By Jamie Womble, Scott Gende (both National Park Service)
Why have harbor seals declined so precipitously in Glacier Bay?
Program has been ongoing since 1975 under several different researchers
Glacier Bay historically supported one of the largest aggregations of harbor seals in Alaska. Harbor seals are still the most numerous marine mammal in the park, but the population has declined by up to 75% since 1992. Meanwhile, most other populations in Southeast Alaska have been stable or increasing. Scientists are still struggling to understand the factors that have contributed to the precipitous decline of this important member of the marine community in Glacier Bay.
The downward trajectory of harbor seals contrasts sharply with that of another signature marine mammal in Glacier Bay. Steller sea lions in the Glacier Bay area have recently experienced the most rapid growth for the species in Alaska. It is not known what relationship, if any, there is between the contrasting trajectories of these two charismatic species, but they illustrate the highly dynamic nature of the Glacier Bay marine ecosystem.
The primary objectives of this study are to continue long-term population monitoring of harbor seals in Glacier Bay National Park to (1) provide information regarding the status, trend, and distribution of seals in Glacier Bay, (2) provide data on the spatial and temporal distribution of harbor seals which can be used to inform management decisions intended to protect seals and (3) assess the availability of glacial ice as habitat for harbor seals.
Harbor seals in Glacier Bay and adjacent Icy Strait were recently designated by the National Marine Fisheries Service as one of twelve stocks of harbor seals in Alaska based primarily on genetics, telemetry data, and population trends. This designation underscores the importance of understanding the status and trends of seals in Glacier Bay.
Harbor seals are surveyed in Glacier Bay using aerial photographic surveys at terrestrial haulout sites and at the primary glacial ice site in Johns Hopkins Inlet. Survey methods differ for the two substrate types. In Johns Hopkins Inlet a very precise method developed by the National Marine Mammal Laboratory, designed to ensure that each seal is counted once and only once, is used. The method involves conducting systematic transects and collecting images using a high-resolution belly-mounted digital camera. For each survey, approximately 1,250 non-overlapping digital images are collected. Back at the office, the images are imported into ArcGIS for counting.
Population monitoring data from Glacier Bay are shared with the National Marine Mammal Laboratory and are used for estimating Alaska-wide trends in harbor seals. Data are also used to inform the National Marine Fisheries Stock Assessments for harbor seals in Alaska, as required by the Marine Mammal Protection Act of 1972.
At terrestrial sites, the long-term trend for harbor seals remains negative after incorporating the past five years of surveys (1992-2011; -9.27 % / year). However, the five-year trend from 2007 to 2011 suggests stabilization (3.52 % / year). There is a glimmer of hope in this latest data. In Johns Hopkins Inlet, where methods transitioned from ground-based to aerial-based surveys beginning in 2007, abundance estimates are not yet available.
A suite of recent studies suggests that (1) harbor seals in Glacier Bay are not significantly stressed due to nutritional constraints, (2) the clinical health and disease status of seals within Glacier Bay is not different than that of seals from other stable or increasing populations, and (3) disturbance by vessels does not appear to be a primary factor driving the decline. A recent study showing that harbor seals from Glacier Bay travel widely during the non-breeding season highlights the importance of understanding the threats that seals may face outside of the park for understanding population dynamics of Glacier Bay seals.
Check out the Glacier Bay National Park seal webpage.