New Zealand fur seals in the Kaikoura region: colony dynamics, maternal investment and health

Abstract

Colony dynamics, maternal investment, and indicators of health were investigated for the New Zealand fur seal (Arctocephalus forsteri) over four austral summers, 2001- 2005. Effort was focused at the Ohau Point seal colony, north of Kaikoura. Two colonies at Banks Peninsula were included for comparisons of colony growth and pup condition. A range of other colonies were also included for making comparisons about colony dynamics and health indices. Colony dynamics were investigated through mark-recapture estimates of pup production and daily census of all individuals at the Ohau Point colony. Maternal attendance patterns were observed through behavioural observations of known females (n = 120), the use of VHF radio transmitters (n = 33), and female mass and body condition estimates (n = 51). Maternal investment was also investigated through longitudinal sampling of pup mass and growth rates. Parameters used to indicate colony health were: body condition, growth, presence of parasites, and the levels and common causes of mortality. The influence of parasites on pup growth was tested using treatment of selected pups with Ivermectin anti-helmentic medication, and mortality in the region was investigated through reports of dead individuals, and post mortems of those found fresh. The Ohau Point colony is in an exponential state of growth, and pup mass and condition was higher and responded to changes in environmental variables differently than at the Banks Peninsula colonies. Lactation lengths were consistently longer at Ohau Point than is typically reported for the species (323-355 days vs. 285 days). Maternal investment strategies were indicative of a close, reliable food source, and showed flexibility between years through extension of foraging trip durations and the increased use of overnight foraging trips. Individual strategies did not significantly influence pup growth. However, increased maternal condition and the ability to respond to inter-annual changes in resource availability resulted in accelerated pup growth even during an El Niño event. The incidence of pups with intestinal parasites was low at Ohau Point, and the average mass of treated and non-treated pups did not differ. Pup mortality in the region was low (3% to 50 days old), however, mortality of older pups was greatly influenced by the proximity of humans, with 2/3 of pup mortality observed between the age of 50 days and weaning being caused by car collisions. The results suggest that population dynamics and maternal investment in the region are greatly influenced by local variables, notably the presence of an accessible food source within close proximity to the colony. Various indicators of health reflect a growing colony in good condition, and the presence of a reliable food source may influence the maximum density and carrying capacity the colony is able to sustain. However, some concerns are raised about the influence of human interactions in the region, and how this may affect mortality and colony dynamics in the future. Extra fencing along the Ohau Point colony is recommended to provide added protection from the road. Continued monitoring of mortality and health indices in the region is also recommended for comparison with other colonies as Ohau Point reaches carrying capacity and density-dependent pressures increase.