Introduction & Objectives

Harbor seals (Phoca vitulina) and gray seal (Halichoerus grypus atlantica) are year-round coastal inhabitants in eastern Canada and New England, and occur seasonally in the mid-Atlantic United States (U.S.) between September and May (Hayes et al. 2022). 

Individuals of both species move to northern areas for mating and pupping in the spring and summer, and return to southerly areas in the fall and winter. In previous years, there was some debate about the southern range extent for both of these 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 have reported that harbor and gray seal distribution along the U.S. Atlantic coast appears to be expanding or shifting (den Heyer et al. 2021, DiGiovianni et al. 2011, Johnston et al. 2015, DiGiovianni et al. 2018). In Virginia, reports from local anglers, Chesapeake Bay Bridge Tunnel (CBBT) staff, and the Virginia Aquarium & Marine Science Center have indicated that seals have been using the CBBT rock armor or "islands" to haul out on for more than a decade. Additionally, annual pinniped stranding numbers have generally increased in Virginia since the early 1990s, with 2020 being a lower than average stranding year (Costidis et al. 2021). The apparent range expansion of harbor seals may be due to the rapid growth of gray seal populations in Canada and the Northeastern U.S. (den Heyer et al. 2021, Wood et al. 2022), which could be causing the displacement of harbor seals at haul-out sites (Cammen et al. 2018; Pace et al. 2019; Wood et al. 2019).

Within the last decade, harbor seals have been observed returning seasonally, from fall to spring, to haul-out locations in coastal Virginia, and gray seals are also occasionally observed during the winter, but not on a consistent basis (Ampela et al. 2023; Jones and Rees 2022). NOAA SARs now indicate the southern extent for the harbor seal population range is North Carolina. However, the geographic range for the gray seal population remains the same (Hayes et al. 2022).

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 continues to provide valuable baseline information for the future assessment of seal movement, site fidelity and abundance in the mid-Atlantic region.

Technical Approach

A series of systematic, vessel-based 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. Haul-out surveys will start in the fall (November) and end in the spring (April/May) to ensure documentation of seal arrival and departure for the season. During each survey, the number of seals hauled out and in the water will be recorded with associated environmental data (e.g. air and water temperature). The use of unmanned aircraft systems (e.g. drones) at the Eastern Shore survey area was added in 2018 to help improve count accuracy during vessel-based point counts and assess the distribution of pinnipeds in the region. Photographs of seals will be collected between counts for photo-ID for a mark-recapture study to estimate the local population abundance and to develop a local catalog.  A second approach for estimating abundance will also be attempted using seal count data from the CBBT and Eastern Shore survey areas as well as satellite telemetry data on harbor seal activity in Virginia waters (Ampela et al. 2023).

Progress & Results

As of the end of the 2023/2024 field season, a total of 145 survey days have been conducted across 10 field seasons (2014-2024) at the CBBT survey area. Seals have been consistently recorded from mid-November to April. For the Eastern Shore survey area, a total of 94 survey days have been conducted across eight field seasons (2016-2024) and seals have been recorded from early November to 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, although not on a consistent annual basis. For the CBBT, gray seal sightings were recorded for the 2014/2015 (n=1), 2015/2016 (n=2), 2020/2021 (n=1), and 2022/2023 (n=2) field seasons. For the Eastern Shore, gray seal sightings were recorded for the 2017/2018 (n=1), 2018/2019 (n=2), 2019/2020 (n=1), 2020/2021 (n=4), 2022/2023 (n=4), and 2023/2024 (n=2) field seasons; one gray seal was sighted off effort during the 2021/2022 season.

Since the start of the study in 2014, there has been a fluctuation in seal presence for the CBBT survey area, with an increasing trend in the average number of seals observed per survey day and maximum seal count for a single survey day from 2014-2018, followed by a decrease from 2018 to 2020. For the 2020/2021 season, seal presence appeared to rebound with an increase in average and maximum seal count. A slight decrease in average and maximum seal count were observed for the 2021-2023 seasons followed by an increase in these summary statistics for the 2023/2024 season. A similar fluctuation in seal presence was observed for the Eastern Shore survey area, with an increase in maximum seal count over the first two field seasons (2016-2018) and again for the 2020-2023 field seasons, followed by a slight decrease for the 2023/2024 season. The average seal count also increased from 2016-2018, but decreased to 15 seals for the 2018/2019 season. Average seal count started to increase again in the 2019/2020 season with some fluctuation over the 2020-2024 seasons. Some of the lowest total, maximum, and average seal counts for the CBBT and Eastern Shore survey areas were reported for the 2018 to 2020 seasons as well as the 2021/2022 season. Additionally, a statistically significant difference was observed between the average seal counts across the 10 field seasons (2014-2024) for the CBBT survey area. The drop in maximum and average seal count for the 2018 to 2020 seasons as well as the 2021/2022 season for the Eastern Shore survey area was not as substantial compared to the CBBT for these seasons, and the difference between average seal counts across the eight field seasons (2016 to 2024) was not statistically different. Potential trends will continue to be evaluated as more data is collected in the coming years.

Photo-ID analysis results identified 198 harbor seals and 4 gray seals from the 2015-2023 field seasons. Of the 198 individuals, 96 (48%) were observed only once and 102 (52%) were determined to be present in the study area on more than one occasion across the eight field seasons, indicating a degree of seasonal site fidelity in the lower Chesapeake Bay and coastal Virginia waters. More than half of the identified harbor seals (53%) have been sighted at only the CBBT survey area, with a smaller percentage (36%) sighted at only the Eastern Shore survey area. Twenty-one harbor seals were re-sighted at both survey areas on separate survey days within a season and across seasons. These results indicate that harbor seals make localized movements throughout the region during their seasonal occupancy and that while some seals may be utilizing a particular haul-out site within a given season, others may utilize multiple haul-out sites within a season.

A population abundance for harbor seals was estimated for the study area using mark-re-capture data and the Lincoln-Peterson model. A total of 150 individuals were estimated as the average abundance across eight seasons (2015-2023). Abundance estimates were also calculated for each field season from 2015-2023 using the mark-recapture data; estimates ranged from 81 (95% CI: 44.14–117.19) to 242 (95% CI: 91.35–392.65) individual harbor seals. A fluctuation in abundance estimates occurred across seasons. There was an overall increase from the 2015/2016 to 2018/2019 field seasons, with the exception of the 2017/2018 season, in which a decrease in abundance (N=135 individuals) was observed. Abundance decreased after the 2018/2019 season, but seemed to remain relatively stable from the 2019/2020 to 2021/2022 seasons, with a decrease in abundance for the 2022/2023 season (N=85 individuals).Regression analysis results indicate there is not a statistically significant trend in population abundance. Therefore, there is reason to believe that the population of animals utilizing the lower Chesapeake Bay and Eastern Shore, Virginia may be relatively stable.

Photo-ID data collected for the 2023/2024 field season at both the CBBT and Eastern Shore survey areas are currently being analyzed and additional survey work is planned for the 2024/2025 season.