Long term trends in mercury and PCB congener concentrations in gannet (Morus bassanus) eggs in Britain

Gannet (Morus bassanus) eggs from Bass Rock (North Sea) and Ailsa Craig (eastern
Atlantic) were monitored for PCB congeners (1990-2004) and total mercury (1974-2004).
Congener profiles for both colonies were dominated by PCBs 153, 138, 180, 118 and 170.
All declined in concentration at Ailsa Craig but some (153, 170, 180) remained stable or
increased slightly at Bass Rock. Egg congener concentrations at Bass Rock were typically
10-fold higher than at Ailsa Craig by 2002, and Principal Component Analysis indicated
that colony differences were driven by the dominant congeners. Egg mercury
concentrations were significantly lower at Bass Rock than Ailsa Craig and temporal trends
differed, there being a significant decline at Ailsa Craig but a marginal increase at Bass
Rock. Our results suggest there may be differences in contamination between the eastern
Atlantic and North Sea and/or there are colony differences in prey selection and associated
contaminant loads.