Metals in shellfish

Metals in fish

Organochlorines in shellfish

Organochlorines in fish liver

Organochlorines and mercury in gannet eggs

Data summarised from the 'National Monitoring Programme Survey of the Quality of UK Coastal Waters' (MPMMG 1998))

Two shellfish species were specified for use in the NMP survey, the common mussel Mytilus edulis (L.) and the horse mussel Modiolus modiolus.

Data were submitted for metals in shellfish for 26 NMP sites around the UK coasts and estuaries. The NMP survey concluded that, while there were some gaps in the available dataset, on the basis of the available data, contamination by the metals examined was not a significant problem in UK estuaries.

Median cadmium concentrations ranged from 0.96 mg kg^-1 dry weight at two sites on the Welsh side of the Dee estuary to 9.78 mg kg^-1 dry weight in the Severn estuary. The range of median concentrations reported in the NMP survey was stated to be consistent with those previously reported for UK estuaries and coastal sites.

The Joint Monitoring Programme (JMP) of the Oslo and Paris Commissions has adopted arbitrary descriptive guidelines for cadmium in mussels of up to 2 mg kg^-1 dry weight as in the "lower" level, between 2-5 mg kg^-1 as the "medium" level, and >5 mg kg^-1 as the "upper" level (MAFF, 1987). In this context, the highest cadmium concentrations reported for the UK NMP survey, in the Severn estuary, fall into the Aupper" category. Results for one site in each of the Clyde, Mersey, and Severn estuaries fall into the "medium" category, while results for the remaining 11 sites are in the "lower" category.

In Scotland, a recommended standard of 15 mg kg^-1 dry weight has been set for cadmium in mussels under the terms of the EC Shellfish Growing Waters Directive, specifically to ensure that "the concentration in the shellfish water or in the flesh must not reach or exceed a level which gives rise to harmful effects on the shellfish or their larvae" (ADRIS, 1982). The NMP survey concluded that the results presented by the NMP survey suggested that harmful effects due to cadmium contamination were unlikely in any of the UK estuaries surveyed.

Median mercury concentrations ranged from 0.084 mg kg^-1 dry weight in Southampton Water to 0.804 mg kg^-1 at the intermediate site off the Lune and Wyre estuaries. These concentrations are lower than levels reported for contaminated estuaries elsewhere in the UK (0. 12-2.3 mg kg^-1) and for European shores (0.012-1.02 mg kg^-1), but similar to levels found at UK coastal sites (0.11-0.38 mg kg^-1).

The ADRIS (Association of Directors and River Inspectors in Scotland) recommended standard set under the EC Shellfish Growing Waters Directive is 3 mg kg^-1, well above the highest reported concentration. The NMP survey concluded from the results presented that harmful effects on mussels from mercury contamination were unlikely.

Median lead concentrations ranged from 3.01 mg kg^-1 dry weight at the intermediate site in the Firth of CIyde to 13.5 mg kg^-1 at Broughty Castle near Dundee in the Tay estuary. The elevated concentrations in the Tay estuary mussels were said to be consistent with high lead levels reported for sediments collected at the mid-channel NMP site off Broughty Castle. Concentrations above 10 mg kg^-1 were also recorded for one site in each of the Forth and Clyde estuaries, which have been subject to historical industrial contamination.

Lead levels at the NMP sites were found to be slightly lower than median concentrations previously reported for contaminated estuaries in the UK (3.5-39 mg kg^-1) but similar to levels reported for UK coastal sites (<3.0-29 mg kg^-1) and European shores (0.6-27.6 mg kg^-1). Standards are not available for lead in mussels, although the ADRIS recommended standard for shellfish growing waters has been set at 50 mg kg^-1 dry weight which the NHP survey suggests /indicates that shellfish are not at risk.

Median zinc concentrations ranged from 84-90 mg kg^-1 dry weight at Selsey Bill and in the Solent to 204-209 mg kg^-1 at the outer most sites in the Tyne and Humber estuaries. The NMP survey reports these to be broadly similar to levels reported at coastal sites in the UK (58-303 mg kg^-1) and Europe (55-294 mg kg^-1), and to results for UK estuaries (91-330 mg kg^-1).

The NMP survey suggests that the high concentrations reported for mussels in the Mersey should be investigated since they approach the ADRIS recommended level of 500 mg kg^-1 set under the terms of the EC Directive on Shellfish Growing Waters to protect shellfish and their larvae.

The NMP determinands were mercury and arsenic in fish muscle tissue and cadmium and lead in fish liver. The coverage achieved in the survey, particularly for intermediate and offshore sites, was considered to represent the best attempt to date to obtain a synoptic survey across the whole of the UK. Only in some estuaries are there significant gaps.

The NMP survey states that the results are reassuring in that they generally confirm those from previous surveys, which have demonstrated that contamination is confined to a few well known geographical locations.

Results were generally as would have been expected from previous surveys, with highest concentrations found in Liverpool Bay and Morecambe Bay, areas that have been subject to considerable inputs of mercury via discharges from the chloro-alkali industry. The results presented confirm those from more specific local studies, e.g. the Mersey.

Levels were relatively high in some other inshore areas, e.g. the Thames estuary, with an observed general reduction in concentrations offshore. One unexpected result was the mercury concentrations in dab from inshore Cardigan Bay, which were relatively high in both CEFAS and Environment Agency samples.

Concentrations were generally low around the coast of Scotland, with only one value, from the Minches, exceeding 0.1 mg kg^-1 wet weight. This was for megrim, which cannot be compared directly with other species. Plaice from the same station contained low concentrations of mercury (0.03 mg kg^-1).

Median mercury concentrations in dab ranged from 0.01- 0.02 mg kg^-1 in Dundrum Bay and off the Tamar in the English Channel, to 0.26 mg kg^-1 in the Burbo Bight off the Mersey. No median values therefore fell in the "higher" level category (>0.3 mg kg^-1) designated by the Joint Monitoring Group of the Oslo and Paris Commissions and virtually all were 50% or less of the maximum limits for mercury in fishery products (0.5 mg kg^-1) designated in European Community Commission Decision 93/351/EEC.

The range of the mean concentrations of mercury in those dab samples collected from North Sea NMP stations during the present survey was 0.03-0.11 mg kg^-1 wet weight, generally similar to that (0.05-0.12 mg kg^-1) found in dab samples collected in the 1989-91 period under the Monitoring Master Plan designed to provide data for the North Sea Quality Status Report.

Median concentrations in the essentially estuarine samples of flounder ranged from <0.01 to 0.08 mg kg^-1.

There appeared to be little obvious spatial pattern in the concentrations observed. The highest levels in dab (~20 mg kg^-1) were found off the Tees, Tay and Forth estuaries. However, concentrations were almost as great in the Moray Firth area, indicating that the levels found are not likely to be entirely due to anthropogenic inputs. The highest arsenic concentration found in fish muscle was in the Minches, which would be expected to be relatively free of anthropogenic contamination. This was, however, in non-recommended species, megrim, and it is known that very considerable differences occur between species in the accumulation of arsenic in fish (Falconer et al., 1983).

Concentrations of arsenic in flounder were relatively low, with a maximum concentration of ~6 mg kg^-1.

Unfortunately, arsenic has not yet been included in most international programmes, so data for comparison purposes from previous monitoring are limited.

Concentrations appear to be highest off the east coast of Scotland, with a maximum median of around 0.8 mg kg^-1 wet weight off the Tay. The maximum median concentration in English waters (0.37 mg kg^-1) was offshore of the Tyne-Humber , previously identified in the sampling carried out under the North Sea Monitoring Master Plan as an area where cadmium concentrations were above the upper quartile of the survey results.

The maximum values found in the earlier North Sea survey was 0.96 mg kg^-1 cadmium, with the median concentration 0.18 mg kg^-1.

Cadmium concentrations were relatively low in the Northern Irish samples and in all samples of flounder.

The range of concentrations of lead found in dab liver was relatively small, from 0.03 mg kg^-1 in the Moray Firth to a maximum of 0.58 mg kg^-1 off the coast of Northern Ireland. Generally low concentrations were recorded in dab taken from Scotland, but plaice taken off the Tay had the highest level for all the fish sampled 0.62 mg kg^-1.

There are unfortunately few earlier data that could be used for comparative purposes, since in previous years, the methods generally employed were rarely able to provide low enough detection limits to yield positive results. In the early 1990s, for example, concentrations in dab liver in the samples collected under the North Sea Monitoring Master Plan were all <0.6-< 1.0 mg kg^-1.

NMP specified the determination of dieldrin, aldrin, endrin and isodrin. The most common compound in biological samples is dieldrin. Other drins tend to revert to dieldrin in the natural environment and are unlikely to be found unless the organism has been recently exposed.

Only data for dieldrin were reported at concentrations greater than the limit of detection. Dieldrin was detected at five sites at concentrations ranging from 0.006 to 2 mg kg^-1. All data were below the ADRIS "no harmful effects" concentration of 100 mg kg^-1.

More than half the data reported for pp-DDT was below the limit of detection of the analysis. Positive data for pp-TDE were reported at both sites in the Forth and one site in the Humber. The concentration found in the Forth is approximately an order of magnitude less than that found in the Humber. Data for pp-DDE were reported at eight sites at concentrations ranging from 0.007-6.5 mg kg^-1. All data were below the ADRIS "no harmful effects" concentration of 100 mg kg^-1.

Data greater than the limit of detection were reported at five sites for gamma-HCH . Concentrations were low (0.25-5.5 mg kg^-1) and well below the ADRIS "no harmful effects" concentration of 30 mg kg^-1. Positive data for %-HCH were reported at only one site (Selsey Bill).

NMP specified a range of congeners for analysis. Data are presented for PCB 153 as this is the congener for which most data were available. Positive data were reported at seven sites, two in the Forth, two in the Humber and three in Belfast Lough. The highest concentrations occurred in the Humber (maximum 12.8mg kg^-1). Concentrations reported in Belfast Lough (0.006 mg kg^-1) were below the limit of detection of all other sites. Belfast Lough and the Forth samples fall into the lower contamination range for PCBs in molluscs (JMP guideline values) whereas the Humber sites fall into the medium range.

Positive data were only reported in the Humber (26.9 mg kg^-1) and the Forth (0.31 mg kg^-1). The sediments of the Forth have been shown to be contaminated with HCB as a result of past discharges (Harper et al, 1992) and provide a source of HCB to the resident biota.

Only two sets of data were returned for PCP and both sets had more than 50% of data less than the limit of detection. PCP is known to metabolise rapidly and is consequently difficult to analyse for in mussels.

The NMP determinands were 11 individual polychlorinated biphenyl congeners (PCBs 28, 52, 101, 105, 118, 128, 138, 153, 156, 170 and 180), dieldrin, aldrin and endrin and three DDT group compounds (pp-DDT, pp-TDE and pp-DDE). As with the metals the first choice species was dab with flounder as an alternative.

PCB 153 (2,2',4,4',5,5'- hexachlorobiphenyl) is generally considered to be highly persistent in marine biota and therefore is most often the dominant congener in a typical chlorobiphenyl profile. This, coupled with the relative ease of measurement, results in it often being used as an indicator chlorobiphenyl and this is the case for the purposes of the NMP report.

PCB 153 was detected at more than 90'% of the 39 stations for which data were presented, with more than 50% of the stations having values exceeding 20 mg kg^-1. Seven stations exceeded 50 mg kg^-1, while the highest median values (in excess of 100 mg kg^-1) were reported for the three stations; the Forth at Kingston Hudds Liverpool Bay at Burbo Bight and Liverpool Bay, offshore with values of 120, 160 and 170 mg kg^-1 respectively. The last two values are consistent with previous data that have put contamination in the medium and upper categories of the JMP guideline levels for PCBs in fish tissue. Although concentrations generally declined offshore, PCB 153 was present at all stations. The relatively high levels in Liverpool Bay are consistent with the results of more localised studies in that area.

Dieldrin was detected in more than 90% of the stations for which data were reported. Highest median values were reported for Liverpool Bay; Off Lune/ Wyre ; Thames and offshore Moray Firth, with median values of 35, 22, 50 and 72 mg kg^-1 respectively

The results for the nine stations for which data were presented were all less than the limit of detection, which is not surprising as aldrin is readily converted to dieldrin in many environmental compartments.

The coverage for endrin was wider than for aldrin, with data for 13 stations being produced. However, only two stations, Moray Firth and Belfast Lough yielded positive results, with median values of 5 and 6 mg kg^-1 respectively . Although endrin is highly toxic, it is much less persistent than dieldrin and its occurrence was not expected to be widespread.

Three DDT compounds were required to be determined, namely, pp-DDE , pp-TDE and pp-DDT. The highest median level concentrations for all three compounds were found at the sites off the north- west of England with median values of 15-91 mg kg^-1 for pp-DDE; 23-130 mg kg^-1 for pp-TDE and 13-28 mg kg^-1 for pp-DDT. These values are within the "expected range" for these compounds in fish liver, with the stable metabolite pp-DDE being the dominant component. The concentrations of these compounds generally decreased at offshore sites, with stations off Wales and the south-west coast of England being close to or below the limit of detection. The concentration patterns were similar for pp-DDE and pp-TDE. However, four stations off the east coast of England appear to have higher levels with median values ranging from 6.5 to 20 mg kg^-1 for pp-DDT. No corresponding data were presented for the estuarine sites in this area.

During 1971-1997, eggs have been obtained from up to eight different gannet colonies around Britain and Ireland as part of an on-going monitoring programme funded by the Joint Nature Conservation Committee (JNCC). Although residues of DDE, HEOD and PCB have generally declined, some sites have shown an increase in residues, particularly of PCBs and mercury, at least over part of this period (Newton et al 1999).

References