Using photo-ID data to investigate population parameters and social structure of sperm whales visiting Kaikōura

Abstract

In order to effectively manage and protect populations, it is crucial that long-term trends in abundance and population structure are understood. Kaikōura is one of the few places worldwide where sperm whales can be found only a few miles offshore. Although whales are present nearly all year round, individuals come and go from the study area, and hence none are truly resident. They are the focus of an economically important whale watching industry and may also indicate changes in the ecosystem, driven by climate change or other human impacts. Previously, a decline in individuals using the study area had been detected from 1991-2007. My research used photo-ID capture-recapture techniques to: 1) Investigate long-term trends in abundance of sperm whales at Kaikōura with Cormack-Jolly-Seber (CJS) modelling to determine if the previously detected decline has continued, 2) Contrast these analyses with an alternative, and potentially more appropriate, approach to modelling abundance (the Robust design), which allows for temporary emigration of whales from the study area, and 3) Explore the social structure of male sperm whales visiting Kaikōura by investigating ways to define associations and testing for preferred associations between individuals. The CJS modelling showed that there was a significant decline in the number of individuals using the study area, from 89 (95% CI: 60-131) individuals in 1991 to 40 (95% CI: 33-49) in 2017. The Robust design analysis indicated that this decline was driven by summer abundance with a significant decline from 43 individuals (95% CI: 12-148) in 1990 to 23 individuals (95% CI: 16-32) in 2017. There was no significant trend in winter abundances over time. Preferred and/or avoided companions, and the temporal patterning of associations were investigated using the program SOCPROG. Potential associations among individual whales were investigated over three time-spans; associated if seen on the same day, associated if seen within two consecutive days, and associated if seen over a four day time-span. Preferred associations were detected in all three scenarios, with preferred associations lasting approximately four years. This is the first statistical evidence of preferential associations in male only groups of sperm whales. Continued research on this population and the potential causes of the decline will aid in determining appropriate management for this economically, ecologically and culturally important population.