Short-beaked common dolphin (Delphinus delphis) total length estimation using laser photogrammetry off the southwest coast of Ireland

Abstract

Measurements from 106 stranded short‐beaked common dolphins along the Irish coast were taken between March 2017 and March 2023. Data were collected from the Irish Necropsy Project and Irish Cetacean Stranding Scheme. Total length measurements were gathered from 103 individuals where the tail flukes were still attached. These ranged between 96–238cm, mean 185.7cm, SD 31.43cm. Males (n = 58) ranged from 96–238cm, averaging 189cm, SD 34.64cm. Females (n = 40) measured 117.5–231cm, averaging 181cm, SD 25.80cm. Scaled dorsal fin photos were taken from stranded dolphins to formulate an equation to estimate the total length of live dolphins. The total lengths of 29 live common dolphins were estimated off the southwest coast of Ireland using laser photogrammetry and dorsal fin dimensions between March 2018–April 2019. Total lengths for three stranded dolphins with amputated tail flukes were also estimated. Dorsal fin base lengths were the most accurate predictor of total length R2 = 0.78. Total length estimates ranged between 143.77–242.25cm, averaging 194.78cm, SD 20.05cm. The adoption of laser photogrammetry as a measurement tool warrants further exploration as a means to reduce potential disruption from aerial systems and enhance the utility of behavioural and photo‐ID images. This study describes a non‐invasive technique with a range of possible applications for understanding pod size structure and seasonality due to this species’ approachable behaviour and inquisitive nature.