Hawaii Pelagic-Seabird Biology

Information about Hawaii's seabird species
and their environment
From Seabirds of Hawaii; Natural History and Conservation
by Craig S. Harrison

Surface water temperatures near the main Hawaiian Island average about 25 to 26 degrees centigrade during summer and two or three degrees lower in winter. All Hawaiian waters have a permanent thermocline which acts as a physical barrier to mixing between the warm surface water lens and the great masses of deep, cold North Pacific water. The thickness of the warm surface layer varies with the location and season but is usually 300 feet near the main islands.

Surface salinity in Hawaii is typical of that of subtropical waters ranging from 35.0 to 35.4 parts per thousand. Sunlight is an important factor in every marine ecosystem because photosynthesis is the ultimate source of most life. In Hawaiian waters the light penetrates to more then 300 feet. The longest day in Honolulu lasts between 13-14 hours and the shortest about 11 and as a result photosynthesis can occur all year whereas it cannot in more northern waters.

Most surface water in Hawaii are deficient in nutrients because the lens of warm surface water rarely mixes with the cooler, nutrient-rich deep water. However some areas are enriched by upwellings, eddies and fronts. On a localized scale, eddies and upwellings are common on the leeward sides of the islands and over submerged banks, where deep-water currents collide with submerged volcanic ridges.

Floating microscopic phytoplankton or algae forms the base of food production in the ocean. Productivity is somewhat higher in the summer then in the winter. The annual primary productivity of the open ocean near Hawaii may be only one - third of that of temperate coastal zone areas and one - tenth of that in open - ocean upwellings. The smallest of the life forms that makes up plankton, from protozoans to larval fishes, graze on the algae in the ocean water. These primary consumers are in turn eaten by secondary consumers, either carnivorous zooplankton or fish. The position of any species in a food web - whom it eats and who eats it - defines its trophic level. Tropical and subtropical waters are usually considered to have five trophic levels.

Most blue offshore waters in Hawaii are an impoverished biological desert where food chains tend to be long, complex and inefficient. Each transfer of energy between trophic levels in Hawaii squanders nine-tenths of the original energy, in part because predatory animals spend much more time and energy searching for food then their counterparts in more productive waters. As a result, of every 10,000 units of energy produced by Hawaiian phytoplankton , only one unit winds up in a yellowfin tuna after four energy transfers through five trophic levels. In contrast, productive areas off the coast of California with higher ecological efficiencies and shorter food chains produce four hundred times more energy to top level predators, such as large fishes, seabirds, porpoises, and humans.

An important phenomenon in the Hawaiian food chain is the daily vertical migration in the water column of planktonic creatures. Unlike phytoplankton, zooplankton is not restricted to the sunlit upper 300 feet of the ocean but can be found at all depths. Many of these creatures undertake extensive migrations, usually moving towards the surface during the night and descending hundreds of feet during the day. Migrations enable the organisms to feed in the productive euphotic zone yet hide in deep water during the day, when the risk of becoming a meal is greatest.

Only a few of the 700 species of fish and thousands of other marine organisms that live in Hawaiian waters are directly important to foraging Hawaiian seabirds. The amount of fish larvae in surface waters seems to influence seabird life cycles. Fish larvae ,especially those of inshore and reef species, are most abundant in surface waters during summer, an indication that fish in Hawaii tend to spawn during the summer months.

The nine flyingfish species in Hawaii are of special interest because they are commonly consumed by many Hawaiian seabirds. All flying fish are migratory. These migrations bring many of them to Hawaiian waters in the summer but far fewer are found in winter. All three species of mackerel scads in Hawaii are eaten by Hawaiian seabirds. These spindle-shaped fishes are resident, remaining in Hawaiian waters throughout the year.

Ommastrephid squids are a surface-dwelling family that are common and widespread in all warm oceans. These long, slender mollusks are migratory traveling thousands of miles into warming waters during summer, then retreating towards the equator in winter. These squid are a very important component of the Hawaiian marine ecosystem.

Many Hawaiian seabirds feed in association with skipjack, yellowfin or little tunas. Tunas forage during the day and apparently make prey available to birds by driving it to the surface, flying fish and flying squid leap from the water to avoid the tunas.

Only about 260 species of seabirds exist today if loons, grebes and sea ducks are eliminated. Of the 55 species of shearwaters and true petrels in the world today,6 breed in Hawaii. 2 of the 20 storm-petrel species breed in Hawaii. 2 of the 3 species of tropicbirds breed in Hawaii. 9 species of boobies or gannets exist and 3 nest in Hawaii. None of the world's 47 species of gulls breeds in Hawaii. 6 of the world's 42 species of terns and noddies breed in Hawaii. Many Hawaiian seabird species are at the northern limit of their breeding ranges. About 6 million seabirds breed in Hawaii, including nonbreeding birds, the total population is about 15 million.

Hawaiian seabird populations are dynamic – they vary with season and year. Breeding season for seabirds in temperate and cold waters is related to food supply. Far more fish and other prey are available to seabirds during spring and summer than in winter, when marine productivity is low. Another important factor for seabirds to breed in spring and summer is the additional time available for feeding in the longer summer days. However five species of Hawaiian seabirds breed during the winter; Laysan Albatross, Bonin Petrel, Sooty Storm-petrel, all of which feed at night, and the Black-footed Albatross and Black Noddy.

About 95% of all marine bird species nest in colonies. Many species use visual clues from neighbors to locate food. Each morning Sooty terns, Wedge-tailed shearwaters, and Brown noddies scatter over the ocean to forage, and when they see other birds aggregated in a feeding flock,they converge from afar to take advantage of the ephemeral food source.

Competition within the Hawaiian seabird community has resulted in partioning of food resources, and a comparison of seabird diets indicates that each species has developed its own individual food spectrum. The 22 seabirds that breed in Hawaii feed on a wide variety of shoaling fishes, squid, and crustaceans. Hawaiian seabirds probably eat several hundred species of marine organisms. Many Hawaiian seabird diets include between 20-40 families of prey species.

Hawaiian seabirds can be divided
into five feeding guilds:

1. Albatrosses
Each of the three North Pacific albatrosses feeds by sitting on the surface of the water and seizing prey, often with other albatrosses but rarely with other types of seabirds.Laysans tend to feed at night and black-foots during daylight hours.

2. Pelecaniformes
In Hawaii this group includes 3 boobies,two tropicbirds, and the great frigatebird. Boobies and tropicbirds plunge-dive to pursue underwater fish and squid to depths of several meters. Brown boobies and tropicbirds are strictly solitary feeders.Most of these birds feed in deep water, but brown boobies forage inshore.

3. Tuna Birds
This guild includes 7 species; sooty terns, wedge-tailed shearwaters, Christmas shearwaters,Newell's shearwaters, brown noddies, black noddies, and white terns. Tuna birds forage in large flocks over feeding schools of tunas, dolphinfish, porpoises, whales, and other large predators that drive smaller prey organisms to the surface. Sooty terns and shearwaters can forage farthest offshore. Black noddies and white terns usually feed inshore.

4. Nocturnal Petrels
Bonin petrels, dark-rumped petrels,Bulwer's petrels, Harcourt's storm-petrels, and sooty storm-petrels seem to feed extensively at night. Nocturnal petrels feed offshore, usually alone but occasionally in the company of other seabirds.

5. Neuston-feeding Terns
The feeding habits of gray-backed terns and blue-gray noddies are unlike those of any other Hawaiian seabirds. Their diets are somewhat similar and consist of small prey obtained close to the islands.They feed by plunging.

Hawaiian seabirds consume over 400,000 metric tons of fish, squid, crustacea, and other food sources each year. About half of the 22 breeding Hawaiian seabirds are year-round residents. The remainder are migrants, wanderers, or nomads, and leave the waters adjacent to the colonies each breeding season after their young have fledged. Albatross, shearwaters, petrels, and storm petrels tend to depart from Hawaiian waters during their nonbreeding months.

Wedgetails begin to reenter the North Pacific in march and are common within 50 miles of the Hawaiian Islands from April to November. Christmas and Newell's shearwaters arrive in Hawaiian waters in march, attain peak populations in May, and leave the waters adjacent to their breeding grounds in fall. Red-tailed tropicbirds also return in March and depart in October.Although some white terns remain year round on Oahu, their numbers substantially decline in winter. Hawaiian boobies, frigatebirds, blue-gray noddies and black noddies tend to be sedentary , roosting on the colonies each night all year.

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