Stable isotopes and mercury in blood of harbour seals (Phoca vitulina) caught on a German sandbank

More than 21 500 harbour seals were killed by Phocine Distempter Virus in the North Sea and adjacent waters in 2002. After the second seal die-off had ceased, seals were captured alive (and then released) on a sandbank (Schleswig-Holstein, Germany) for health and ecotoxicological investigations. Stable carbon and nitrogen ratios (δ13C and δ15N) were measured by mass spectrometry in clotted blood cells of 24 harbour seals captured between 2002 and 2003. Total mercury (Hg) concentrations were determined by spectrometric absorption in the whole blood of 8 harbour seals and compared to the Hg level measured in blood from 8 seals found stranded along the southern North Sea coast. The average isotopic composition measured in the blood cells was –15.6 ± 0.3 0/00 and 18.7 ± 0.6 0/00 for δ13C and δ15N respectively, similar to that obtained previously in muscle of stranded individuals, confirming the high position of the harbour seal in the trophic food chain. δ13C and δ15N values did not differ significantly between seals caught in November 2002, April 2003 and September 2003 suggesting similar diet throughout these 3 periods. The average concentration of mercury in whole blood of living and stranded harbour seals did not differ significantly (94 ± 41 vs 146 ± 71 µg.l-1 respectively). No biological parameters (weight, length, age status and stable isotopes ratios) seemed to influence these concentrations. Blood is known to reflect recent exposure to mercury through the diet. These preliminary results suggest that blood is an interesting substrate for both trophic studies and pollutant long-term monitoring of the harbour seal in the North Sea.