Habitat selection in a coastal dolphin species (Cephalorhynchus hectori)

Hector's dolphin (Cephalorhynchus hectori) is a small New Zealand delphinid with a coastal distribution. Within a strip of I km from shore, the present study quantified the habitat used by the dolphins (n=461 groups) over a 19-month period (216 field days with 966 survey hours) by recording the abiotic factors sea surface temperature (SST), water depth and water clarity. Resource selection functions were used to distinguish the properties of 461 used sites (dolphins present) from 425 unused sites (no dolphins present) in six different study areas. Most dolphins were encountered in waters < 39 in depth, with < 4 in Secchi disk visibility and > 14degreesC temperature. The preference of Hector's dolphins for warm and turbid waters was tested using eight models. Water depth, water clarity, SST and the study area explained dolphin presence to a very significant degree (p<0.001), and the model allowed the creation of probability plots for a variety of combinations of the variables. Habitat selection by dolphins differed between study areas, particularly between east and west coasts, in summer (December-February) and winter (June-August). Dolphin abundance appeared to change seasonally in some study areas, possibly due to a more offshore distribution of their prey in the winter, with its lower SSTs. This was so especially in summer (the main reproductive season), when dolphins (frequently with calves) occupied shallow and turbid waters, whereas in winter less use was made of this habitat.