Use of Admiralty Bay, New Zealand by Sympatric Marine Mammal Species

Abstract

Understanding how an organism uses its environment can provide useful information regarding its resource requirements and how it may interact with other species. Consistent relationships between the range and distribution of a species with environmental variables enables investigation of the potential for direct species interactions, providing insight into how species may partition their ecological niches in order to successfully coexist. This thesis provides the first examination of the temporal and spatial relationships between sympatric groups of bottlenose, common and dusky dolphins and New Zealand fur seals in Admiralty Bay, with reference to physical environmental variables, and investigates the existence and extent of ecological niche partitioning between these species. Three sampling periods were undertaken during 2011, and 2012. Land‐based surveys collected data on the distribution and relative abundance of species, and boat surveys were used to photo‐identify individual bottlenose and dusky dolphins. Temporal patterns of species relative abundance within the Bay varied, with significant differences in the numbers of common and dusky dolphins and fur seals sighted by month and season, and bottlenose dolphins sighted by month. A total of 130 individual bottlenose dolphins were identified, with 118 frequent users of the Bay and a group of 23 core users. The majority, 171 of 217 total individual dusky dolphins identified were rare users. Due to the large numbers of zeros involved with species’ spatial abundance data, a novel analytical technique called zero‐inflated negative binomial regression was used to test for spatial relationships between species’ abundance and selected temporal and environmental parameters. No significant spatial relationships with dusky and common dolphins and the tested parameters were found, but significant temporal effects were indicated with bottlenose dolphins, and temporal factors, space and slope significantly affected fur seal abundance patterns. Niche partitioning was assessed with Pianka’s niche overlap index using the same temporal, spatial and environmental parameters. Overall, the four marine mammal species showed relatively high, albeit not complete niche overlap; with the highest degree of multi‐dimensional niche overlap between the two most ecologically similar species, common and dusky dolphins. Spatial and temporal niches were the most commonly partitioned between species. The inclusion of dynamic physical and biological variables, comparison of photos with existing catalogues and year‐round, longer term future research will help build a more comprehensive picture of how sympatric marine mammal species share such a small space as Admiralty Bay.