Abstract
Bycatch of marine mammals occurs in fishing operations worldwide. Substantial numbers of New Zealand fur seals (Arctocephalus forsteri) and New Zealand sea lions (Phocarctos hookeri) are captured incidentally each year. Lack of information on population biology and population size over the duration of the major bycatch fisheries means that bycatch impacts cannot be estimated for either species. In an attempt to redress this, I studied the population biology and dynamics of New Zealand fur seals and New Zealand sea lions.
Growth and reproductive biology of both species were studied by examining animals killed incidentally in fishing gear in New Zealand waters in 1996. Tooth sections were used to age the animals, and male and female reproductive organs were examined macroscopically and histologically. One hundred and twenty seven New Zealand fur seals were examined. The maximum age observed in the sample was 22 years for females and 12 years for males. Males were significantly larger than females, but growth was similar up to five years. Males reached sexual maturity between five and nine years of age, whereas females did so between four and six years. The pregnancy rate in females was approximately 69%.
The first steps towards modelling population growth of New Zealand fur seals are taken in Chapter 3. These models use the biological data from fur seals but rely on data from other species where such data are not yet available. For this reason model results should be considered indicative rather than definitive. The models use a risk analysis program to incorporate uncertainty in the input, and provide a distribution of estimates of population growth rather than a single point estimate. They illustrate a productive approach and outline possible population trajectories. They also indicate what data are most urgently needed to provide more realistic modelling.
Stomach and lower intestine contents were examined for 112 of the fur seals. Remains of 19 taxa were identified. Seals were targeting similar prey species to the fishery in which they were captured. However, it was clear that, at least in the west coast South Island fishery, seals took much smaller hoki than the fishery. A surprising result was that extremely small fish (< 12 cm) are taken directly, and in quantity, by seals. There has been doubt in previous studies as to whether these remains have occurred indirectly from stomachs of other species eaten. Comparison of stomach versus colon contents showed differences in digestion of prey species and only smaller items appeared to pass through to the colon. This suggests that larger indigestible items are regurgitated by the seals, resulting in misleading results from studies based on scats or regurgitates only. It is therefore advisable that studies using these types of remains are based on both scats and regurgitates.
The sample size available for New Zealand sea lions was much smaller than for fur seals, with only 30 sea lions dissected. The maximum age observed in the sample was 21 years for females and 12 years for males. As with other seal species, males reached larger sizes than females. Males appeared to reach sexual maturity between five and nine years of age, as is the case for New Zealand fur seals. Maturity in females seemed to occur between three and five years. Due to the season of capture it was not possible to establish pregnancy reliably. However, the data do suggest a delay in implantation of the embryo of at least three months. A larger sample size of New Zealand sea lions is necessary to further confirm these results.