Blue Whale (Balaenoptera musculus) Behavior and Group Dynamics as Observed from an Aircraft off Southern California

Abstract

Group behavior and interactions of endangered blue whales (Balaenoptera musculus) have not been systematically studied. Such behavioral data are often overlooked when assessing anthropogenic effects. Yet behavioral data are necessary to compare "normal" behaviors with behavior affected by anthropogenic factors of concern relative to effective management and recovery of blue whales. For a baseline study, we hypothesized that the response variables sighting rate, group size, calf presence and group cohesion (i.e., spacing between individuals within a group) differed according to the spatio-temporal explanatory variables behavioral state, time of day, season, water depth and distance from shore. To address our hypotheses, we flew systematic line transect surveys in southern California and collected focal group data. Two sets of data were separately analyzed using different sampling approaches: (1) point sample data associated with the first sighting of a blue whale(s), and (2) extended all-occurrence focal group behavioral sampling data (i.e., focal follows) collected on a subsample of all sightings while the aircraft circled at a radial distance of approximately 0.5-1 km and an altitude of 1,500 m for extended periods of 5 -60 minutes. Chi-square contingency table and G² analyses were used to assess statistical relationships between response and explanatory variables. We conducted 18 one-week-long aerial surveys spanning October 2008 through May 2013 (at least once during every month except December), totaling 87,555 km of observation effort. Seventy blue whale sightings (117 individuals) were seen, ranging in size from 1 -6 whales, and focal follow was performed on over half (55%) of these sightings. Results supported our hypotheses that blue whale group characteristics were related to behavioral state and spatio-temporal variables. Sighting rates were significantly highest during summer followed by spring, fall, and winter. Group type differed significantly by season: groups (at least three individuals) were seen only during summer but singles and pairs were seen from spring through fall. Noncalf groups and mother/calf pairs engaged predominantly in mill and slow travel/rest, while singles and pairs were only observed in medium/fast travel. Behavioral state differed significantly by time of day, group type, water depth and distance from shore. On average mother/calf pairs (n = 3) and singles (n = 43) were found in deeper offshore waters than groups (n = 8) and pairs (n = 16). Group cohesion and group size were negatively correlated: as group size increased whales became less cohesive and more spread out. Our study suggests that group characteristics and behavior are sensitive to spatio-temporal factors. These data are important in providing the first systematic quantitative study of blue whale behavior, providing a baseline against which to assess potential impacts of increasing anthropogenic marine activities in the study area and elsewhere. Results further indicate that social, behavioral and spatio-temporal fa...

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