NATURALIST’S NOTEBOOK
North Carolina’s Year-Round Residents
BY SARA MIRABILIO
Many vacationers to the Outer Banks have been lucky enough to spot dolphins in the sounds that separate the islands from the mainland. Researchers estimate that 500 dolphins migrate to the Outer Banks to give birth during May through September. When autumn arrives, many of the dolphins migrate southward. But what about the year-round residents? What does that population of dolphins look like?
The northernmost boundary for year-round-resident bottlenose dolphins once was believed to be Beaufort, North Carolina — but research has shown two estuarine resident populations, with a mixing area between the two around Beaufort.
In estuarine and coastal ocean habitats, dolphins face numerous threats, such as fishing gear, vessel strikes, environmental contaminants, and more. Understanding where clusters of dolphins live throughout the year, so we can estimate their populations, informs management and conservation.
The Northern North Carolina Estuarine System Stock (NNCESS) and Southern North Carolina Estuarine System Stock (SNCESS) currently define two resident populations of bottlenose dolphins whose ranges overlap (in yellow). Credit: PLoS ONE journal & the study authors.
DOLPHINS ON CAMERA
Researchers have developed techniques for identifying individual dolphins by the scars and notches acquired on their dorsal fins through photo identification (Photo-ID).
With funding from NOAA National Marine Fisheries Service, Aleta A. Hohn led a research team that photographed all the dolphins they saw, noting the number of barnacles on dorsal fins, a feature that sheds light on habitat use. There are several factors that indicate whether dolphins are migratory— for instance, when individual dolphins have a medium to heavy barnacle load, or when a group of dolphins has barnacles on more than 70% of its fins.
The team compared photographs from individually identified dolphins to a long-term Photo-ID catalog at the NOAA Beaufort Laboratory in North Carolina. This catalog includes photographs of 2,423 different dolphins from Cape May, New Jersey, to Georgetown, South Carolina, from 1995 to 2018.
The scientists identified 547 distinct dolphins — nearly three times higher than the 2006 estimate of fewer than 200 — and the long-term Photo-ID catalog yielded matches with 228 dolphins. Of those, 65 had been included in the 2013 abundance estimate for the northern North Carolina estuarine stock.
WHERE THEY CLUSTER
To look at social structure, the team examined 95 individual dolphins for which there were repeat sightings. Three large clusters characterized general differences in the locations of their primary sightings: the Pamlico Sound cluster (northern and estuarine), the Beaufort cluster (central), and the southern North Carolina cluster.
Barnacle infestations, a possible indicator of cluster membership based on habitat use, differed among clusters. The findings indicate that the structure of estuarine bottlenose dolphins in North Carolina is more complex than previously drawn. The three primary clusters of resident dolphins are inconsistent with current designations.
New approaches are necessary to determine how best to obtain reliable estimates of dolphin populations — and to understand how human activity influences the mortality of dolphins.
the full study in PLoS ONE
“Patterns of Association and Distribution of Estuarine-Resident Common Bottlenose Dolphins (Tursiops truncatus) in North Carolina”
lead photo credit: NASA.
Sara Mirabilio is co-curator with Scott Baker of North Carolina Sea Grant’s award-winning Hook, Line & Science series, which originally published this story. HookLineScience.com.