Graduate Research Assistant - Sarah Hardee
NOAA/Padilla Bay NERR Research Assistantship Recipient
Harbor Seal Movements and Home Ranges
Sarah Hardee
M.Sc. Recipient, Western Washington University
Department of Biology
Bellingham, WA
Movements and Home Ranges of Harbor Seals (Phoca vitulina) in the Georgia Basin
Background
Harbor seals (Phoca vitulina) are a common sight in Padilla Bay. Often they are spotted at haul-outs, locations where seals leave the water to rest, molt, mate, and give birth. Although not the largest of the pinnipeds (a family which includes walruses and sea lions), harbor seals are numerous, year-round residents of the Georgia Basin, an area encompassing the Puget Sound, San Juan Islands, Canadian Gulf Islands, Strait of Juan de Fuca, and Strait of Georgia. These top predators feed on more than 20 species of fish and invertebrates. Their varied diet, abundance, and year-round residency in the region make harbor seals an important component of the local ecosystem with a potentially large effect on populations of their prey species.
Rockfish (Sebastes spp.) are not only a part of harbor seal diets; they are also fished commercially for human consumption. Long-lived but slow to reproduce, rockfish are suffering sharp population declines due to overfishing and habitat loss. Recent efforts by the Skagit County Marine Resources Committee to establish marine reserves in the eastern San Juan Islands are aimed at protecting rockfish habitat with the goal of restoring populations to healthy numbers.
Despite their potential impact on rockfish and other threatened fish species, little is known about harbor seal home ranges and movements in the Georgia Basin, and it is unknown to what extent the seals utilize the proposed MPAs while foraging. Sarah Hardee, a Master’s student at Western Washington University, completed a research project in June 2008 that focused on several aspects of seal behavior, including patterns of movement, size of home range, and how often a seal returns to the same haul-out (Fig. 1). By determining how these behaviors relate to the two types of haul-out habitats found in the Georgia Basin (muddy intertidal and rocky reef) , Sarah helped shed light on how harbor seals utilize their environment and what pressures the local seal population may place on proposed marine reserves.
Fig. 1. Sarah Hardee conducting aerial population counts of hauled-out harbor seals.
- What is the population of seals in the San Juan Islands, Padilla Bay, and surrounding bays?
- What are the movement patterns and home range sizes of individual seals?
- How often do satellite-tagged seals utilize proposed marine reserves, and what foraging pressures will the entire local seal population place on proposed reserves?
- Do seals haul out primarily in one area, or do they utilize multiple haul-out sites?
Fig. 2. Map showing the locations of captured harbor seals.
Methods
Population counts were conducted during pupping season (late July through mid-August), when the largest number of seals are hauled-out. In collaboration with the Washington Department of Fish and Wildlife, Sarah surveyed the seals from a small airplane through a combination of visual counts and digital photographs (Fig. 1). A correction factor was applied to account for seals in the water.
Satellite and time-depth recorder tags were attached to six seals captured at Padilla Bay, six seals captured at Bird Rocks, and eight seals captured in the Belle Chain Islets (Figs. 2 and 3). With these tags, a seal’s location and whether or not a seal was on land could be monitored by satellite (Fig. 4). Seals were tagged in March of 2007, and data were collected until the seals molted in September or October.
Extensive data analyses were performed to determine the tagged seals’ movement patterns, home ranges, and use of proposed marine reserves.
Fig. 3. Captured harbor seal outfitted with monitoring tag.
Conclusions
The population of harbor seals in the San Juan Islands, Padilla Bay, and surrounding bays was approximately 8,100 in 2007.
Sarah’s work showed that seals regularly travel farther than had previously been observed in this region. Several traveled to the Pacific coast of Washington and Canada and back during the course of the study, a distance of over 200 km. Seals living in Padilla Bay’s protected estuarine environment remained closer to their capture sites than the Bird Rocks and Belle Chain Islets rocky reef-dwelling seals. Padilla Bay seals had an average straight-line distance from their haul-out site of only 9 km, compared to 48 km for Bird Rocks seals and 23 km for Belle Chain seals. Home ranges also differed based on habitat type, with Padilla Bay seals having smaller home ranges than Bird Rocks or Belle Chain seals, which often had fragmented home ranges.
The tagged harbor seals placed little pressure on the proposed marine reserves during the study, with only 4.5% of transmissions from the satellite tags occurring within 3 km of the reserve boundaries. However, this research shows that harbor seals in the Georgia Basin have complex movements and home ranges, do not limit their feeding to a small area, and might place foraging pressure on protected areas in the future. Harbor seals will need to be monitored after the establishment of the marine reserves to ensure the success of rockfish recovery.
Fig. 4. Harbor seal outfitted with monitoring tag, swimming away after being released.
Biographical Information
Sarah Hardee completed her M.Sc. degree at Western Washington University in June 2008. Her advisor was Dr. Alejandro Acevedo, a professor in the Biology Department. Sarah received a Padilla Bay Research Assistantship in 2007 to help sponsor her M.Sc. research. Her thesis, which she presented at Padilla Bay in July 2008, was entitled “Movements and Home Ranges of Harbor Seals (Phoca vitulina) in the Inland Waters of the Pacific Northwest.”