Pacific White-Sided Dolphin

The Pacific white-sided dolphin Lagenorhynchus obliquidens Gill, 1865, is endemic to the North Pacific and one of the smallest marine delphinids occurring there It attains adult size at less than 25 m The Japanese common name kama-iruka [sickle dolphin] came from the falcate dorsal fin with gray trailing edge The adult male has a dorsal fin with a round leading edge and drooping tip described in this chapter

Maximum sea surface temperatures are reached in the northern North Pacific in August-September, which is about one month after the peak in terrestrial air temperature During this time of the year, Pacific white-sided dolphins inhabit latitudes of 40°N-47°N with sea surface temperatures of 10°C-19°C in the central North Pacific (Iwasaki and Kasuya 1997) During the rest of the year, they shift their range to the south The coldwater Dall’s porpoise is found to the north of this latitudinal range and warmwater species such as the striped dolphin to the south The habitat preference of the northern right whale dolphin is the same as that of the Pacific white-sided dolphin in the longitudinal range during that time of year However, it tends to inhabit more offshore waters and is not usually found in coastal waters like the Pacific white-sided dolphin Sea surface temperatures inhabited by these species are listed in Table 101

The Pacific white-sided dolphin inhabits Japanese coastal waters of the East China Sea coast of Kyushu, Sea of Japan coasts of Honshu and Hokkaido, Okhotsk Sea coast of Hokkaido, Pacific coast of Hokkaido, and south to central Honshu around the Kii Peninsula (33°26′N-34°15′N, 135°05′E-136°15′E) It is common within the range, but an apparently seasonal visitor to southern and northern part of the range

The small cetaceans spend the spring to autumn seasons off the Sea of Japan coast of Japan, probably winter in waters around Iki Island (33°47′N, 129°43′E) and in the East China Sea This was deduced from the predominance of Dall’s porpoises in winter in the southern Sea of Japan off the coasts of Yamaguchi Prefecture (33°55′N-34°40′N, 130°50′E-131°40′E) (Section 9311) and the shift of the former hand-harpoon fishery from the Dall’s porpoise to the Pacific white-sided dolphin in middle May off Kinosaki (35°39′N, 134°50′E) (Noguchi 1946, in Japanese), although small numbers of Pacific white-sided dolphins could remain in this area in February and March as evidenced by a few individuals hunted during the Dall’s porpoise season The Pacific white-sided dolphins that winter in Tsushima Strait and the East China Sea start moving in the spring into the Sea of Japan along the southern coast of the sea

Pacific white-sided dolphins are known to occur in the Tsugaru Strait (41°30′N, 140°40′E), through which the Sea of

Japan opens to the Pacific, during April to July with a peak density in June (Kawamura et al 1983, in Japanese), but their migration route is unknown

Along the Pacific coast of Japan, the Pacific white-sided dolphin is known from the coast of the Kii Peninsula to the southern Kuril Islands, or in the approximate latitudinal range of 33°26′N-46°N (Hayano et al 2004) This species is almost replaced by Dall’s porpoise in winter in the northern habitat north of Choshi Point (35°42′N, 140°51′E) The pattern of north-south segregation of these two species is probably similar to that observed in the offshore central North Pacific, although the absolute latitudinal range might not be the same Pacific white-sided dolphins are known to occur in the southern Okhotsk Sea in summer

Hayano et al (2004) identified a distribution gap around the longitude of 150°E or between Japanese coastal waters and the offshore western and central North Pacific, identified genetic differences between a Japanese coastal sample (Iki Island area, the Sea of Japan, the Tsugaru Strait, and the Pacific coasts of Hokkaido and Honshu) and an offshore sample from longitudes 160°E-160°W, and concluded that these two geographical samples came from different populations However, it seems premature to me to conclude that the species around Japan constitutes a uniform single population The question remains whether dolphins in the southern Sea of Japan and off the Pacific coast of central Japan really belong to a single population A larger sample covering a broader geographical distribution and seasonal range is needed for further clarification of the genetic structure of the species around Japan Hayano et al (2004) stated that the low genetic difference could be due to a short time since reproductive isolation between the geographical populations off Japan

Even if they are not genetically indistinguishable, it does not necessarily mean that individuals in the two wintering grounds-Iki-East China Sea and the Pacific coast of central Japan-have opportunities to freely intermingle and interbreed, which is key information needed for management Under such a situation, genetics will be of limited use for the purpose of studying population structure Studies on the geographical movement of individual dolphins or mixing of schools using natural marks or radio transmitters will offer more information

14.3.1.1 Materials Materials were obtained from dolphins killed at Katsumoto on Iki Island as a part of a culling operation on dolphins led by Iki fishermen (Section 34) A total of 441 Pacific

white-sided dolphins were taken in four drives in February and March in 1979 and 1981; 177 were randomly sampled (Kasuya 1995) (Table 141) These schools were found on a bank called Shichiriga Sone (33°56′N, 129°30′E) located between Iki Island and the southern tip of Tsushima Island, which extends from 34°07′N, 129°10′E to 34°42′N, 129°24′E Thus, the sample represents part of the putative southern Sea of Japan population spending the winter near the northern range of its winter distribution

The sample was collected almost randomly with the cooperation of volunteers while the fishermen killed the dolphins and processed the carcasses for disposal This explains the low sampling rate No particular sex or growth stages were selected A similar type of sampling was experienced for the common bottlenose dolphin in the Iki Island area (Chapter 12) Details of the culling operation and our research activities are in Kasuya and Miyazaki (1981, in Japanese) and Kasuya (1985) Our sampling activity at Iki Island ended with the 1981 winter season and was followed by operation of a Fisheries Agency team that was established in April 1981 and started sampling in winter 1982 Some biological information on the species from our study was given to the Fisheries Agency team at their request and analyzed by combining it with materials collected by the team (Takemura 1986, in Japanese) Takemura (1986, in Japanese) is occasionally cited in the following section to supplement my data

14.3.1.2 Male Sexual Maturity Male maturity was determined through histology of a testicular sample about 1 × 1 cm collected from the center of either the right or left testis A seminiferous tubule containing any stage of reproductive cells from spermatocytes to spermatozoa was identified as spermatogenic, and four maturity stages were determined using the proportion of spermatogenic tubules in the histological slide: “immature” (0% spermatogenic), “early maturing” (>0% and <50% spermatogenic), “late maturing” (≥50% and <100% spermatogenic), and “mature” (100% spermatogenic) The maturity stages are the same as those used for the common bottlenose dolphin (Chapter 11), shortfinned pilot whale (Chapter 12), and Baird’s beaked whale (Chapter 13)

The testis was weighed after removing the epididymis

Female maturity was determined from the evidence of ovulation, that is, presence of a corpus luteum or albicans in the ovaries If the ovaries were not available for examination, presence of lactation or pregnancy was used as evidence of sexual maturity However, absence of these indicators was not used as evidence of immature status; the ovaries had to be examined for the identification of immature females The consequences of such maturity determination are stated in Section 1342

14.3.1.4 Age Determination The process of age determination was almost the same as for the striped dolphin (Chapter 10), common bottlenose dolphin (Chapter 11), and short-finned pilot whale (Chapter 12) The thickness of the tooth preparation, which was a function of tooth size or more directly of thickness of the growth layers, was close to that for the striped dolphin A polished thin section of a mandibular tooth was decalcified and stained with hematoxylin for the examination of growth layers in the dentine and cementum, and the middle figure of three independent counts was used If the pulp cavity was considered closed and the cemental tissue indicated a larger age, the number of cemental growth layers was used as the age of the individual For other cases, the number of dentinal layers was used as the age Accumulation of growth layers was assumed to be annual

14.3.1.5 Measurement of the Dorsal Fin Three measurements were taken: (1) height from the base of the fin to the tip measured perpendicular to the base, (2) total height of the fin measured from the base to the highest point, and (3) maximum depth of the posterior concavity of the fin measured in parallel with the base The difference between (1) and (2) was used as an indicator of downward drooping of the fin tip

Maximum body length of 235-239  cm in males and 225229  cm in females (Figure 141) was the same as given by Takemura (1986, in Japanese) for the same data plus data from two additional drives examined by him in 1983 and 1984 (one dolphin examined from another drive was ignored) These data indicated that males reach a body size about 10 cm greater than that of females

The modal body length was 195-199  cm in males and 220-224 cm in females (Figure 141) This was considered not to represent the body-length composition of the population for three reasons One reason was that the female mode in body length exceeded that of males, which was inconsistent with the observations on maximum body size The second was that the location of the male mode agreed with the peak length of “nonmature” individuals, which could be explained by assuming that a significant proportion of “mature” individuals were missed in the sample The third reason was that more reasonable modes, 200-209  cm for

Number of Pacific White-Sided Dolphins Analyzed for Growth and Dorsal Fin Morphology

males and 190-199  cm for females, appeared in the bodylength composition data reported by Takemura (1986, in Japanese) using a larger sample His data showed great variation in the sex ratio among four drives, ranging from 1:56 to 1:04 (F:M) This suggests a social structure where groups are established by age, sex, and maturity, as suggested for striped dolphins (Section 10514) and common bottlenose dolphins (Section 1144)

Thus, the body-length composition of the sample used here (Figure 141) should be considered skewed with a possible deficiency of mature males as well as of immature and mature females, or a surplus of subadults

Iwasaki and Kasuya (1997) presented the body-length composition of a population of the same species incidentally taken in the squid drift-net fishery in the offshore North Pacific in longitudes between 170°E and 145°W Their maximum body lengths of males and females were both in a range of 230-239 cm and were not different from the figures from the Iki Island area

Mean neonatal length for the offshore population was calculated at 918  cm (Ferrero and Walker 1996) or 937 cm (Iwasaki and Kasuya 1997), which were not statistically different We do not have an estimate of mean neonatal body length of the Pacific white-sided dolphins off Japan, but it is expected to be similar to the figure estimated for the offshore population judging from the similarity in adult size

In the offshore drift-net ground, full-term fetuses and neonates measuring 90-140 cm were obtained from May to September, within which period full-term fetuses occurred only in July and earlier months This suggested that the parturition season was from May to August (Iwasaki and Kasuya 1997) Fetuses in the Iki Island area ranged from 30 to 50 cm in

1986, in Japanese), suggesting that these months were neither mating nor parturition seasons of the species in the Iki Island area Fetal growth has not been analyzed

Although there were sexually mature females in the sample of Kasuya (1995), females with estimated ages were limited to immature females of ages below 9 years Males with estimated ages covered all the maturity stages of testicular histology The oldest male was aged 44 The maximum age of the same species from the offshore North Pacific was 32-36 years for males and 27-40 years for females (Ferrero and Walker 1996; Iwasaki and Kasuya 1997) These figures obtained from limited samples suggest that the maximum longevity is around 45 years for both sexes

Pacific white-sided dolphins have great individual variation in adult size, which is evidenced by a body-length range of 175-230 cm among males over 10 years old (Figure 142) Takemura (1986, in Japanese) reached the same conclusion and obtained the following equations between body length (L, cm) and age (t, year);

Male: L = 20098(1 − exp(−0228(t + 5679)))

Female: L = 19697(1 − exp(−0292(t + 4362)))

These equations have an asymptote at 201 cm (males) and 197 cm (females) Extrapolation of the equations to age zero has the risk of extrapolating outside of the data range

Growth equations obtained by Ferrero and Walker (1996) and Iwasaki and Kasuya (1997) for the Pacific white-sided dolphin in offshore waters are similar The following are

with less of a skewed age composition

Male: L = 937 exp(06796(1 − exp(−09451t)))

Female: L = 937 exp(06709(1 − exp(−12045t)))

The figure of 937 is their mean neonatal length in cm Males have an asymptote at 1849 cm and females at 1833 cm The equations are based on good samples of ages below 5 years but small samples of higher ages, that is, only 11 males and 10  females aged over 10  years The validity of these equations, particularly for higher ages, should be reevaluated with larger samples

Takemura (1986, in Japanese) concluded that females attained sexual maturity at body length of 170-220  cm (average 1830  cm) and at ages 6-9  years (average 82) The corresponding figures for males were 170-220 cm (average 1845) and ages 7-9 years (average 83) How he determined male maturity is not explained

For the offshore North Pacific, Ferrero and Walker (1996) and Iwasaki and Kasuya (1997) estimated male age at the attainment of sexual maturity to be 10-11  years and 9-12 years, respectively They judged male maturity visually from the relationship between testis weight and age, and their sample was biased to younger individuals, presumably due to gear selection or segregation by age of animals These factors, particularly the latter, result in overestimation of average age at the attainment of sexual maturity They did not estimate female age at the attainment of sexual maturity

Figure 143 shows relationships between testis weight, maturity, and age Maturity correlates well with age; “immature” males occurred below age 10 years, “early maturing” in an age range of 7-13 years, “late maturing” 9-13 years, and “mature” males 7 years or over Puberty seemed to be entered at around 7 years of age, and all males attained the “mature” stage by age 14 We are uncertain about the histological stage where males achieve the physiological ability to reproduce However, in view of the extremely rapid testicular growth of males in the pubertal stage (Figure 143), it is safe to estimate that they attain reproductive ability at ages 7-13 years, with an average of about 10  years This estimate is likely to be an overestimate to some degree due to underrepresentation of mature males in the sample and is closer to the estimates of Ferrero and Walker (1996) and Iwasaki and Kasuya (1997) than to that of Takemura (1986, in Japanese) based on different maturity criteria

With the exception of one outlier, “immature” testes weighed 70 g or less, “early maturing” 60-185 g, “late maturing” 100-190 g, and “mature” 125 g and over All the testes weighing over 190 g were safely identifiable as “mature” Testis weight increase between ages 7 and 13 is rapid

Correlation between testicular maturity and body length is poor (Figure 144) “Immature” males occurred at body

length of 209 cm or below, “early maturing” at 180-210 cm, “late maturing” at 185-207 cm, and “mature” at 183 cm and over Males attain the “mature” stage at a broad body-length range of 183-210 cm

All the testes weighing 190 g or more were found to be “mature” Weight of these testes did not show correlation with either age (Figure 143) or body length (Figure 144) and showed broad individual variation of 200-400 g The sample covered the two months of February and March, which were

son for such broad individual variation in testicular weight has not been investigated

Males change the shape of their dorsal fin with growth (Figure 145) The dorsal fin of males with “immature” testes has a slightly convex leading edge and the tip pointing posterior-dorsally These features are similar to those of immature and mature females Males with “mature” testes have a strongly convex or rounded leading edge of the dorsal fin, with the tip drooping posterior-ventrally This morphology results in the tip of the dorsal fin being situated below the highest point of the fin This change is examined in the following in comparison with sex, maturity, and body length Data are not available for this analysis on age and on

“early maturing” and “late maturing”

Figure 146b presents the total height of dorsal fin in relation to sex, maturity, and body length Height increases almost linearly with body length until 190 cm and then increases at a decreasing rate Females are likely to have dorsal fins of lesser height, but the sexual difference is small This measurement is unsuitable for expressing sexual dimorphism of the dorsal fin as shown in Figure 145

Figure 146a shows height from the base to the tip of the dorsal fin The measurement increases linearly with increasing body length of immature individuals of both sexes but decreases in sexually “mature” males of over 215  cm Some sexually mature females have a similar feature but of lesser magnitude

Figure 146c further examines the change mentioned earlier using the value of (b-a), or the degree of droop of the dorsal fin tip This parameter is small at less than

2 cm in “immature” males and females of all the growth stages but is great at 4-7  cm in “mature” males “Early maturing” and “late maturing” males were not available for these analyses

Concavity of the trailing edge of the dorsal fin increases in individuals over 210 cm, but the magnitude is greater in males (Figure 147)

Significant morphological change of the dorsal fin is known in various cetaceans, such as the killer whale, short-finned pilot whale, spinner dolphin, and Dall’s porpoise, and offers a key for the identification of sex and maturity The analysis presented earlier proved that the Pacific white-sided dolphin is such a species Among Pacific white-sided dolphins in the Iki Island area, only males measuring 215 cm or over had a dorsal fin with round leading edge and the tip situated more than 3 cm below the highest point of the fin Such a dorsal fin was not seen on any males below 210 cm or females of any growth stage The sample did not cover the body-length range of 210-215 cm

This analysis of the morphology of the dorsal fin had two limitations One was the lack of age data for direct comparison with the morphology of the dorsal fin, and the other was the lack of males determined with testicular histology at the “early maturing” and “late maturing” stages Noting that “maturing” males are known in the body-length range of 180-210 cm and that all the males over 210 cm were classified as “mature,” I would judge that three males of unknown testis maturity and measuring over 215  cm in Figure 146 were “mature” and two males at 200-210  cm were in the “immature” or “maturing” stages Thus, it is concluded that the character of the dorsal fin described earlier is limited to sexually mature males and is not seen on pubertal and

some exceptions

Van Waerebeek (1993) reported a round dorsal fin on male dusky dolphins, Lagenorhynchus obscurus Walker et  al (1986) reported a similar character of the dorsal fin, as described earlier, on Pacific white-sided dolphins in the eastern North Pacific Although they were unable to analyze the character against sex or growth stage, they suggested the possibility that it indicated a state of physical maturity Age or body length at physical maturity has not been examined for the species from the Iki Island area, but limited increase in body length of this species after sexual maturity suggests that the body-length difference between attainment of sexual maturity and physical maturity is small and the time between the two stages short It is my conclusion that the character of a round dorsal fin can be used as a sign of sexual maturity in males This character has the potential to contribute to behavioral study of the species

Pacific white-sided dolphins around Japan have been hunted by historical drive and hand-harpoon fisheries, by various types of dolphin fisheries during the peri-World War II period (Chapters 2 and 3), and by culling of a total of 4600 in the 1980s as a countermeasure against a dolphin-fishery conflict in the area of Iki Island in Nagasaki Prefecture (Sections 33 and 34) Subsequent low-level culling continued in Nagasaki Prefecture until 1995, but not of Pacific white-sided dolphins (Tables 35 and 36) After 1980, the species was not a target of directed hunting for 17  years However, in 2007, the Fisheries Agency of Japan placed it on a list of species allowed for the dolphin fisheries and set an annual quota of 360 (Tables 63 and 64) Allocation of the quota to fishery types and locations was 154 for the handharpoon fishery in Iwate Prefecture on the Pacific coast in 38°59′N-40°27′N, 36 for the drive fishery on the coast of the Izu Peninsula (34°36′N-35°05′N, 138°45′E-139°10′E) on the Pacific coast of Shizuoka Prefecture, 36 for the hand-harpoon fishery, and 134 for drive fisheries in Wakayama Prefecture (33°26′N-34°19′N, 135°00′E-135°59′E) on the Pacific coast Reasons for this action and the basis for the quota have not been given The quota was left almost unused by the fisheries that received the allocations The only possible reason for the policy could be demand from aquariums and the desire for live-capture fisheries If drive fishermen have the opportunity, they are likely to drive schools of Pacific white-sided dolphins for selective transfer to aquariums