NEW ZEALAND FUR SEALS
Fur seals were hunted for their fur extensively from 1792 until populations began to collapse in 1815. By then fur seals had been exterminated from the Antipodes Islands and other locations. Fur seals have since been fully protected by the Marine Mammal Protection Act of 1978. As a result, fur seals have begun to recolonise their original range and their numbers are increasing. Although the current total abundance of New Zealand fur seals is not known, estimates based on pup counts and rates of population increase suggest that New Zealand fur seal numbers are nearing 100,000. Foraging behaviour of the various species of fur seals has been under investigation for a number of years. A study on a male Australian fur seal showed no sign of a diurnal pattern with approximately 43% of dives occurring at night. However, studies on dive behaviour in Galapagos fur seals (A. gaZapagoensis) and Antarctic fur seals (A. gazella) suggest most dives occur at night and either side of the hours of darkness. A similar diurnal pattern with shallow dives occurring through the night and the deepest dives at dawn and dusk was observed in New Zealand fur seals. Dietary analysis using otoliths in scats indicate that fur seals frequently feed on deep-sea species that would be beyond their diving abilities in the daytime.
New Zealand fur seals are an opportunistic species and feed on a number of prey items. Several studies have looked at the prey species of fur seals around New Zealand, however, different methodologies show slightly different results. Street (1964) examined stomach contents and concluded that fur seals were eating banacouta (Thyrsites atun) , octopus (Octopus maorum) and arrow squid (Nototodarus sioanilJ. Twelve fish species were identified with the most common prey species being lantern fish (Myctophidae), anchovy (Engraulis australis), ahuru (Auchenoceros punctactus), and hoki. The primary prey species for various sites depended on the distribution and abundance of available prey and the seab;’ ability to shift prey species as needed. The primary prey species for seals on the Kaikoura coast were lantern fish. Here the Kaikoura Canyon makes deep-sea species more accessible. When not foraging, seals come ashore for body maintenance, to rest, give birth, and mate. Fur seals are a polygynous species where one male holds a territory over several females. Females come ashore to give birth in late November, and pups are born from late November to early December. Due to the relatively small size of fur seal females, they must forage during lactation in order to maintain condition and nurse their young. Fur seal mothers from colonies with a nearby food source have the ability to alternate between overnight foraging trips (less than a day in duration) and extended foraging trips (more than one day). This pattern was seen in Antarctic (A. gazella) and subAntarctic fur seals (A. tropicalis) at Maquarie Island. The observed increase in time mothers spend at sea foraging and the subsequent decrease in time they spent ashore as lactation progresses, was suggested by Oftedal (1984) to be a result of the female meeting the greater demand for food by older pups. Evidence of reduced growth rates and/or increased mortality in relation to mothers spending extended periods at sea has been seen in Galapagos fur sea1s, Antarctic fur seals, and New Zealand fur seals. In a similar study, Ono et aZ. In general, otariid mothers have three options to increase nutritional transfer to pups during times of food shortages:
1) to increase diving effort while at sea and dive at all times of the day;
2) increase time spent at sea; or
3) a combination of both.
California sea lions and Northern fur seals (Callorhinus ursinus) tend to adopt the first option, however they are not specialist feeders and they can dive deeper than many of the smaller otariid species.