Habitat differentiation between sei (<i><scp>B</scp>alaenoptera borealis</i>) and <scp>B</scp>ryde's whales (<i><scp>B</scp>. brydei</i>) in the western <scp>N</scp>orth <scp>P</scp>acific

Abstract

Abstract Two closely related baleen whale species, sei and B ryde's whales, in the western N orth P acific were studied to identify differences in habitat use. Data were obtained from M ay to A ugust 2004 and 2005. This study examined the relationship between oceanographic features derived from satellite data and the distribution of sei and B ryde's whales using basic statistics. We investigated oceanographic features including sea surface temperature ( SST ), sea surface chlorophyll a ( C hl‐ a ), sea surface height anomalies ( SSHA s), and depth of the habitat. These two whale species used habitats with different SST , C hl‐ a , and SSHA ranges. The 0.25 mg m −3 C hl‐ a contour (similar to the definition of the T ransition Z one C hlorophyll F ront) was a good indicator that separated the habitats of sei and B ryde's whales. Then generalized linear models were used to model the probabilities that the whale species would be present in a habitat and to estimate their habitat distribution throughout the study area as a function of environmental variables. The potential habitats of the two species were clearly divided, and the boundary moved north with seasonal progression. The habitat partitioning results indicated that SST contributed to the patterns of habitat‐use and might reflect differences in prey species between the two whales. This study showed that the habitats of the sei and B ryde's whales were clearly divided and their potential habitat‐use changed seasonally.