The reproduction and reproductive seasonality of 365 Danish harbour porpoises (Phocoena phocoena (L.)) incidentally caught in various gill nets or stranded, was studied. The animals were collected from all Danish waters throughout the year from 1985 to 1991.
Ovulations were only found in the left ovary whilst the right was permanently sub mature. Large vesicular follicles are found in July, with the largest size (14 mm) in the end of July. Developing corpora lutea with mean diameters of 13.5 mm were found in early August. These findings suggest that ovulation and conception occur in late July and early August. Testicular mass and spermatogenic activity peak in July.
Foetuses are first detectable in early September. Gestation lasts for approximately 10.5 months and parturition occurs in the middle of June and early July. Age at sexual maturity (ASM) was estimated by the sum of fraction immature method for both females and males. The average age at attainment of sexual maturity for females was 3.64 yrs and for males 2.93 yrs. Pregnancy rate was estimated using two methods. By considering the entire sample and using the presence of a corpus luteum as indicative of pregnancy, and by considering the sample collected after implantation and using the foetus as the criterion of pregnancy. Estimated pregnancy rate using these two methods were 0.61 and 0.73, respectively.