Life history and multivariate analyses of habitat selection patterns among small cetaceans in the central North Pacific Ocean

Abstract

Habitat selection and life history patterns were compared among three species of small cetaceans in the central North Pacific Ocean: Dall's porpoise (Phocoenoides dalli), northern right whale dolphin (Lissodelphis borealis) and Pacific white-sided dolphin (Lagenorhynchus obliquidens). Biological samples and fisheries data used for analysis of habitat preferences were obtained by observers monitoring Japanese high seas squid driftnet operations between 36$\sp\circ$N and 46$\sp\circ$N, and 152$\sp\circ$E and 150$\sp\circ$W during 1990 and 1991. Supplementary data used for analysis of Dall's porpoise life history were collected between 46$\sp\circ$N and 53$\sp\circ$N and 168$\sp\circ$E and 175$\sp\circ$E in the Japanese salmon mothership fishery from 1981 to 1987.The main objective was to determine if habitat partitioning mechanisms could be detected among the three marine mammal species. Two approaches were taken. First, reproductive patterns were compared, with special attention to calving seasonality. The timing and location of calving activity were indicated as possible factors differentiating habitat use, operating: (a) spatially between P. dalli and L. borealis, (b) temporally between L. borealis and L. obliguidens, and (c) both spatially and temporally between L. obliquidens and P. dalli.Second, canonical correspondence analysis (CCA) was used: (a) to compare species locations in ordination space along observed environmental gradients (i.e., habitat features), and (b) to compare community association patterns among the three marine mammal species and other marine organisms caught in the same driftnets. Monthly records, from June to September were examined separately and compared. Sea surface temperature was the most influential habitat parameter examined, with L. borealis occupying the warmest waters, P. dalli the coolest, and L. obliquidens in between, but with greater preference overlap with P. dalli. Habitat partitioning was best expressed by mature female L. borealis, in July, during their calving period. Mature female L. borealis associated with a consistent assemblage of other marine organisms during July and August while associations among other species were more varied. Collectively, these findings suggest that habitat preference patterns for these three species may be specific to reproductively active females, while coincident habitat use among other species constituents is common.