This paper presents the results of the first-ever study on the accumulation of cadmium in different parts of fish scales during exposure to cadmium in water.
As the scale grows, its diameter increases as a result of sclerite formation under its entire surface, where it adheres to the dermis. The outermost region (ring) is entirely new and contains microelements incorporated over the most recent period in the fish’s life – and this is where the highest increase in the concentration of microelements to which the fish is currently exposed is expected. However, the inner part of the scale may also contain new microelements that accumulate in the bottom layer during scale growth. Therefore, the concentration of Cd in this part of the scale was also expected to increase, but to a lesser extent compared with the outer part.
The analysis of the results obtained showed that the concentration of cadmium in the scales of Prussian carp exposed to Cd at a concentration of 0.4 mg/L for a period of 42 days increased statistically significantly both in the inner parts of scales, where a six-fold increase in Cd concentration was observed, and in the outer parts of scales, where a 13-fold increase was observed. In turn, in fish exposed to 4.0 mg Cd/L, a 48-fold increase in Cd concentration was observed in the inner parts of scales and a 68-fold increase in the concentration of the metal was found in the outer parts of scales, which confirms the sensitivity of scale tissue to the presence of cadmium in water. The results of the present study confirm that fish scales can accumulate cadmium and can thus be used as a bioindicator tissue. In another study, which investigated the effect of chronic exposure of sea bass (Dicentrarchus labrax) to 0.5 µg Cd/L (for 8 consecutive days, 4 hours per day), a significant increase in the concentration of cadmium (from 0.052 to 0.147 µg/g) was observed in the scales after three days of exposure. Moreover, the concentration of cadmium in the scales of the exposed fish remained significantly higher compared with control fish until day 21 of exposure. In contrast, the exposure to cadmium did not have a significant impact on the concentration of the metal in the gills of the fish studied (Faucher et al. 2008). A study by Rashed (2001) on the concentration of various elements in the tissues of Nile tilapia (Tilapia nilotica) found that the level of accumulation of the elements differed between tissues. In that study, the highest concentrations of zinc, copper and cobalt were found in the liver, the highest levels of iron and manganese – in the tissues of the digestive tract, and the highest levels of Cr, Ni and Sr – in the scales of the fish. The gills of Nile tilapia showed the lowest levels of the elements analysed, even though they serve as the first barrier between the environment and the fish.
The analysis of the present findings showed that while the concentration of cadmium in the scales of Prussian carp exposed to the higher concentration of Cd was higher, significantly lower BCF values, both for the inner and outer parts of scales, were found in those fish after 42 days of exposure (Table 2). The results indicate that fish accumulate higher proportions of cadmium when exposed to lower concentrations of this metal. It is very important to ensure that low levels of heavy metals in the environment, those that slightly exceed the permitted thresholds, are not disregarded during the monitoring of the chemical status of open waters in the belief that fish can handle them. The findings from the present study indicate that chronic exposure to heavy metals also results in their accumulation. This is confirmed even more strongly by the increase in Cd concentration in the scales of control fish, which were kept in mains-water without the addition of cadmium. The increase in Cd concentration in the scales collected from those fish was probably due to the trace levels of Cd present in the standard feed given to the fish. A study by Łuszczek-Trojnar and Nowacki (2021) confirms that the concentration of heavy metals in the scales of cyprinids increases with time, even in those fish that are not experimentally exposed to heavy metals.
In the present study, a further increase in Cd concentration was observed after 84 days of exposure in group Cd1 and group Cd2. It was only in the case of the inner parts of the scales of fish in the control group and group Cd2 that the increase in Cd concentration was not statistically significant (Table 1). The results obtained indicate that the accumulation of cadmium in fish scales is correlated with both Cd dose and the duration of exposure. This is also confirmed by the highly statistically significant correlation coefficients between the concentration of cadmium in scales and the exposure concentration of the metal, which were 0.75 and 0.87 (p < 0.0001), respectively, for the inner and outer parts of scales, and the highly statistically significant correlation coefficients between Cd concentration in scales and the duration of exposure, which were 0.67 and 0.82 (p < 0.0001), respectively, for the inner parts of scales collected from fish in groups Cd1 and Cd2, and 0.5 and 0.92 (p < 0.0001), respectively, for the outer parts of scales collected from fish in groups Cd1 and Cd2 (Table 3).
The bioconcentration factor increased with the duration of exposure to cadmium. The increase was statistically significant in each of the groups studied. However, differences in the BCF values between day 42 and 84 of exposure were more statistically significant for fish in group Cd2 (p < 0.0001). Differences in the BCF values between the Cd1-in and Cd2-in subgroups and between the Cd1-out and Cd2-out subgroups ceased to be statistically significant, which indicates that the longer the period of exposure, the more cadmium accumulated in fish from group Cd2.
It seems that metal concentration in the outer parts of scales reflects the recent status of environmental pollution, but is also subject to dynamic changes where the contamination continues. The analysis of two consecutive 42-day periods of exposure of Prussian carp to different concentrations of cadmium in water showed that the increase in the concentration of the metal in the scales analysed was not proportional to either the dose of the metal or the duration of exposure. Despite the same dose and a further 42-day period of exposure, the increase in the concentration of cadmium was lower than at baseline. A study by Łuszczek-Trojnar et al. (2013), in which fish were exposed to different concentrations of lead in feed, found that the accumulation of the metal in scales was highest at the beginning of the period of exposure and that the levels of the metal in particular tissues stabilised in the subsequent months of chronic exposure, depending on the concentration of the metal in the environment. The significant decrease in the level of cadmium accumulation observed in the present study after 84 days of exposure indicates that the concentration of the metal in the fish studied was approaching a plateau and that a steady, yet statistically insignificant increase in cadmium levels in scales would have been observed if the period of exposure had been extended.
Studies by other authors confirm the usefulness of fish scales as a bioindicator of environmental contamination by heavy metals (Rauf et al. 2009; Valova et al. 2012, Zubcov et al. 2012). Łuszczek-Trojnar and Nowacki (2021) investigated the concentration of selected heavy metals in particular annuli of the scales of carp from two pond farms in southern Poland. They found that the concentration of the metals differed between particular parts of the scales, increasing towards the outer growth rings. Understanding the concentrations of heavy metals in particular annuli of the scales of fish from unpolluted environments should be part of basic knowledge, necessary to determine reference values which could be used in the future as a benchmark for metal concentration thresholds in the scales of fish living in the natural environment. The concentrations of cadmium found by Łuszczek-Trojnar and Nowacki (2021) in the scale annuli of carp formed in the first, second and third year of life were 0.2, 0.25 and 0.48 mg/kg, respectively. The difference in cadmium concentration between the annuli formed in the first and third year of life was statistically significant.
Cadmium accumulates in scales to a much lower degree compared with other recognised indicator tissues, such as the liver or kidney. Studies by Drąg-Kozak et al. (2018 and 2019) found that the concentration of cadmium in the kidneys of Prussian carp exposed to 0.4 mg Cd/L for a period of 7 weeks was 16.1 ± 1.25 mg Cd/kg and increased to 23 ± 1.41 mg/kg after the exposure period was extended to 13 weeks. In contrast, in fish exposed to 4.0 mg Cd/kg, the concentration of cadmium did not increase from the level of 112.5 ± 3.92 mg Cd/kg recorded after 7 weeks of exposure when the exposure period was further extended to 13 weeks. The kidney is a recognised bioindicator tissue of cadmium accumulation as it is highly sensitive to the presence of the metal in the fish body. This is confirmed by the nearly 300 times higher concentration of Cd found in the kidneys of fish exposed to cadmium at a concentration of 4.0 mg/L compared with control fish. However, kidney tissue does not grow with time and no further increase in the accumulation of cadmium in the tissue is observed once it has become saturated with Cd, despite continued exposure to the metal. Similarly, it was found that the concentration of Cd in the blood of fish exposed to 4.0 mg Cd/L for a period of 13 weeks was 1.13 ± 0.08 mg/kg and did not differ significantly from the blood concentration of the metal recorded after 7 weeks of exposure (1.23 mg Cd/kg) (Drąg-Kozak et al. 2019). In turn, in the present study, the concentration of Cd in scales increased continuously until day 84 of exposure (Table 1).
The results of the present study on Prussian carp (Carassius gibelio B.) showed that Cd can accumulate in fish scales and that its concentration in scales is correlated with the exposure dose of the metal and the duration of exposure, which might argue in favour of using fish scales as a bioindicator of cadmium contamination in aquatic ecosystems. Fish scales seem to be an excellent indicator tissue as they reflect very well the status of environmental pollution and are easy to sample non-lethally. Scales can be collected for analysis during the standard fish sampling procedure to assess the ecological status of waters under the National Surface Water Quality Monitoring programme. Each scale collected for analysis is replaced by a new scale, which prevents potential bacterial, viral and parasitic infections that could be caused through the contact of the skin with water. The protective barrier of the tegument remains intact and the fish whose scales have been collected recovers quickly and returns to feeding.