Spatiotemporal variations in habitat utilization patterns of four Delphinidae species in the western North Pacific, inferred from carbon and nitrogen stable isotope ratios

Abstract

Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in skin biopsy samples of four species in the Delphinidae family collected in 1993–2015 in the western North Pacific were investigated. Within the four species—common bottlenose, pantropical spotted, striped, and short-beaked common dolphins—δ13C and δ15N were highest in common bottlenose dolphins, and lowest in short-beaked common dolphins. Standard ellipse areas describing the likely range of the δ13C and δ15N, estimated using Bayesian inferences, indicated wider isotopic niche width in common bottlenose and pantropical spotted dolphins. Although spatial distribution patterns have been known to be similar among striped and short-beaked common dolphins, isotopic niche of the two species less overlapped. The stable isotope ratios were then analyzed in relation to space (longitude and latitude) and time (season) using generalized linear mixed models. The δ13C and δ15N values tended to be higher in lower longitudes for all four species, while δ15N tended to be lower in lower latitudes for common bottlenose and pantropical spotted dolphins, but lowest in higher latitudes for striped dolphins. These variations are considered to relate to north–south and coastal–offshore variations in nitrogen and carbon sources and nitrogen fixation in Kuroshio waters. This theory was further supported by analyzing stable isotope ratios in relation to distance to coast and sea surface temperature. Seasonal variations in δ13C and δ15N values were recognized for pantropical spotted and striped dolphins.