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Behavioral Ecology of Deep-Diving Odontocetes in the Bahamas
Ms. Diane Claridge | Bahamas Marine Mammal Research Organisation
Odontocete cetaceans are known for the diversity and complexity of behavioral ecology exhibited across this diverse taxonomic suborder. Characterization of such population-specific traits is required for assessing and mitigating the potential impacts of anthropogenic activities. The Great Bahama Canyon, in the northern Bahamas, was the site of a multi-species atypical stranding of cetaceans associated with the use of Navy sonar. More recently, dedicated studies of beaked whales (Family Ziphiidae) in this area documented behavioral responses of these deep-diving odontocetes to sonar exposure at the United States (U.S.) Navy’s Atlantic Undersea Test and Evaluation Center (AUTEC). The AUTEC range, and the Great Bahama Canyon in general, is known to be used by a number of odontocete species in addition to beaked whales, requiring data on species-specific vulnerabilities.
In this study, an interdisciplinary set of individual-based data was used to provide baseline information on the behavioral ecology of six Department of Defense (DoD) priority species of odontocete cetaceans that occur sympatrically throughout the Great Bahama Canyon. The six species are taxonomically diverse and include two species of delphinids (melon-headed whales Peponocephala electra, Pe; short-finned pilot whales, Globicephala macrorhynchus, Gm), three species of beaked whales (Blainville’s, Mesoplodon densirostris, Md; Gervais’, Mesoplodon europaeus, Me; Cuvier’s, Ziphius cavirostris, Zc) and the sperm whale (Physeter macrocephalus, Pm). The six study species range in adult size from less than 3 m to greater than 15 m, all are thought to be deep diving, and all are found in deep-water habitats that overlap with Navy sonar use. The project’s goal was to characterize and compare habitat use and behaviors of these species and to assess potential vulnerabilities to disturbance from Navy activities.
Data acquired through individual photo-identification, molecular genetics and chemical biomarkers from tissue biopsies, satellite telemetry, and acoustic recordings were integrated to characterize the population structure and movement patterns, social organization, foraging behavior, and habitat of each species. Three month-long shipboard surveys of the Great Bahama Canyon were conducted annually between 2011 and 2013. Data were also leveraged from studies conducted prior to, and concurrent with the Strategic Environmental Research and Development Program (SERDP) project, resulting in a total dataset of over 60,000 photographs from 913 encounters, 407 biopsy samples, 95 acoustic detections comprising over 200 hours of recordings, and 74 tag deployments. Encounters were distributed across seven geographically-defined strata that divided the Great Bahama Canyon and surrounding waters.
Photo-identification and satellite telemetry revealed ranging patterns that varied greatly across species. Gm exhibited relatively widespread movements across strata throughout the canyon, and some groups were tracked into Gulf Stream waters off the coast of Florida. Together with an extremely low re-sighting rate over multiple years from photo-identifications and population genetic analyses, these data suggest a population that ranges well beyond the Bahamas and is likely part of a stock recognized in U.S. waters. The movements of Pe also were relatively expansive within the canyon, but the distinctly seasonal pattern to their encounters (April – September) and the anomalous signatures of persistent organic pollutants measured in blubber biopsies suggest they may be seasonal migrants into the study area from elsewhere. In contrast to these two delphinid species, Pm exhibited evidence of sex-based habitat partitioning. Genetically-confirmed and tagged sub-adult males ranged more widely than adult females with the majority of young males tracked primarily within the Tongue of the Ocean (containing the AUTEC range) either solitarily or in small bachelor groups, in contrast to adult female groups and their calves that rarely used this area. Photo-identification data documented multi-year re-sightings of individuals within both demographic groups, suggesting a vulnerability of the sub-adult males that may be repeatedly disturbed by sonar exposure at AUTEC. Genetic analyses revealed that bachelor males were more closely related to one another than to adult females, suggesting these males may be immigrants from areas outside the canyon rather than originating from local matrilines. In contrast to the other tagged species, telemetry data showed beaked whales (Md and Zc) exhibit a high level of site-fidelity on a small-scale. This was strongly supported by photo-identification analysis for Md, which documented high site fidelity of adult females to local sampling strata, including AUTEC, spanning more than a decade. This suggests that beaked whales, particularly Md, may be vulnerable to repeated disturbances due to their limited ranging patterns.
These new data fill key data gaps and provide necessary information for decision-making to mitigate impacts of Navy activities on vulnerable species and populations. Despite melon-headed whales being a highly mobile species, re-sightings across multiple years of over a third of individuals photo-identified at AUTEC suggests that ongoing monitoring is required to assess and identify population-level impacts from repeated disturbance at this site. Similarly, estimates of future Navy “takes” of the Gm stock off the U.S. southeast coast likely need to include takes that occur at AUTEC due to new data documenting the movement of individuals between regions. The integrated results for Md emphasize a particular vulnerability for these animals at AUTEC and a precautionary need for this distinct subpopulation to be considered a separate conservation unit. Although data are more limited, resource managers should also consider other beaked whales in the Bahamas particularly vulnerable to disturbance because of similar foraging constraints and apparently limited ranges. Future work in the Bahamas should focus on developing longitudinal datasets for Zc and Me in the Tongue of the Ocean (TOTO) and surrounding waters of sufficient resolution to allow monitoring of these potential effects. To develop an effective management strategy for sperm whales in the Bahamas, it is necessary to investigate the stock identity for whales using TOTO through an expanded study of population structure and gene flow throughout the wider Caribbean. To this end, the sperm whale photo-identification catalogue and acoustic recordings of codas compiled during this project will be shared with collaborators in the North Atlantic, and genetic data could contribute to an expanded population study.