Stable isotope composition and parasitic infections of harbor seal young‐of‐the‐year used as prey‐based diet indicators

The stable isotope composition (δ¹³C and δ¹⁵N values) of harbor seals (Phoca vitulina) is influenced by their diet. Young‐of‐the‐year during lactation and postweaning fast are expected be enriched in ¹⁵N compared to foraging seals. We studied the temporal variation of stable isotope composition of young‐of‐the‐year and adults to determine from which point in time the young‐of‐the‐year tissues (i.e., muscles and vibrissae) are influenced by independent foraging only. These results were compared with the development of trophically transmitted parasitic infections. The δ¹⁵N values in young‐of‐the‐year muscles decreased from June (20.3‰ ± 0.5‰) to October (18.5‰ ± 0.4‰), while those of foraging seals were all year long below 19.2‰. This decrease coincides with the increase of parasitic infections in young‐of‐the‐year, reflecting a shift to fish diet. Together these results suggest that the muscles of the young‐of‐the‐year older than 5–6 mo reflect independent foraging and that they can therefore be used in community diet studies. The nursing signal in vibrissae was unclear, as the δ¹⁵N values of young‐of‐the‐year were stable over time, whereas those of adults varied seasonally. However, δ¹⁵N values of nursing pups were significantly higher than those of adults in May and June, maybe due to their reliance on milk.