Phenotypic changes in reproductive traits with changes in stock size of the starspotted smooth-hound Mustelus manazo in Tokyo Bay, Japan

摘要

Reproductive traits are among the primary determinants of population dynamics, and their changes could affect stock size. We investigated differences in reproductive traits of the starspotted smooth-hound Mustelus manazo in Tokyo Bay between the 1990s (a low-stock period) and the 2000s (a high-stock period). Peak timings of mating (May–July), ovulation (May–June), and parturition (May–June) were similar between the two periods. However, the size at first maturity in the 2000s was smaller than that in the 1990s for both sexes. In addition, embryo-related parameters differed greatly between the two periods. Slopes of size–fecundity regression in the 2000s decreased to 43–73 % of the 1990–1996 values, suggesting a substantial decline in fecundity during the 2000s. Frequency distribution of the maximum ovum diameter during the ovulation season showed two distinct two modes in 1990s while it showed a substantial dispersion during the 2000s, implying that changes occurred in the developmental pattern of ova in the ovary. Undeveloped eggs were present in the uteri with low frequency from September to May during the 2000s, but they were totally absent from September through May in the 1990s. The increased number of remnant undeveloped eggs that did not become embryos may explain the fecundity decline during the 2000s.