Reproduction, growth and migrations of sei whales Balaenoptera borealis off the west coast of South Africa

Abstract

Results of the examination of 1 062 sei whales Balaenoptera borealis landed at the whaling station at Donkergat, Saldanha Bay, South Africa, in the 1962 and 1963 whaling seasons are presented. Sei whales were usually encountered off the edge of the continental shelf, with males being caught closer inshore than females and mature females furthest of all from the station. Females reached puberty at an average length of 46.1 ft (14.1 m) and an age of 8.2 (95% CI = 7.3, 9.0) years (assuming one growth layer group is formed in the ear plug per year). Males reached sexual maturity at an average length of 45.3 ft (13.8 m) and an age of 8.6 (95% CI = 7.8, 9.4) years. Mean lengths at which growth ceased were 52 ft (15.8 m) in females and 48.6 ft (14.8 m) in males. Most (~90%) conceptions occurred over a 70-day period with a peak in June, and primigravid females conceived six days later than multigravid females. Observed pregnancy rates were as high as 86.1%, but with ovulation rates averaging only 0.47 a year, the catch could not have been fully representative of the population. There was no significant decline in the observed pregnancy rate with age. During the northward migration (May–July), fewer whales were taken in water shallower than 2 000 m than in the southern migration, and the catch was largely composed of immatures. The few adult males taken at that time of year had significantly heavier testes than males of an equivalent size on the southern migration. The southward migration (August–October) was markedly structured, such that pregnant females and immatures of both sexes were in the vanguard, followed by mature males and lastly lactating females and calves. The availability of sei whales off Donkergat declined rapidly from 1965 to 1967, following an episode of massive catching by pelagic whalers in higher latitudes.

Keywords: growth, migration, reproduction, sei whales, South Africa

African Journal of Marine Science 2002, 24: 111–133