Glacier Bay National Park & PreserveСтраница 18 из 97
How Boats and Cruise Ships Disturb Kittlitz's Murrelets in Glacier Bay
Glacier Bay National Park and Preserve holds a large share of the world's Kittlitz's murrelets during their summer breeding season, and the tidewater glaciers where the birds like to feed are the same ones that draw cruise ships, tour boats and kayakers. Alison M. Agness reports in Alaska Park Science on a study of what those vessels do to the birds: they push them out of an area for a while and cost them energy, and they matter most for birds that are raising chicks.
A Kittlitz's murrelet holding a fish, a behavior that shows it is rearing a chick (NPS photo).
A rare seabird in decline
The Kittlitz's murrelet is a rare seabird that spends most of its time at sea. It is a candidate for listing under the Endangered Species Act because of dramatic declines documented over three decades across its range, including Southeast Alaska. Possible causes include oil pollution, fisheries bycatch, food limits and climate change. The article says vessel disturbance is not a likely cause of the decline but adds to the threats the birds face. The birds favor the marine waters near tidewater glaciers and the outflow of glacial streams.
The Park Service regulates vessel traffic in Glacier Bay to protect sensitive wildlife while providing for visitor access and a wilderness experience. Daily vessel quotas in summer and operating rules such as speed and area restrictions apply, and the park's vessel management plan allows both to be changed. The research aimed to learn about a little-known species and its dealings with vessels, cruise ships included, in order to inform vessel management in the park and elsewhere in Alaska.
How the study worked
A small field crew watched murrelets at seven sites in Glacier Bay, through daylight hours at regular intervals, with and without vessels present. When a vessel crossed a site, observers recorded its size and speed, how nearby birds responded and the distance between vessel and bird, along with tides, currents, weather and time of day. The behaviors they logged were loafing, diving, flying, holding a fish and flying while holding a fish. A bird holding a fish is feeding a chick, so observers counted those birds as breeding; birds not holding a fish were labeled non-breeders, although their breeding status cannot be known. Diving while holding a fish, which is not typical for the species, showed up in response to vessels.
Statistics tested whether effects were immediate, short term (after 30 minutes) or long term (over a day). Modeling then asked whether a bird's flight from a vessel could cost enough energy to affect its fitness, with daily energy costs rising from more than 0% to as much as 50%, and whether the costs would be chronic or rare.
A cruise ship (left), kayak (middle) and tour boat (right) in the West Arm of Glacier Bay, amid prime Kittlitz's murrelet habitat (NPS photo).
What vessels did to the birds
| Measure | Finding |
|---|---|
| Numbers | Thirty minutes after a vessel event, murrelet density was down by an average of 40%. Over a day, density depended more on environmental and biological variables than on vessels, and it recovered; it was even higher on days with more vessel traffic, for reasons that remain unclear |
| Flight | None of the birds were flying with no vessel present, and 30% flew when a vessel was immediately near |
| Diving | Birds dove three times more on days with vessel activity, although the overall dive response during vessel events did not change significantly, except for breeding birds |
| Breeding birds | Most likely to dive in response to vessels, which fish-holding birds do not normally do and which was not seen without vessels. They also flew away with their fish, most often when slow vessels (under 10 miles per hour) approached from far away, 0.25 to more than 0.5 miles |
| Non-breeding birds | Most likely to fly from large vessels, cruise ships and tour boats, whatever their speed or distance |
| Extra energy cost | Non-breeding birds: under 10% extra on 86% of days and between 10% and 30% extra on 2% of days. Breeding birds: under 10% extra on 26% of days |
What it means for the birds and the park
The study showed that murrelets are displaced by vessels, cruise ships included, but only briefly: density rebounded within a day, so displacement does not mean a lasting loss of preferred habitat in the park. The author sees breeding birds as highly sensitive, even to slow boats far away. Raising a chick is already energetically costly, so even an extra cost under 10% may affect a breeding bird's fitness, and a dive can cost a held fish: a dropped or eaten fish means a lost chick meal and extra effort to catch another. Non-breeding birds are less sensitive and likely have more room in their energy budgets, and can probably cope with an extra cost under 10% on a given day. Whether they can cope with even small costs almost every day is still questionable.
Based on this, the author suggests the park could consider:
- Area restrictions. Less vessel traffic during the chick-rearing season, from about June 21 to July 15 in Glacier Bay, especially at known hot spots for the birds.
- Speed limits. These might reduce dive responses in those areas but would not prevent flight responses, and both carry possible fitness costs for breeding birds.
- Route changes. Looking at standard cruise ship routes, and defining tour boat routes, to lessen encounters with murrelets, since non-breeding birds' flights from large vessels appear to occur chronically.
- More research. Surveys of murrelets from cruise ships along their usual routes, to judge whether route changes are needed, and a study of tagged birds, because the recorded flight durations are minimums: observers stopped timing when a bird flew out of sight of the land-based viewing stations.
A large cruise ship in the upper fjords of Glacier Bay National Park and Preserve (NPS photo).
Sources
- Alison M. Agness, Vessels disturb Kittlitz's murrelets in Glacier Bay National Park and Preserve, Alaska Park Science, National Park Service; rewritten in hubnx's own words.