US Navy


Marine Species Monitoring

Haul-Out Counts and Photo-Identification of Pinnipeds in Chesapeake Bay, Virginia

Introduction & Objectives

There has been some debate in recent years about the southern range extent for harbor (Phoca vitulina) and gray seal (Halichoerus grypus atlantica) stocks in the Western North Atlantic. Until 2018, NOAA Stock Assessment Reports (SARs) indicated that the gray and harbor seal populations range from New Jersey to Labrador; with scattered sightings and strandings reported as far south as North Carolina for gray seals and Florida for harbor seals (Hayes et al. 2018). Other researchers, report that harbor and gray seal distribution along the U.S. Atlantic coast appears to be expanding or shifting (DiGiovianni et al. 2018; Johnston et al. 2015), possibly due to the rapid growth of gray seal populations in Canada and Northeastern U.S. (Cammen et al. 2018; Wood et al. 2019). More recently, NOAA SARs indicate the southern extent for the harbor seal population range is now North Carolina. However, the geographic range for the gray seal population, mentioned above, remains the same (Hayes et al. 2019; NOAA NMFS-NEFSC 2019). Observations from Virginia, by Chesapeake Bay Bridge Tunnel (CBBT) staff and local anglers, indicate that seals have been using the CBBT islands to haul out on for many years, but that the number of animals appears to be increasing. Additionally, annual pinniped stranding numbers have increased in Virginia since the early 1990s (Costidis et al. 2019).

This study aims to document seal presence at select haul-out locations in the lower Chesapeake Bay and coastal waters of Virginia, which are important areas to Navy training and testing activities. Haul-out counts and photo-identification (photo-ID) methods are being utilized in order to acquire a better understanding of the seals’ seasonal occurrence, habitat use, and haul-out patterns in this area. This study will provide valuable baseline information for the future assessment of seal movement, site fidelity and relative abundance in the mid-Atlantic region.

Technical Approach

A series of systematic, counts of all seal species will be conducted at two different survey areas; 1) in the lower Chesapeake Bay along the CBBT, on the four “islands” (referred to as CBBT 1, CBBT 2, CBBT 3, and CBBT 4), and 2) on the southern tip of the Eastern Shore, which is comprised of about five main haul-out locations. Vessel-based counts will be conducted at the CBBT (in collaboration with Virginia Department of Game and Inland Fisheries and HDR, Inc.) and Eastern Shore (in collaboration with The Nature Conservancy) survey areas. The number of seals hauled out and in the water will be recorded during each count throughout the season. Photographs of seals will be collected between counts for photo-ID for a mark-recapture study to estimate the local population abundance (N), and to develop a local catalog. The use of unmanned aircraft systems (e.g. drones) at the Eastern Shore survey area was added onto the research permit in 2018 to help improve count accuracy during vessel-based point counts and assess the distribution of pinnipeds in the region.

Progress & Results

A total of 88 survey days have been conducted across 5 field seasons (2014-2019) at the CBBT survey area. Seals have been consistently recorded from mid-November to early May, with most sightings (85.8%) recorded at the CBBT 3 haul-out site. For the Eastern Shore survey area, a total of 31 survey days have been conducted across 3 field seasons (2016-2019). Seals have been recorded from early November to early April. The majority of seals observed at both survey areas were harbor seals. Gray seals have been occasionally sighted during the winter at both survey areas, but not on a consistent annual basis. For the CBBT, gray seal sightings were recorded for the 2014/2015 (n=1) and 2015/2016 (n=2) field seasons. For the Eastern Shore, gray seal sightings were recorded for the 2017/2018 (n=1) and 2018/2019 (n=2) field seasons. 

Once seals arrived, animals were recorded on a fairly consistent basis until departure for both survey areas; 69 out of 88 survey days (78.4%) for the CBBT and 26 out of 31 survey days (83.9%) for the Eastern Shore. The number of seals at both CBBT and Eastern Shore survey areas appeared to be increasing occurred through the 2017/18 season based on maximum and average survey counts, however, a drop in both max and average counts occurred for the 2018/2019 season at both locations.  The difference between the mean counts across the 5 field seasons at CBBT was statistically significant, specifically between the 2017/2018 and 2018/2019 seasons. The difference between the mean counts across the 3 field seasons at the Eastern Shore site was not statistically significant. These trends will continue to be evaluated as more data is collected over the coming years. Abundance estimates calculated from the Lincoln-Peterson model have increased from 2015/2016 (88 individuals) to 2018/2019 (221 individuals), however  regression analysis results suggest the population does not appear to be increasing or decreasing, and may be stable. With the abundance showing a fluctuation across seasons and no discernable trend, a mean abundance estimate for all 4 seasons was calculated, N=159 individuals (95% CI: 148.61-168.96). Given the CI, this estimate may be a fairly reliable representation of the number of harbor seals using both the CBBT and Eastern Shore survey areas.

Photo-ID analysis has identified 112 unique individuals through the 2018-19 field season. Of the 112 individuals, 72 were observed only once and 40 were re-sighted both within a season and across multiple seasons, with one individual being sighted across 4 different field seasons, indicating at least some degree of seasonal site fidelity in the lower Chesapeake Bay and coastal Virginia waters. The majority of identified individuals (n=71) have been sighted at only the CBBT survey area, with 34 being sighted at only the Eastern Shore survey area. However, 7 identified seals have been sighted at both survey areas on separate survey days.

More detail on the 2018/2019 field season is provided in the 2018/2019 progress report, and progress through the 2016-17 field season for the CBBT haul-out sites was presented at the 22nd Biennial Conference on the Biology of Marine Mammals in Halifax, Nova Scotia. Haul-out counts and photo-ID data collection are continuing for the 2019/2020 field season at both the CBBT and Eastern Shore survey areas.


2017 SMM Biennial Seal Poster_10-12-2017.jpg