Quantifying habitat selection and foraging ecology of broadnose sevengill sharks (Notorynchus cepedianus) in southern New Zealand

Abstract

Investigating distribution of apex predators is essential for understanding their habitat requirements and can provide insights into community ecology. The aim of the present study was to quantify the habitat selection and foraging ecology of broadnose sevengill sharks (Notorynchus cepedianus) in southern New Zealand. Baited remote underwater videos (BRUVs) were deployed during seven sampling trips in Te Whaka ā Te Wera (Paterson Inlet), Rakiura (Stewart Island) between February 2018 and October 2019. The number of unique individual sharks observed per deployment was used as an index of relative abundance. Habitat data hypothesised to influence shark distribution were gathered in situ and related to relative shark abundances in a multi-model inference framework using a suite of Poisson generalised additive models. Based on minimum Akaike Information Criteria (AIC), the best fitting model (deviance explained = 61%, r2 = 0.72) included positive associations with (i) water temperature, (ii) distance from the seaward entrance to the inlet, (iii) prey abundance, and (iv) coarse sediment type, and negative associations with (v) ambient light, (vi) depth, and (vii) water visibility. These results corroborate previous findings of strong seasonal patterns in abundance of sevengill sharks, with higher densities in warmer summer months, and lower densities in cooler winter months in California, Argentina, Tasmania, Washington and New Zealand.

The food web of sevengill sharks was analysed using stable isotope analysis. Isotopic ratios of carbon and nitrogen were measured from white muscle tissue of nine bycaught specimens, and compared to potential prey and primary producers from southern New Zealand. Mean proportion of organic matter derived from coastal macroalgae (65%) was significantly higher than mean proportion of organic matter derived from pelagic suspended particulate organic matter (SPOM; 35%), suggesting a preference for food webs linked to coastal macroalgal dominated habitats. Females held significantly higher proportions of macroalgae-derived carbon (67%) than male sharks (61%), indicating the potential residence in coastal habitats by females, corroborating studies from Tasmania, Washington and California. Trophic level estimates based on nitrogen isotopes ranged from trophic levels 3.5 – 4 (mean = 3.9), similar to previously identified high trophic levels for this species. No significant geographic or ontogenetic variability was identified using stable isotope analysis.

These results suggest a combination of multiple abiotic and biotic variables influence distributions of broadnose sevengill sharks in southern New Zealand. Therefore, future management should prioritise an ecosystem-based approach, protecting potential prey species and different habitats important for these sharks. The dominant contribution of macroalgae-derived organic matter in broadnose sevengill shark diets reiterates their likely important interaction with the wider coastal communities they inhabit in southern New Zealand. These findings will be useful for predicting the distribution and critical habitats of broadnose sevengill sharks elsewhere, in order to identify and prioritise potential areas for management and conservation priority.