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Abundance and Movements of the Northern South Carolina Estuarine System Stock of Bottlenose Dolphins (Tursiops truncatus) (USA)

Dani Silva1, Brianna Tramutolo1, Erika DeSalvio1, Todd Speakman2, and Rob Young1

1Department of Marine Science, Coastal Carolina University, Conway, South Carolina, U.S.A.

2National Marine Mammal Foundation, Charleston, South Carolina, U.S.A.

Abstract In the USA, federal law requires managing anthropogenic threats to

bottlenose dolphins at the stock level. The Northern South Carolina Estuarine System Stock (NSCESS) lacks estimates of abundance and potential biological removal required for management. Moreover, the southern stock boundary of the NSCESS is shared with the northern boundary for the Charleston Estuarine System Stock (CESS). The goals of this study were to investigate the empirical location of the southern stock boundary and to generate the first estimate of abundance for the NSCESS. Mark-recapture photo-identification surveys including one ‘mark’ and two ‘recapture’ sessions were completed between August and early October 2016. Predefined survey tracks approximately 245 km in length covered the described range for the NSCESS and an additional 11 km southwest of the currently defined southern stock boundary. Long term movement patterns were also investigated via comparison with historical catalogs from the NSCESS and CESS regions. Observed movement patterns did not suggest a clear revision to the southern stock boundary, thus sightings within the defined NSCESS stock boundaries were used to estimate abundance. Dorsal fin photographs were scored for suitable quality and distinctiveness using the program FinBase, and stock abundance was estimated with closed population models using the package Rcapture for program R. The best fitted model, Mth, was selected based on the lowest AIC value and yielded

an abundance estimate of 453 dolphins (95% CI = 265-773, CV = 0.28). This study contributes to the sustainable management of the NSCESS and informed response to anthropogenic and natural threats.

[JMATE 2019;11(1):8-18]

Keywords: photo-ID; mark-recapture; closed population

Since then, stock descriptions for the coastal morphotype evolved from a proposed single coastal migratory stock to a total of 16 stocks: 5 coastal and 11 estuarine (28, 40, 41). Coastal stocks inhabit primarily coastal waters whereas estuarine stocks are found pri-marily in waters inshore of the coastline, though some overlap occurs in nearshore coastal waters (19, 40).

One of these estuarine stocks, the Northern South Carolina Estuarine System Stock (NSCESS), was first described in 2013 but is listed as data deficient with no estimate for population size or PBR as required under the MMPA1b (40). Based on various studies within this area the NSCESS is described as dolphins that inhabit estuarine and coastal waters within 1 km of the shoreline from Murrells Inlet to Price Inlet, South Carolina (SC) (Figure 1) (8, 32, 40). The northern stock boundary was defined based on the lack of significant estuaries for over 50 km north of Murrells Inlet, SC and on a single dolphin sighted in coastal waters off Murrells Inlet and Cape Romain (Figure 1) (40). The southern stock boundary was defined by the northern boundary for the Charleston Estuarine System Stock (CESS) at Price Inlet (40). This was the northern extent of the CESS surveys and no published dolphin surveys have investigated the estuarine waters for 20 km north of this boundary (34).

Mark-recapture methodologies using photo identification techniques can be used to investigate stock boundaries and to estimate population abundance (15). Boat-based mark-recapture surveys have been extensively used to estimate abundance of bottlenose dolphins in estuarine systems along the Southeast US (6, 13, 25, 33, 39). During boat-based surveys, individuals are photographed and subsequently identified based on the pattern of natural markings found on dorsal fins (45). The geographical locations and history of individual sightings provide evidence of

Submitted January 8, 2019; Final Acceptance June 4, 2019 Correspondence: Daniela da Silva Department of Marine Science, Coastal Carolina University PO Box 261954, Conway, South Carolina, U.S.A. 29528-6054. Telephone: 843-349-2277 Email: [email protected]

Journal of Marine Animals and Their Ecology Copyright © 2008 Oceanographic Environmental Research Society

Vol 11, Issue 1, 2019 Printed in Canada

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Introduction Along the east coast of the USA, bottlenose

dolphins (Tursiops truncatus) are divided into two morphotypes: offshore and coastal and are federally protected under the Marine Mammal Protection Act (MMPA)1a (7, 16, 27). The coastal morphotype was classified as “depleted” following a large die-off in 1987/19881b (28). Depleted stocks are considered strategic and require current estimates of population parameters to calculate potential biological removal (PBR)1c. These federal designations prompted scientific efforts to examine bottlenose dolphin stock structure and seasonal movements along the US east coast (17, 37).

Footnote: 1MMPA 16USC §1361-1407 at https://www.fws.gov/international/pdf/legislation-marine-mammal-protection-act.pdf (a)§1361 et seq; (b) §1362; (c) §1386

occurrence, movements, and distribution patterns (30). Sighting histories of marked individuals analyzed using closed population mark-recapture models yield estimates of abundance (15, 29).

The objectives of the current study were 1) to employ mark-recapture photo-identification methods to provide a comprehensive abundance estimate for the NSCESS stock and 2) investigate dolphin movements within the described stock boundaries and across the under-investigated southern boundary.

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Abundance and Movements of the NSCESS (USA) Vol 11, Issue 1, 2019 Printed in Canada

Figure 1: Currently defined boundaries for the Northern South Carolina Estuarine System Stock.

Methods Survey Area

The survey track included approximately 245 km of salt marsh creeks, small bays, and parts of the Intracoastal Waterway (ICW) from North Inlet/Winyah Bay to Dewees Inlet (Figure 2). Coastal waters were excluded from these surveys to minimize encounters with coastal stock dolphins. Although the published northern boundary of the NSCESS is Murrells Inlet, this small salt marsh system does not support a resident dolphin population (Coastal Carolina University, unpublished data), hence the North Inlet/Winyah Bay system was the appropriate northern terminus for estuarine surveys. The southern terminus at Dewees Inlet was approximately 11 km southwest of the currently defined NSCESS southern boundary at Price Inlet, allowing an investigation of potential movements across the stock boundary (Figure 2).

Figure 2: Mark-Recapture Survey Routes. Each panel represents a ‘mark’ or ‘recapture’ session along with associated sighting locations indicated by yellow stars. Panel A labels show stock boundaries, panel B includes survey boundaries and panel C labels locations mentioned in the text.

Water temperature averages in South Carolina vary from warm (21-29 °C) between May and late-October to cool (9-20 °C) between November and April, hereafter referred to as warm and cool seasons (3). Previous studies within the NSCESS boundaries reported lower abundance and/or encounter rates during the cooler months (8, 32). Moreover, the distribution of estuarine dolphins in the Carolinas shifts toward inlets and coastal waters during the cool season (8, 31). Therefore, surveys were conducted during the warm season when estuarine stock dolphins are most likely to stay within the estuary. Photo-Identification Mark-Recapture Surveys

Boat-based mark-recapture photo-identification surveys were carried out according to established methodology aboard a 5.5 m aluminum john boat with a 60 hp outboard engine (27, 44). One ‘mark’ and two

scored for photographic quality using the following parameters: contrast, focus, angle of the fin, and whether the entire fin was visible. Photographs that did not meet the threshold for quality (i.e. PQ score > 12) were excluded from subsequent analysis (25, 35). Dorsal fin distinctiveness was scored as high (D1), average (D2), low (D3) or not distinct (D4) based on the number and size of natural marking such as nicks, notches, rake marks, and skin disorder (25, 44, 45). High and average distinct fins were designated as marked, and low or not distinct fins were designated as unmarked (45, 35). Dolphin Movements Short-term movements of marked individuals were investigated by plotting all sighting locations of dolphins seen on two or more sampling days. Individual movements were depicted by connecting sighting locations with a straight line using the ‘point-to-line’ tool (Data Management Toolbox) in ArcMap 10.3. Dolphin movements across longer time-scales were analyzed by comparing the dorsal fin catalog created during this study to historical catalogs compiled either within or near our study area, including nearshore coastal waters. These included previous studies at Coastal Carolina University (CCU) as well as catalogs found in the Mid-Atlantic Bottlenose Dolphin Catalog (MABDC), an online collaborative tool to investigate the stock structure of bottlenose dolphins along the US east coast (14, 37). The CCU dorsal fin catalog includes over 18 years of photo-identification data from studies in estuarine and coastal waters off northern South Carolina, but comparisons were restricted to images collected in North Inlet/Winyah Bay (2011-2016) and Cape Romain (2012). The Sloan catalog, listed as SC-PS Bulls Bay in the MABDC, includes 104 dorsal fin images collected between 2003 and 2005 from estuarine waters near

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‘recapture’ survey sessions were completed between August 11 and October 2, 2016, each requiring 4 to 5 days to complete (Table 1). An interval of at least seven days between survey sessions was observed to allow for population mixing (43). Pre-defined track lines were followed at 10-14 kt until a dolphin was spotted. For each dolphin encounter, GPS locations were recorded upon sighting and upon approach using a Garmin 76c hand-held GPS. We aimed to photograph every dolphin, regardless of visible marks, using a DLSR Canon Rebel Ti2 camera equipped with a 100-400 mm adjustable lens (45). Group size and composition were estimated as a consensus among the researchers aboard the vessel and reviewed after photo analysis. Observations on the conditions of individual dolphins were recorded such as the presence of the barnacle Xenobalanus globicipitis on the dorsal fins, wounds from recent shark bites, and signs of fisheries interactions. Predominant behavior and the cohesiveness of the group were also recorded. GPS coordinates and environmental conditions, including Beaufort Sea State (BSS), water temperature, and salinity were recorded at the end of the sighting prior to returning to the survey track. Photo Analysis

Dorsal fin photographs were sorted by survey date and sighting encounter. Photographs were selected and manipulated following the recommendations of Urian et al. (35). High quality photos were cropped to include the dorsal fin of an individual dolphin and image characteristics such as lighting, exposure, and contrast were improved using Microsoft Photo Gallery (2012). Survey and sighting data along with respective photographic data were entered into FinBase, a modified Microsoft Access database for managing photo-identification data (1). The best right and/or left image of each individual in each sighting encounter was

Survey Session Sampling Days km on-effort Marked Dolphins Recaptured Dolphins

Mark 08/11-08/13

08/15, 08/17 254 40 N/A

Recapture I 08/27

09/09-09/11 240 22 8

Recapture II 09/23-09/25

09/30, 10/02 243 18 5

Table 1: Summary of mark-recapture field effort in estuarine waters of northern South Carolina (Aug 11-Oct 2, 2016).

Cape Romain but specific sighting locations were unavailable (32, 36). The Charleston catalog (1994-2011), shared and maintained by Speakman and listed as SC-NOS (National Ocean Service) in the MABDC, includes 2,248 dorsal fins from coastal and estuarine waters surrounding the Charleston Harbor (36).

The NSCESS southern stock boundary was investigated by examining short-term and long-term movements of marked dolphins across the defined boundary at Price Inlet. Documented dolphin movements, or lack thereof, were used to define the appropriate area from which sighting histories were compiled to estimate stock abundance. Abundance estimates

Abundance is commonly estimated using mark-recapture closed population models which require that the following assumptions are met: 1) the population is demographically and geographically closed, 2) marks are not lost or misread, and 3) every individual has equal and independent probability of capture within a capture occasion (2, 43). These assumptions have been explored by Read et al. who concluded that studies conducted over a short time frame reduce violations of demographic closure and minimize violations of mark loss (25). In addition, mark recognition error is minimal when good quality photographs and individuals with at least two distinctive marks are used in the analysis (25). Conversely, geographic closure and equal probability of capture may be violated if the range of the study subjects is unknown and if individuals behave differently towards the research vessel, respectively (25). Capture independency may be violated if dolphins have preferred associates, however the highest rates of association are among mom/calf pairs, thus this violation can be reduced by removing calves and neonates from the marked dataset (23, 33).

Therefore, assumptions for closed population models are potentially met when surveys are conducted within a relatively short time frame, the study area covers the entire range of the stock in question, the photographic dataset includes only good to high quality images of marked individuals, calves and neonates are excluded from the dataset, and the investigated population models address potential variations in capture probability which ‘relaxes’ the equal catchability assumption (27, 33, 43). Of the traditional closed population models, model Mt accounts for temporal variations in capture probability that may

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occur between the mark and the recapture sessions, model Mh accounts for individual heterogeneity in capture probability, and model Mth explores a combination of temporal and individual variations in capture probabilities (2, 22, 43).

Data from on-effort events during each of the three survey sessions (i.e. ‘mark’, ‘recapture I’, ‘recapture II’) were pooled and sighting histories for each marked dolphin were compiled as a string of 0s (not observed) and 1s (observed). The package Rcapture in Program R was used to estimate marked population abundance under the null, Mt, Mh, and Mth closed population models (4, 24). The best fitting model was selected based on the lowest AIC value (10). Models that included differences in capture probability as a function of behavioral responses after first capture (i.e. trap-happy or trap-shy) were not considered for the current study given that photographic capture is a non-invasive procedure with no documented behavioral response toward the research vessel (25, 43, 45). When the probability of capture was below 0.3, the bias corrected version of the best fitted model was selected (22, 26). Marked abundance estimates derived from this method were then divided by the proportion of marked dolphins (θ) to generate total population abundance (44). The proportion of marked dolphins (θ) was calculated as an average of the ratio of the total number of marked individuals in a group divided by the total number of individuals in the group (44). This ratio was calculated for encounters wherein every individual in the group was photographed (44). The 95% confidence intervals were calculated using the log-normal approximation given that confidence intervals calculated using log-linear profile likelihood are not suitable for closed population models (20). The total population variance was calculated using the delta method (44) as:

and the 95% confidence limits for the total population were calculated as lower limit = Ntotal/C and upper limit = Ntotal x C, where C was calculated with the following formula (20):

Results The three mark-recapture surveys were completed

in 14 survey days over a 52-day period (Table 1). Track lines were slightly adjusted depending on the tidal cycle (Figure 2). Water temperature during dolphin encounters ranged from 26.3 to 32.4 °C and salinity ranged from 3.4 to 36.4. Salinities below 15 were only recorded in upper Winyah and Santee Bays, and 74 % of all sightings were in salinities above 30. A total of 63 dolphin groups were encountered, and 237 dolphins were photographed, of which images of 96 individuals met the criteria for distinctiveness and photographic quality. Sighting locations from each survey and associated place names are plotted on Figure 2. Group sizes ranged from 1 to 17 with an average of 5.5 individuals per group.

Dolphin Movements

Dolphin movements were investigated within the NSCESS’s range and across the defined southern stock boundary at Price Inlet. During our 52-day survey period, 35 individuals were re-sighted on two or more survey days, representing 1/3 of all marked dolphins. Time between re-sightings ranged from 1 to 42 days, and the median linear distance between re-sightings was 4.2 km, with a maximum distance of 19.6 km. Two individuals were sighted both northeast and southwest of Price Inlet, and thus crossed the southern stock boundary, while two others were sighted directly on the boundary as well as southwest of Price Inlet (Figure 3).


Although movements between the Cape Romain salt marsh system and the North Inlet/Winyah Bay system were not observed during the current study, comparisons with historical catalogs identified nine dolphins who traveled between these systems over a longer time interval (from four months to five years). Sixteen dolphins from the present study were previously sighted by CCU researchers between 2011 and 2016, eight of which were seen in both sub-areas (Figure 4A). The maximum linear distance between re-sighting locations was 36 km. A comparison with Sloan’s Cape Romain catalog (2003-2005) also identified matches with dolphins from this study, including two who were re-sighted to the northeast in Winyah Bay or the Intracoastal Waterway adjacent to the bay, four re-sighted within the Cape Romain system, and four re-sighted to the southwest between Bulls Bay and Price Inlet (Figure 4B). Five of these ten dolphins were also matched to the historical CCU catalog, including one well-known male that has been sighted > 100 times in North Inlet/Winyah Bay since 1998. A comparison with the historical Charleston cata-log identified four matches with the current study, all of which were sighted on or southwest of Price Inlet and thus remained within the defined boundaries of the CESS (Figure 5). Three were historically sighted between Price Inlet and Charleston Harbor or along the northeastern side of the Harbor, while the remaining dolphin, a freeze-branded female, has been historically sighted throughout the Charleston Harbor and in nearshore coastal waters (> 50 sightings in the Charleston catalog). Abundance Estimates

Our survey tracks extended beyond the southern boundary for the NSCESS. Although two dolphins in the current study were observed to cross the Price Inlet stock boundary, no known CESS dolphins matched to the Charleston catalog were observed to cross the boundary. In the absence of compelling evidence to revise the stock boundary, estimates of stock abundance were calculated using the currently described stock range from Murrells Inlet to Price Inlet, and all sightings southwest of the Price Inlet boundary were eliminated from the analysis.

Capture histories of 67 marked dolphins photographed during on-effort encounters were analyzed using package Rcapture for Program R. The best fitted model was the bias corrected Chao Mth model (Δ AIC = 8.13) which includes both individual

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Figure 3: Dolphin movements during the 2016 summer surveys. Note most recaptures occurred within the same sub-area. Four dolphins were sighted either on or across the currently defined stock boundary at Price Inlet and six dolphins were only sighted southwest of Price Inlet.

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movements associated with the southern stock boundary and finding insufficient evidence for a revision, stock abundance during the warm season of 2016 was calculated using the existing stock boundaries, yielding an estimate of 453 individuals (95% CI = 265-773). In comparison with neighboring stocks, this value is greater than the summer 2014 abundance estimate of 272 (95% CI 189-390) for the Southern North Carolina Estuarine System Stock2 (SNCESS) and is comparable to the average abundance estimate for the Charleston Estuarine System Stock of 498 from the summers of 2004 through 2006 (range 370-652) (33).

Bias in abundance estimates could arise from low probability of capture or violations of model assumptions (22, 43). The average capture probability was 0.15, which may cause a moderate upward bias in the abundance estimate and associated confidence interval (22). However, significant bias is not expected because capture probabilities were estimated at 0.1 or greater for all three sampling periods and the selected model was designed to account for low capture probabilities and therefore low recapture rates, proving a conservative estimate preferable for management purposes (11, 22, 26). Despite the 7.5-week time frame for this study, the assumption of geographical closure was likely met because of the stable population utilizing estuarine waters during the warm months. Violations of demographic closure were also unlikely because

heterogeneity and temporal differences in capture probability and provides a lower bound estimate. Abundance for marked individuals was estimated at 163 individuals (95% CI = 110-282). This estimate was then divided by the proportion of marked individuals (θ = 0.36) to estimate total population abundance. Stock abundance was estimated at 453 individuals (95% CI = 265-773, CV = 0.28).

Figure 4: Dolphin movements within the NSCESS range identified by comparing the current study with historical catalogs. (A) Dolphin movements between North Inlet/Winyah Bay and Cape Romain identified from comparisons with historical CCU catalogs (2011 to 2016). Sighting locations are color-coded for each dolphin and movements are indicated by colored lines. (B) Sighting locations of dolphins from the current study matched to the Sloan catalog (2003-2005). Historical sighting coordinates were unavailable, but the Sloan survey area is indicated by an open box.

Discussion This study represents the first investigation of the

abundance and movements for the entire Northern South Carolina Estuarine System Stock (NSCESS) of bottlenose dolphins. After investigating dolphin

Figure 5: Catalog matches between the current study and previous studies in Charleston. Circles represent sightings from this study, stars represent historical sightings from the Charleston catalog, and each color represents a single dolphin.

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Re-sightings of marked individuals suggest the movements of most NSCESS dolphins traverse only a small proportion of their total stock range, with a maximum linear distance between re-sightings of 20 km and most distances less than 10 km. The maximum distance increased to 36 km when historical catalogs were used to examine patterns over a multi-year time span. These patterns revealed numerous re-sightings between the North Inlet/Winyah Bay and Cape Romain sub-areas, though the lack of historical catalogs between Cape Romain and Price Inlet prevented investigation of additional movements. The NSCESS ranging patterns contrast with the movement patterns of estuarine dolphins in North Carolina, where individual dolphins commonly use much of the stock range, even within a single season. In surveys of both the SNCESS and what would later be named the Northern North Carolina Estuarine System Stock (NNCESS), numerous individual dolphins were re-sighted at linear distances of 90 to 120 km over summer survey periods comparable in duration to the current study2 (25, 31).

Differences in stock ranging patterns likely reflect differences in habitat. The SNCESS habitat is long and thin, dominated by the linear Intra-Coastal Waterway which connects relatively small salt marsh systems and some larger river mouths and sounds. The NNCESS habitat, in contrast, is much larger and is dominated by the large open sounds of North Carolina and their associated river mouths. Finally, the NSCESS habitat from the current study includes bays and river mouths but is primarily defined by large, contiguous salt marshes with highly dendritic creek systems that provide an abundance of shoreline edge habitat. This type of estuarine habitat defines most of the coast of South Carolina and Georgia. Dolphins in this habitat can travel a substantial distance through meandering creeks without getting far from their starting point when measured by straight-line distance.

The spatiotemporal mixing of more than one stock can misrepresent the abundance estimate for the stock in question if dolphins from a neighboring coastal or estuarine stock are counted within the survey area. The timing and location of our surveys were designed to minimize and potentially eliminate possible encounters with dolphins from other stocks. Specifically, we maximized the likelihood of encountering estuarine stock dolphins and minimized potential encounters with coastal stock dolphins by running surveys during the warm season and only in estuarine waters. Previous studies in North Inlet observed a stable closed population of estuarine dolphins during the warm season (May

dolphins have low reproductive rates and low mortality rates, with the highest mortality rates among calves and neonates, which usually have not acquired distinctive marks and were not included in the ‘marked’ abundance estimate (33, 42, 44). It is unlikely that new additions to the marked population are significantly higher than natural mortalities within this two-month study. Potential misidentifications were unlikely given that a grading scale for photographic quality and dorsal fin distinctiveness was used, the dorsal fin catalog was cross-checked by at least two researchers, and identification marks such as notches and fin deformities are permanent features and should outlast the survey time frame (44). Dolphins can be misidentified if dorsal fins are severely damaged obscuring previous marks within a survey period, however this is a rare occurrence (33, 44).

Variations in capture probability were included in the model as a function of time and individual heterogeneity. It is reasonable to assume that individual dolphins will behave differently towards the research vessel. For example, begging behavior was observed on three of the 26 dolphin encounters in Cape Romain, possibly increasing the probability of capture for ‘beggar’ dolphins. In addition, time variations in capture probability allow for cyclical changes in habitat use such as those affiliated with tidal cycles. In North Inlet, dolphins utilize smaller side creeks during high tides, potentially resulting in lower encounter rates (46).

Additional bias can occur if NSCESS dolphins spend a large proportion of their time outside the target survey area (in coastal waters or estuarine waters of a neighboring stock). However, our surveys were temporally designed to reduce this bias as dolphins tend to spend most of their time in the estuary during the warm months, therefore potentially available for capture (8). If random movements outside of the target survey area occurred, closed population models are still valid, but the abundance estimate applies to the super-population and estimated capture probability is lowered, reducing the precision of abundance estimates (18). Movements into estuarine waters outside of the stock range could have occurred, as we documented two dolphins crossing the stock boundary. Nonetheless, documented movement patterns suggest that NSCESS dolphins utilize a small portion of their range, at least during the warm months, and our extended survey area did not find evidence of wide-spread movement into estuarine waters outside of the stock range. Thus, we believe that most or all of the stock was available for capture during our surveys.

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through October) with few sightings near inlets, and infrequent forays into the coastal ocean (8, 46). In contrast, estuarine dolphin encounter rates were reduced in winter with increased sightings near inlets (8). Similar seasonal patterns in abundance and/or encounter rates have been observed for estuarine dolphins in South Carolina, North Carolina2, and Georgia (6, 31-33). Results from summer and winter surveys in both estuarine and coastal waters in southern North Carolina documented an increased use of coastal waters by estuarine stock dolphins during the winter2.

Based on nearly a decade of coastal and estuarine photo-ID surveys near Charleston, SC, Speakman et al. concluded that incursions by coastal dolphins into surrounding estuaries were limited to the waters immediately adjacent to the coast and that the vast majority of these incursions occurred in Charleston Harbor rather than the smaller inlets and river mouths nearby (34). To limit encounters with coastal dolphins, our estuarine surveys did not extend beyond the front beach at inlets and river mouths and did not include any systems with broad mouths like Charleston Harbor. Coastal dolphins have been successfully distinguished from estuarine dolphins based on a combination of extensive sighting histories, occurrence patterns of the pseudo-stock barnacle Xenobalanus globicipitis on dolphin dorsal fins, and/or associations with shrimp trawlers (12, 33, 38). Associations with shrimp trawlers were not observed in this study and only one dolphin was observed with a barnacle on its dorsal fin, suggesting that coastal stock dolphins were rare or absent in our sightings.

In contrast, potential encounters with dolphins from neighboring estuarine stocks could only be identified through comparison with neighboring catalogs. The northern stock boundary is well established, with no substantial estuarine systems supporting dolphins for approximately 80 km north of North Inlet. However, Price Inlet to the south is the shared boundary with the CESS and does not represent an obvious break or change in habitat. A comparison with the Charleston catalog found no examples of CESS dolphins crossing the Price Inlet boundary, though most contributions to this catalog were almost 10 years old. In our surveys, however, two dolphins were sighted on both sides of the boundary, extending up to 6 km to the northeast and 9 km to the southwest of the line. It is unclear whether these minor incursions warrant a revision of the stock boundary. However, this 17 km displacement is among the longest movements observed by any dolphin in our surveys (Figure 3), and if one

were to suggest a stock boundary based simply on the movements observed in this study, Price Inlet would not be a candidate.

In short, the current and historical sighting data do not suggest an obvious location for the southern stock boundary and no genetic studies have been conducted to investigate the current boundary. Ideally, management-defined stocks are representative of true biological populations. In this case, we have chosen to use only sightings within the defined stock boundaries to estimate the stock abundance for management purposes, but without an unequivocal boundary, it is unclear whether this estimate applies to a true biological population. The US MMPA mandates periodic evaluations on the status of each marine mammal stock primarily as a comparison between population parameters and anthropogenic removals. Between 2009 and 2013, a single fishery mortality was attributed to a NSCESS dolphin for an interaction with the Southeast Atlantic inshore gillnet fishery, and interactions with the Atlantic blue crab trap/pot fishery have occurred in the CESS but have not been reported for the NSCESS (40). The crab pot fishery is historically the main cause of entanglement cases for stranded bottlenose dolphins in the state (9). An Unusual Mortality Event (UME) was declared in South Carolina during the winter of 2011, with one carcass recovered from the NSCESS range, but the cause of the UME remains undetermined (40). More recently, an UME caused by a Morbillivirus outbreak occurred along the Mid-Atlantic east coast between the summers of 2013 and 2015. This UME is thought to have impacted mostly coastal stock dolphins and no carcasses were assigned to the NSCESS, but stock identity of infected carcasses needs to be further investigated (21). The estimate of NSCESS dolphin abundance and movements provided by this study will enable sustainable stock management and response to these and other anthropogenic and natural threats.

Acknowledgements

This research was conducted under Marine Mammal Protection Act Permit Number 14450 issued to the National Marine Fisheries Service, Southeast Fisheries Science Center (Dr. Keith D. Mullin, PI). Funding for this project was generously provided by The Rare Species Fund. We wish to thank the following individuals: Peggy Sloan for sharing her photo-identification catalog with the Mid-Atlantic Bottlenose Dolphin Catalog (MABDC); former CCU graduate students Jamie Brusa and Jessica Conway for sharing photo-identification and sighting data; Kim Urian, who confirmed all catalog matches found via the MADBC;

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