Dynamics of two populations of the humpback whale, Megaptera novaeangliae (Borowski)

Abstract

Results of studies of the structure and dynamics of two humpback whale stocks of the southern hemisphere (group IV, 70º E.-130°E.; group V, 130º E.-170º W.) are drawn together. Estimates are made of recruitment and mortality rates, and an assessment is made of the yields to be taken from these stocks under various conditions.

The two stocks are shown to be, in the main, independent of one another although there is a negligible sporadic exchange between them. The group V stock is shown to fragment, but probably randomly, in its northern migration.

Reproduction, nutrition, and growth are described. Birth rate of females is estimated to be 0.186, and since the sex ratio is approximately 1, the total birth rate is about 0.37. Parameters (von Bertalanffy) for growth are L_∞ 42.58 ft for males, 45.21 ft for females; k(male) = 0.266, k(female): = 0.205.

The history of exploitation is reported.

Population structure is described from evidence drawn from examination of commercial catches; substantial changes in recent years (reduction of the numbers in older groups) are described.

Measurement of effort, and an analysis of variations in selectivity of the killings are reported in detail.

Decline in the abundance of these groups, group IV steadily since 1954 and group V sharply since 1959, is described.

Total mortality, natural mortality, fishing mortality, and recruitment rates are estimated and are used in estimating stock numbers and sustainable yields. The group IV stock probably consisted of 12,000-17,000 individuals in its unfished state, of about 10,000 individuals in 1949, and no more than 800 in 1962. The group V stock probably contained about 10,000 individuals in its unfished state, but only 500 or less in 1962. In its present state, group IV could give a sustainable yield of 18 (range 4-32) whales, and group V of 12 (range 3-21) whales. The maximum yields these stocks could sustain in completely regenerated state are: group IV, 390 whales per year; group V, 330 whales per year. Group IV would require 28-49 years to reach that state, group V would require 36-63 years.