**3.3 Ontogenetic changes in δ13C and δ15N values in** *Dalli***-type Dall's porpoises**

*Dalli*-type DP (odontocete) is widely distributed in the northern part of the North Pacific and move from south to north seasonally in the North Pacific Ocean [75]. According to Kasuya [77], the calves are born from August to September at 100 cm BL and nursed for approximately 2 years. In addition, sexual maturity is attained in males at the age of 7.9 years at 195.7 cm BL and in females at 6.8 years at 186.5 cm BL. Huang et al. [31] estimated the weaned BL of DPs to be 135 cm. We analyzed the δ15N and δ13C values and Hg concentrations of muscle samples of DPs stranded along the coast of Hokkaido.

An <sup>δ</sup>15N-enriched peak at 115 cm BL owing to lactation was found in the muscle samples of DPs (**Figure 5**), similar to that observed in the mysticetes BWs and MWs (**Figures 3** and **<sup>4</sup>**). The <sup>δ</sup>15N values following the peak gradually declined at 160 cm BL, then increased slightly and plateaued. The BL at the δ15N-enriched peak could imply the BL at the onset of weaning, although available information on the onset of weaning in DPs is lacking. The <sup>δ</sup>15N-depleted peak at 160 cm BL may be related to BL at complete weaning, although the reported BL of DP at weaning DP is 135 cm [31]. The DP sample shown in **Figure 5** was biased and did not include the DP samples at 130–150 cm BL at which weaning was reported to occur.

Although the δ13C values varied considerably, small δ13C-enriched and <sup>δ</sup>13C-depleted peaks were found at 130 cm BL and 190 cm BL, respectively, and the small δ13C-enriched peaks may be related to weaning. No correlation was found between the δ13C and δ15N values of DPs, similar to that of the mysticetes (**Figures 3** and **4**). We believe that the large variability in δ15N and δ13C values between 180 and 230 cm BL may be due to sex-related differences, but this was not clear (males and females are not shown separately in **Figure 5**). Ontogenetic changes in δ15N and δ13C profiles in DP samples (**Figure 5**) were similar to those in BW samples (**Figure 3**).

The Hg concentrations in DPs increased with increases in BL, and were markedly higher than those of MWs (**Figure 4**), reflecting the higher trophic position of DPs.

The ontogenetic signatures of δ15N and δ13C in KWs were investigated by Newsome et al. [33] using the dentin growth layer of teeth. They reported a decrease in δ15N values related to weaning in those dentins with no δ15N-enriched peaks related to nursing, and a gradual and continuous increase in δ15N values after weaning. This post-weaning increase in δ15N values in KWs is more prominent than that in DPs (**Figure 5**), ringed seals [56], and the beluga whales [55], and similar to that in harbor porpoises [14] and South American fur seals [57]. We believe that the increase in δ15N values after weaning may be correlated with the increase in trophic level of juvenile animals owing to growth.

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

**Figure 5.** *Ontogenetic changes of δ15N and δ13C signatures and Hg concentration in muscle of Dall's porpoises stranded in Hokkaido, Japan.*
