Factors Affecting the Occurrence and Demographics of Common Dolphins (<I>Delphinus</I> sp.) in the Hauraki Gulf, New Zealand

Abstract

The common dolphin (Delphinus sp.) is the most frequently observed cetacean species in the Hauraki Gulf, a large shallow body of water on the northeastern coastline of North Island, New Zealand. Herein, we present the first data relating to the occurrence and distribution of common dolphins in this region and assess the possible effects of abiotic parameters on the demographics of this population. The presence of associated marine species is quantified, and differences in the occurrence and demographics of single and multi-species groups are examined. Sightings data were collected between February 2002 and January 2005 during boat-based surveys. We recorded 719 independent encounters with common dolphins, involving 1 to > 300 animals. Dolphin presence was significantly affected by month, latitude and depth. Group size varied significantly by month, season, depth, sea surface temperature (SST) and latitude, and was highly skewed towards smaller groups made up of < 50 animals. Larger aggregations were most frequent during the austral winter when nutrient upwelling typically leads to increased prey availability within the region. Over 70% of groups encountered contained immature animals and 25% of groups included neonates. Calves were observed throughout the year but were most prevalent in the austral summer months of December and January. Month, season, depth, and SST significantly affected group composition. Common dolphins were observed in association with four cetacean and eight avian species, most frequently with the Australasian gannet (Morus serrator) and the Bryde’s whale (Balaenoptera brydei). The distribution of dolphin-only groups differed significantly from that of dolphin-whale groups, with mono-specific groups found on average in waters that were 3.6 m shallower and 3.1oC warmer. The year-round occurrence and social organisation of common dolphins in Hauraki Gulf waters suggest this region is important both as a calving and nursery ground.