Population demographics of sevengill sharks (Notorynchus cepedianus) in Paterson Inlet, Stewart Island.
The broadnose sevengill shark (Notorynchus cepedianus) is an abundant large marine predator found around coastal New Zealand. Data on the ecology of the species are severely limited and anthropogenic impacts are unquantified. To partially address this, a study of the demographics of a sevengill shark population was conducted in Sawdust Bay, a shallow coastal embayment within Paterson Inlet, Stewart Island/Rakiura. Underwater video of the sharks was captured for photo-identification (photo-ID) capture-recapture and photogrammetry.Sevengill sharks were detected on all but one of 133 deployments, resulting in 349 separate encounters. Extracted images were subjected to strict image quality criteria resulting in usable images of 149 individuals that were included in the analysis. Capture-recapture analysis of the photo-ID data was conducted using a Huggins Robust Design. Abundance varied seasonally from 34 (95% CI = 21 - 55) during winter 2016, to 94 (95% CI = 44 - 199) during spring 2017. These matched seasonal trends seen in other studies of areas with high sevengill shark abundance around Tasmania. Temporary emigration rate was greatest before autumn and winter (0.721 and 0.752 respectively) and lower before spring and summer (0.331 and 0.409 respectively). Apparent annual survival rate was estimated as 0.51, but with low precision (95% CI = 0.16 - 0.86).The observed sex ratio was heavily biased, with females making up 95% of the population in winter 2016 and 73% in summer 2017. Using stereo-photogrammetric methods, 78 of the 149 individuals were measured using the image analysis software Vidsync 1.1661. Total lengths ranged from 149.0 to 276.5 cm (mean = 211.4, SE = 2.97). There was no apparent difference among seasons in the size classes that used the study area. All measured males (n = 15) were found to be at sizes consistent with maturity (>150 cm), while females (n = 75) consisted of a mix of juvenile (60%) and mature (40%) females. Not all individuals measured over the duration of the study appeared to grow, probably because sevengill sharks are slow growing when mature, and potential growth may have been swamped by measurement error.This study presents the first data on abundance, apparent annual survival, sex and size structure of broadnose sevengill sharks in New Zealand. The methods used here will be continued in a wider study with the aim of quantifying demographics and ecological role of the species.
Advisor: Rayment, Will; Dawson, Steve
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Keywords: New Zealand; Robust Design; capture recapture; Stewart Island; broadnose sevengill shark; stereo-photogrammetry
Research Type: Thesis