Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS

puffin home

Harriman Expedition Retraced

Home

expedition log
Expedition
Log

back

""

Vivian M. Mendenhall

Seabirds in the Marine Environment

More than 100 million seabirds occupy the Bering Sea and Gulf of Alaska each summer. They are an important component of the marine ecosystem; they also provide great pleasure for lovers of nature, and they are a vital component of Native subsistence traditions. I reviewed the seabird species of the Bering Sea and Gulf of Alaska in my first lecture; here I will discuss their ecology.

Like us, scholars on the George W. Elder were impressed by the beauty, variety, and abundance of Alaskan seabirds 102 years ago. Yet at that time it never occurred to anyone to estimate the numbers of birds, or even that they could be estimated; and the concept of ecological relationships was in its infancy. Scientists studied birds in 1899 by shooting them, measuring and painting them, and filing them away in museum drawers.

We now know vastly more about seabirds and their environment -- although we still don't know enough. We have estimates of bird numbers in breeding colonies and at sea, population trends, how far they fly and how deep they dive, how many young birds they raise each year, and what they eat. We are helped by tools that would have seemed like science fiction to the Elder scholars: computers, miniature radios that allow us to follow birds via satellite, recorders of diving depth, measurements of metabolism. And we are sobered now by the drastic ways in which humans can alter the environment, which a few on the Elder were just beginning to glimpse. We also know that the sea undergoes natural changes in temperature and currents. However, we still understand too little how marine organisms, including seabirds, and their environment are interrelated (read more about the oceanography of the Bering Sea as described by Vera Alexander).

Seabirds depend on the marine environment. Although they nest on land, they spend almost their whole lives at sea. They can fly and swim for hours or days, protected from the cold by thick down under water-repellant feathers. Seabirds obtain almost all their food from the sea -- primarily small fish and squid (less than 6 inches long) for larger birds, tiny shrimp-like zooplankton for small ones. Some species feed on the ocean surface, others by diving; some feed near shore (within 5 kilometers), others far offshore (30 kilometers or more). Feeding characteristics of seabirds are summarized in Table1 below.


Pigeon Guillemot

Pigeon Guillemot bringing fish to nest. (Photo by D.L. Hayes, U.S. Fish & Wildlife Service).
Click image for a larger view.

Most seabirds prefer prey that are concentrated in dense schools (which are easiest to find), and that are oily (because they are energy-rich and repay the effort of catching them). This means that they depend on only a few species of Alaskan fish, such as capelin (Mallotus villosus), sand lance (Ammodytes hexaptera), and young herring (Clupea harengus). If energy-rich fish are not available, birds will increase their intake of low-fat fish such as walleye pollock (Theragra chalcogramma). A few birds spice their diet with non-fish foods: gulls may take other species' young, jaegers take mice and sometimes other birds' food, parakeet auklets eat jellyfish. A few species eat garbage if it's available, or try to grab the bait from fishermen's hooks. Birds that eat zooplankton (see Table 1) also depend on certain species.

Feeding Characteristics of Seabirds
Principle
Diet
Foraging
Method
Maximum
Distance to Feed
Seabird
Groups

Fish, squid

Grab from surface

Near shore

Terns

Offshore

Gulls
Kittiwakes
Jaegers
Fulmar
Albatrosses

Dive and chase

Near shore

Cormorants
Guillemots

Offshore

Murres
Murrelets
Puffins
Sooty Shearwaters

Zooplankton

Grab from surface

Near shore

(None)

Offshore

Storm-petrels

Dive and chase

Near shore

(None)

Offshore

Auklets
Short-tailed Shearwaters

 
Birds do not search for food at random -- they look for places where they can find a lot of it predictably. They flock to fish schools that may form in bays during summer. Currents may cause prey to converge near headlands or between islands. Food also is found at upwellings, where water is forced from the nutrient-rich ocean floor to the surface at undersea ridges, the shelf edge, and the 50-meter depth on the Bering Sea shelf. Various seabird species seek specific feeding conditions: auklets congregate between islands in the Aleutians, albatrosses, kittiwakes and murres favor the shelf edge, murres and shearwaters at forage at upwellings on the shelf. The Clipper Odyssey passed large groups of birds feeding in Unimak Pass, south of Bogoslof Island, around Hall Island, and in bays.


Feeding flock

Feeding flock of kittiwakes and murres. (Photo by B. Fadely, U.S. Fish & Wildlife Service).
Click image for a larger view.

When seabirds nest in spring, they must be close to their marine feeding grounds and protected from predators such as foxes. These criteria are met in dense colonies on cliffs and offshore islands. (I mentioned each species' nesting behavior in my first lecture). Birds arrive at colonies in April through June and usually use the same site for years. Reproductive rates of seabirds are low: many species, including fulmars, murres, puffins, and auklets, lay only one egg each year; others may lay two or three eggs. Eggs hatch in about a month, and chicks are big enough to leave the colony in three to six weeks (depending on species). In an average year parents may succeed in raising a full-grown young bird from 1/3 to 1/2 of their eggs. However, in some summers, most birds in a colony lose their chicks. Reasons for failure can include storms, problems caused by man, or lack of food.

Many people (conservationists, biologists, and Natives) are concerned about declines in Alaskan seabirds. The U.S. Fish and Wildlife Service has been monitoring these populations since 1976. Kittiwakes, murres, and other species did decline in many places between about 1976 and 1982, at the time of a "regime shift" when ocean temperatures increased. However, populations at most colonies are now stable; the principal exceptions are the Pribilof Islands, Prince William Sound, and a few smaller colonies. Most declines have occurred because birds could not find enough suitable food, particularly energy-rich small fish. Birds need these foods to feed their young adequately, and some may need them to survive the winter. The reasons for changes in fish populations are complex and not fully understood. Warmer water can cause fish to avoid some areas or to decline in numbers. Alaska's large commercial fisheries have been blamed for reductions in prey of birds. However, fisheries do not catch the fish that most birds eat (capelin, sand lance, and juvenile pollock), so they probably have had no direct effect on prey numbers. Fisheries may have had indirect ecological effects on populations of small fish; however, we cannot determine this until we know much more about the marine ecosystem.

Seabirds are vulnerable to other man-caused problems. Oil spills have killed birds, notably the Exxon Valdez spill in 1989. Most species have recovered from that spill, but a few have not because of a combination of food scarcity and residual oil contamination. Smaller oil spills threaten seabirds somewhere in Alaska every few years, most recently fuel from a fishing vessel that sank in Prince William Sound on August 4, 2001. Commercial fishing gear (trawl, long line, and gill net) catches seabirds unintentionally; federal agencies monitor this "bycatch" and now require fishermen to use special gear to reduce the numbers of birds caught in longlines. Logging can destroy nesting sites for the marbled murrelet, which breeds in old-growth forests. Seabirds can lose their eggs or chicks if they are frightened from the colony by people, boats, or airplanes approaching too close.

Perhaps the worst modern threat to seabirds is the introduction of rats to nesting islands. Natural predators are not a serious problem, but seabirds have no defense against new ones. Rats have gotten onto some islands from ships and often cannot be removed. Their introduction to seabird islands would be a disaster for the birds. Rats are good climbers and prey voraciously on eggs and chicks. Communities and the U.S. Fish and Wildlife Service are working hard to kill rats that might spread onto shore in harbors and from shipwrecks. The only safe solution, however, is for every vessel to get rid of its own rats.

There is an urgent need for more research on seabirds and their environment. We especially need some data on where seabirds are during winter, what they eat then, and the ecological relationships that affect their prey. Better information would enable us to improve the chances of healthy seabird populations for the next century.


(top)

""

 

For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu

Home | 2001 Expedition | 1899 Expedition | Maps | Log | Educators and Students | Film | Century of Change | After Expedition | About This Site