Long-term monitoring of sperm whales in Kaikoura, New Zealand : data-management, abundance and population dynamics

• Main Author: Van der Linde, Miranda
• Other Authors: Dawson, Steve
• Format: Thesis
• Language: English
• Published: University of Otago 2020
• Subjects: Handle The; New Zealand; Physeter macrocephalus
• Online Access: http://hdl.handle.net/10523/10453

Male sperm whales (Physeter macrocephalus) can be found year-round within a few nautical miles off the coast of Kaikoura, New Zealand. Systematic sighting surveys were carried out in 2007, contributing to the long-term study of these individuals. The combined data (spanning 17 years of research) include information from 4,845 encounters with sperm whales. The objectives of this study were to create a relational database to handle the large amounts of encounter and photographic data, and to analyse the long-term data for trends in abundance and related population parameters. An existing film-based fluke catalogue was digitised and updated. The new digital catalogue, encounter data, daily encounter photographs, sound recording information, and boat track data were integrated into a relational database that can be used to search for specific information in the long-term dataset. A total of 232 individuals (comprising 112 returning residents and 120 transients) were identified over the entire study period. Mark-recapture estimates indicated that abundance of sperm whales in Kaikoura waters has declined significantly from 97 individuals (95% CI: 62 to 153) in 1991 to 46 individuals (95% CI: 36 to 60) in 2007. Similarly, annual abundance of the resident portion of the population declined significantly from 89 individuals (95% CI: 62 to 128) in 1991 to 50 individuals (95% CI: 40 to 62) in 2007. Pradel's reverse-time capture-recapture models were used to assess changes in population growth rate in relation to recruitment (addition of individuals through immigration) and 'apparent' survival rates (removal of individuals through emigration and death). Apparent survival was found to contribute 8-9 times more to changes in population growth rate; however there were no detectable trends in survival or recruitment which can be related to the decline in abundance. There are several factors that could explain a change in the number of individuals coming into Kaikoura's coastal waters, and these have been discussed. In conclusion, the apparent decline in abundance, combined with our limited knowledge of the cause of the decline suggests that a cautious management approach is warranted.