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Variability in the edible fraction content of 60Co, 99Tc, 110mAg, 137Cs and 241Am between individual crabs and lobsters from Sellafield (north eastern Irish Sea)
Journal of Environmental Radioactivity 54 (2001) 311–326
Variability in the edible fraction content of Co, 99Tc, 110mAg, 137Cs and 241Am between individual crabs and lobsters from Sellafield (north eastern Irish Sea)
60
D.J. Swift*, M.D. Nicholson The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK Received 17 February 2000; received in revised form 26 June 2000; accepted 6 July 2000
Abstract We investigated the variability in anthropogenic radionuclide content of the edible fractions of individual edible crabs (Cancer pagurus L.) and European lobsters (Homarus gammarus L.) caught commercially in the Sellafield offshore area. Sixteen female and 18 male crabs and 20 female and 17 male lobsters were selected from commercial catches made between 25 May and 5 June 1997. Each gender group was selected to be within the known weight range for commercially caught crustacea from the area. Four artificial radionuclides (60Co, 110mAg, 137 Cs or 241Am) were detected by g -spectrometry. The edible fraction content of these radionuclides between males and females for either species were not statistically significantly different. 99Tc was analysed by chemical separation and b -counting. 99Tc concentrations in female crabs tended to be higher (172 205 (16) Bq kg ÿ 1(wet); mean standard deviation (n samples)) than those in males (85 58 (18) Bq kg ÿ 1 (wet)), although this was not a statistically significant difference. For both male and female crabs, 99Tc concentrations tended to decrease with increasing whole live weights. For 99Tc in lobsters the picture is less clear. Female lobsters contained more activity (14800 7400 (20) Bq kg ÿ 1 (wet)) than males (7100 3900 (17) Bq kg ÿ 1 (wet)). The results were used to discuss the implications for sampling and monitoring. Crown Copyright # 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Radioactivity; Sellafield; Crabs; Lobsters; Variability
*Corresponding author. Tel.: +44-1502-52-4445; fax: +44-1502-513-865. E-mail address: [email protected] (D.J. Swift). 0265-931X/01/$ - see front matter Crown Copyright # 2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 3 2 - 6
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1. Introduction Commercially caught crustacea from the Sellafield coastal area are regularly sampled and analysed for radioactivity as part of the Ministry of Agriculture, Fisheries and Food’s (MAFF) statutory surveillance of authorised radioactive liquid waste discharges from the Sellafield nuclear fuel reprocessing plant (Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency, 1999). Edible crabs (Cancer pagurus L.) and European lobsters (Homarus gammarus L.) have been monitored routinely for a range of radionuclides, including transuranics, annually as part of this programme since the 1960’s. In 1994 the Enhanced Actinide Removal Plant (EARP) came on stream at Sellafield. The introduction of this new stage in the reprocessing scheme has greatly reduced the discharges of the most radiologically significant nuclides, such as 106Ru, 137 Cs and the transuranics, in liquid waste to the sea. However, the new plant’s efficiency for removing some other radionuclides, such as 90Sr and 99Tc, is low (Hunt, Smith & Camplin, 1998). From early 1994, decay-stored, medium activity liquors have been reprocessed through EARP and these liquors contain large amounts of 99Tc. As a consequence of the low recovery of 99Tc, discharges of this radionuclide to the sea have increased. Annual 99Tc discharges in liquid waste before 1994 were relatively small averaging 4.1 1.4 TBq between 1981 and 1993. However, with the start of reprocessing stored liquors through EARP the annual discharges increased substantially: 72 TBq in 1994, 192 TBq in 1995, 155 TBq in 1996, 84 TBq in 1997 and 53 TBq in 1998 (Hunt et al., 1998; Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency, 1999). A consequence of the increased 99Tc discharges to the sea after 1993 has been a sharp increase in 99Tc concentrations measured in some marine biota. This is especially so for some seaweed and some crustacea species although these increases are of negligible radiological significance (Hunt et al., 1998). Of the commercially important food animals caught off Sellafield, the increased activities recorded in lobsters have attracted the most attention. This species has an anomalously high 99Tc accumulation capacity compared with other crustacea, such as the edible crab, a phenomenon first observed and investigated in the 1980s (Masson, Aprosi, Laniece, Guegeniat & Belot, 1981; Pentreath, 1981; Swift, 1985). Consequently, the increased 99 Tc content in the edible fraction of lobsters, and to a lesser extent crabs, after increased discharges of 99Tc had been predicted and an allowance made in setting new Sellafield 99Tc discharge authorizations to cover the planned reprocessing of stored liquors through EARP. The radioactivity content of marine biota derived from radioactive liquid waste discharges to the sea is a function of many factors. These include the concentration and discharge rate of the radionuclides in the effluent, their subsequent degree of dilution in the sea and possible chemical speciation changes in the sea due to geochemical factors before interaction. Biological factors influencing radionuclide bioconcentration include physiological and biochemical differences due to genetic
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313
effects between species and biometric differences, such as in age, size and gender between individuals within a species. For MAFF’s aquatic biota surveillance programme, radionuclides are measured in a bulked, edible fraction sample of sufficient size to complete all of the radiological analyses. Since each animal’s live weight is different, the number sampled to make up the required edible fraction weight varies at each time of sampling. Although consumers cannot usually select by gender, for example mollusca or fish fillets, selection by size may occur with individual crabs or lobsters. Crustacea monitoring samples are therefore collected without regard to gender but do cover the size range available to consumers at the time. However, information on the gender, number and weight of the individual crustacea making up a routine surveillance sample is not usually recorded. With the current interest in 99Tc accumulation by crustacea, it was felt that information on the variability of both 99Tc and other radionuclides with gender and size in lobsters and crabs should be obtained. This information could also be used as a check on the robustness of the current sampling protocol. This project was therefore set up to analyse the edible fractions of individual lobsters and crabs for 99 Tc and g -emitting radionuclides and to statistically analyse any apparent correlations with biometric factors, such as body weight and gender.
2. Materials and methods The statistical design of the sampling required the analysis of a minimum of 32 lobsters and 32 crabs sampled from commercial catches and divided half-and-half male and female. In turn, each gender group was to be selected to cover the known weight range of commercially caught crustacea from the area. The effect of temporal variation in the individual radionuclide concentrations at source, especially of 99Tc, had also to be considered. Unfortunately, the pattern of radionuclide discharges from the Sellafield site on a day to day basis is unknown. Discharge data for the Sellafield plant are published but they are only available at some time after the discharge has occurred. For 99Tc only monthly discharge values are available. Since we had no way of predicting when particular radionuclides would be discharge or in what quantities, we had to complete the sampling in the shortest possible time. We also had to contend with the unpredictability of the fishing effort needed to supply the required number of individuals of the required sex and weight range at any particular time. A sampling period of approximately two weeks was achieved. Two local commercial fishermen who regularly fish for crustacea in areas near the end of the Sellafield pipeline were employed. Suitable samples were selected from catches made between 25 May and 5 June 1997. Potential sampling areas were a fishing area close to the end of the Sellafield pipeline (station A), about 2.5 km offshore, and another to the south east of this, off the Kokoarrah Rocks (station B). Both fishing areas are used to supply crustacea for the Sellafield component of the aquatic surveillance programme conducted by CEFAS for MAFF.
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During the selected sampling time neither fishing area alone yielded enough animals of the required weight range and gender to satisfy the sampling requirements. The total numbers for each species therefore included animals from both areas. After selection by CEFAS staff at Whitehaven, the animals were weighed and measured and then killed. The animals were cooked using the normal sample protocol for the MAFF surveillance programme of boiling in salted freshwater for twenty minutes. The sex, the carapace length for lobsters or width for crabs (mm) and the whole body live and cooked wet weights (g) were recorded for each animal. The animals were dissected individually. The brown meat and white meat fractions were removed separately, weighed and recorded. The brown meat fraction for both crabs and lobsters consisted of the digestive gland only. The white meat fraction consisted of the claw muscle and body cavity muscle for crabs and the claw and tail muscle for lobsters. Muscle tissue from the walking legs was not included for either species. The two fractions for each individual were then combined and the cooked wet weights of the individual total edible fraction recorded. The labelled samples were deep-frozen for transport to CEFAS, Lowestoft for radioanalysis. On receipt the samples were individually dried, homogenized and prepared in an appropriate geometry for g -spectrometry. Analysis by g -spectrometry was carried out using a GeLi counter with an enhanced low-energy response to improve detection of 241Am (limit of detection 0.4 Bq kg ÿ 1; absolute counting efficiency 18.9% at 59.54 keV for the given geometry and counting time; FWHM of 0.83 keV). After g -spectrometry sub-samples were dissolved in acid and, following a series of chemical separation steps, the 99Tc activity was estimated by b -counting using a gas proportional detector. All activity results were reported as Bq kg ÿ 1 (wet weight) of the cooked edible fraction. The extent to which variation in whole body weight may, through sampling, have become inadvertently associated with gender, sampling site or sampling association was evaluated simply by making Student’s t-tests to compare the corresponding mean whole body weights. Gross summaries (means, standard deviations and coefficients of variation) were computed for each radionuclide. Potential outliers were identified from either the scaled distance from the mean, or from scatterplots between each radionuclide and the weights of various body fractions. Outliers for which there were no transcription or other identified errors were noted, but retained in the data set. The relationship between radionuclide concentration and the weight of the edible fraction was tested by fitting logðconcentrationÞ ¼ a þ b logðedible fraction weightÞ þ noise;
ð1Þ
where the noise term was assumed to have a Normal distribution with zero mean and constant variance. These assumptions were validated by a subsequent examination of the residuals from the fitted model. An estimated value of b =0 implies that concentration does not vary with the weight of the edible fraction. Further, since load is obtained by multiplying the
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edible fraction concentration by the edible fraction wet weight, i.e. concentration ¼
load ; edible fraction weight
we can write log ðloadÞ ¼ log ðconcentrationÞ þ log ðediblefractionweightÞ: Then, from Eq. (1), log ðloadÞ ¼ a þ ðb þ 1Þ log ðedible fraction weightÞ and hence a value of b = ÿ 1 in Eq. (1) implies that load does not vary with edible fraction weight. Radionuclide levels expressed as either concentration or load vary with edible fraction weight for values of b other than zero or unity. To minimise the influence of any outliers, the model was fitted using a robust method. Observations with large residuals were down-weighted, the model refitted, residuals re-calculated, and so on until a stable fit was achieved. The model was further tested to see if the intercept, a , depends on gender. i.e. that, for an animal of a given weight of the edible fraction, the mean radionuclide concentration (or load) might be higher for males or females. Initial statistical analysis was carried out using Microsoft-Excel TM(Microsoft Corporation, Seattle) spreadsheets. Further statistical analysis was carried out using the statistical package S-PLUS TM(MathSoft, Inc., Seattle).
3. Results 3.1. Edible crabs 3.1.1. Biometric data A total of 34 crabs were sampled, 20 from station B and 14 from station A. Of the total 16 were female and 18 male (Table 1). The mean whole body wet weight for all crabs was 490 141 (34) g. The mean carapace width was 141 12 (34) mm. There were no statistically significant differences between the mean weight and carapace width of the crabs collected from the two sampling stations. There were statistically significant differences between the sexes for some biometric factors (Table 1). Male crabs were on average heavier and had a significantly greater mean white meat weight compared with those for females although there was no significant difference between the mean weights of the brown meat fractions (Table 1). Analysis of variance showed a small but statistically significant difference in the mean body weight between samples taken different sampling days. However, the 12 day sampling interval was judged to be sufficiently short that any associated differences in radionuclide concentrations could most likely be attributed to weight and not to temporal changes in the source levels. The data were therefore combined without regard to capture date or station.
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Table 1 Mean crab biometric data Carapace width (mm)
Whole live weight (g)
Whole cooked weight (g)
Brown meat weight (g)
All Mean s n
141 11.8 34
490 141 34
439 122 34
59.4 22.5 34
90.6 29.5 34
148 40.9 34
Females Mean s n
140 14.9 16
419 134 16
384 126 16
65.9 28.8 16
70.0 16.9 16
133 42.7 16
Males Mean s n
142 8.5 18
552 118 18
488 98.9 18
53.6 13.0 18
109 26.2 18
161 35.1 18
t-test p-value
nsa
50.01
50.02
ns
50.001
50.05
a
White meat weight (g)
Edible fraction weight (g)
ns: not statistically significant.
3.1.2. g -spectrometry results Four artificial radionuclides: 60Co, 110mAg, 137Cs and 241Am (Table 2) were detected by g -spectrometry. The natural radionuclide 40K was also detected. Since potassium in biota is invariably under homeostatic control, there should be little variation in the concentration of 40K between individuals of the same species and sex. The 40K values have therefore been included as a possible physiological marker against which to assess any variability in the content of the other nuclides with sex or body size. The four artificial radionuclides detected by g -spectrometry varied considerably in concentration between individuals (Table 2). 110mAg showed the largest variability (coefficient of variation (CV)=79%) and 241Am the smallest (CV=5%). For 40K (CV=13%) there was one anomalously low value for a male crab. However, in other respects the biometric and radionuclide data for this individual were unexceptional and the analytical data were included in further statistical analysis. 3.1.3. 99Tc results 99 Tc concentrations varied more between individuals (CV=120%) than did the other radionuclides (Table 2). For the 34 crabs sampled the minimum concentration detected was 22 Bq kg ÿ 1 and the maximum 905 Bq kg ÿ 1 with a mean of 126 151 (34) Bq kg ÿ 1. The large standard deviation was due to the maximum concentration measured. This value is well over four standard deviations from the mean value and was considered to be an outlier. When removed from the calculation, the mean
317
D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 2 Mean crab radioanalytical data (Bq kg ÿ 1 (wet)) 40
K
60
Co
99
110m
Tc
Ag
137
241
Cs
Am
All Mean s n
93.0 12.5 34
1.9 1.1 34
126 151 34
4.2 3.3 34
3.1 0.8 34
1.7 0.8 34
Females Mean s n
87.8 7.5 16
2.0 1.2 16
172 205 16
3.9 3.9 16
3.1 0.9 16
1.7 0.9 16
Males Mean s n
97.7 14.3 18
1.8 0.9 18
85 58 18
4.4 2.8 18
3.2 0.8 18
1.7 0.8 18
t-test p-value
50.02
ns
ns
ns
ns
a
nsa
ns: not statistically significant.
becomes 102 63 (33) Bq kg ÿ 1. However, the crab for which this value occurred, a female, also had the highest concentrations of 110mAg, 60Co and 137Cs, although these did not appear to be outliers. The 40K concentration was close to the mean value. As there was nothing exceptional about the biometric values for this crab, the analytical data were retained in subsequent analysis. 3.1.4. Regression and further statistical analysis Possible relationships between the content of each radionuclide and the weight of the edible fraction were looked for by fitting a robust linear regression between log concentration and log edible fraction weight. The calculated slope values and their 95% confidence limits are shown in Fig. 1 for each nuclide. For 40K the regression slope was zero, implying no variability in concentration with edible fraction weight, as expected for an element under homeostatic control. For 241Am and 137Cs the picture was less clear with slopes intermediate between zero and ÿ 1. For 60Co, 99Tc and 110mAg the regression slopes were closer to ÿ 1. This suggests that the load of these nuclides in crabs did not vary with edible fraction weight and consequently whole body weight. The possible variation in activity concentration with gender was examined. The crab radiological data were divided by sex, and Student’s t-test applied to the means for the concentration results (Table 2). This showed no statistically significant variation between males and females for 60Co, 110mAg, 99Tc, 137Cs or 241Am. However, there was a significant difference (P50.02; n=32) between the higher 40K
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Fig. 1. Crab data: estimated slopes 95% confidence limits for log (nuclide concentration) on log (edible fraction weight).
Table 3 Mean crab radioanalytical data as load by gender (load (Bq)) 40
K
60
Co
99
110m
Tc
Ag
137
Cs
241
Am
Females Mean s n
11.5 3.3 16
0.25 0.13 16
19.5 18.7 16
0.46 0.4 16
0.39 0.09 16
0.21 0.13 16
Males Mean s n
15.8 4.1 18
0.27 0.14 18
13.2 10 18
0.68 0.36 18
0.51 0.15 18
0.26 0.11 18
t-test p-value
50.02
ns
ns
a
nsa
50.01
ns
ns: not statistically significant.
content in males compared with that for females. This may possibly have been due to physiological factors, such as hormonal effects on cellular Na+/K+ ion transport, following moulting or as part of the reproductive cycle. A generally similar picture was seen when the measured activities were expressed in terms of load (Table 3). However, a small but significant difference in 137Cs content (P50.01; n=32) was observed with male crabs having marginally higher values than females.
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319
Part of any differences in concentrations (or loads) between males and females may be due to differences in the weight of the edible fraction. The log–log regression analysis of concentration on weight of edible fraction was therefore extended to allow for different intercepts for males and females, a M and a F, respectively. From these, we can calculate ð1 ÿ eaM ÿaF Þ100 giving the percentage by which either concentration or load in males exceeds that in females at a common weight of the edible fraction. This is given in Table 8 together with: M C 1 ÿ 100; CF i.e. the excess in males in terms of the average concentrations in the edible fraction for males and females given in Table 2, and M L 1 ÿ 100; LF i.e. the excess in males in terms of the average loads in the edible fraction given in Table 3. Whereas the first equation measures the excess in males relative to females of the same weight, the second and third equations measure the excess in concentration or load for males and females at the corresponding average weights observed in the sample } 161 g for males, 133 g for females. Differences in the gender effect expressed in the three measures depend on the nature of the relationship between concentration and weight in the edible fraction. For example, for 40K, concentration tended to be constant over the range of weight of edible fraction ( b =0). Hence, the percentage excess of concentration in males tends to be the same at both a common weight of edible fraction and at the average weights of edible fraction for males (161 g) and females (133 g) observed in the sample. However, the excess in males is increased when levels are expressed as loads, since 161>133 g. 3.1.5. Conclusions In the crabs no convincing statistical evidence was found to suggest that either the sex or the live weight of the animals sampled influenced the edible fraction concentrations of 60Co, 137Cs or 241Am. However, this conclusion may apply only to crabs sampled in May–June (as in this project), since seasonal effects could also influence radionuclide accumulation at other times of the year. There was some evidence that levels of 40K and 110Ag tend to be higher in males, although in the latter case this was only significant when expressed at a common weight of the edible fraction. The 99Tc concentration in the edible fractions of female crabs tended to be higher than that in males. However, this was not a statistically significant difference when expressed as either concentration, load, or at a common weight of edible fraction.
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For both male and female crabs, the edible fraction 99Tc concentrations tended to decrease with increasing edible fraction weight, and hence whole body weight. 3.2. Lobsters 3.2.1. Biometric data Thirty-seven lobsters were sampled, 20 females and 17 males (Table 4). Of the total, 16 came from station A and 21 from station B. The minimum lobster live weight was 353 g and the maximum 1878 g and the mean 654 286 (37) g. The average whole live weight (Table 4) for male lobsters, 807 351 (17) g, was statistically significantly higher than that for females, 523 110 (20) g. However, there was no significant difference between the weights of individuals caught at either of the sampling stations. A significant difference was found between weights of samples caught on different days. Again however, the 12 day sampling interval was judged to be sufficiently short that any associated differences in radionuclide concentrations could most likely be attributed to weight and not to temporal changes in the source levels, a factor which, unfortunately, was uncontrollable and unknowable. There was a statistically significant difference between the mean edible fraction weight for male lobsters, 246 122 (17) g, and that for females, 160 34 (20) g. This was due to the greater average white meat weight for males compared with that for females. There was no significant difference in the mean weights for the brown meat fractions (Table 4).
Table 4 Mean lobster biometric data Carapace length (mm)
Whole live weight (g)
Whole cooked weight (g)
Brown meat weight (g)
White meat weight (g)
Edible fraction weight (g)
All Mean s n
96 10 37
654 286 37
552 243 37
21.9 17.1 37
178 82.8 37
199 95.3 37
Females Mean s n
92 5.1 20
523 110 20
448 88 20
19.4 9.9 20
142 35 20
160 34 20
24.8 22.9 17
222 101 17
246 122 17
50.01
50.01
Males Mean s n
101 12 17
807 351 17
673 308 17
t-test p-value
50.01
50.01
50.01
a
ns: not statistically significant.
nsa
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D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 5 Mean lobster radioanalytical data (Bq kg ÿ 1 (wet)) 40
K
60
99
Tc
110m
All Mean s n
92 9.7 37
1.3 0.6 34
11300 7120 37
Females Mean s n
91.4 9.7 20
1.5 0.6 20
Males Mean s n
92.2 10.0 17
1.1 0.4 14
t-test p-value a
nsa
Co
ns
137
241
9.0 5.4 36
5.0 0.9 37
8.3 11.3 35
14853 7382 20
7.6 3.9 20
5.0 0.9 20
5.5 4.4 18
7131 3904 17
10.7 6.5 16
5.0 1.0 17
11.4 15.3 17
ns
ns
ns
50.001
Ag
Cs
Am
ns: not statistically significant.
3.2.2. g -spectrometry results As was found for the crab samples, g -spectrometry of the lobster samples detected only four artificial radionuclides, 60Co, 110mAg, 137Cs and 241Am (Table 5). The natural radionuclide 40K was also detected and has been included here as a marker as described previously for the crab results. For these artificial radionuclides the highest variability was seen for 241Am concentrations (CV=137%) and the least for 137Cs (CV=18%). For comparison the CV for 40K was 10%. 3.2.3. 99Tc results Technetium concentrations in the lobster samples (Table 5) ranged from a minimum of 964 to a maximum of 36,600 Bq kg ÿ 1 with a mean of 11,300 7100 (37) Bq kg ÿ 1. 3.2.4. Regression and further statistical analysis Regression analysis on the concentration and edible fraction weight data, as applied to the crab results, was repeated for the lobster data. The slopes for plots of log (concentration) for the six radionuclides against log (edible fraction weight) were calculated and the slopes and their 95% confidence limits plotted (Fig. 2). The results for lobsters lacked the ambiguities seen for crabs. For all of the radionuclides except 99Tc, the estimated slopes were close to zero (Fig. 2). This implies that in lobsters the activity concentrations for these nuclides did not vary significantly with the edible fraction weight. For 99Tc the estimated slope was close
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Fig. 2. Lobster data: estimated slopes 95% confidence limits for log (nuclide concentration) on log (edible fraction weight).
to ÿ 1. Hence, the 99Tc concentration decreased with increased edible fraction weight or, expressed in terms of load rather than concentration, the 99Tc load in lobsters remained constant with changing weight of the edible fraction. No statistically significant differences were seen between the female and male lobster mean edible fraction concentrations for any of the radionuclides with the exception of 99Tc (Table 5). This was in spite of the statistically significant difference between the mean body weights of males and females already mentioned. However, 99 Tc concentration showed a statistically significant difference, with females having a higher mean edible fraction concentration than males. This was despite the females being statistically significantly lighter and yielding lighter edible fraction weights. The data were also analysed when expressed as radionuclide load values (Table 6). The results indicated that statistically significant differences existed between the mean values for the sexes for all of the analysed nuclides except 60Co. The log–log regression analysis of concentration on weight of edible fraction was again extended to allow for different intercepts for males and females and provide estimates of the percentage by which either concentration or load in males exceeds that in females at a common weight of the edible fraction. These are given in Table 8 together with the corresponding estimated excesses based upon average concentrations or loads derived from Tables 5 and 6. With the exception of 99Tc, there was no evidence of any relationship between nuclide concentration and weight of edible fraction. Hence, excess in males computed at a common weight of edible fraction and computed from the average concentration in the sample tend to be similar. However, the excesses based on average load for all nuclides except 60Co tend to be significantly different, induced by the higher weights of edible fraction in males (246 g) compared with females (160 g).
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D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 6 Mean radioanalytical data expressed as load for lobsters by gender (Bq) 40
K
60
Co
99
Tc
110m
Ag
137
Cs
241
Am
Females Mean s n
14.8 4.1 20
0.23 0.12 20
2367 1390 20
1.21 0.71 20
0.81 0.27 20
0.86 0.78 18
Males Mean s n
22.6 11.7 17
0.24 0.10 14
1584 918 17
2.29 1.60 16
1.14 0.35 17
2.68 3.57 17
t-test p-value
50.01
50.02
50.01
50.05
a
nsa
50.05
ns: not statistically significant.
Table 7 Analysis of variance of log (load) on lobster edible fraction weight and gender
40
K Co 99 Tc 110m Ag 137 Cs 241 Am 60
a
Edible fraction weight
Sex
50.001 ns ns ns 50.001 ns
nsa ns 50.05 50.05 ns ns
ns: not statically significant.
3.2.5. Conclusions Statistical analysis suggested that the concentrations of 40K, 60Co, 110mAg, 137Cs and 241Am in the lobsters did not vary significantly with the gender of the lobster sampled. For 99Tc, there was evidence that female lobsters contained significantly higher activity concentrations than males. Significant differences between male and female loads tended to be induced by the corresponding larger weight of the edible fraction in the males (Table 7). There was no evidence that nuclide concentrations varied with the weight of the edible fraction, with the exception of 99Tc, where concentration decreased with the weight of the edible fraction such that the load tended to be constant.
4. Discussion For the artificial g -emitting radionuclides detected (60Co, 110mAg, 137Cs and Am), no statistically significant differences were found between the radionuclide
241
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Table 8 Percentage excess of male to female radionuclide levelsa A
B
C
Crabs K 60 Co 99 Tc 110m Ag 137 Cs 241 Am
18b 12 ÿ 21 102b 15 32
11b ÿ 10 ÿ 51 13 3 0
37b 8 ÿ 32 48 31b 24
Lobsters 40 K 60 Co 99 Tc 110m Ag 137 Cs 241 Am
0 ÿ 20 ÿ 44b 49 6 94
1 ÿ 27 ÿ 52b 41 0 107
53b 4 ÿ 33b 89b 41b 212b
40
a A, concentrations or loads at a common weight of the edible fraction; B, the mean concentrations, C, the mean loads. b Significantly different from zero at 5% level.
content of males and females of either species over the body weight ranges studied. There was no statistically significant evidence that selection by gender or edible fraction, and hence body weight, for bulk sampling need be stratified in any way. This is less clearly seen with the crab results, however. For 99Tc the position was less clear. Female crabs contained more but not significantly more than males while female lobsters contained significantly more activity than males. For both crabs and lobsters, there was also a general trend in both sexes for 99Tc edible fraction concentrations to decrease with increasing body weight (Table 8). It must also be remembered that gender differences may be more highly correlated with seasonal effects, a factor not considered in this study. The lobster results suggest that for 99Tc selective sampling strategies would give different analytical results. For instance, sampling only small animals could give an estimate of the highest mean activity that consumers might receive. Stratified sampling in turn would increase the precision of the estimate and would provide some control over potential bias in the selection of animals, where collectors may not be truly randomly sampling, either deliberately or subconsciously. However, as we are more concerned with the average concentration in the landed lobsters for radiological dose assessments, random sampling would be appropriate. An appropriate sample size (n) can be calculated for a given or estimated index of precision (D) if the arithmetic mean ( x) and standard deviation (s) of the population is known (Elliott, 1979). Hence, n¼
s2 : D2 x2
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Fig. 3. Numbers of animals (n) in a random sample of lobsters or crabs corresponding to a target precision D.
For example, Fig. 3 shows n plotted against the target D for 99Tc in crabs or lobsters. For surveillance purposes, a precision of 0.2, that is of being within 20% of the population mean concentration is generally considered acceptable. To achieve this precision in estimating 99Tc in lobsters, the data in Table 5 give the value of n as 10. For the other radionuclides, a similar analysis shows that sampling 10 lobsters gives a similar or better precision for all except 241Am (Table 9). To achieve a precision of 0.2 for 241Am would require a sample of about 46 lobsters, which for practical and financial reasons would be unacceptable. For crabs the index of precision for 99Tc can be calculated from the data given in Table 2. The precision of the mean for the 99Tc analysis, using figures for the complete data set, is 0.21. This implies that the crab sample size needs to be of the same order as in this study, namely 34. However, the data in Table 2 include a possible outlier value of 905 Bq kg ÿ 1 that inflates the standard deviation. The original high datum figure was left in the original statistical analysis since there were no convincing reasons to leave it out. Using the original data, the theoretical precision for 99Tc would be about 0.38 for samples of 10 crabs, although D would be less than 0.2 for the other radionuclides, except 110mAg (Table 9). Without further data on the distribution of 99Tc in crabs off Sellafield, it is difficult to resolve the true nature of the outlying value. Since the value was over four standard deviations from the mean, the pragmatic approach, at least for determining 99 Tc, would be to ignore it and recalculate the mean and standard deviation, giving 102 63 (33). The overall index of precision is now 0.11 and a minimum of 10 crabs would be needed to achieve a precision of 0.2. Nevertheless, it must be borne in mind
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Table 9 Index of precision (D) for analysis based on a sample size of 10 animals 60
99
Tc
110m
137
241
Crab All Females Males
0.18 0.18 0.17
0.38 0.38 0.22
0.25 0.32 0.20
0.08 0.09 0.08
0.16 0.17 0.15
Lobster All Female Male
0.14 0.14 0.10
0.20 0.16 0.17
0.19 0.16 0.19
0.06 0.06 0.06
0.43 0.25 0.42
Co
Ag
Cs
Am
that by accepting a recommendation to analyse samples of 10 crabs the precision might actually range from about 20% to about 40%. Seasonal effects were not addressed by this study. Given the known annual physiological cycles occurring in crustacea, especially moulting, and the controlling influence of environmental (seasonal) factors, such as temperature and daylight length, on these processes, further study of seasonal effects on radionuclide accumulation by crustacea should be considered.
References Elliott, J. (1979). Some methods for the statistical analysis of samples of benthic invertebrates (160pp). Freshwater Biological Association, Scientific Publication No. 25, Second impression. Hunt, G. J., Smith, B. D., & Camplin, W. C. (1998). Recent changes in liquid radioactive waste discharges from Sellafield to the Irish Sea: monitoring of the environmental consequences and radiological implications. Radiation Protection Dosimetry, 75, 149–153. Masson, M., Aprosi, G., Laniece, A., Guegeniat, P., & Belot, Y. (1981). Approches experimentales de l’e´tude des transferts du techne´tium a` des se´diments et a` des espe`ces marines benthiques. In Impacts of radionuclide releases into the marine environment, Vienna, IAEA, STI/PUB/565 (pp. 341–359). Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency (1999). Radioactivity in food and the environment 1998. RIFE-4. MAFF and SEPA, London. Pentreath, R.J. (1981). The biological availability to marine organisms of transuranium and other longlived nuclides. In Impacts of Radionuclide Releases into the Marine Environment, Vienna, IAEA, STI/ PUB/565 (pp. 241–272). Swift, D. J. (1985). The accumulation of 95mTc by juvenile lobsters (Homarus gammerus L.). Journal of Environmental Radioactivity, 2, 229–243.
Variability in the edible fraction content of Co, 99Tc, 110mAg, 137Cs and 241Am between individual crabs and lobsters from Sellafield (north eastern Irish Sea)
60
D.J. Swift*, M.D. Nicholson The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK Received 17 February 2000; received in revised form 26 June 2000; accepted 6 July 2000
Abstract We investigated the variability in anthropogenic radionuclide content of the edible fractions of individual edible crabs (Cancer pagurus L.) and European lobsters (Homarus gammarus L.) caught commercially in the Sellafield offshore area. Sixteen female and 18 male crabs and 20 female and 17 male lobsters were selected from commercial catches made between 25 May and 5 June 1997. Each gender group was selected to be within the known weight range for commercially caught crustacea from the area. Four artificial radionuclides (60Co, 110mAg, 137 Cs or 241Am) were detected by g -spectrometry. The edible fraction content of these radionuclides between males and females for either species were not statistically significantly different. 99Tc was analysed by chemical separation and b -counting. 99Tc concentrations in female crabs tended to be higher (172 205 (16) Bq kg ÿ 1(wet); mean standard deviation (n samples)) than those in males (85 58 (18) Bq kg ÿ 1 (wet)), although this was not a statistically significant difference. For both male and female crabs, 99Tc concentrations tended to decrease with increasing whole live weights. For 99Tc in lobsters the picture is less clear. Female lobsters contained more activity (14800 7400 (20) Bq kg ÿ 1 (wet)) than males (7100 3900 (17) Bq kg ÿ 1 (wet)). The results were used to discuss the implications for sampling and monitoring. Crown Copyright # 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Radioactivity; Sellafield; Crabs; Lobsters; Variability
*Corresponding author. Tel.: +44-1502-52-4445; fax: +44-1502-513-865. E-mail address: [email protected] (D.J. Swift). 0265-931X/01/$ - see front matter Crown Copyright # 2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 3 2 - 6
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1. Introduction Commercially caught crustacea from the Sellafield coastal area are regularly sampled and analysed for radioactivity as part of the Ministry of Agriculture, Fisheries and Food’s (MAFF) statutory surveillance of authorised radioactive liquid waste discharges from the Sellafield nuclear fuel reprocessing plant (Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency, 1999). Edible crabs (Cancer pagurus L.) and European lobsters (Homarus gammarus L.) have been monitored routinely for a range of radionuclides, including transuranics, annually as part of this programme since the 1960’s. In 1994 the Enhanced Actinide Removal Plant (EARP) came on stream at Sellafield. The introduction of this new stage in the reprocessing scheme has greatly reduced the discharges of the most radiologically significant nuclides, such as 106Ru, 137 Cs and the transuranics, in liquid waste to the sea. However, the new plant’s efficiency for removing some other radionuclides, such as 90Sr and 99Tc, is low (Hunt, Smith & Camplin, 1998). From early 1994, decay-stored, medium activity liquors have been reprocessed through EARP and these liquors contain large amounts of 99Tc. As a consequence of the low recovery of 99Tc, discharges of this radionuclide to the sea have increased. Annual 99Tc discharges in liquid waste before 1994 were relatively small averaging 4.1 1.4 TBq between 1981 and 1993. However, with the start of reprocessing stored liquors through EARP the annual discharges increased substantially: 72 TBq in 1994, 192 TBq in 1995, 155 TBq in 1996, 84 TBq in 1997 and 53 TBq in 1998 (Hunt et al., 1998; Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency, 1999). A consequence of the increased 99Tc discharges to the sea after 1993 has been a sharp increase in 99Tc concentrations measured in some marine biota. This is especially so for some seaweed and some crustacea species although these increases are of negligible radiological significance (Hunt et al., 1998). Of the commercially important food animals caught off Sellafield, the increased activities recorded in lobsters have attracted the most attention. This species has an anomalously high 99Tc accumulation capacity compared with other crustacea, such as the edible crab, a phenomenon first observed and investigated in the 1980s (Masson, Aprosi, Laniece, Guegeniat & Belot, 1981; Pentreath, 1981; Swift, 1985). Consequently, the increased 99 Tc content in the edible fraction of lobsters, and to a lesser extent crabs, after increased discharges of 99Tc had been predicted and an allowance made in setting new Sellafield 99Tc discharge authorizations to cover the planned reprocessing of stored liquors through EARP. The radioactivity content of marine biota derived from radioactive liquid waste discharges to the sea is a function of many factors. These include the concentration and discharge rate of the radionuclides in the effluent, their subsequent degree of dilution in the sea and possible chemical speciation changes in the sea due to geochemical factors before interaction. Biological factors influencing radionuclide bioconcentration include physiological and biochemical differences due to genetic
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313
effects between species and biometric differences, such as in age, size and gender between individuals within a species. For MAFF’s aquatic biota surveillance programme, radionuclides are measured in a bulked, edible fraction sample of sufficient size to complete all of the radiological analyses. Since each animal’s live weight is different, the number sampled to make up the required edible fraction weight varies at each time of sampling. Although consumers cannot usually select by gender, for example mollusca or fish fillets, selection by size may occur with individual crabs or lobsters. Crustacea monitoring samples are therefore collected without regard to gender but do cover the size range available to consumers at the time. However, information on the gender, number and weight of the individual crustacea making up a routine surveillance sample is not usually recorded. With the current interest in 99Tc accumulation by crustacea, it was felt that information on the variability of both 99Tc and other radionuclides with gender and size in lobsters and crabs should be obtained. This information could also be used as a check on the robustness of the current sampling protocol. This project was therefore set up to analyse the edible fractions of individual lobsters and crabs for 99 Tc and g -emitting radionuclides and to statistically analyse any apparent correlations with biometric factors, such as body weight and gender.
2. Materials and methods The statistical design of the sampling required the analysis of a minimum of 32 lobsters and 32 crabs sampled from commercial catches and divided half-and-half male and female. In turn, each gender group was to be selected to cover the known weight range of commercially caught crustacea from the area. The effect of temporal variation in the individual radionuclide concentrations at source, especially of 99Tc, had also to be considered. Unfortunately, the pattern of radionuclide discharges from the Sellafield site on a day to day basis is unknown. Discharge data for the Sellafield plant are published but they are only available at some time after the discharge has occurred. For 99Tc only monthly discharge values are available. Since we had no way of predicting when particular radionuclides would be discharge or in what quantities, we had to complete the sampling in the shortest possible time. We also had to contend with the unpredictability of the fishing effort needed to supply the required number of individuals of the required sex and weight range at any particular time. A sampling period of approximately two weeks was achieved. Two local commercial fishermen who regularly fish for crustacea in areas near the end of the Sellafield pipeline were employed. Suitable samples were selected from catches made between 25 May and 5 June 1997. Potential sampling areas were a fishing area close to the end of the Sellafield pipeline (station A), about 2.5 km offshore, and another to the south east of this, off the Kokoarrah Rocks (station B). Both fishing areas are used to supply crustacea for the Sellafield component of the aquatic surveillance programme conducted by CEFAS for MAFF.
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During the selected sampling time neither fishing area alone yielded enough animals of the required weight range and gender to satisfy the sampling requirements. The total numbers for each species therefore included animals from both areas. After selection by CEFAS staff at Whitehaven, the animals were weighed and measured and then killed. The animals were cooked using the normal sample protocol for the MAFF surveillance programme of boiling in salted freshwater for twenty minutes. The sex, the carapace length for lobsters or width for crabs (mm) and the whole body live and cooked wet weights (g) were recorded for each animal. The animals were dissected individually. The brown meat and white meat fractions were removed separately, weighed and recorded. The brown meat fraction for both crabs and lobsters consisted of the digestive gland only. The white meat fraction consisted of the claw muscle and body cavity muscle for crabs and the claw and tail muscle for lobsters. Muscle tissue from the walking legs was not included for either species. The two fractions for each individual were then combined and the cooked wet weights of the individual total edible fraction recorded. The labelled samples were deep-frozen for transport to CEFAS, Lowestoft for radioanalysis. On receipt the samples were individually dried, homogenized and prepared in an appropriate geometry for g -spectrometry. Analysis by g -spectrometry was carried out using a GeLi counter with an enhanced low-energy response to improve detection of 241Am (limit of detection 0.4 Bq kg ÿ 1; absolute counting efficiency 18.9% at 59.54 keV for the given geometry and counting time; FWHM of 0.83 keV). After g -spectrometry sub-samples were dissolved in acid and, following a series of chemical separation steps, the 99Tc activity was estimated by b -counting using a gas proportional detector. All activity results were reported as Bq kg ÿ 1 (wet weight) of the cooked edible fraction. The extent to which variation in whole body weight may, through sampling, have become inadvertently associated with gender, sampling site or sampling association was evaluated simply by making Student’s t-tests to compare the corresponding mean whole body weights. Gross summaries (means, standard deviations and coefficients of variation) were computed for each radionuclide. Potential outliers were identified from either the scaled distance from the mean, or from scatterplots between each radionuclide and the weights of various body fractions. Outliers for which there were no transcription or other identified errors were noted, but retained in the data set. The relationship between radionuclide concentration and the weight of the edible fraction was tested by fitting logðconcentrationÞ ¼ a þ b logðedible fraction weightÞ þ noise;
ð1Þ
where the noise term was assumed to have a Normal distribution with zero mean and constant variance. These assumptions were validated by a subsequent examination of the residuals from the fitted model. An estimated value of b =0 implies that concentration does not vary with the weight of the edible fraction. Further, since load is obtained by multiplying the
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315
edible fraction concentration by the edible fraction wet weight, i.e. concentration ¼
load ; edible fraction weight
we can write log ðloadÞ ¼ log ðconcentrationÞ þ log ðediblefractionweightÞ: Then, from Eq. (1), log ðloadÞ ¼ a þ ðb þ 1Þ log ðedible fraction weightÞ and hence a value of b = ÿ 1 in Eq. (1) implies that load does not vary with edible fraction weight. Radionuclide levels expressed as either concentration or load vary with edible fraction weight for values of b other than zero or unity. To minimise the influence of any outliers, the model was fitted using a robust method. Observations with large residuals were down-weighted, the model refitted, residuals re-calculated, and so on until a stable fit was achieved. The model was further tested to see if the intercept, a , depends on gender. i.e. that, for an animal of a given weight of the edible fraction, the mean radionuclide concentration (or load) might be higher for males or females. Initial statistical analysis was carried out using Microsoft-Excel TM(Microsoft Corporation, Seattle) spreadsheets. Further statistical analysis was carried out using the statistical package S-PLUS TM(MathSoft, Inc., Seattle).
3. Results 3.1. Edible crabs 3.1.1. Biometric data A total of 34 crabs were sampled, 20 from station B and 14 from station A. Of the total 16 were female and 18 male (Table 1). The mean whole body wet weight for all crabs was 490 141 (34) g. The mean carapace width was 141 12 (34) mm. There were no statistically significant differences between the mean weight and carapace width of the crabs collected from the two sampling stations. There were statistically significant differences between the sexes for some biometric factors (Table 1). Male crabs were on average heavier and had a significantly greater mean white meat weight compared with those for females although there was no significant difference between the mean weights of the brown meat fractions (Table 1). Analysis of variance showed a small but statistically significant difference in the mean body weight between samples taken different sampling days. However, the 12 day sampling interval was judged to be sufficiently short that any associated differences in radionuclide concentrations could most likely be attributed to weight and not to temporal changes in the source levels. The data were therefore combined without regard to capture date or station.
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Table 1 Mean crab biometric data Carapace width (mm)
Whole live weight (g)
Whole cooked weight (g)
Brown meat weight (g)
All Mean s n
141 11.8 34
490 141 34
439 122 34
59.4 22.5 34
90.6 29.5 34
148 40.9 34
Females Mean s n
140 14.9 16
419 134 16
384 126 16
65.9 28.8 16
70.0 16.9 16
133 42.7 16
Males Mean s n
142 8.5 18
552 118 18
488 98.9 18
53.6 13.0 18
109 26.2 18
161 35.1 18
t-test p-value
nsa
50.01
50.02
ns
50.001
50.05
a
White meat weight (g)
Edible fraction weight (g)
ns: not statistically significant.
3.1.2. g -spectrometry results Four artificial radionuclides: 60Co, 110mAg, 137Cs and 241Am (Table 2) were detected by g -spectrometry. The natural radionuclide 40K was also detected. Since potassium in biota is invariably under homeostatic control, there should be little variation in the concentration of 40K between individuals of the same species and sex. The 40K values have therefore been included as a possible physiological marker against which to assess any variability in the content of the other nuclides with sex or body size. The four artificial radionuclides detected by g -spectrometry varied considerably in concentration between individuals (Table 2). 110mAg showed the largest variability (coefficient of variation (CV)=79%) and 241Am the smallest (CV=5%). For 40K (CV=13%) there was one anomalously low value for a male crab. However, in other respects the biometric and radionuclide data for this individual were unexceptional and the analytical data were included in further statistical analysis. 3.1.3. 99Tc results 99 Tc concentrations varied more between individuals (CV=120%) than did the other radionuclides (Table 2). For the 34 crabs sampled the minimum concentration detected was 22 Bq kg ÿ 1 and the maximum 905 Bq kg ÿ 1 with a mean of 126 151 (34) Bq kg ÿ 1. The large standard deviation was due to the maximum concentration measured. This value is well over four standard deviations from the mean value and was considered to be an outlier. When removed from the calculation, the mean
317
D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 2 Mean crab radioanalytical data (Bq kg ÿ 1 (wet)) 40
K
60
Co
99
110m
Tc
Ag
137
241
Cs
Am
All Mean s n
93.0 12.5 34
1.9 1.1 34
126 151 34
4.2 3.3 34
3.1 0.8 34
1.7 0.8 34
Females Mean s n
87.8 7.5 16
2.0 1.2 16
172 205 16
3.9 3.9 16
3.1 0.9 16
1.7 0.9 16
Males Mean s n
97.7 14.3 18
1.8 0.9 18
85 58 18
4.4 2.8 18
3.2 0.8 18
1.7 0.8 18
t-test p-value
50.02
ns
ns
ns
ns
a
nsa
ns: not statistically significant.
becomes 102 63 (33) Bq kg ÿ 1. However, the crab for which this value occurred, a female, also had the highest concentrations of 110mAg, 60Co and 137Cs, although these did not appear to be outliers. The 40K concentration was close to the mean value. As there was nothing exceptional about the biometric values for this crab, the analytical data were retained in subsequent analysis. 3.1.4. Regression and further statistical analysis Possible relationships between the content of each radionuclide and the weight of the edible fraction were looked for by fitting a robust linear regression between log concentration and log edible fraction weight. The calculated slope values and their 95% confidence limits are shown in Fig. 1 for each nuclide. For 40K the regression slope was zero, implying no variability in concentration with edible fraction weight, as expected for an element under homeostatic control. For 241Am and 137Cs the picture was less clear with slopes intermediate between zero and ÿ 1. For 60Co, 99Tc and 110mAg the regression slopes were closer to ÿ 1. This suggests that the load of these nuclides in crabs did not vary with edible fraction weight and consequently whole body weight. The possible variation in activity concentration with gender was examined. The crab radiological data were divided by sex, and Student’s t-test applied to the means for the concentration results (Table 2). This showed no statistically significant variation between males and females for 60Co, 110mAg, 99Tc, 137Cs or 241Am. However, there was a significant difference (P50.02; n=32) between the higher 40K
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Fig. 1. Crab data: estimated slopes 95% confidence limits for log (nuclide concentration) on log (edible fraction weight).
Table 3 Mean crab radioanalytical data as load by gender (load (Bq)) 40
K
60
Co
99
110m
Tc
Ag
137
Cs
241
Am
Females Mean s n
11.5 3.3 16
0.25 0.13 16
19.5 18.7 16
0.46 0.4 16
0.39 0.09 16
0.21 0.13 16
Males Mean s n
15.8 4.1 18
0.27 0.14 18
13.2 10 18
0.68 0.36 18
0.51 0.15 18
0.26 0.11 18
t-test p-value
50.02
ns
ns
a
nsa
50.01
ns
ns: not statistically significant.
content in males compared with that for females. This may possibly have been due to physiological factors, such as hormonal effects on cellular Na+/K+ ion transport, following moulting or as part of the reproductive cycle. A generally similar picture was seen when the measured activities were expressed in terms of load (Table 3). However, a small but significant difference in 137Cs content (P50.01; n=32) was observed with male crabs having marginally higher values than females.
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319
Part of any differences in concentrations (or loads) between males and females may be due to differences in the weight of the edible fraction. The log–log regression analysis of concentration on weight of edible fraction was therefore extended to allow for different intercepts for males and females, a M and a F, respectively. From these, we can calculate ð1 ÿ eaM ÿaF Þ100 giving the percentage by which either concentration or load in males exceeds that in females at a common weight of the edible fraction. This is given in Table 8 together with: M C 1 ÿ 100; CF i.e. the excess in males in terms of the average concentrations in the edible fraction for males and females given in Table 2, and M L 1 ÿ 100; LF i.e. the excess in males in terms of the average loads in the edible fraction given in Table 3. Whereas the first equation measures the excess in males relative to females of the same weight, the second and third equations measure the excess in concentration or load for males and females at the corresponding average weights observed in the sample } 161 g for males, 133 g for females. Differences in the gender effect expressed in the three measures depend on the nature of the relationship between concentration and weight in the edible fraction. For example, for 40K, concentration tended to be constant over the range of weight of edible fraction ( b =0). Hence, the percentage excess of concentration in males tends to be the same at both a common weight of edible fraction and at the average weights of edible fraction for males (161 g) and females (133 g) observed in the sample. However, the excess in males is increased when levels are expressed as loads, since 161>133 g. 3.1.5. Conclusions In the crabs no convincing statistical evidence was found to suggest that either the sex or the live weight of the animals sampled influenced the edible fraction concentrations of 60Co, 137Cs or 241Am. However, this conclusion may apply only to crabs sampled in May–June (as in this project), since seasonal effects could also influence radionuclide accumulation at other times of the year. There was some evidence that levels of 40K and 110Ag tend to be higher in males, although in the latter case this was only significant when expressed at a common weight of the edible fraction. The 99Tc concentration in the edible fractions of female crabs tended to be higher than that in males. However, this was not a statistically significant difference when expressed as either concentration, load, or at a common weight of edible fraction.
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For both male and female crabs, the edible fraction 99Tc concentrations tended to decrease with increasing edible fraction weight, and hence whole body weight. 3.2. Lobsters 3.2.1. Biometric data Thirty-seven lobsters were sampled, 20 females and 17 males (Table 4). Of the total, 16 came from station A and 21 from station B. The minimum lobster live weight was 353 g and the maximum 1878 g and the mean 654 286 (37) g. The average whole live weight (Table 4) for male lobsters, 807 351 (17) g, was statistically significantly higher than that for females, 523 110 (20) g. However, there was no significant difference between the weights of individuals caught at either of the sampling stations. A significant difference was found between weights of samples caught on different days. Again however, the 12 day sampling interval was judged to be sufficiently short that any associated differences in radionuclide concentrations could most likely be attributed to weight and not to temporal changes in the source levels, a factor which, unfortunately, was uncontrollable and unknowable. There was a statistically significant difference between the mean edible fraction weight for male lobsters, 246 122 (17) g, and that for females, 160 34 (20) g. This was due to the greater average white meat weight for males compared with that for females. There was no significant difference in the mean weights for the brown meat fractions (Table 4).
Table 4 Mean lobster biometric data Carapace length (mm)
Whole live weight (g)
Whole cooked weight (g)
Brown meat weight (g)
White meat weight (g)
Edible fraction weight (g)
All Mean s n
96 10 37
654 286 37
552 243 37
21.9 17.1 37
178 82.8 37
199 95.3 37
Females Mean s n
92 5.1 20
523 110 20
448 88 20
19.4 9.9 20
142 35 20
160 34 20
24.8 22.9 17
222 101 17
246 122 17
50.01
50.01
Males Mean s n
101 12 17
807 351 17
673 308 17
t-test p-value
50.01
50.01
50.01
a
ns: not statistically significant.
nsa
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D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 5 Mean lobster radioanalytical data (Bq kg ÿ 1 (wet)) 40
K
60
99
Tc
110m
All Mean s n
92 9.7 37
1.3 0.6 34
11300 7120 37
Females Mean s n
91.4 9.7 20
1.5 0.6 20
Males Mean s n
92.2 10.0 17
1.1 0.4 14
t-test p-value a
nsa
Co
ns
137
241
9.0 5.4 36
5.0 0.9 37
8.3 11.3 35
14853 7382 20
7.6 3.9 20
5.0 0.9 20
5.5 4.4 18
7131 3904 17
10.7 6.5 16
5.0 1.0 17
11.4 15.3 17
ns
ns
ns
50.001
Ag
Cs
Am
ns: not statistically significant.
3.2.2. g -spectrometry results As was found for the crab samples, g -spectrometry of the lobster samples detected only four artificial radionuclides, 60Co, 110mAg, 137Cs and 241Am (Table 5). The natural radionuclide 40K was also detected and has been included here as a marker as described previously for the crab results. For these artificial radionuclides the highest variability was seen for 241Am concentrations (CV=137%) and the least for 137Cs (CV=18%). For comparison the CV for 40K was 10%. 3.2.3. 99Tc results Technetium concentrations in the lobster samples (Table 5) ranged from a minimum of 964 to a maximum of 36,600 Bq kg ÿ 1 with a mean of 11,300 7100 (37) Bq kg ÿ 1. 3.2.4. Regression and further statistical analysis Regression analysis on the concentration and edible fraction weight data, as applied to the crab results, was repeated for the lobster data. The slopes for plots of log (concentration) for the six radionuclides against log (edible fraction weight) were calculated and the slopes and their 95% confidence limits plotted (Fig. 2). The results for lobsters lacked the ambiguities seen for crabs. For all of the radionuclides except 99Tc, the estimated slopes were close to zero (Fig. 2). This implies that in lobsters the activity concentrations for these nuclides did not vary significantly with the edible fraction weight. For 99Tc the estimated slope was close
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Fig. 2. Lobster data: estimated slopes 95% confidence limits for log (nuclide concentration) on log (edible fraction weight).
to ÿ 1. Hence, the 99Tc concentration decreased with increased edible fraction weight or, expressed in terms of load rather than concentration, the 99Tc load in lobsters remained constant with changing weight of the edible fraction. No statistically significant differences were seen between the female and male lobster mean edible fraction concentrations for any of the radionuclides with the exception of 99Tc (Table 5). This was in spite of the statistically significant difference between the mean body weights of males and females already mentioned. However, 99 Tc concentration showed a statistically significant difference, with females having a higher mean edible fraction concentration than males. This was despite the females being statistically significantly lighter and yielding lighter edible fraction weights. The data were also analysed when expressed as radionuclide load values (Table 6). The results indicated that statistically significant differences existed between the mean values for the sexes for all of the analysed nuclides except 60Co. The log–log regression analysis of concentration on weight of edible fraction was again extended to allow for different intercepts for males and females and provide estimates of the percentage by which either concentration or load in males exceeds that in females at a common weight of the edible fraction. These are given in Table 8 together with the corresponding estimated excesses based upon average concentrations or loads derived from Tables 5 and 6. With the exception of 99Tc, there was no evidence of any relationship between nuclide concentration and weight of edible fraction. Hence, excess in males computed at a common weight of edible fraction and computed from the average concentration in the sample tend to be similar. However, the excesses based on average load for all nuclides except 60Co tend to be significantly different, induced by the higher weights of edible fraction in males (246 g) compared with females (160 g).
323
D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326 Table 6 Mean radioanalytical data expressed as load for lobsters by gender (Bq) 40
K
60
Co
99
Tc
110m
Ag
137
Cs
241
Am
Females Mean s n
14.8 4.1 20
0.23 0.12 20
2367 1390 20
1.21 0.71 20
0.81 0.27 20
0.86 0.78 18
Males Mean s n
22.6 11.7 17
0.24 0.10 14
1584 918 17
2.29 1.60 16
1.14 0.35 17
2.68 3.57 17
t-test p-value
50.01
50.02
50.01
50.05
a
nsa
50.05
ns: not statistically significant.
Table 7 Analysis of variance of log (load) on lobster edible fraction weight and gender
40
K Co 99 Tc 110m Ag 137 Cs 241 Am 60
a
Edible fraction weight
Sex
50.001 ns ns ns 50.001 ns
nsa ns 50.05 50.05 ns ns
ns: not statically significant.
3.2.5. Conclusions Statistical analysis suggested that the concentrations of 40K, 60Co, 110mAg, 137Cs and 241Am in the lobsters did not vary significantly with the gender of the lobster sampled. For 99Tc, there was evidence that female lobsters contained significantly higher activity concentrations than males. Significant differences between male and female loads tended to be induced by the corresponding larger weight of the edible fraction in the males (Table 7). There was no evidence that nuclide concentrations varied with the weight of the edible fraction, with the exception of 99Tc, where concentration decreased with the weight of the edible fraction such that the load tended to be constant.
4. Discussion For the artificial g -emitting radionuclides detected (60Co, 110mAg, 137Cs and Am), no statistically significant differences were found between the radionuclide
241
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D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326
Table 8 Percentage excess of male to female radionuclide levelsa A
B
C
Crabs K 60 Co 99 Tc 110m Ag 137 Cs 241 Am
18b 12 ÿ 21 102b 15 32
11b ÿ 10 ÿ 51 13 3 0
37b 8 ÿ 32 48 31b 24
Lobsters 40 K 60 Co 99 Tc 110m Ag 137 Cs 241 Am
0 ÿ 20 ÿ 44b 49 6 94
1 ÿ 27 ÿ 52b 41 0 107
53b 4 ÿ 33b 89b 41b 212b
40
a A, concentrations or loads at a common weight of the edible fraction; B, the mean concentrations, C, the mean loads. b Significantly different from zero at 5% level.
content of males and females of either species over the body weight ranges studied. There was no statistically significant evidence that selection by gender or edible fraction, and hence body weight, for bulk sampling need be stratified in any way. This is less clearly seen with the crab results, however. For 99Tc the position was less clear. Female crabs contained more but not significantly more than males while female lobsters contained significantly more activity than males. For both crabs and lobsters, there was also a general trend in both sexes for 99Tc edible fraction concentrations to decrease with increasing body weight (Table 8). It must also be remembered that gender differences may be more highly correlated with seasonal effects, a factor not considered in this study. The lobster results suggest that for 99Tc selective sampling strategies would give different analytical results. For instance, sampling only small animals could give an estimate of the highest mean activity that consumers might receive. Stratified sampling in turn would increase the precision of the estimate and would provide some control over potential bias in the selection of animals, where collectors may not be truly randomly sampling, either deliberately or subconsciously. However, as we are more concerned with the average concentration in the landed lobsters for radiological dose assessments, random sampling would be appropriate. An appropriate sample size (n) can be calculated for a given or estimated index of precision (D) if the arithmetic mean ( x) and standard deviation (s) of the population is known (Elliott, 1979). Hence, n¼
s2 : D2 x2
D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326
325
Fig. 3. Numbers of animals (n) in a random sample of lobsters or crabs corresponding to a target precision D.
For example, Fig. 3 shows n plotted against the target D for 99Tc in crabs or lobsters. For surveillance purposes, a precision of 0.2, that is of being within 20% of the population mean concentration is generally considered acceptable. To achieve this precision in estimating 99Tc in lobsters, the data in Table 5 give the value of n as 10. For the other radionuclides, a similar analysis shows that sampling 10 lobsters gives a similar or better precision for all except 241Am (Table 9). To achieve a precision of 0.2 for 241Am would require a sample of about 46 lobsters, which for practical and financial reasons would be unacceptable. For crabs the index of precision for 99Tc can be calculated from the data given in Table 2. The precision of the mean for the 99Tc analysis, using figures for the complete data set, is 0.21. This implies that the crab sample size needs to be of the same order as in this study, namely 34. However, the data in Table 2 include a possible outlier value of 905 Bq kg ÿ 1 that inflates the standard deviation. The original high datum figure was left in the original statistical analysis since there were no convincing reasons to leave it out. Using the original data, the theoretical precision for 99Tc would be about 0.38 for samples of 10 crabs, although D would be less than 0.2 for the other radionuclides, except 110mAg (Table 9). Without further data on the distribution of 99Tc in crabs off Sellafield, it is difficult to resolve the true nature of the outlying value. Since the value was over four standard deviations from the mean, the pragmatic approach, at least for determining 99 Tc, would be to ignore it and recalculate the mean and standard deviation, giving 102 63 (33). The overall index of precision is now 0.11 and a minimum of 10 crabs would be needed to achieve a precision of 0.2. Nevertheless, it must be borne in mind
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D.J. Swift, M.D. Nicholson / J. Environ. Radioactivity 54 (2001) 311–326
Table 9 Index of precision (D) for analysis based on a sample size of 10 animals 60
99
Tc
110m
137
241
Crab All Females Males
0.18 0.18 0.17
0.38 0.38 0.22
0.25 0.32 0.20
0.08 0.09 0.08
0.16 0.17 0.15
Lobster All Female Male
0.14 0.14 0.10
0.20 0.16 0.17
0.19 0.16 0.19
0.06 0.06 0.06
0.43 0.25 0.42
Co
Ag
Cs
Am
that by accepting a recommendation to analyse samples of 10 crabs the precision might actually range from about 20% to about 40%. Seasonal effects were not addressed by this study. Given the known annual physiological cycles occurring in crustacea, especially moulting, and the controlling influence of environmental (seasonal) factors, such as temperature and daylight length, on these processes, further study of seasonal effects on radionuclide accumulation by crustacea should be considered.
References Elliott, J. (1979). Some methods for the statistical analysis of samples of benthic invertebrates (160pp). Freshwater Biological Association, Scientific Publication No. 25, Second impression. Hunt, G. J., Smith, B. D., & Camplin, W. C. (1998). Recent changes in liquid radioactive waste discharges from Sellafield to the Irish Sea: monitoring of the environmental consequences and radiological implications. Radiation Protection Dosimetry, 75, 149–153. Masson, M., Aprosi, G., Laniece, A., Guegeniat, P., & Belot, Y. (1981). Approches experimentales de l’e´tude des transferts du techne´tium a` des se´diments et a` des espe`ces marines benthiques. In Impacts of radionuclide releases into the marine environment, Vienna, IAEA, STI/PUB/565 (pp. 341–359). Ministry of Agriculture, Fisheries and Food and Scottish Environment Protection Agency (1999). Radioactivity in food and the environment 1998. RIFE-4. MAFF and SEPA, London. Pentreath, R.J. (1981). The biological availability to marine organisms of transuranium and other longlived nuclides. In Impacts of Radionuclide Releases into the Marine Environment, Vienna, IAEA, STI/ PUB/565 (pp. 241–272). Swift, D. J. (1985). The accumulation of 95mTc by juvenile lobsters (Homarus gammerus L.). Journal of Environmental Radioactivity, 2, 229–243.