Growth and reproduction of the northern right whale dolphin, Lissodelphis borealis, in the offshore waters of the North Pacific Ocean

Abstract

Biological samples and data were obtained from 229 northern right whale dolphins, Lissodelphis borealis (99 males and 130 females), taken in Japanese squid driftnets in the central North Pacific Ocean, May – November of 1990 and 1991. Length and sex were recorded for an additional 188 males and 272 females. Ages were determined by counting dentinal growth layer groups. Female reproductive status was determined by macroscopic examination of ovaries; 67 were mature. Sixteen percent were pregnant, 3% were pregnant and lactating, 33% were post partum, 24% were lactating (no recent pregnancy), 10% were resting, and 14% were of unknown condition. Testis and epididymis were examined for evidence of spermatogenesis; 28 were mature. Three estimates of length at birth were 99.7, 100.6, and 103.8 cm. The gestation period was 12.1 – 12.3 months. The sex ratio at birth was not significantly different from 1.0; the male component decreased with age. Estimates of the average age at sexual maturation among males were 9.9 and 10.1 years; average length at sexual maturation was estimated at 215.1 and 214.7 cm. Mature testis masses ranged from 117.4 to 1300 g. Estimates of the average age at sexual maturation among females were 9.7 and 10.4 years; the average length at sexual maturation was estimated at 201.1 and 199.8 cm. Males reach an asymptotic length of 265 cm, females 210 cm. Ovulation rates were < 1.0/year. Calving appeared to peak in July and August; the minimum calving interval was 2 years.