Using aerial photogrammetry and sexually dimorphic measurements to investigate seasonal differences in school composition of Delphinus spp. off Southern California
Date of Award
Thesis: Open Access
MS Marine Science
Environmental and Ocean Sciences
Lisa T. Ballance
John W. Durban
Drew M. Talley
Insights into school composition can provide a means to understand basic biology and ecology, including reproductive patterns. They can also be applied to conservation assessments, allowing for better understanding of the potentially differential vulnerability of demographic groups to natural or anthropogenic disturbances that may influence their populations. However, the two subspecies of common dolphins in the waters off California (Delphinus delphis delphis and D. d. bairdii) form large, energetic groups that make characterization of school composition difficult. Remotely controlled drones now offer the opportunity for the study of school composition in Delphinus spp., allowing for precise morphometric measurements of individuals in large schools. This thesis is a collection of two papers aimed at: 1) improving methods we can use to determine school composition of California Delphinus spp. in aerial photographs, and 2) applying these methods to further understanding of school composition in D. d. bairdii in waters off Southern California.
In Chapter 1, I use data from stranded and fisheries bycaught common dolphins to develop sexually diagnostic measurements of D. d. delphis and D. d. bairdii that can be applied to aerial images to infer the sex of free-ranging dolphins. We chose three morphometrics based on criteria designed to ensure accurate measurability in aerial images: 1) standard total body length (hereafter “total length”); 2) an “anterior” length, from tip of the rostrum to anterior insertion of the dorsal fin; and 3) a “posterior” length, from anterior insertion of the dorsal fin to fluke notch. All three measurements exhibited interspecific differences, but they varied in the extent of sexual dimorphism within species. For both species, posterior length showed the greatest degree of sexual dimorphism, with 58.7% and 37.3% of adult D. d. bairdii and delphis males, respectively, larger than 95% of adult females. Total length showed the next greatest degree of sexual dimorphism, due largely to the contribution from posterior length, and anterior length showed the lowest. We demonstrated the utility of these sexually dimorphic features in photogrammetry measurements of an example aerial image of D. d. bairdii taken by a drone at an altitude of 58m (190ft), illustrating their value for identifying likely large adult males in aerial images.
In Chapter 2, I demonstrate how drones can be used to obtain precise measurements of individual D. d. bairdii. Eastern North Pacific long-beaked common dolphins (Delphinus delphis bairdii) often form large, energetic schools that make characterization of school composition difficult. Here we demonstrate how drones can be used to obtain precise measurements of individual D. d. bairdii. Additionally, we apply the recently identified sexually dimorphic measurements of the subspecies (specifically “posterior” length, or the length from the anterior insertion of the dorsal fin to fluke notch) to distinguish adult males of a large size class from visually similar adult females. Using aerial images from eight schools of D. d. baiirdi collected during June and October (2017-2019), we obtained total length estimates of 693 dolphins and posterior length estimates of 768 dolphins in waters off Southern California. In June, measured dolphins were on average 7-8cm longer in both total and posterior length. In addition, there were proportionally fewer calves and more large adult males measured during June compared to October. In a Bayesian mixture model, two June schools were best described by a size distribution cluster with a higher proportion of large individuals than the cluster that best described all October schools and two of four June schools, which had a higher proportion of small individuals. Our results suggest that some schools sampled in June have a different composition than the other six sampled schools, and likely represent either “bachelor” groups, consisting of all males, or reproductive schools, in which adult females are also present. Insights into school composition can provide a means to understand basic biology and ecology, including reproductive patterns. They can also be applied to conservation assessments, allowing better understanding of the potentially differential vulnerability of demographic groups to natural or anthropogenic disturbances.
I analyzed data from hundreds of stranded and bycaught individuals, as well as hundreds of measurements collected from over 11,000 aerial images. This work furthers our knowledge on the basic biology and ecology of Delphinus spp. in the waters off California. Additionally, it shows the utility of methods that can be broadly applicable to free-ranging cetacean populations around the world, providing a non-invasive monitoring tool that can be integrated into management and conservations plans of vulnerable species.
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Digital USD Citation
Leander, Samantha GM, "Using aerial photogrammetry and sexually dimorphic measurements to investigate seasonal differences in school composition of Delphinus spp. off Southern California" (2021). Theses. 45.
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This work is licensed under a Creative Commons Attribution 4.0 License.
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