Introduction & Objectives

Bottlenose dolphins (Tursiops truncatus) are common in Chesapeake Bay and off the Virginia coastline, which is an area also heavily utilized by the U.S. Navy. There are known to be significant seasonal fluctuations in bottlenose dolphin distribution and numbers in this area, with peak abundance occurring in late summer/early autumn when water temperatures peak (Barco et al. 1999). Although previous work has investigated metrics to estimate bottlenose dolphin abundance in this region (e.g., Baker 2000, Barco et al. 1999, Blaylock 1988), the actual abundance estimated in the mid-Atlantic region is not thoroughly understood.

Potential exists for interaction with dolphins during pier construction, ship movements, and Naval training exercises. A more complete assessment of the seasonal occurrence of bottlenose dolphins in the area (including calculated densities) will allow for more informed decisions regarding U.S. Navy activities in these areas, while baseline occurrence information can be used to minimize potential affects on the marine mammals utilizing the area.

A monitoring program was initiated in August 2012 in order to provide quantitative data and information on the seasonal occurrence, distribution and density of marine mammals in the coastal waters around Naval Station Norfolk (NSN), Joint Expeditionary Base Little Creek (JEB-LC), Joint Expeditionary Base Fort Story (JEB-FS) and the Virginia Beach waterfront, including the Virginia Capes (VACAPES) W-50 Mine Neutralization Exercise (MINEX) training area. Effort was dedicated to working with local researchers and employing proven marine mammal monitoring and research techniques to address the following three goals:

1. Conduct monthly systematic vessel line-transect surveys to determine distribution of marine mammals in the vicinity of NSN, JEB-LC, JEB-FS, and the W-50 MINEX area.
2. Conduct monthly photo-ID vessel surveys during summer months to determine the site fidelity of marine mammals utilizing these areas.
3. Monitor for dolphin echolocation clicks by deploying and retrieving four C-POD recording devices in areas specified by NAVFAC Atlantic.

Technical Approach

Standard distance sampling techniques are employed to generate bottlenose dolphin density estimates for two study areas:

INSHORE/COASTAL – a 310.4 km2 area covering a strip extending from shore out to 3.7km (2 NM). The area includes the Chesapeake Bay waters near NSN, past JEB-LC and JEB-FS, and extends down the Atlantic coast towards the Virginia/North Carolina border.  

OFFSHORE/MINEX – a 909.6 km2 area covering Atlantic waters 3.7km (2NM) to 33.3km (18NM) from shore. The box includes the entire VACAPES MINEX W-50 training area.

These vessel line transect surveys are completed monthly to allow seasonal comparisons. Monthly small-vessel photo-identification surveys are also conducted to assess the site fidelity and movement patterns of individual dolphins utilizing the Chesapeake Bay waters near NSN, JEB-LC, and JEB-Fort Story. During photo-identification surveys, the emphasis is on spending time with dolphin groups to obtain as many dorsal fin photographs as possible using a Canon 7D camera with 100-400mm lens. C-POD dolphin click detectors are also deployed in the Chesapeake Bay near NSN and JEB-LC to allow a continuous representation of dolphin occurrence in those areas to complement the visual surveys.

Progress & Results

A combination of visual line-transect survey, photo-identification (photo-ID), and automated acoustic monitoring methods was used to gather important baseline information on the occurrence, distribution, and density of marine mammals near Naval Station Norfolk (NSN) and adjacent areas. The study area was designed to cover areas where United States Navy activity is substantial, including Chesapeake Bay waters near NSN and Joint Expeditionary Base Little Creek-Fort Story, as well as a Mine Exercise (MINEX) Area (W-50) in the Atlantic Ocean off the coast of Virginia Beach, Virginia. Sixty-one line-transect surveys were completed in two zones (INSHORE and MINEX) between August 2012 and August 2015, with 6,550 kilometers (km) and 349.6 hours completed on-effort. The majority of sightings were of bottlenose dolphins (Tursiops truncatus), although humpback whales (Megaptera novaeangliae), harbor porpoises (Phocoena phocoena), and short-beaked common dolphins (Delphinus delphis) were also sighted in the study area on occasion. In addition, loggerhead sea turtles (Caretta caretta), leatherback sea turtles (Dermochelys coriacea), and a Kemp’s ridley sea turtle (Lepidochelys kempii) were sighted during surveys. Conventional line-transect analysis of bottlenose dolphin sightings showed both spatial and seasonal variation in density and abundance, with greatest density in the INSHORE zone during fall months. Densities in the INSHORE zone were calculated as 3.88 individuals per square kilometer (km2) (abundance[N]=1,203) in fall, 0.63 individuals per km2 (N=195) in winter, 1.00 individuals per km2 (N=311) in spring, and 3.55 individuals per km2 (N=1,101) in summer. Densities in the MINEX zone were calculated as 2.14 individuals per km2 (N=1,277) in fall, 0.06 individuals per km2 (N=37) in winter, 1.53 individuals per km2 (N=913) in spring, and 1.39 individuals per km2 (N=829) in summer.

Twenty-seven photo-ID surveys were completed, and a photo-ID catalog was created using photos taken during both dedicated photo-ID and line-transect surveys through May 2014; it contains 878 identified individuals to date. Subsequent photos will continue to be added and analyzed. One hundred ten individuals were re-sighted; however, most re-sightings were less than 4 months and 30 km apart. Additional survey effort and further analysis will be required before any clear movement patterns can be determined.

C-POD acoustic data-loggers were initially deployed at four sites throughout the study area to cover areas of high United States Navy activity. Bottlenose dolphins were detected in each deployment location during all deployments from August 2012 to December 2015. Though deployments did not provide consistent coverage in all seasons for all sites due to loss of gear, results from two deployment sites nearest to NSN showed a greater level of occurrence during fall months, and a diel pattern of occurrence with increased detections during nighttime hours for three deployment sites.