Ecology of Hector's dolphin (Cephalorhynchus hectori): Quantifying diet and investigating habitat selection at Banks Peninsula

Abstract

Hector’s dolphins (Cephalorhynchus hectori) are endangered and endemic to New Zealand. Although their distribution and abundance have been thoroughly described, little is known about Hector’s dolphin ecology. Being small cetaceans living in a cold water environment, Hector’s dolphins have high energy requirements, and prey abundance and movements are often assumed to be a major driver of distribution patterns. This thesis aims to increase our understanding of Hector’s dolphins in a wider ecosystem context, by quantifying their dietary preferences and investigating the influence of prey availability on their habitat selection.

Using stomach contents collected from beachcast and bycaught dolphins throughout New Zealand between 1984 and 2006 (n=63), the diet of Hector’s dolphin was quantified for the first time. The dolphins were found to feed throughout the water column, with 29 prey taxa identified in total. Red cod (Pseudophycis bachus), ahuru (Auchenoceros punctatus), arrow squid (Nototodarus sp.), sprat (Sprattus sp.), sole (Peltorhamphus sp.) and stargazer (Crapatalus sp.) were most commonly consumed. Red cod contributed most in terms of mass (37%), while ahuru and Hector’s laternfish (Lampanyctodes hectoris) were consumed in large numbers. Prey length ranged from <1 cm to over 60 cm, but the majority were <10 cm long. To help overcome inherent biases of stomach content analysis, complementary stable isotope analysis was used to examine long-term dietary preferences on the South Island east coast. Comparison of δ13C and δ15N signatures of dolphins (n=42) and potential prey (n=19 species) using mixing models, revealed that bony fish contributed most to the dolphins’ diet. While stomach content results showed demersal prey to contribute most over spring and summer seasons, stable isotope analysis suggested that epipelagic fish have a greater long-term contribution. The influence of prey availability and oceanography on Hector’s dolphin habitat selection was assessed at Banks Peninsula, a stronghold for this species on the South Island east coast. Concurrent dolphin, demersal prey, and oceanographic surveys were carried out offshore, alongshore, and within Akaroa harbour. For all three regions, generalised additive models indicated that Hector’s dolphins strongly correlated with areas of greater red cod mass. The offshore distribution of dolphins was also associated with areas of higher chlorophyll-a and salinity. Alongshore, dolphins were found in areas of higher prey diversity, chlorophyll-a, salinity, and warmer water. The performance and feasibility of two methods for sampling the demersal prey of Hector’s dolphins were assessed. Fish traps sampled a greater number of species, and were more effective at sampling the important prey, red cod. Baited stereo-video sampled greater fish diversity, but was limited by the prevailing water clarity. This study represents the first attempt to study Hector’s dolphin ecology in relation to their prey, addressing fundamental knowledge gaps in what the dolphins eat, and the role of prey availability in their habitat selection. The continued survival of Hector’s dolphins relies upon the success of conservation protection measures, and an increased understanding of their ecology and habitat use will help inform spatial management objectives and future conservation tools.