A length-structured population model for southern fin whales and a test for density dependence

• Published in: ICES Journal of Marine Science
• Main Author: Sampson, David B.
• Format: Text
• Language: English
• Published: Oxford University Press 1990
• Subjects: Articles; Balaenoptera physalus; baleen whales
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/46/3/249; https://doi.org/10.1093/icesjms/46.3.249

It is reasonable to assume that the populations of baleen whales in the Southern Hemisphere are limited by prey availability more than by disease or predation and that the increased food per capita, a likely result of the rapid removal of animals by whaling operations during this century, may have affected the whale populations'rates of birth, juvenile mortality, and growth. This paper assesses the validity of these assumptions by attempting to detect changes in these rates and to ascertain which of them could have been responsible for density-dependent regulation in two populations of southern fin whales ( Balaenoptera physalus ). It is assumed that the birth and death rates for animals in these populations depend on the size of the individuals; larger females are on average more frequently pregnant; infants and juveniles which are unusually small or slow growing may be subject to higher levels of natural mortality; animals less than the minimum size are not harvested by whaling operations. The rate of progression from birth to adulthood and reproduction is governed by the rate of growth-in-length. An increase in the rate of recruitment will cause a decrease in the average length. A length-structured population model is developed from Deriso's delay-difference equation model; two difference equations describe the annual changes in the numbers of females greater than the legal minimum length and the annual changes in their combined lengths. Reproduction is modelled as a function of average length. Parameter estimates are obtained from observed catches, times spent whaling, average lengths of the catches, and the fractions pregnant. For the period examined. 1931–1975. more animals were caught than were produced; the populations declined steadily. It is assumed that if there were temporal trends in the population parameters during this period in response to declining density, they were approximately linear. Density dependence is measured by changes in goodness-of-fit for the model with and without trends in the ...