Temporal Trends in Contaminants in Herring in the Baltic Sea in the Period 1980–2002

	Author: ICES/ACME

Key message

Temporal trend analyses of heavy metal concentrations in herring muscle and liver in the Baltic Sea showed twenty significant trends (nine upward and eleven downward) out of fifty tests. The remarkable upward trends were for cadmium (3), zinc (4), and copper (1). The downward trends were for lead (5), mercury (4), and cadmium (2). 

Temporal trend analyses of concentrations of PCBs (polychlorinated biphenyls) and Lindane (?-hexachlorocyclohexane) in herring muscle showed 32 significant trends (all downward) out of 86 tests.

Results and assessment

Temporal trend analyses of heavy metal concentrations in herring tissues showed twenty significant trends (nine upward and eleven downward) out of fifty tests. For cadmium, three areas (Hanöbukt, East of Stockholm, and Gävlebukt) showed a strong significant upward trend, while two areas (Inner Gulf of Finland and Bothnian Bay) showed a downward trend. In areas showing an upward trend, the chemical analyses were conducted on liver, while in areas showing a downward trend, analyses were conducted on muscle tissue. For lead, five areas showed a downward trend, while one area showed an upward trend. The pattern of tissues analysed and the trends were the opposite for cadmium: the downward trends were based on liver tissue, while the upward trends were based on muscle tissue. Mercury showed a significant downward trend in four areas. Four upward trends were found for zinc and one upward trend for copper. These trends are documented in Table A10.1 and are summarized in Figures 1 and 2.

Temporal trend analyses of PCB and Lindane concentrations in herring muscle showed 32 significant trends (all downward) out of 86 tests. These trends are shown in Table A10.2 and are summarized in Figures 3 to 5.

The observed upward trends in the concentrations of cadmium and copper in herring are comparable to observations in other biotic matrices. However, the temporal trends of cadmium and copper in sea water are generally downward. This remarkable difference between matrices has so far no clear explanation.

The downward trends of lead in the majority of the areas studied here are probably the result of the reduced atmospheric input of anthropogenic lead due to the removal of lead from petrol. The upward trend in lead in herring around Åland is probably an artefact of changes in the detection limit of the analytical method.

The causes of the upward trends in copper and zinc are not clear. The upward trend of copper in herring could be a result of the phasing out of TBT as an antifoulant on ships, which is then substituted by new antifouling substances containing copper. It is, however, difficult to explain why this effect should be so pronounced in Gävlebukt.

Both copper and zinc are involved in many metabolic processes and carry out specific physiological roles, which means that the observed changes can be related to natural variability as well as to anthropogenic activities.

The very pronounced reduction in the concentration of PCBs in many areas is probably a result of the regulations and bans on the use of PCBs in the countries around the Baltic Sea.

The downward trend in the concentration of Lindane in herring in five areas is probably a result of regulations and bans on the use of this substance in countries around Baltic Sea, leading to a reduced emission of Lindane.

Tables A10.1 and 10.2 show the results of the temporal trend analysis using a non-parametric Mann-Kendall test. The results of the test are given for each area and substance. For the heavy metals, values for analyses in both liver and muscle are provided. All chemical analyses for PCBs and Lindane were conducted on muscle tissue. In the tables, r is the Kendall tau correlation coefficient, P is the significance level, and n is the number of observations (years) in the time series. The significant upward trends are indicated by P-values shown in red, while significant downward trends are indicated by P-values in green. A significance level of P<0.05 was used.

	Click here to see the results of temporal trend analyses of data on heavy metal concentrations in herring liver and muscle using a non-parametric Mann-Kendall test.   Click here to see the results of temporal trend analyses of data on PCB and Lindane concentrations in herring muscle using a non-parametric Mann-Kendall test.

Figure 1. Temporal trends in concentrations of the metals cadmium (blue), lead (red), and mercury (green) in herring during the period 1980–2002 from different areas in the Baltic Sea. The results of the statistical tests are shown in Table A10.1. X: no significant trend; Ý: significant upward trend; ß: significant downward trend.

Figure 2. Temporal trends in concentrations of the metals copper (red) and zinc (blue) in herring during the period 1980–2002 from different areas in the Baltic Sea. The results of the statistical tests are shown in Table A10.1. X: no significant trend; Ý: significant upward trend; ß: significant downward trend.

Figure 3. Temporal trends in concentrations of the chlorobiphenyls CB28 (blue), CB52 (red), and CB101 (green) in herring muscle during the period 1980–2002 from different areas in the Baltic Sea. The results of the statistical tests are shown in Table A10.2. X: no significant trend; Ý: significant upward trend; ß: significant downward trend.

Figure 4. Temporal trends in concentrations of the chlorobiphenyls CB118 (blue), CB138 (red), and CB153 (green) in herring muscle during the period 1980–2002 from different areas in the Baltic Sea. The results of the statistical tests are shown in Table A10.2. X: no significant trend; Ý: significant upward trend; ß: significant downward trend.

Figure 5. Temporal trends in concentrations of CB180 (blue), the sum of the seven CBs (red), and Lindane (green) in herring muscle during the period 1980–2002 from different areas in the Baltic Sea. The results of the statistical tests are shown in Table A10.2. X: no significant trend; Ý: significant upward trend; ß: significant downward trend.

Metadata

Data on various contaminants (cadmium, lead, mercury, copper, zinc, PCBs, and Lindane) in herring have been collected from several areas of the Baltic Sea. Based on extractions from the ICES database on contaminants in biota, ten areas with sufficient temporal coverage were identified. The areas are listed in Tables A10.1 and A10.2. The length of the time series is rather variable. Some time series extend back to 1980, while others cover only the very recent years. Only time series of more than five years have been considered in this analysis, but the number of years covered in each time series is indicated in the tables. The tissue analysed differs from area to area for most of the heavy metals and this is reflected in Table A10.1.

Temporal trend analyses were made using the non-parametric Mann-Kendall test using a two-sided test with a significance level of 5%. The analyses revealed some significant trends in various contaminants. The detailed results of the analyses can be found in Tables A10.1 and A10.2. In total, 52 of the 138 trend tests showed a significant trend. Even though repetitive testing increases the risk of making a Type I error (finding a significant trend when there actually is no trend), the number of significant trends is well over what would be expected (approximately seven) from this error. The time frame for this analysis is the period 1980–2002. Data from some years (especially 2002) are lacking in some areas.