Exploring the murky world of the sevengill shark, Notorynchus cepedianus, in southern New Zealand

Abstract

The broadnose sevengill shark, Notorynchus cepedianus, is an important marine apex predator found in temperate coastal regions worldwide. Despite being identified as an abundant, high trophic level species, there is a paucity of ecological research dedicated to sevengill sharks and their use of marine systems. In particular, there is a lack of specific research on the distribution and demography of sevengill shark populations. This study represents the first systematic data on the seasonal distribution, habitat use and genetic structure of sevengill shark populations in Aotearoa, New Zealand. During 71 sampling trips conducted from July 2013 to May 2015, sharks were attracted to coastal sampling sites at Ōtākou (Otago Harbour) and Te Whaka ā Te Wera (Paterson Inlet) using chum. Sharks were implanted with stainless steel dart tags (n = 55) and photographs of unique dorsal markings (n = 23 unique individuals) were obtained. At Ōtākou, a clear seasonal pattern of sevengill shark sightings emerged. Sharks were detected on 86% of survey trips in summer, whilst no sharks were detected in winter or spring. At Te Whaka ā Te Wera, sharks were sighted throughout all seasons, but a decline in shark encounters occurred during winter. Both male and female sevengill sharks were encountered, and individuals ranged between 1.5 m and 2 m total length. Using a logistic regression model, water temperature was identified as a key predictor of sevengill shark presence at Ōtākou and Te Whaka ā Te Wera. In addition, location, cloud cover, and sea state were also identified as influential predictors. On supplementary sampling trips, two individual sevengill sharks were re-sighted using recognition of tags, and three individuals were re-sighted using photo-ID. Long-term stability of natural marks and higher re-sight rates suggest photo-ID is an effective, less invasive alternative to physical tagging in sevengill sharks. Instances of tag shedding and bio-fouling further support photo-ID as a more robust means to studying sevengill shark demographics. Individual sevengill sharks showed some fidelity to coastal areas, but low re-sight rates suggest large population sizes, and/or high levels of migration among populations are occurring. Phylogenetic relationships among sevengill shark populations were also explored, using tissue samples extracted from free-swimming sevengill sharks in conjunction with previously collected samples. Mitochondrial DNA sequencing detected no differentiation in mtDNA COI (n = 41) and ND4 (n = 42) at a national scale. COI sequences also detected no genetic structure among sevengill shark populations from Argentina, Australia, and New Zealand. These results suggest that sevengill sharks in New Zealand display low breeding site fidelity, and high mobility among sites. The findings of this study provide some of the first data to help comprehend the role of sevengill sharks in marine coastal systems in New Zealand. This information will be useful for current and future ecological assessments of sevengill shark populations, and the coastal communities of which they reside.