Abstract
Many species of marine top-predator have ‘hotspots’ in their distribution. Hotspots can be defined as areas within the distribution of a population that are used disproportionately more than other locations. Usually identified as consistent, high density aggregations, hotspots are generally locations that are important for certain life history processes or key behaviours such as foraging. Knowledge on what drives the existence of hotspots is crucial for the conservation of species and for understanding ecosystems. New Zealand’s only endemic cetacean, Hector’s dolphin, is endangered and sparsely distributed. This thesis investigates hotspots in the distribution of Hector’s dolphins at Banks Peninsula in order to determine what makes these locations unique and thus to appraise how human disturbance may threaten the ecology of these areas. Three simple questions were considered: Where and when do hotspots exist? Why do the dolphins use these areas? What habitat features make these locations unique?
Hotspot locations were defined using Kernel density analysis (KDe) of a visual sightings database from standardized, boat-based surveys. The analyses showed that fifty percent of sightings, made over 29 years and weighted by search effort, were clustered into only 21% of the study area. The seasonal pattern of hotspots strongly reflected summer distribution patterns, but several hotspots were also important in spring and autumn. Locations of hotspots were consistent over almost three decades. Passive acoustic monitoring showed the highest rates of foraging buzzes at hotspots; suggesting that foraging opportunities shape distribution in this species. The temporal distribution of foraging was complex, with substantial differences among locations over seasonal, diel and tidal cycles.
Data from hydro-acoustic surveys of epipelagic fish showed strong overlap between dolphins and their prey. The depth of prey schools was also important. Prey were generally more abundant, and shallower, at hotspots compared to reference areas. A broad range of habitat variables were pooled from several sources to determine the best predictors of habitat use and the characteristics of hotspots. Covariates were considered that define the physical and biological features of habitat that may be correlated with distribution. Variables significantly related to the relative abundance of dolphins included prey abundance, mud coverage, reef coverage, depth, current velocity, salinity, fluorescence and thermocline depth. However, only the preferred values of prey, depth, dominant habitat type, and to a lesser extent, reef coverage were more common at hotspots.
Confirmation of the locations of hotspots, their stability over time and their importance for foraging provides candidates for areas deserving more protection. Hector’s dolphins in this area face threats associated with fisheries bycatch, vessel strike and noise pollution. Further, information on the characteristics of hotspots provides management with opportunities to prevent degradation of the features that make good quality habitat. With spatially explicit management that focusses on the full range of threats, populations of this ecologically important, taonga species may recover to previous, un-impacted levels.