Spatial Ecology of Cetaceans in the Hauraki Gulf, New Zealand

Abstract

Understanding how environmental variables influence the distribution of species is essential for conservation management; to mitigate anthropogenic effects, protect critical habitats, and for our general understanding of the species’ ecology. Bottlenose dolphins (Tursiops truncatus), common dolphins (Delphinus delphis) and Bryde’s whales (Balaenoptera edeni) are the most frequently sighted cetaceans in the Hauraki Gulf, New Zealand and occur year-round. This thesis examines the spatial distribution of these cetaceans in relation to physical and biological environmental variables. Nine replicate aerial line-transect surveys were conducted during the warm-water season, between December 2013 and April 2014 to collect data on the distribution and group size of cetaceans together with concurrent observations on near-surface aggregations of potential prey: plankton and fish. The spatial overlap between cetaceans and their potential prey was investigated by analysing the data in the form of sightings per unit effort (SPUE) and as presence points. A strong correlation between Bryde’s whales and plankton was found but there was only a weak correlation between the cetaceans and the distribution of fish. Physiographic and biological variables including prey SPUE were used to model the distribution patterns of each species using generalised additive models (GAMs). Three types of model were built to investigate differences in spatial scale and explanatory variables: (A) physiographic variables at 420 m scale; (B) physiographic and prey variables at 420 m scale; (C) physiographic, prey and remote-sensed variables at 2,775 m scale. The GAMs indicated that depth and distance to shore were the most important factors driving the spatial distribution of cetaceans. The inclusion of plankton at the 420 m scale also influenced the distribution of Bryde’s whales. Habitat partitioning was observed between the different species with bottlenose dolphins preferring shallow waters close to shore, common dolphins preferring deeper waters further from shore and Bryde’s whales occupying an intermediate habitat. This study demonstrated the potential of aerial surveys to collect data on cetaceans and prey efficiently over a wide area. Further replicate surveys during the cold-water season will enable an improved understanding of the ecology and distribution of cetaceans in the Hauraki Gulf and how these might change over time.