Environmental correlates of nearshore habitat distribution by the Critically Endangered M(a)over-barui dolphin

Effective management of space-use conflicts with anthropogenic activities is contingent upon reliable knowledge of a species' ecology. The M (a) over bar ui dolphin Cephalorhynchus hectori maui is endemic to New Zealand and is listed as Critically Endangered, mainly as a result of fisheries bycatch. Despite conservation efforts, the population was estimated at 55 animals in 2011. Here we investigate environmental correlates of M (a) over bar ui dolphin nearshore distribution, using 119 encounters with M (a) over bar ui dolphin groups during boat-based, coastal surveys across 4 summers (2010, 2011, 2013, 2015). We describe the nearshore distribution using a kernel density analysis with differential smoothing on the x- and y-axes to account for the nearshore preference of the dolphins and the survey design. In all years, dolphins were encountered consistently in a restricted area (4 year area of overlap: 87.3 km(2)). We modelled habitat preference with boosted regression trees, using presence/absence of dolphins relative to static and dynamic environmental predictors. An index of coastal turbidity was created based on a near-linear relationship between Secchi disk measurements and log-transformed remotely sensed chl a concentration. Sea surface temperature (SST; 22.6% contribution), turbidity (22.2%), distance to major watersheds (17%), depth (14.5%), distance to minor watersheds (13.3%) and distance to the coast (10.4%) partly explained M (a) over bar ui dolphin distribution. We detected a match between predicted areas of high nearshore habitat suitability around North Island and historical sightings (76.2% overlap), thus highlighting potential areas of M (a) over bar ui dolphin recovery. Our study presents methods broadly applicable to distribution analyses, and demonstrates an evidence-based application toward managing M (a) over bar ui dolphin habitat.