Now Dr Brittany Graham of the National Institute of Water and Atmospheric Research (Niwa) hopes to shed light on the issue by drilling into tiny samples of sea lion teeth.
Dr Graham said sea lion teeth had bands that grew annually - similar to the rings of tree trunks - and by taking tiny samples and preparing them for isotype analysis, she hoped to see what the sea lion ate, where it foraged and whether ocean conditions had changed over its lifetime.
After the tooth is cut in half lengthwise, the pieces are polished to remove any contaminants, before a 2mm-thin slice is removed and mounted on a slide. Once a series of high-resolution images are taken, identifying which bands to micro-drill, the samples are prepared for isotope analysis in a complex process involving different chemicals, temperatures and extractions.
The final material is then placed into equipment that obtains isotope signatures of individual compounds.
"These teeth will provide information not only on whether ocean conditions have changed, but also whether there have been changes to the structure of the marine food web," Dr Graham told Niwa's Water and Atmosphere magazine. "In other words, this is a new tool that provides a glimpse at the past 20 years from a sea lion's perspective."
Under threat:
• The New Zealand sea lion once numbered in the hundreds of thousands, and their habitat extended throughout New Zealand.
• However, in the 19th century, the species was decimated for its blubber and skins, and in 1997, it was declared a threatened species under the Marine Mammals Protection Act.
• The New Zealand sea lion breeds mainly on the Auckland Islands (70 per cent of the species) and Campbell Island (30 per cent), with small numbers found on Stewart Island.
• While the Campbell Island population is faring well, the Auckland Islands colony has been declining rapidly for a number of years, with several research programmes now under way to find out why.
