**3.4 Ontogenetic changes in δ13C and δ15N values in kurill harbor seals**

HSs are distributed on the eastern coastline of Hokkaido, Japan. According to Naito and Nishiwaki [78], HSs is are born at 98 cm BL after a 9-month gestation period, and weaning occurs at approximately 4 weeks after birth at 110 cm BL. It is unknown whether weaning occurs suddenly or gradually, but the weaning is likely rapid, considering the brief lactation period. Males attain sexual maturity at 140 cm BL (4 years) and females at 133 cm BL (3 years), and the maximum BL observed was 191 and 175 cm in males and females, respectively. HSs feed on prey in inshore areas, such as cephalopods, crustaceans, and small fish. The competition between HSs and coastal fisheries has recently become a serious problem in Hokkaido because of the increased number of HSs. Tissue samples were collected from stranded HSs in salmon set nets. **Figure 6** shows our data on HS males and females.

A δ15N-depleted peak was found at 120–130 cm BL of males, and the δ15N values increased thereafter, whereas a small δ13C-depleted peak was found at 120 cm BL, and δ13C values gradually increased thereafter. In contrast, a δ15Ndepleted peak was observed in females, whereas the δ13C-depleted peak and following <sup>δ</sup>13C-enriched peak were clearly found at 125 and 135 cm BL, respectively. The small HSs at 110 cm BL, shown in **Figure 6**, could be just-weaned animals who might forage independently of their mothers, and be stranded in salmon set nets.

#### **Figure 6.**

*Ontogenetic changes of δ15N and δ13C signatures and Hg concentration in muscle of male and female harbor seals stranded in Hokkaido.*

*Typical Changes in Carbon and Nitrogen Stable Isotope Ratios and Mercury Concentration… DOI: http://dx.doi.org/10.5772/intechopen.103067*

The δ15N-enriched peaks due to the lactation were not observed in HS samples, as our samples did not include nursing pups whose BL was less than 110 cm. Sharp decreases in δ15N and δ13C values were found in male and female pups at 110–120 cm BL after weaning, which may reflect their weaning processes: The feeding sifts from milk to solid food and turnover rate of δ15N signature may be extremely first in HS muscle.

Sex-related differences in δ15N and δ13C profiles were observed in the animals. Pregnancy and lactation of females may be the reasons for the differences in δ15N and δ13C signatures. Sex-related differences in δ15N and δ13C signatures were not found in the MWs (**Figure 4**) and were unclear in DPs (**Figure 5**), which migrate annually in the waters around Japan.

#### **3.5 Isotopic segregation of calves of cetacean species stranded in Hokkaido**

Stable isotope analyses of carbon and nitrogen in the tissues of suckling offspring, before weaning, when milk constitutes the entire diet, are increasingly used as proxies for maternal forging habits and trophic levels [54, 60, 79]. This indirect approach is useful when sampling from females is difficult or has a high potential risk. In this section, we compare the δ13C and δ15N values of stranded calves (nursing and weaning stages) from mysticete species (MW) and two odontocete species (DP and KW) inhabiting the waters around Hokkaido, Japan, and the North Pacific Ocean, to investigate whether these calves reflect their maternal foraging habits and trophic levels.

**Figure 7** shows the δ13C and δ15N values of MW, DP, and KW calves stranded along the coast of Hokkaido. MW is opportunistic feeders that temporally and

**Figure 7.**

*Isotopic discrimination of calves of common mink whale, Dall's porpoise and killer whale stranded in Hokkaido. MW: Common minke whales (see Figure 4, n = 12), DP: Dall's porpoises (see Figure 5, BL* ≥*130 cm, n = 11), KW: killer whales (see Table 1, n = 3), HS: harbor seals (see Figure 6, n = 32).*

regionally adapt to prey items (zooplankton and small fish), whereas KW and DP are odontocetes, and the KW is a cetacean located at the top of the marine food chain.

The δ15N and δ13C values of KW calves and MW calves were the highest and lowest, respectively, and those values of DP calves were intermediate, reflecting the trophic positions of their mothers.

Stable isotope signatures of HSs (immature and mature animals shown in **Figure 6** (pinniped) were compared with those of MW, DP and KW calves (**Figure 7**). According to the inshore prey comprising cephalopods, crustaceans, and fish, the δ13C values of HSs were the highest among the marine mammals tested and δ15N values of HSs were higher than those of DP and MW calves.
