Post-breeding season migrations of a top predator, the harbor seal (Phoca vitulina richardii), from a marine protected area in Alaska

Abstract

Marine protected areas (MPAs) are increasingly being used as a conservation tool for highly mobile marine vertebrates and the focus is typically on protecting breeding areas where individuals are aggregated seasonally. Yet movements during the non-breeding season can overlap with threats that may be equally as important to population dynamics. Thus understanding habitat use and movements of species during the non-breeding periods is critical for conservation. Glacier Bay National Park, Alaska, is one of the largest marine mammal protected areas in the world and has the only enforceable protection measures for reducing disturbance to harbor seals in the United States. Yet harbor seals have declined by up to 11.5%/year from 1992 to 2009. We used satellite-linked transmitters that were attached to 37 female harbor seals to quantify the post-breeding season migrations of seals and the amount of time that seals spent inside vs. outside of the MPA of Glacier Bay. Harbor seals traveled extensively beyond the boundaries of the MPA of Glacier Bay during the post-breeding season, encompassing an area (25,325 km(2)) significantly larger than that used by seals during the breeding season (8,125 km(2)). These movements included the longest migration yet recorded for a harbor seal (3,411 km) and extended use (up to 23 days) of pelagic areas by some seals. Although the collective utilization distribution of harbor seals during the post-breeding season was quite expansive, there was a substantial degree of individual variability in the percentage of days that seals spent in the MPA. Nevertheless, harbor seals demonstrated a high degree of inter-annual site fidelity (93%) to Glacier Bay the following breeding season. Our results highlight the importance of understanding the threats that seals may interact with outside of the boundaries of the MPA of Glacier Bay for understanding population dynamics of seals in Glacier Bay.

Figure 1. Study site in Glacier Bay National Park, Alaska, United States of America.

The map includes harbor seal haulout sites (black circles), closures associated with harbor seal haulout sites (red areas), non-motorized area closures (green areas), dates of closures for each area, and the boundary of marine protected area of Glacier Bay National Park (grey outline). Harbor seals were tagged at the glacial ice haulout site in Johns Hopkins Inlet.

Figure 2. Utilization distribution of female harbor seals (Phoca vitulina richardii) during September and October.

Boundary of the marine protected area (MPA) of Glacier Bay is shown as black line. JHI indicates tagging location in Johns Hopkins Inlet.

Figure 3. Utilization distribution of female harbor seals (Phoca vitulina richardii) during November and December.

Boundary of the marine protected area (MPA) of Glacier Bay is shown as black line. JHI indicates tagging location in Johns Hopkins Inlet.

Figure 4. Utilization distribution of female harbor seals (Phoca vitulina richardii) during January and February.

Boundary of the marine protected area (MPA) of Glacier Bay is shown as black line. JHI indicates tagging location in Johns Hopkins Inlet.

Figure 5. Utilization distribution of female harbor seals (Phoca vitulina richardii) during March and April.

Boundary of the marine protected area (MPA) of Glacier Bay is shown as black line. JHI indicates tagging location in Johns Hopkins Inlet.

Figure 6. Percentage of tagged harbor seals in Glacier Bay National Park.

The percentage of tagged harbor seals in the marine protected area (MPA) of Glacier Bay decreased in mid- to late September in 2007 (A) and in 2008 (B). The percentage of tagged harbor seals in Glacier Bay began to increase starting in late April and early-May in 2008 (A) and in mid-May in 2009 (B).

Figure 7. Utilization distribution of female harbor seals (Phoca vitulina richardii) during May and June (breeding season).

Boundary of the marine protected area (MPA) of Glacier Bay is shown as black line. JHI indicates tagging location in Johns Hopkins Inlet.

Figure 8. Percentage of days spent inside and outside of the marine protected area (MPA) of Glacier Bay National Park by harbor seals (Phoca vitulina richardii) during the post-breeding season.

There was a substantial degree of individual variability in the percentage of days that harbor seals spent in the MPA of Glacier Bay. Some harbor seals were more resident to Glacier Bay spending the majority of the post-breeding season inside the MPA whereas other seals were more non-resident spending extended periods of time outside of Glacier Bay.

Figure 9. State-space modeled track for adult female harbor seal #PV08GB21.

Seal #PV08GB21 spent >200 days in the eastern Gulf of Alaska region and exhibited a high degree of fidelity to a region approximately 95 km from shore on the continental shelf from September 2008 to February 2009. Beginning in late February, PV08GB21 transitioned to a nearshore area at the Alsek River where eulachon (Thaleichthys pacificus) aggregate for spawning. Seal #PV08GB21 was tagged in Johns Hopkins Inlet in September of 2008.