Patterns of association and distribution of estuarine-resident common bottlenose dolphins (Tursiops truncatus) in North Carolina, USA

Abstract

The social structure of estuarine-resident bottlenose dolphins is complex and varied. Residing in habitats often utilized for resource exploitation, dolphins are at risk due to anthropogenic pressures while still federally protected. Effective conservation is predicated upon accurate abundance estimates. In North Carolina, two estuarine-resident stocks (demographically independent groups) of common bottlenose dolphin have been designated using spatiotemporal criteria. Both stocks are subjected to bycatch in fishing gear. The southern North Carolina estuarine stock was estimated at <200 individuals from surveys in 2006, which is outdated per US guidelines. Thus, we conducted a new capture-mark-recapture survey in 2018, identifying 547 distinct individuals, about three times higher than the prior abundance estimate. We compared those individuals to our long-term photo-identification catalog (1995–2018, n = 2,423 individuals), matching 228 individuals. Of those 228, 65 were also included in the 2013 abundance estimate for the northern North Carolina estuarine stock. Using sighting histories for all individuals in the long-term catalog, we conducted a social network analysis, which is independent of a priori stock assignments. The three primary clusters identified were inconsistent with current stock designations and not defined by spatiotemporal distribution. All three clusters had sighting histories in the estuary and on the coast, however, that with the highest within-cluster associations appeared to use estuarine waters more often. The within-cluster association strength was low for one cluster, possibly due to only part of that cluster inhabiting the southern North Carolina estuarine system. Between-cluster differences occurred in infestation rates by the pseudostalked barnacle, Xenobalanus globicipitis, but that did not predict clusters. We suggest the need to re-evaluate the stock structure of estuarine-resident common bottlenose dolphins in North Carolina and currently have insufficient information to assign an abundance estimate to a currently designated stock.

Associations and ranging patterns of common bottlenose dolphins near Brunswick, Georgia, USA

“…Titcomb et al (2015) conducted a study on a population of estuarine bottlenose dolphins in the Indian River Lagoon, Florida, organized into 6 distinct social clusters. The same number of clusters ( n = 6) was reported by Hohn et al (2022) in North Carolina. Urian et al (2009) reported that dolphins in Tampa Bay, Florida formed 5 clusters.…”

“…Dolphins from SGES that comprised small clusters associated loosely with the large clusters, both according to the sociogram (Figure 4) and based on the social metric strength values measured between clusters (Table S2). A social network analysis of estuarine dolphins in North Carolina, USA, resulted in nearly identical delineations as the SGES stock where they reported 3 large clusters (23-35 individuals) and 3 small clusters (1-3 individuals; Hohn et al 2022). Differences observed among clusters, based on data from other populations, could be related to the physical environment and habitat complexity (Karczmarski et al 2005, Wiszniewski et al 2009), resource availability (Lusseau et al 2003), predation risk (Wilkinson et al 2017), foraging strategy (Torres and Read 2009), and possibly anthropogenic activities (Greenfield et al 2021).…”

“…There are a number of possible explanations for the occurrence of the small clusters within the SGES stock. One possibility is that the individuals contained within these clusters could have lower site fidelity (Defran and Weller 1999, Zanardo et al 2016) within the boundaries of the SGES and only be observed seasonally while primarily using adjacent BSE or coastal habitats (Hohn et al 2022). This was not supported by any matches to the GDEP or NEFL dorsal fin catalogs.…”

“…Several studies identify distinct communities within dolphin populations that have emerged as a result of habitat preference (Kiszka et al 2012) or behavior (Chilvers and Corkeron 2002). Recently, social network analyses have provided greater insight into fine‐scale dolphin social structure (Chabanne et al 2017, Hohn et al 2022, Speakman et al 2022). For example, Brusa et al (2016) reported that dolphins in the North Inlet‐Winyah Bay estuarine system in South Carolina, USA, associated nonrandomly and identified 4 social communities with overlapping ranges but unique core use areas.…”