Chasing rays: Distribution and habitat-use of mobulid rays in the northeastern continental shelf of Aotearoa New Zealand

Ozaki R (2023) Chasing rays: Distribution and habitat-use of mobulid rays in the northeastern continental shelf of Aotearoa New Zealand. Masters Thesis, The University of Auckland

Endangered mobulid rays have a conservative life history associated with long reproductive cycles and subsequent low population growth rates. Targeted fisheries as well as bycatch threaten the survival of mobulid rays. Although attempts have been made to reduce overlap between mobulid rays habitat and fisheries, the lack of robust knowledge surrounding rays distribution and basic ecology remains a hindrance to effective protection. This thesis investigated the spatio-temporal distribution and habitat use of the only two mobulid rays that are known to occur in the northeastern (NE) coast of Te Ika-a-Māui North Island of Aotearoa New Zealand: Spinetail devil ray (Mobula mobular) and Oceanic manta ray (Mobula birostris), with the overarching aim to contribute this information to conservation and management. Fisheries and citizen science occurrence data were used in a Boosted Regression Tree (BRT) model to investigate mobulid rays’ habitat suitability in Aotearoa. Along with a spatial model, using long-term averages of environmental conditions (i.e., static model), this study assessed temporal changes in habitat suitability by considering different years separately (i.e., dynamic model). For both species, the 200m depth contour line outlining the continental shelf edge was predicted to be an important feature – for Spinetail devil rays demarcating their inshore extent and for Oceanic manta rays, their offshore extent. As large filter feeders, this is likely related to prey availability and potential evidence of trophic separation, in that despite presumed similar habitat requirements, there is clear segregation in their distribution. For both static and dynamic models, sea surface temperature and chlorophyll-a concentrations were consistently prevalent. Although the dynamic models demonstrated that there was a change in habitat suitability occurring over time, some of this change was unable to be captured by the variables in the model, especially for Oceanic manta rays. Through this study, important baseline information has been identified in the NE coast. The high coupling of prey availability and distribution for both species poses questions on how their distribution may change with climate change and proposed poleward movement of prey. Further, the contraction of trophic separation with predicted reductions in prey availability create further uncertainties in the future that require additional investigation. Increased data availability and deeper understandings of the processes that drive mobulid distribution and habitat use are required for their protection and conservation in the wake of current and future threats.