Nuclear reprocessing-related radiocarbon ( 14 C) uptake into UK marine mammals

To evaluate the transfer of Sella ﬁ eld-derived radiocarbon ( 14 C) to top predators in the UK marine environment, 14 C activities were examined in stranded marine mammals. All samples of harbour porpoise ( Phocoena phocoena ) obtained from the Irish Sea showed 14 C enrichment above background. Mammal samples obtained from the West of Scotland, including harbour porpoise, grey seals ( Halichoerus grypus ) and harbour seals ( Phoca vitulina ) showed 14 C enrichment but to a lesser extent. This study demonstrates, for the ﬁ rst time, enriched 14 C is transferred through the marine food web to apex predators as a consequence of ongoing nuclear reprocessing activities at Sella ﬁ eld. Total Sella ﬁ eld 14 C discharge activity 24 months prior to stranding and, in particular, distance of animal stranding site from Sella ﬁ eld are signi ﬁ cant variables a ﬀ ecting individual 14 C activity. 14 C activities of West of Scotland harbour porpoises suggest they did not forage in the Irish Sea prior to stranding, indicating a high foraging ﬁ delity.


Introduction
During reprocessing of nuclear materials at the Sellafield Ltd. facility ( Fig. 1A), low-level radioactive waste, including 14 C (half-life 5730 years), is discharged to the Northeast Irish Sea, primarily as dissolved inorganic carbon (DIC; Begg et al., 1992, Cook et al., 1995. Dissolved 14 C is subject to solution transport and largely dispersed northwards from the Irish Sea by prevailing currents through the North Channel (Gulliver et al., 2001) and around the Scottish coastline to the North Sea . 14 C enters the marine food web via the efficient uptake of soluble 14 C in DIC during photosynthesis by primary producing organisms, i.e. phytoplankton (Cook et al., 1995;Cook et al., 1998;Cook et al., 2004;Muir et al., 2017;Tierney et al., 2017). In the UK, Sellafield discharges of 14 C have dominated enriched activities in the marine environment. Although Amersham International plc (now GE Healthcare), Cardiff, was an additional source causing localised enriched 14 C activities (Cook et al., 1998), the 14 C discharge activity from this site was minimal between 2000 and 2010 and negligible since 2010 (RIFE, 2016).
Since the early 1990s there have been significant changes in Sellafield 14 C discharges to the Irish Sea as described in detail by Muir et al. (2017). Briefly, the average discharged 14 C activity from 1984 to 1993 was 1.78 Tera Becquerels per year (TBq year − 1 ). An increase in the volume of waste reprocessed and a change in discharge policy in 1994, from an atmospheric route to marine discharge routes, resulted in an increase in marine 14 C discharges. The annual discharged activity peaked in 2003 at 16.87 TBq and remained high relative to pre-1994 releases with an average of 7.63 TBq year − 1 until the end of 2015 (RIFE, 2016;Muir et al., 2017). Recent studies of Sellafield 14 C discharges have considered the accumulation of 14 C within intertidal environments Muir et al., 2015;Tierney et al., 2016) and the biological uptake and transfer of 14 C through a major part of the marine food webs of the Irish Sea and West of Scotland Tierney et al., 2017). The latter studies reported enriched activities in a range of marine species occupying the lowest (phytoplankton) to middle-upper (e.g. piscivorous fish) trophic levels and described the trophic transfer of Sellafield-derived 14 C previously observed for intertidal organisms . Here we examine 14 C activities in marine mammals that occupy the upper trophic levels of the UK marine environment, and which are Fig. 1. A. Map of UK and Ireland indicating study areas (Irish Sea and West of Scotland) and the location of the Sellafield nuclear fuel reprocessing facility. B. Maps of study areas and stranding locations in the southern Irish Sea (A), the transition area between the Irish Sea and West of Scotland (B) and the West of Scotland with additional sites on the Scottish east coast (C).

K.M. Tierney et al.
M a r in e P o llu t io n B u lle t in 1 2 4 ( 2 0 1 7 ) 4 3 -5 0 4 4 potentially at risk from increased radioactive dose due to uptake of bioavailable contaminant radionuclides. Harbour seals (Phoca vitulina) are locally resident and typically forage within 40 km of their haul-out sites (Thompson et al., 1998). The foraging range of grey seals (Halichoerus grypus) can be much larger but individuals will always return to the same breeding site and they are resident to the British Isles (McConnell et al., 1999). Less is known about the distribution and behaviour of harbour porpoise (Phocoena phocoena) in UK waters, however, population structure analysis of the northeast Atlantic has indicated that there is a subpopulation in British waters (De Luna et al., 2012) and 137 Cs measurements of their tissues suggest regional residency around the UK (Berrow et al., 1998;Watson et al., 1999). Resident mammals from the Irish Sea and West of Scotland (defined here as the area located to the north of the North Channel) will be susceptible to 14 C enrichment as they spend most or all of their foraging time in waters enriched in 14 C and consequently, containing prey species enriched in 14 C. In these regions, Sellafield 14 C will be transferred through the food chain to marine mammals as has been observed for other species Tierney et al., 2017).
Harbour porpoise, harbour seal and grey seal are generalist predators, although with some dietary specialisations. Overlaps in their diets have been observed in Irish coastal waters where harbour seals and grey seals predate on a number of the same species, as do grey seals and harbour porpoises (Hernandez-Milian, 2014). However, there may be little direct competition between these mammal species as they target prey of different sizes (Hernandez-Milian, 2014). Dietary differences have been observed between porpoise populations in the Irish Sea and the West of Scotland. Harbour porpoises in Irish Sea waters, for example, show a higher presence of pelagic fish such as herring (Clupea harengus) in their diet (Hernandez-Milian, 2014), whereas Scottish coastal harbour porpoises predate more on sandeels (e.g. Hyperoplus spp.; Santos et al., 2004), although gadoid species are important prey species for both. This difference in diet could be due to prey availability in different environments but it could also indicate a change in diet during the period between studies. Some genetic research has indicated that the Irish Sea harbour porpoise may be a sub-population within the UK population (Andersen et al., 2001), although a review of recent literature has found no clear evidence for distinct populations on the west coast of Britain (IAMMWG, 2015) and Fontaine et al. (2017) showed that there is a genetic continuum in UK waters.
The samples described in this study come from animals that were found dead, or died at the stranding site. Studies of 137 Cs activities (a radionuclide that was historically discharged to the Irish Sea from Sellafield) in marine mammals stranded in the UK and Ireland have shown that Celtic Sea activities are significantly lower than Irish Sea activities (Berrow et al., 1998) and that activities decrease with distance of stranding site from Sellafield (Watson et al., 1999). These findings demonstrate that anthropogenic radionuclides are transferred through the food chain to marine mammals and suggest that stranding sites are approximately within the same region in which the animal has been foraging.
The aims of this study were to: 1) evaluate the transfer of Sellafieldderived 14 C to top predators in the UK marine environment; 2) examine the spatial distribution of 14 C relative to dilution with distance from Sellafield; and 3) determine the effect of temporal changes in Sellafield 14 C discharge activities and subsequent transfer through the food chain to marine mammals.

Methods
Access to the Scottish Marine Animal Stranding Scheme (SMASS) and Cetacean Strandings Investigation Programme (CSIP) sample archives provided the opportunity to consider samples from different mammal species at various locations over a relatively long time-period. The species of interestharbour porpoise, grey seal and harbour sealwere selected as they represent resident UK marine mammal species, of which a number of samples were available. Muscle tissue samples of stranded mammals from the Irish Sea and the West of Scotland (Fig. 1B) were identified formally by CSIP and SMASS respectively (Tables 1 and  2). Three time-periods were significant: 1) Pre-1994, when 14 C discharges were relatively low, 2) 2001-2004, which encompasses the period of peak 14 C discharge, and 3) 2011-2015, to examine contemporary 14 C activities in marine mammals. Most of the samples came from these three time-periods, although additional Irish Sea samples were analysed from other years (Tables 1 and 2). Three samples from the Scottish east coast (Firth of Forth) were also analysed to identify the extent and influence of Sellafield discharges at greater distances from the facility. The analytical techniques employed are described in detail in Muir et al. (2017) and are briefly summarised here. Muscle tissue samples from each organism were freeze-dried and approximately 15 mg were combusted (850°C) in sealed quartz tubes, according to the method of Vandeputte et al. (1996) to liberate CO 2 gas. CO 2 was cryogenically purified (under vacuum) and converted to graphite (Slota et al., 1987) prior to 14 C/ 13 C isotope ratio measurement by accelerator mass spectrometry (AMS). Sub-samples of CO 2 were collected to determine the δ 13 C value ( 13 C/ 12 C stable isotope ratio) for calibration of natural fractionation of measured 14 C. Reported AMS fraction modern results were converted to specific activities (Bq kg − 1 C) using the regime described by Mook and van der Plicht (1999). Error bars are omitted from figures, as analytical uncertainties for AMS measurements at SUERC are typically < 0.5% of the measured activity, and therefore indistinguishable in measured values. Statistical analyses and modelling were conducted using the software package R (R Development Core Team, 2016). Generalised least squares (GLS) regression models were used to identify significant variables and model fit was compared using the corrected Akaike information criterion (AICc; Venables and Ripley, 2002).
Due to natural production of 14 C and the legacy of 14 C from atmospheric testing of atomic weapons during the 1950s and 1960s, a baseline (or background) activity was quantified to determine enriched activities resulting from Sellafield discharges. Cook et al. (1998) defined the UK 14 C coastal marine background as 248 ± 1 Bq kg − 1 C from west coast of Ireland samples that are free of UK coastal influences, i.e. Sellafield radionuclide discharges. Tierney et al. (2016) presented a new, but near-identical, UK 14 C background activity of 249 ± 1 Bq kg − 1 C which has been used in subsequent studies Tierney et al., 2017) and is used here.

Results
Analytical results for samples obtained from CSIP and SMASS are listed in Tables 1 and 2, respectively. Of the 56 marine mammal samples analysed, three were from grey seals, six from harbour seals and 47 from harbour porpoises. Two samples (46 and 50) were measured with 14 C activities below that of the current UK coastal marine background.
These came from porpoises stranded in the West of Scotland in 2014 (238 ± 1 Bq kg − 1 C and 242 ± 1 Bq kg − 1 C). The highest measured (gross) activities were from two porpoises (samples 6 and 7) that stranded in the Irish Sea in 2002 (674 ± 3 Bq kg − 1 C and 657 ± 3 Bq kg − 1 C). One harbour seal sample and two grey seal samples (52, 53 and 54) from animals that died at approximately the same time in Loch Fyne (West of Scotland) showed similar 14 C activities (262 ± 1 Bq kg − 1 C, 271 ± 1 Bq kg − 1 C and 265 ± 1 Bq kg − 1 C), respectively. Conversely, two young male harbour porpoises (samples 22 and 23) that stranded at Porth Dafarch (North Wales) on the Southern Irish Sea coastline at a similar time had a relatively large difference in measured activity (524 ± 2 Bq kg − 1 C and 315 ± 2 Bq kg − 1 C). The average 14 C activity across all Irish Sea samples was 388 Bq kg − 1 C, compared to a significantly lower 14 C activity of 285 Bq kg − 1 C for West of Scotland samples. Three samples from the Scottish east coast (39, 40 and 43) also show enriched 14 C activities (253 ± 1, 262 ± 1 and 264 ± 1 Bq kg − 1 C).
No seal samples were obtained for the Irish Sea area and the number of seal samples was low overall, relative to that of harbour porpoise, resulting in a large activity difference between the ranges observed for seal (254-284 Bq kg − 1 C) and porpoise (238-674 Bq kg − 1 C) 14 C activities. As there was no significant difference between seal and porpoise 14 C activity in the West of Scotland, these species were grouped for statistical analysis.
A number of variables were considered to explain the measured 14 C activities, including distance (measured as distance from Sellafield by  1 2 4 ( 2 0 1 7 ) 4 3 -5 0 sea in km), sex, age class (neonate, juvenile, sub-adult and adult), level of decomposition (freshly dead, slight and moderate decomposition), month of stranding and year of stranding. The data were explored prior to statistical analyses and linear model assumptions checked. As correlation in the data residuals was detected, generalised least squares (GLS) regression was used with a simple correlation structure (AR1) and model descriptions, and AICc scores are given in Table 3. Initial model fitting of 14 C activity found the best fit (lowest AICc score) when only including the predictor variable distance (Model 1). However, the relationship between sample activity and distance was not significant (p > 0.01). Distance appears to have an exponential influence on activity as stranding site gets closer to Sellafield (Fig. 2). Model fitting of log-transformed 14 C activity found distance significantly (p < 0.001) affected 14 C activity and the model with the lowest AICc score also included year, despite this variable having little significance (p > 0.01; Model 2). It is likely that year improved model fit due to the temporal changes in Sellafield 14 C discharges having some effect on individual 14 C activities, although this is not obvious in Fig. 3. A new variable was considered describing discharge activity prior to stranding (pDischarge), where pDischarge is the sum of total monthly 14 C activities discharged from Sellafield (a) for a number of months (n) prior to the month of stranding (s; Eq. (1)). Periods of 6, 12, 24 and 36 months were considered and 12 (Model 3) and 24 months (Model 4) were found to improve model fit, however, pDischarge had little significance (p > 0.01). Log-transformed 14 C activity with variables: distance from Sellafield (p < 0.0001) and pDischarge 12 months prior to stranding (p > 0.01).

Model 4
Log-transformed 14 C activity with variables: distance from Sellafield (p < 0.0001) and pDischarge 24 months prior to stranding (p > 0.01). 143.9 Model 5 Log-transformed 14 C activity with variables: distance from Sellafield (p < 0.0001) and pDischarge 24 months prior to stranding including delay factor of 12 months for West of Scotland mammals (p < 0.01).

139.2
Model 6 Log-transformed 14 C activity with variable distance from Sellafield only (p < 0.0001).

Model 7
Log-transformed 14 C activity with variable pDischarge only (p < 0.01). 166.0 Sellafield discharges will not reach the West of Scotland environment immediately. Estimates for transit times of other highly soluble radionuclides discharged from Sellafield ( 134 Cs, 137 Cs and 99 Tc) range from 3 to 18 months (Jefferies et al., 1973;Kershaw and Baxter, 1995;Kershaw et al., 2004). To account for transit time in calculating pDischarge for West of Scotland samples, a delay factor (d) was used (Eq. (2)). Although a number of delay factors were considered, a factor of 12 months was statistically significant (p < 0.01), and improved model fit when included with distance (p < 0.0001; Model 5). A delay factor of 12 months meant pDischarge for West of Scotland samples was the total 14 C discharge activity from 12 to 36 months prior to stranding.
Distance alone (Model 6) did not improve model fit and although pDischarge was significant, there is no obvious correlation between 14 C activity and pDischarge (Fig. 4) and pDischarge alone (Model 7) did not improve the model fit. Therefore, the overall best model fit for mammal 14 C activity included the predictor variables of: distance of stranding from Sellafield and total Sellafield 14 C discharge activity 24 months prior to stranding, including a 12-month delay for West of Scotland animals (Model 5). For every kilometre increase away from Sellafield, this model predicts an estimated 0.3% decrease in sample activity. For every TBq increase in discharged 14 C activity during the 24 months prior to stranding, the model predicts an estimated 6.5% increase in sample activity. The combined effect of distance and prior Sellafield discharges on sample 14 C activity is illustrated in Fig. 5 where distance is normalised to pDischarge and the scatter again indicates an exponential relationship.

Discussion
The two West of Scotland samples (46 and 50) with below UK coastal marine 14 C background activities (238 ± 1 Bq kg − 1 C and 242 ± 2 Bq kg − 1 C) were from young female porpoises that stranded hundreds of kilometres apart (approximately 178 km and 596 km from Sellafield respectively). Similar activities were observed in phytoplankton and zooplankton in the West of Scotland . It is not clear why these 14 C activities would be below this background value, however, the depleted plankton activities were linked to a possible source of older water, possibly derived from upwelling of deep Atlantic water or another Atlantic source, reducing ambient 14 C activities . Natural 14 C activities are not homogenous and the cited UK coastal marine 14 C background activity does not represent the "oceanic background" for the entire northeast Atlantic. This may vary as 14 C produced by atomic weapon testing decays in different hydrographic and biogeochemical settings (Scourse et al., 2012). The lower activities could result from the animals previously inhabiting a region with a lower ambient 14 C activity before stranding at these sites.
The relatively high 14 C activities (674 ± 3 and 657 ± 3 Bq kg − 1 C) observed in harbour porpoises stranded close to Sellafield in 2002 (samples 6 and 7) coincides with the period of peak 14 C discharges. Although peak 14 C discharge occurred in 2003 (17 TBq), cumulative increases in 14 C discharge were made to the Irish Sea in both 2001 (9.5 TBq) and 2002 (13 TBq). Similarly, higher activities (608 ± 3 Bq kg − 1 C and 609 ± 3 Bq kg − 1 C) were measured in two harbour porpoises that stranded in different areas of the Irish Sea in 2010 (samples 14 and 16). Although the 2010 annual discharge (4.4 TBq) was low relative to the peak discharges, the 2009 annual discharge (8.2 TBq) was the highest between 2006 and 2015. However, a porpoise (sample 17) that stranded relatively close to Sellafield in 2010 (74 km away) had a comparatively low activity (308 ± 2 Bq kg − 1 C). This individual died from starvation and an extended period of limited foraging with little or no food intake from the eastern Irish Sea may help explain this individual's anomalously low 14 C activity.
It is reasonable to assume that samples obtained from animals of the same or similar species that stranded in the same location at the same time of year, would have comparable 14 C activities. This was observed in three seal samples (52, 53 and 54) from Loch Fyne (West of Scotland; 262 ± 1 Bq kg − 1 C, 271 ± 1 Bq kg − 1 C and 265 ± 1 Bq kg − 1 C). The fact that two porpoise samples (22 and 23) from Porth Dafarch (North Wales) had significantly different activities (524 ± 2 Bq kg − 1 C and 315 ± 2 Bq kg − 1 C) illustrates the difficulty in making assumptions based on stranding location alone. Animals that inhabit different areas might strand in the same area due to a number of variables including changes in water masses, wind patterns, and bloating of animal carcasses prior to stranding. However, diet source will directly affect the individual's 14 C activity. Studies of shark age, using the radiocarbon bomb peak (from atomic weapons testing), found that changes in diet could affect shark vertebrae 14 C activity (Campana et al., 2002). Sharks feeding on longer-lived species during the bomb peak could have a relatively lower 14 C activity because of integration with lower activities Fig. 4. Sample 14 C activities as a function of Sellafield 14 C discharge activity for 24 months prior to stranding, including a 12-month delay factor for West of Scotland samples.

K.M. Tierney et al.
M a r in e P o llu t io n B u lle t in 1 2 4 ( 2 0 1 7 ) 4 3 -5 0 from before the bomb peak (Campana et al., 2002;Kerr et al., 2006). Integration of higher 14 C activities from Sellafield discharges have also been linked to higher activities in longer-lived species at specific sites in the Irish Sea . Of the two Porth Dafarch porpoise samples, the higher 14 C activity came from a juvenile porpoise, whereas the lower activity was from a neonate. The 14 C activity of the neonatal porpoise is likely a result of transfer from mother to calf. It could be inferred, therefore, that its mother had been foraging in an area of lower ambient activity relative to the sampled juvenile. It is equally possible that the mother of the neonatal porpoise had a lower 14 C activity due to a longer integration period covering a previous period of lower prey 14 C activity and this was transferred to the calf. During data analysis, species type did not have an impact on sample 14 C activity. However, harbour seal and grey seal samples were only available from the West of Scotland where there was little observed variation in 14 C activities relative to Irish Sea mammals, and concurs with the relative homogeneity in 14 C activities of other marine species in this area . In order to determine whether diet and life history influence mammal 14 C activities between species, it would be necessary to analyse seal samples from the Irish Sea.
Across all the data, several other variables including sex, age class (neonate, juvenile, sub-adult and adult) and level of decomposition (freshly dead, slight and moderate decomposition), showed no significance with 14 C activity. However, distance of stranding site from Sellafield and the Sellafield 14 C discharge activity prior to stranding (pDischarge) were significant. The best model fit predicted that for every 1 km increase in distance away from Sellafield there would be an estimated 0.3% decrease in mammal 14 C activity. This is significant as the samples analysed in this study came from mammals that stranded in the range of 10-1000 km from Sellafield. It indicates that stranding site is a reasonable approximation for the area an individual has been foraging in, as 14 C activities in the UK marine environment reduce with distance from Sellafield (Begg et al., 1992, Cook et al., 1998., Muir et al., 2015., Tierney et al., 2016. Within the Irish Sea and, in particular, at stranding distances of < 200 km from Sellafield, there was a wide range of 14 C activities (280-674 Bq kg − 1 C). At distances > 200 km a general tail of decreasing 14 C activity is apparent and a distinct reduction in maximum activity exists between Irish Sea and West of Scotland samples. Three samples from the Scottish east coast show slight enrichments above 14 C background activity (253-264 ± 1 Bq kg − 1 C). This could indicate the long distance dispersion of 14 C from Sellafield to the North Sea, as has been noted before (Cook et al., 1998;Gulliver et al., 2004). The reduction in 14 C activity with distance from Sellafield is due to dilution and subsequently lower activities within prey species  as discussed below.
The best model fit also predicted that for every 1 TBq increase in total Sellafield 14 C discharge activity for the period of 24 months prior to stranding, mammal 14 C activity would increase by an estimated 6.5%. This confirms that Sellafield is the source of 14 C enrichment in these samples. Furthermore, it also indicates the complex nature of 14 C transfer to these animals through the food web and shows the persistence of enriched 14 C within the marine environment, despite dispersion and dilution. Adding a delay factor of 12 months for West of Scotland samples improved the overall model fit. It suggests that the sampled West of Scotland mammals have spent little or no time foraging in the Irish Sea during the 12 months prior to stranding. This is expected of harbour seals as they typically only forage within 40 km of their haul-out site (Tierney et al., 2016). However, the number of harbour seal samples analysed was low (6) so this increased significance is unlikely to be attributable to these samples alone. In addition, few grey seal samples were analysed (3), therefore it is likely that the increased model significance is proportionally weighted toward the porpoise samples, which made up the bulk of the samples analysed (47). By removing the seal samples from the model fitting process, a similar level of significance for pDischarge (p < 0.01) was found. The model fit suggests that the sampled West of Scotland porpoises fed mainly in areas other than that of the Irish Sea for (at least) 12 months prior to stranding. This indicates a high foraging fidelity for harbour porpoises in the West of Scotland.
Herring, sandeel and gadoid species such as haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) are important prey for harbour porpoise (Santos et al., 2004;Hernandez-Milian, 2014). Herring activity (from a bulk sample) in the eastern Irish Sea in 2014 was reported at 274 ± 1 Bq kg − 1 C, sandeel 314 ± 1 Bq kg − 1 C and Irish Sea haddock ranged between 293 and 469 Bq kg − 1 C . The average 14 C activity of the six Irish Sea porpoise samples from animals that stranded between 2014 and 2015 was 329 Bq kg − 1 C (range 288-428 Bq kg − 1 C) and agrees well with the 14 C activities of their prey species. This is expected as 14 C is transferred through the food web without any bioaccumulation or concentration effect. Measurements of West of Scotland fish demonstrated 14 C activity ranges of 282-284 Bq kg − 1 C in herring, 286-296 Bq kg − 1 C in haddock, and 288-413 Bq kg − 1 C in whiting . These activities fit reasonably well with the range of porpoise and seal activities between 2014 and 2015 porpoise Fig. 5. Sample 14 C activity as a function of stranding distance from Sellafield, normalised by the discharge activity for the 24 months prior to individual stranding (note that no delay factor for West of Scotland samples is included here).

K.M. Tierney et al.
M a r in e P o llu t io n B u lle t in 1 2 4 ( 2 0 1 7 ) 4 3 -5 0 (254-398 Bq kg − 1 C), after exclusion of the two individuals with belowbackground activities. The average mammal 14 C activity of these ten samples, 280 Bq kg − 1 C, is at the lower end of the prey species activity range. However, the mammal samples come from a much wider area, including north of the fish sample sites, where 14 C activities in other benthic species are lower . It is apparent that trophic transfer of enriched 14 C from prey species is the cause for enriched activities found in mammals. The significant relationship that exists between Sellafield 14 C discharges and mammal 14 C activity, and the comparable activities between predator and prey, demonstrate the transfer pathway in its entirety as a trophic level flow of 14 C from source to top marine predators.

Conclusions
Enriched 14 C activities were found in almost all marine mammal samples from the west coast of the British Isles. The highest activities were from harbour porpoises that stranded within the Irish Sea, although enriched 14 C activities were also observed in the West of Scotland and in three samples from the Scottish east coast. 14 C activities vary greatly both temporally and spatially. They correlate significantly with: 1) the distance the animal stranded from the Sellafield nuclear fuel reprocessing facility; and 2) the total 14 C activity discharged from Sellafield to the Irish Sea for a period of 24 months prior to stranding.
West of Scotland marine mammal 14 C activities correlate significantly with discharges made between 12 and 36 months prior to the animal stranding. This indicates the time taken for Sellafield 14 C discharges to be transported to the West of Scotland environment and become fully integrated in prey species. The model fit also suggests that West of Scotland harbour porpoises did not forage in the Irish Sea and have a high foraging fidelity to the West of Scotland. 14 C activities in samples from 2014 and 2015 are similar to 14 C activities measured in typical prey species showing that transfer of enriched 14 C from prey to predator occurs without any concentration or bioaccumulation effect. Although the 14 C activities presented in this study do not pose any radiological risk to the individual, it is clear that 14 C enrichment in marine mammals result from 14 C transfer from prey species, and that distance and discharge activity from Sellafield are key factors in determining an individual's muscle 14 C activity. Sellafield is one of a number of facilities that continue to release low-level radioactive material, such as 14 C, into the marine environment. This study demonstrates, for the first time, the transfer of nuclear industry derived, 14 C through the entire marine food web to top predators, and highlights the necessity for continual monitoring of the fate of 14 C and other bioavailable radionuclides in marine ecosystems. Future work includes measuring 14 C concentrations in seals from the Irish Sea and addressing differences in 14 C transfer in relation to dietary preferences.