Population biology and conservation of Hector's dolphin

During the past decade, Hector's dolphins, Cephalorhynchus hectori, have suffered an alarming level of mortality due to entanglement in commercial and amateur gill nets. In this paper we study two Leslie matrix population models that incorporate known features of dolphin fertility and mortality, focussing on the information they provide regarding age distributions and maximum population growth rates. The simplest model specifies constant survival rates over many age-classes. The second model uses more realistic curves of age-specific survival rates. The results indicate that Hector's dolphin, like most other small cetaceans, has a low potential for population growth. Growth rates of 1.8–4.9% per year are likely to be the maximum possible for Hector's dolphin populations, and C. hectori (and C. commersonii) populations are likely to be declining under recent levels of net entanglement. Survival rate estimates from free-living populations, subject to natural and net-entanglement mortality, showed decreasing populations. Even with the most optimistic reproductive parameters, survival rates would need to be some 5–10% higher than those observed in populations subject to gill-net entanglement before population growth could occur. The likely consequences of a reduction in entanglement mortality through conservation management are explored using the survivorship curve model. These simulations show that the age structure of the population can have an important effect on changes in the size and growth rate of the population during the recovery phase following a reduction in entanglement mortality.