic chain, and with the types of food the fish consume (Quedrago and Amyot, 2013). Large, predatory fish accumulate the most mercury (Kehring et al., 2008). Fish on lower levels of the trophic chain, which are present in great number in the Baltic, such as herring, sprat, and flatfish, contain lower mercury concentrations (Polak-Juszczak, 2015). Mercury readily undergoes biochemical transformations in the aquatic environment, and its most toxic form, MeHg, is created during methylation. Mercury enters the aquatic food chain in this form, and concentrations of it increase in subsequent links. Nearly all the mercury that occurs in fish, particularly that in the muscles, is in its organic form of MeHg (Mason et al., 2006). Fish muscle is a source of MeHg, and many researchers have concluded that concentrations of this metal in humans are the result of fish consumption (Bjornberg et al., 2003; Kehrig et al., 2008). MeHg is a strong neurotoxin, which, at high concentrations, has a devastating impact on the nervous system and brain cells, and it can disrupt the senses of sight, hearing, and speech. This is why studies of MeHg in fish permit assessing the potential risk it poses to consumer health. Studies to date of mercury in the fish available for purchase on the Polish market have been focused mainly on THg. The first study of MeHg in fish available on the Polish market was conducted in 1998 and 1999 (Barska and Skrzyński, 2003) and included six fish species from the Baltic (cod, herring, sprat, flounder, eel, and perch). Subsequent studies by Kwaśniak et al. (2012) in 2010 examined the concentrations of MeHg in the muscles of five fish species from the Baltic. A review of the available literature indicates that these were the only studies of MeHg concentrations in fish from the southern Baltic. In 2010, MeHg was tested in fish caught in the North Sea and on fish purchased on the German market (Kubala et al., 2011).

The aim of the study was (1) to determine the dependence between MeHg concentrations in fish muscle tissues and fish length and weight; (2) to determine the percentage share of MeHg in THg for fish species; (3) to present current data on the concentrations of MeHg and THg in commercially important fish species; (4) to assess risks to consumer health; (5) to present information for consumers regarding safe fish consumption in terms of fish species and specimen size that will help them to make informed decisions when purchasing and consuming fish. The justification for undertaking this study is also the lack of information available on the concentrations of MeHg in freshwater fish and farmed and imported fish, which have come to dominate the Polish consumer market in recent years.

Materials and methods

Sample collection

The study material was fish caught in the central middle coast of the Baltic Sea in 2015–2016: small-sized fish (herring, sprat, and flatfish), large-sized fish (cod, salmon, and sea trout) in Puck Bay (garfish), in the freshwaters of the Vistula Lagoon (roach, pikeperch, bream, perch, and burbot), Szczecin Lagoon (European perch), imported fish sold on the Polish market (panga, carp, tilapia, walleye, pollock, mackerel, and sole), and farmed fish (silver carp). Assays of THg and MeHg were performed on 184 samples of muscle tissue collected from 18 species of fish.

Analysis of total mercury concentration

Total mercury (THg) content was assayed with the cold vapor atomic absorption method in an AMA 254 mercury analyzer (ALTEC Ltd.). The analyses were conducted according to the following procedure. Tissue samples of about 100 mg were placed in the combustion chamber of the analyzer where they were dried and then burned at a temperature of 600°C under an oxygen atmosphere. The measurements were conducted as follows programs: fish muscle tissue—drying time 70 s, decomposition time 120 s, and waiting time 50 s; fish liver tissue—drying time 100 s, decomposition time 160 s, and waiting time 60 s. Each series of analyses was preceded by measurements of mercury in reference materials of a similar matrix.

Analysis of methylmercury concentration

Methylmercury (MeHg) content was determined according to the method described by Barska and Skrzyński (2003), Maggi et al. (2009), and Tong et al. (2012). In brief, the procedure was as follows: from 1 to 2.0 g of homogenized eel muscle tissue was weighed out and placed in 50-ml test tubes for centrifugation; 5 ml of hydrochloric acid (18% v/v) and 5 ml toluene were added; the test tubes were placed in an ultrasound water bath for 30 min; then the test tubes were centrifuged for 30 min at 3500 r min⁻¹. The toluene layer was moved to 10-ml test tubes, while 5 ml of toluene was added to the remaining solution in the test tubes, which were again placed in an ultrasound bath for 30 min, after which they were centrifuged again, as described above. After centrifugation, the upper layer of toluene was separated and added to the previously separated toluene, and 1 ml of cysteine hydrochloride solution (1% cysteine hydrochloride solution in a 20% sodium citrate solution) was added to the combined toluene layers, and this was placed in an ultrasound bath for 30 min, and then centrifuged again, as described above. Before measurements, the upper layer of toluene was removed with a syringe. The content of MeHg was measured in the prepared samples with an AMA 254 mercury analyzer.

Measurement quality control

The accuracy of the chemical analysis was verified using reference material before every measurement series. The following materials were used for THg: TORT-2 Lobster Hepatopancreas (National Research Council of Canada, Canada) at a concentration of THg 0.27 ± 0.06 mg kg⁻¹ and BCR-422 cod muscle (Joint Research Centre Institute for Materials and Measurements, Geel, Belgium) with a concentration of THg of 0.559 ± 0.016 mg kg⁻¹. The following reference material was used for measurements of MeHg: TORT-2 Lobster Hepatopancreas with a concentration of MeHg 0.152 ± 0.013 mg kg⁻¹ and BCR-463 tuna fish (Joint Research Centre Institute for Materials and Measurements) with a concentration of MeHg 3.04 ± 0.16 mg kg⁻¹. The recovery ranges were from 90% to 110%. During validation, the limit of detection for THg was 0.5 µg kg⁻¹ and for MeHg it was 5 µg kg⁻¹. All samples were analyzed in duplicate. The results presented are arithmetic averages with a standard deviation of less than 10%.

Consumer health risk assessment

The risk posed to Polish consumers by the ingestion of THg and MeHg with fish consumption was assessed by comparing the estimated weekly intake (EWI) for THg and MeHg with the tolerable weekly intake (TWI) and target hazard quotient (THQ). The EWI for THg and MeHg was estimated based on the mean concentrations of these elements in fish muscles (mean concentrations for species are used widely in risk assessment) and the quantity of these fish that were consumed weekly. Fish consumption in Poland, and in most EU countries, with the exception of those on the Mediterranean Sea, in 2015, was 13 kg of live fish per capita annually (Failler et al., 2007). Recalculated as clean meat, this means that Poles consume about 6 kg of fish annually, which is a weekly portion of approximately 100 g (Pieńkowska and Hryszko, 2013).

The THQ was calculated according to the (Bonsignore et al., 2013) method, which is described by the following equation:

THQ = (\frac{EF \times ED \times FIR \times C}{RFD \times WAB \times TA}) \times 10^{- 3}

Where EF is exposure frequency (365 days/year); ED is the exposure duration (70 years), equivalent to the average lifetime; FIR is the food ingestion rate (36 g/person/day) (FAO, 2005); C is the metal concentration in seafood (μg g⁻¹); RFD is the USEPA’s reference dose (0.1 μg Hg kg bw⁻¹ day⁻¹; http://cfpub.epa.gov) or acceptable daily intake determined by WHO (0.23 μg Hg kg bw⁻¹ day⁻¹; http://apps.who.int); WAB is the average body weight (60 kg), and TA is the average exposure time for no carcinogens (365 days/year × ED).

Statistical analysis

Statistical analysis was conducted with STATISTICA, Version 8, Standard Package.

The normality of THg and MeHg data was confirmed with Shapiro-Wilk test. Relationships between THg and MeHg concentrations in the muscle tissues of fish, the length and weight of fish, as well as the correlation between THg and MeHg concentrations were tested using a linear regression model. The level of significance of p < 0.05 was applied. Analysis differences that depended on the different ecological groups of fish (such as carnivorous, omnivorous, and herbivorous fish) were analyzed statistically with one-way analysis of variance (ANOVA) and Tukey’s post hoc test.

Results and discussion

Intra- and interspecific variability in MeHg and THg concentrations

The concentrations of THg in fish caught in Polish waters and in fish imported and sold on the Polish market ranged from 1 to 555 µg kg⁻¹ and concentrations of MeHg ranged from 1 to 506 µg kg⁻¹ (Table 1). Concentrations of MeHg in fish available to consumers on the German market were within a similar range of 20–567 µg kg⁻¹ (Kubala et al., 2011). The current EU limit for THg is 500 µg kg⁻¹. Among the fish caught in the southern Baltic that were tested, this limit was exceeded in only one specimen of cod measuring 110 cm in length the muscle tissues of which contained 555 µg THg kg⁻¹. The bioaccumulation of THg in fish depends on fish species and, also on feeding, position in the trophic chain, and region of occurrence age (Kehring et al., 2008; Mason et al., 2006). Fish age often is represented by specimen length (Bermejo, 2007) To confirm these dependencies for MeHg, assays were performed on species of fish that differed in terms of feeding habits and region in which they were caught. Concentrations of THg and MeHg varied depending on the species, with the highest concentrations of THg and MeHg noted in garfish, cod, flatfish, pikeperch, and perch (Table 1). The mean concentrations of THg and MeHg in the muscles of these fish were 117 and 98 µg kg⁻¹ for garfish, 92 and 78 µg kg⁻¹ for cod, 93 and 78 µg kg⁻¹ for flatfish, 88 and 67 µg kg⁻¹ for pikeperch, and 86 and 62 µg kg⁻¹ for perch, respectively. In comparison with studies of fish caught in the Baltic in 1998–1999 (Barska and Skrzyński, 2003), the concentrations of MeHg in small-sized fish herring, sprat, and flatfish were at similar levels to the results of the present study. MeHg concentrations in fish from the North Sea and from the German market were studied recently, and part of this study examined the same fish species as those of the present study (Kubala et al., 2011). The MeHg concentrations in fish reported by Kubala et al. (2011) are also low at an average content of 38 µg kg⁻¹ MeHg in all samples. MeHg levels in fish from the Polish and German markets are generally low and do not pose health risks to consumers.

Table 1.

The average concentrations of THg (µg kg⁻¹ wet weight) and MeHg (µg kg⁻¹ wet weight) in fish (mean, SD, and range), participation % MeHg in THg, and participation of THg and MeHg in dose TWI estimated based on the average contents, THg and MeHg, in 100 g of meat fish, THQ.^a

Species	N	Length (cm)	THg (µg kg⁻¹ wet weight)	MeHg (µg kg⁻¹ wet weight)	% MeHgin THg	THg	MeHg	THQ (Hg)	THQ (MeHg)
Species	N	Length (cm)	THg (µg kg⁻¹ wet weight)	MeHg (µg kg⁻¹ wet weight)	% MeHgin THg	% TWI	% TWI	THQ (Hg)	THQ (MeHg)
Cod	27	46.4 ± 18.5	92 ± 116	78 ± 108	82.1 ± 9.6	3.8 ± 4.8	8.1 ± 12.0	0.038	0.081
		30–110	25–555	21–506	63.4–99.1	0.9–23.1	2.2–52.7
Herring	16	20.8 ± 2.0	22 ± 13	15 ± 9	69.6 ± 8.6	0.9 ± 0.5	1.5 ± 0.9	0.009	0.016
		18–23	8–45	6–30	54.5–85.7	0.3–1.9	0.6–3.1
Flatfish	16	29.7 ± 4.5	93 ± 5	78 ± 39	83.1 ± 8.7	3.9 ± 1.9	8.1 ± 4.0	0.039	0.081
		25–35	32–191	27–154	65.4–94.1	1.3–8.0	2.8–16
Sprat	11	12 ± 2	16 ± 4	9 ± 2	54.5 ± 5.7	0.7 ± 0.2	0.9 ± 0.2	0.007	0.009
		11–15	10–23	6–11	42.8–61.5	0.4–1.0	0.6–1.1
Garfish	30	72 ± 3.9	117 ± 54	98 ± 47	83.2 ± 7.3	4.9 ± 2.2	10.2 ± 4.9	0.048	0.102
		64–79	45–279	37–248	63–98	1.9–12	3.9–25.8
Salmon	10	82 ± 7	16 ± 11	12 ± 8	71 ± 12.8	0.7 ± 0.5	1.3 ± 0.8	0.007	0.012
		77–87	7–46	4–32	50–93.3	0.3–1.9	0.4–3.3
Sea trout	4	57.5 ± 6.4	42 ± 13	34 ± 11	81.8 ± 0.8	1.6 ± 0.7	3.6 ± 1.2	0.018	0.035
		53–62	32–51	26–42	81.2–82.4	1.1–2.2	2.7–4.4
Mackerel	8	36 ± 5	50 ± 11	37 ± 11	73.7 − 8.3	2.0 ± 0.4	3.8 ± 0.7	0.021	0.038
		35–40	34–67	23–58	64.1–86.7	1.4–2.6	2.4–4.6
Roach	9	25.6 ± 6.3	52 ± 21	43 ± 21	80.4 ± 8.2	2.1 ± 0.9	4.5 ± 2.2	0.022	0.044
		14–32	22–95	18–86	61.9–92.3	0.9–4.0	1.9–9.0
Pikeperch walleye	16	41 ± 11	88 ± 67	67 ± 48	78.4 ± 9.6	3.7 ± 2.8	7.1 ± 5.1	0.037	0.069
Pikeperch walleye		24–58	29–286	24–212	63.2–94.1	1.2–11.9	2.5–22.1
Bream	10	39 ± 17	34 ± 36	28 ± 34	76.1 ± 10.8	1.4 ± 1.5	3.0 ± 3.5	0.014	0.029
		13–57	2–102	2–96	50–94.1	0.1–4.3	0.2–10
Perch	7	23 ± 5.0	86 ± 19	62 ± 15	71.2 ± 8.1	3.6 ± 0.8	6.4 ± 1.6	0.036	0.064
		17–32	53–110	36–88	60.7–85.4	2.2–4.,6	3.8–9.2
Burbot	3	40 ± 5.3	66 ± 10	56 ± 8	84.8 ± 5.1	2.8 ± 1.4	5.8 ± 2.1	0.028	0.058
		38–42	54–63	52–64	80.2–88.3	2.1–3.6	6.4–8.1
Pollock	4	Fillets	2.8 ± 0.7	2.3 ± 0.6	82.6 ± 0.6	0.1 ± 0.03	0.24 ± 0.1	0.001	0.002
			2.3–3.0	1.9–2.0	81.8–82.6	01–0.14	0.2–0.28
Silver carp	3	Fillets	6.7 ± 1.2	3.3 ± 0.6	50.2 ± 2.1	0.3 ± 0.05	0.34 ± 0.1	0.003	0.003
			6–	3–4	49.6–53.1	0.2–0.3	0.3–0.4
Sole	3	Fillets	62 ± 10	54 ± 13	87.3 ± 7.7	2.6 ± 0.4	5.7 ± 1.4	0.026	0.056
			55–69	45–64	81.8–92.8	2.3–2.9	4.7–6.7
Panga	4	Fillets	1.4 ± 0.8	1.0 ± 0.1	55.5 ± 4.5	0.06 ± 0.03	0.1 ± 0.01	0.001	0.001
			0.9–2.3	0.8–1.1	51.1–60.1	0.04–0.1	0.08–0.11
Tilapia	3	Fillets	0.8 ± 0.1	1.0 ± 0.1	52.8 ± 3.9	0.03 ± 0.03	0.1 ± 0.01	0.0003	0.001
			0.8–0.9	0.9–1.0	50–56.5	0.03–0.04	0.09–0.10

TWI: tolerable weekly intake; THg: total mercury; MeHg: methylmercury; THQ: target hazard quotient; Hg: mercury; N: amount of samples.

^aTWI for THg = 4 µg per kg of body weight on week, for person 60 kg TWI = 240 µg Hg week⁻¹ and TWI for MeHg = 1.6 µg per kg of body weight on week, for person 60 kg TWI = 96 µg MeHg week⁻¹ (EFSA, 2012).

Significant differences occurred in the concentrations of MeHg in species that have a wide length distribution (cod, pikeperch, garfish, and bream). Our study indicated that differences in the concentrations of both forms of mercury was dependent on specimen size (Figure 1). This refers mainly to specimens that individuals achieve large sizes and that accumulate high concentrations of mercury compounds along with age. Table 2 presents THg and MeHg concentrations in the muscle tissues of cod from five length classes; in garfish from four length classes; and in perch, pikeperch, roach, and bream from three length classes. Differences in THg and MeHg concentrations within length classes of species were confirmed in cod, garfish, herring, pikeperch, roach, bream, and perch. THg and MeHg concentrations in the muscle tissues of cod from the 30–40 cm length class were sixfold lower than those in specimens from the 60–70 cm length class and approximately tenfold lower than in cod measuring 110 cm (Table 2). In other species (garfish, perch, pikeperch, roach, and bream) concentrations of both forms of mercury in individuals from the smallest and largest length classes differed from three- to fourfold.

Figure 1.

Concentration of THg and MeHg and % share of MeHg in THg depending the length of cod. THg: total mercury; MeHg: methylmercury.

Table 2.

Concentrations of THg (µg kg⁻¹ wet weight) and MeHg (µg kg⁻¹ wet weight) (mean, SD) in the muscles of fish from different classes of length, participation (%) of MeHg in THg, and participation of THg and MeHg in a dose TWI estimated on the basis of 100 grams portions of meat fish.^a

Species	Class of length (cm)	THg (µg kg⁻¹)	MeHg (µg kg⁻¹)	% MeHg	% TWI	% TWI	THQ (Hg)	THQ (MeHg)
Species	Class of length (cm)	THg (µg kg⁻¹)	MeHg (µg kg⁻¹)	% MeHg	THg	MeHg	THQ (Hg)	THQ (MeHg)
	30–40	40 ± 16	32 ± 15	80.6 ± 6.8	1.7 ± 0.6	3.4 ± 1.5	0.017	0.033
Cod	41–50	66 ± 34	51 ± 22	79.9 ± 11.2	2.7 ± 1.5	5.3 ± 2.2	0.027	0.053
	51–60	75 ± 21	61 ± 21	80.2 ± 7.4	3.1 ± 0.9	6.3 ± 2.3	0.031	0.063
	61–70	236 ± 142	215 ± 105	91.1 ± 3.1	9.8 ± 2.9	22.4 ± 10.2	0.098	0.223
	100–110	427 ± 182	395 ± 157	97.1 ± 2.7	17.8 ± 7.6	41.1 ± 16.4	0.178	0.409
Perch	17–19	76 ± 32	50 ± 20	66.2 ± 2.3	3.2 ± 1.4	5.2 ± 2.1	0.032	0.052
	21–25	96 ± 13	64 ± 11	66.4 ± 6.3	3.9 ± 0.6	6.6 ± 1.2	0.04	0.066
	26–32	90 ± 18	74 ± 20	81.0 ± 6/2	3.8 ± 0.7	7.7 ± 2.1	0.038	0.077
Pikeperch	25–35	44 ± 13	38 ± 15	83.9 ± 12.6	1.8 ± 0.6	3.9 ± 1.6	0.018	0.039
	36–45	88 ± 23	68 ± 11	79.5 ± 8/2	3.7 ± 0.9	7.1 ± 1.1	0.037	0.07
	46–55	143 ± 101	104 ± 75	72.7 ± 3.1	6.0 ± 4.2	10.9 ± 7.8	0.06	0.108
Garfish	60–65	53 ± 11	46 ± 1	84.9 ± 2/5	2.2 ± 0.5	4.8 ± 1.1	0.022	0.048
	65–70	98 ± 26	81 ± 21	84.7 ± 7.6	4.0 ± 1.0	8.5 ± 2.1	0.041	0.084
	70–75	104 ± 29	91 ± 27	84.8 ± 4.9	4.5 ± 1.2	9.5 ± 2.8	0.043	0.094
	75–80	180 ± 59	149 ± 58	81.5 ± 8/9	7.5 ± 2.5	15.5 ± 6	0.075	0.154
Roach	14–18	28 ± 8	21 ± 4	76.2 ± 7.9	1.2 ± 0.4	2.2 ± 0.4	0.012	0.022
	23–26	52 ± 11	44 ± 16	82.1 ± 17.5	2.2 ± 0.4	4.6 ± 1.7	0.022	0.046
	30–32	63 ± 23	54 ± 23	81.3 ± 12.5	2.6 ± 0.9	5.6 ± 2.4	0.026	0.056
Bream	13–20	3.3 ± 1.1	2.2 ± 0.7	66.7 ± 14.4	0.1 ± 0.05	0.2 ± 0.08	0.001	0.002
	35–45	18 ± 15	12 ± 10	64.5 ± 5.8	0.8 ± 0.6	1.2 ± 1.0	0.008	0.012
	50–60	70 ± 28	61 ± 31	82.2 ± 12.9	2.9 ± 1.2	6.3 ± 3.3	0.029	0.063
Herring	18–9	11 ± 3	8 ± 2	68.6 ± 9.7	0.5 ± 0.1	0.8 ± 0.,2	0.004	0.008
	23–24	32 ± 9	23 ± 6	70.6 ± 7.8	1.3 ± 0.4	2.4 ± 0.6	0.013	0.024
Flounder	25–28	88 ± 41	75 ± 34	85.3 ± 7.6	3.3 ± 1.7	7.8 ± 3.6	0.036	0.078
	33–35	110 ± 46	93 ± 41	84.0 ± 1.3	4.6 ± 1.9	9.7 ± 4.2	0.046	0.096

TWI: tolerable weekly intake; THg: total mercury; MeHg: methylmercury; THQ: target hazard quotient; Hg: mercury.

^aTWI for THg = 4 µg kg⁻¹ m. b. on week, for person 60 kg TWI = 240 µg Hg week⁻¹ and TWI for MeHg = 1.6 µg per kg of body weight on week, for person 60 kg TWI for THg = 4 µg per kg of body weight on week (EFSA, 2012).

The concentrations of THg and MeHg in the muscles of fish were also correlated positively with the weight of individuals of different species (r ² > 0.9). However, in all the fish species examined, the concentrations of MeHg were strongly, positively correlated with concentrations of THg (r ² > 0.9). This dependence is illustrated for cod and garfish in Figure 2.

Figure 2.

Relationship between the concentration of THg and MeHg in the muscle tissue of cod, garfish. THg: total mercury; MeHg: methylmercury.

Differences in the concentrations of mercury depending on fish food preferences were analyzed in the present study with one-way ANOVA and Tukey’s test. THg and MeHg concentrations in carnivorous fish (cod, garfish, perch, and pikeperch) were significantly different from those in herbivorous fish (sprat, herring, and roach). In contrast, there were no differences in THg and MeHg concentrations between carnivorous and omnivorous (flounder and bream) fish. Differences in MeHg concentrations between carnivorous and non-carnivorous fish are also described by Rivera et al. (2016).

Differences in the percentage of MeHg in THg

Differences in the percentage of MeHg in THg in the muscle tissues of large fish were also confirmed within species (small Figure 1). For cod, the percentage of MeHg in THg ranged from 63.4% to 99.1% and were positively correlated with specimen length (r ² > 0.8). These correlations were not noted in the muscles of small fish, as is indicated by the stable values of the percentage of MeHg in THg. Mean concentration of MeHg constituted more than 80% of the THg in the muscles of cod, garfish, flatfish, sea trout, burbot, and roach; approximately 70% in the muscles of herring, salmon, perch, bream, and mackerel; and less than 60% in sprat, carp, panga, and tilapia (Table 1). The data in the literature reflect a wide range in the percentage of MeHg in THg in the muscles of fish depending on the species: in fish available for purchase on the German market, the mean was 70% within a range of 44–100% (Kubala et al., 2011); in fish caught in the Baltic it was above 82% (Kwaśniak et al., 2012); in predatory fish caught off of the coast of Brazil it was 70% (Kehrig et al., 2008); in commercial fish from Japan it was 67% (Yamashita et al., 2005); in fish caught in the North Sea it was 95% (Baeyens et al., 2003); in fish from Columbia it ranged from 80% to 98% (Alvarez et al., 2012; Marrugo-Negrete et al., 2008); and in fish caught in the vicinity of the Azores it was 88% (Magalhaes et al., 2007).

Consumer health risk assessment

Assessing the risk to consumer health based on comparing the EWI for THg and MeHg with the TWI. The estimated EWI for THg based on these data ranges from 0.08 µg for tilapia to 11.7 µg for garfish (Table 1). The TWI reference dose for THg is 4 µg kg⁻¹ body weight weekly (EFSA, 2012), which means that the TWI for a consumer who weighs 60 kg is 240 µg THg. The EWI for THg comprises a share of the TWI that ranges from 0.03% for tilapia to 4.9% for garfish. The EWI for MeHg ranged from 0.1 µg for tilapia and 9.8 µg for garfish. The TWI reference dose for MeHg is 1.6 µg kg⁻¹ body weight weekly (EFSA, 2012), while for an adult consumer weighing 60 kg it is 96 µg MeHg weekly. The quantity of MeHg ingested with 100 g of fish meat corresponds to a share of the TWI that ranges from 0.1% for tilapia to 10.2% for garfish. These EWI for THg and MeHg represent a small share of the TWI reference dose, which means that the fish available for purchase on the Polish market contains low concentrations of mercury compounds and poses no health risk to consumers. As a reminder, this assessment was made using EWI based on mean concentrations of THg and MeHg for particular species. This assessment did not take into consideration the size of specimens or the resulting increase in concentrations of THg and MeHg with increasing fish length. Assessing risk for small-sized species based on mean concentrations of THg and MeHg in species is precise enough and sufficient for assessing the risks to consumer health. However, in species that have a wide length distribution, error is significant in this type of assessment. Variation within species in concentrations of THg and MeHg in the muscle tissues of fish fundamentally changes the EWI and, consequently, the degree of risk posed to consumer health. This is why the EWI was estimated separately for species with wide length distributions, and specimens of these species were divided into length classes (Table 2), for which the EWI for THg and MeHg was determined. According to these assessments, 100 g of meat from small cod (30–40 cm) contains 3.2 µg MeHg, which is 3.4% of the TWI. However, the same amount of meat from a very large cod (100–110 cm) increases the amount of MeHg to 39.5 µg, which is 41.1% of the TWI. This dose is in excess of 10-fold more than when small specimens are consumed, but it is still safe. Nevertheless, a single portion of fish is generally larger than 100 g, and some consumers eat fish more than once per week. A 200 g portion of meat from a very large cod (100–110 cm) contains 79 µg MeHg, which is 82% of the TWI (the TWI for MeHg is 96 µg/week) and this could pose a risk to consumer health. The same quantity of meat (200 g) from smaller cod (30–60 cm) comprises from 6.6 to 12.6% of the reference dose of MeHg. Consumers also ingest varied doses of MeHg when eating garfish, pikeperch, roach, or bream of different lengths. Health risks linked with MeHg can arise from the frequent consumption of large portions of meat from large fish, but consuming small fish species (flatfish, herring, sprat, and mackerel), even in large quantities, does not pose a health risk with regard to mercury compounds. A 400-g portion of meat from the small species mentioned above contains doses of MeHg that are about 10–30% of the safe dose and do not pose a risk to consumer health even when consumption is frequent. The THQ index is presented as a second indicator for evaluating the impact of mercury on consumer health. The total THQ values calculated using Hg and MeHg base mean concentrations ranged from 0.001 to 0.048 and 0.001 to 0.102, respectively. The THQ values for THg and MeHg in the largest sizes of fish were 0.178 and 0.409, respectively. The estimated values of EWI and THQ do not indicate there is any risk to consumer health posed by the MeHg concentrations in fish muscles.

Generally, it can be concluded that Polish consumer health is not threatened by MeHg exposure from eating fish. Certain precautions are recommended for people who consume fish more than once per week. Statistical data indicate that among Poles, these are mainly people who are either retired or are living on disability benefits. In order to reduce the health threat from exposure to the toxin MeHg, this group of consumers should pay attention to the size and species of fish they consume and also to apply the following principle: if large quantities of fish are consumed frequently, then one should avoid eating large specimens from certain species instead opting for smaller specimens that are more advantageous for one’s health. Another argument in favor of choosing to consume smaller fish is that the selenium they contain has protective properties against the toxicity of mercury (Polak-Juszczak, 2015; Polak-Juszczak and Robak, 2015). The toxicity of mercury is lower when the Se–Hg molar ratio is greater than one (Ralston, 2008; Ralston and Raymond, 2010). The Se–Hg molar ratio in specimens of a given species differs significantly, and it decreases with fish length (Polak-Juszczak, 2015). Consequently, the toxic properties of mercury in small fish are lesser than the results of direct measurements of this element indicate. In fish from the Baltic Sea, the protective properties of selenium occur in flatfish of up to 27 cm in length, in herring up to 26 cm, in cod up to 70 cm, and all sprat (Polak-Juszczak, 2015). Another factor that lowers the risks associated with the toxicity of mercury is its bioavailability after the fish meat is digested in the consumer’s digestive tract. A study by Kwaśniak et al. (2012) for fish from Polish market demonstrated that the share of organic mercury in digested fish meat that is bioavailable and assimilated by the body is 21% for cod, 61% for herring, and 30% for flatfish. The coefficient of organic mercury bioavailability and bioaccessibility depends on the species and precisely on the concentration and protein quality and fat in the fish meat and ranged from 38% to 83% and 9% to 95% respectively (Cabanero et al., 2004, 2007; He and Wang, 2011; Laird et al., 2009; Siedlikowski et al., 2016; Wang et al., 2013). The degree of assimilability of organic mercury from fish meat reduces significantly the dose of mercury absorbed by the body.

Practical applications

Assessing the risk to consumer health based on mean concentrations of MeHg is precise and sufficient enough for small-sized fish species. However, for species of fish that occur within a wide length distribution, the dose of MeHg should be assessed separately in different length classes. Fish consumption of small-sized species poses no health risk from exposure to MeHg. Certain cautionary measures are recommended for consumers who consume fish more than one per week. In order to avoid health risks posed by MeHg, this population group should pay attention to the species and size of the fish they choose to eat and apply the following principle: the consumption of large fish of a give species should be limited, while small fish specimens are a more healthy choice.

Conclusions

Concentrations of MeHg in fish vary depending on age. Large, adult specimens accumulate the most MeHg, and concentrations of it increase with fish length. Differences in concentrations of THg and MeHg within species were confirmed in the muscles of fish of species that have wide length distributions (cod, garfish, herring, pikeperch, roach, bream, and perch). This dependency is less significant in fish of small sizes such as sprat, flatfish, or mackerel. In all of the fish species examined, concentrations of MeHg were strongly, positively correlated with those of THg. Within species, variation in the percentage of MeHg in THg in the muscles of large fish was noted, while this correlation was not noted in small-sized fish. This is why using mean MeHg concentration data from small-sized species is precise and sufficient for consumer health risk assessments. However, for species that have wide length distributions, the amount of MeHg must be assessed separately for different fish length classes.

Generally, fish caught in the Baltic Sea and other fish species available for purchase on the Polish market contain low concentrations of MeHg and are safe for consumption. Nevertheless, certain precautions are recommended for consumers who eat large quantities of fish. They should consider which species of fish and the sizes of fish they consume and keep in mind the principle that the consumption of large specimens of fish (exceeding 70 cm) should be limited.

Footnotes

Acknowledgment

The authors thank Leszek Barcz for assistance in sample preparation and analysis of mercury concentrations.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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