Final Report Issued on Harbor Seal Disturbance in Disenchantment Bay
NMML's Polar Ecosystems Program released a final report on the first year (2002) of research on disturbance of harbor seals by cruise ships in Disenchantment Bay near Yakutat, Alaska. The report summarizes the findings from each of the three spatiotemporal scales at which potential interactions were examined between ice-hauling harbor seals and cruise ships that visit the bay to view Hubbard Glacier. The impetus for the study was a concern expressed by the local Tlingit Tribe that cruise ship visits, which have increased dramatically in frequency over the past two decades, have caused the seal population to decline, perhaps through increased pup mortality or emigration.
The program's study's main findings were:
Seals resting on ice reacted to cruise ships by entering the water with increasing frequency as ships approached within 500 m;
More than 75%-90% of the seals approached by cruise ships within 100 yards (91 m) abandoned the ice on which they are hauled out;
Seals detected when ships approached directly, entering the water at 3-4 times the frequency than when they were passed abeam of the ship;
The relative abundance of harbor seals (and mother-pup pairs) scattered throughout the bay had no apparent relationship with the frequency, duration, or locations of ship visits;
There were pronounced seasonal trends in abundance that did not appear to be related to the onset or incidence of cruise ship; shifts in abundance likely occurred in response to other environmental factors such as prey availability;
The spatial distribution of seals was not related to the frequency or locations of ships' visits, but increased durations of stay at the ships' closest approach to seal habitat were related to tighter distributions of seals; and
The two glacial study sites, with and without cruise ship traffic (Disenchantment and nearby Icy Bay, respectively), exhibited different seasonal trends suggesting that comparable numbers of seals use the two sites during pupping but that as few as one-third of the number of seals use Disenchantment Bay during molting season.
The long-term, population-wide implications of frequent disturbance of these harbor seals cannot yet be determined. Seals that enter the water when disturbed at close range, and those that may redistribute into denser aggregations in response to more distant approach by vessels, will spend more time submerged than undisturbed seals. The energetic cost to remaining in the water can be significant, particularly in ice-filled fjords where water temperatures are just above freezing year-round.
Additional information is needed about possible shifts in haul-out behavior and movement of individual seals in response to vessel disturbance. Such data will help to clarify whether disturbed seals, especially juveniles and pups, are likely to incur an energy deficit sufficient to reduce survival below the rates required to maintain the population. The report can be viewed online at: http://www.afsc.noaa.gov/Publications/ProcRpt/PR%202006-02.pdf
