Changes in diets of individual Baltic ringed seals (Phoca hispida botnica) during their breeding season inferred from stable isotope analysis of multiple tissues

The stable isotope ratios (delta C-13 and delta N-15) of three tissues with different metabolic rates (plasma, liver, and muscle) were used to investigate temporal variation in diet among nine individual Baltic ringed seals (Phoca hispida botnica Gmelin) from the Bothnian Bay, northeast Baltic Sea. The isotope values from plasma should reflect the most recent diet, values from liver the diet of the past weeks prior to sampling, and values from muscle should integrate diet over almost the entire breeding season of the ringed seals. In general, delta C-13 values of liver were more enriched in C-13 than were those of either muscle or plasma, suggesting that the diet of the seals may have included a higher proportion of C-13-enriched benthic prey in April. Females showed more variable delta C-13 values than males, suggesting possible gender differences in diet or in foraging locations. The differences that were apparent between females possibly reflect individual variation in the onset and duration of parturition and lactation, both of which likely restrict female foraging. Previous data from parasite infections and from alimentary tract contents of the same seals were linked to the isotope data to assist in drawing inferences about changes in the diets of individual seals.