Post-Nuchal Depression as an Indicator of Health in Bottlenose Dolphins (Tursiops truncatus)

Abstract

Health assessments for resident dolphins in Sarasota Bay, Florida provide direct measurements of body condition for a subset of animals each year. However, directly handling dolphins can be stressful to the animal, involves risky capture and release techniques, and limits researchers’ ability to study larger populations. Therefore, developing photographic methods to determine the health and nutritional status of free-ranging dolphins is a priority. A simple metric being considered is the presence of a visible area of reduced blubber behind the skull called a post-nuchal depression (PND), which has accurately indicated poor body condition in stranded animals. This study used archived images and data from dolphin population monitoring surveys, prey abundance studies, and health assessments to apply the PND metric to live, free-ranging individuals and assess its utility as an indicator of dolphin health. Images from two seasonal surveys in years of scarce and abundant prey were coded to identify the individual and the post-nuchal region and determine if a PND was measurable. ACDSee was used to score individuals by drawing a line across the dorsal surface to see if a space existed below the line behind the nuchal crest of the skull. Individual dolphins were scored with a 0 (no PND), 1 (visible PND), borderline (BL), or cannot be determined (CBD). PND prevalence was compared between years of differing resource availability as well as by age and sex class. PND prevalence was significantly higher in the year of scarce prey, and PND frequency also varied significantly among age class, but not by sex class. The replicability of this method was probable, but PND determinations from field photographs were sometimes inconsistent with direct measurements of individuals from health assessments conducted in the same season. However, this method may be useful as a basic indicator of overall population health.