Dear members of the MARMAM community, My co-authors and I are pleased to announce the publication of the following paper on sexually dimorphic measurements in *Delphinus *spp. in waters off California:
Leander SGM, Durban JW, Danil K, Fearnbach H, Joyce TW, Ballance LT. Sexually dimorphic measurements from stranded and bycaught specimens contribute to the characterization of group composition in free-ranging common dolphins (*Delphinus *spp.) from aerial images. Mar Mam Sci. 2021;1–7. https://doi.org/10.1111/mms.12804 Summary: Here we 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 of free ranging *Delphinus* spp. to describe demographic composition. Based on criteria designed to ensure accurate measurability in aerial images, three morphometrics were selected for further analysis: standard total body length (hereafter “total length”), an “anterior” measurement of length between the snout to anterior insertion of the dorsal fin, and a “posterior” measurement of length between the anterior insertion of the dorsal fin and the fluke notch. All three measurements exhibited interspecific differences, but they varied in the extent of sexual dimorphism within species. For both species, posterior showed the greatest degree of sexual dimorphism, with 58.7% and 37.3% of adult *D. d. bairdii* and* delphis* males, respectively, were larger than 95% of adult females. Total length showed the next greatest degree of sexual dimorphism and anterior showed the lowest. We then applied these measurements to an aerial image taken at an altitude of 58m (190ft). We were able to successfully measure five flat individuals using a segmented line and determine, using the posterior measurement, that two of the five measured were likely males. Our results will facilitate characterization of group composition in aerially photographed *Delphinus* spp. schools off Southern California (photogrammetric analysis currently underway). Additionally, these methods are 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. Best, *Sam Leander *(*she/her)* Associate Scientist *Southall Environmental Associates, Inc.*
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