Only 9 of 156 avian families are specialized as seabirds. These birds are involved in marine energy cycles during all aspects of their lives except for the 10% of time
they spend in some nesting activities. As marine organisms their occurrence and distribution are directly affected by properties of their oceanic habitat, such as water temperature, salinity, and turbidity. In their trophic relationships, almost all are secondary or tertiary carnivores. As
a group within specific ecosystems, estimates of their feeding rates range between 20 and 35% of annual prey production. Their usual prey are abundant, schooling organisms such as euphausiids and squid (invertebrates) and clupeids, engraulids, and exoccetids (fish). Their high rates of feeding and metabolism, and the large amounts of nutrients they return to the marine environment, indicate
that seabirds are probably important components in pelagic ecosystems. As such they have been strongly affected by human fisheries; for example, decline in the size of Peruvian anchovy and South African sardine populations have led to tremendous reductions in associated bird numbers. Evidence indicates that seabirds could provide an understanding of fish stock dynamics prior to over exploitation by man.
WHAT ARE SEABIRDS?
Avian taxonomists currently recognize about 156 existing families of birds, and only 9 of these are rather
specialized as seabirds: the Speniscida (penquins), Diomedeida (albatrosses), Procellariidae (petrels), Hydrobatida (storm-petrels), Pelecanoidida (diving petrels),
Phathontida (tropicbirds), Sulida (Boobies), Fregatida
(frigatebirds), Alcida (auks), and a few species (terns) of
Larida (gulls and terns). Members of these families
share the following characteristics: they derive all their
food from the sea, they void virtually all their feces into
the sea, and when individuals die they do so at sea. They
are full-time participants of marine energy cycles. Because of these characteristics, and in spite of the fact that
most birds can travel above the sea’s surface instead of
being confined to the water (as are fish or marine mammals) or that they have to spend some time on land to
raise young, these birds should be recognized as true
marine organisms. The time that they do spend away
from the sea is in fact minimal. Using the Adelie Penguin
(Pygoscelis adeliae) as an example, and the data in
Ainley (1 978) and Ainley and DeMaster (in press), it can be
calculated that a typical seabird spends about 85% of its
time annually at sea during its breeding years; over its
entire lifetime, it spends 90% of its time at sea. This
assumes 6 years of breeding and a life span of 10 years.
All of the time on land is devoted to breeding activities,
mainly the incubation of eggs, and during these activities, seabirds rely entirely on fat reserves built up at sea.
Some species that live much longer than Adelie Penguins and that breed every other year, for instance some
albatrosses, probably spend even less time on land over
the duration of their lives.
Oftentimes avian species that undertake even a small
part of their life cycle at sea are also considered to be
“seabirds.” Included are species from 15 other families,
principally the Gaviidae (loons), Podicipedidae (grebes),
Pelecanidae (pelicans), Phalacrocoracidae (cormorants),
Anatida: (ducks and geese), Scolopacidae (shorebirds),
Stercorariidae (skuas), Rynchopidae (skimmers), and the
majority of the Laridae. It is from these groups that the
general but rather misleading conception of a seabird is
derived, the one shared by most people. The “sea gull” is
the typical example. Such birds, like man, are based on
land and from there undertake trips to sea or to terrestrial
and freshwater habitats for food. They are involved only
part time in marine energy cycles; and because they
spend a third or more of their time on land, they remove
energy from marine ecosystems that is not returned.
THE MARINE DISTRIBUTION OF SEABIRDS
Much has been written about the distribution of seabirds at sea, but the factors that affect occurrence are not
well understood. Murphy (1936) was among the earliest
writers to point out that around South America (since
proved to be true elsewhere) some species were confined
to the “blue” oceanic waters offshore whereas others
occurred in more turbid coastal waters. From there,
through the work of many authors (for example WynneEdwards 1935; Jehl 1973) the idea has arisen that seabirds occur in concentric zones spaced outward from
continents and islands. This view explains the occurrence of some species, especially the coastal ones, most
of which are only part-time marine organisms, but it is
biased towaras the land orientation of man and the sea gull
type of seabird. It is further biased to the breeding season
(10% of a seabird’s life) when birds must return repeatedly to their nests.
We are beginning to understand that the distribution of
seabirds is affected by some of the same oceanographic
factors that affect the distributions of what are thought of
as typical marine organisms, such as fish. One such factor
is water temperature. For instance, when the California
Current weakens, as it periodically does (Sette and Isaacs
1960), California waters become warmer and species of
warmwater birds move farther north than they normally
occur in the eastern North Pacific (Ainley and Lewis
1974; Ainley 1976). Since the California Current moves
parallel to the coast and displaces warmer waters westward, warmwater bird species also move closer to shore
in such circumstances. This concept that birds live where
their preferred water occurs is complicated by their abilities to fly over, but still technically occur in, unsuitable
waters. For instance, Leach’s Storm-Petrels (Oceanodroma leucorhoa) prefer warm oceanic waters, but because the only suitable West Coast breeding sites in the
Pacific are coastal islands they must cross cold waters of
the California Current as they fly between feeding and
breeding areas (Ainley et al. 1975; Wiens and Scott
1975; Ainley 1976).
It is rather amazing, in light of the present land-oriented
zonal conception of seabird distribution, that on two
cruises from North America to the New Zealand sector of
Antarctica, during which seabirds and sea temperatures
were monitored continuously, with a temperature range
of 28” to O’C, 1-2°C changes brought about a consistent
turnover of 30-70% (E about 45%) of bird species (Ainley and co-workers, work in progress). Some species were
very precisely related to certain temperature ranges, i.e.
stenothermal, whereas others occurred over larger ranges,
i.e. eurythermal. This same pattern is evident in other
groups of marine organisms.
Brown et al. (1975) recently tried to correlate seabird
occurrence in Chilean fjords with not just temperature but
also salinity profiles. Pocklington (1979) attempted the
same for seabirds in the Indian Ocean. They found several good correlations. It is rather easy to understand how
seabirds might sense temperature changes, but it is not
easy to visualize how they might sense changes in salinity, even though they do drink seawater. It is just that, as
far as we know, they do not fly about continuously sampling salinity as they would be able to “monitor” temperature. Brown et al. and Pocklington tried to explain the
correlation as an indirect one involving the temperature/
salinity profiles of the preferred prey of different bird species. The opportunistic and unspecialized feeding habits
of most seabirds, as reviewed below, would also argue
against this for all but exceptional species.
Other physical oceanographic factors can also affect
seabird occurrence. For instance, some species that employ certain methods of food capture live only where
conditions favor those methods. The prime example is
given by birds that plunge for food (Ashmole 197 1 ), principally the boobies and tropicbirds. Spotting prey as deep
as 10 m or more below the surface while flying 15 m or
more above it, and thereafter using only momentum from
a “fall” to reach the prey, requires water of high clarity.
For this reason deep plungers occur only in tropical/sub
tropical waters where low phytoplankton standing stocks
(compared to those in cooler waters) result in very clear
water (Ainley 1977). Supporting this is the fact that deepplunging species occur most consistently off California
during the later summer and fall when the annual marine
cycle is in its oceanic period (Ainley 1976). At that time
water temperatures reach the subtropical range and phytoplankton standing stocks are lowest for the year (Bolin
and Abbott 1963).
T RO PH I C R E LATlO NS H I PS
Marine biologists, in their discussions of food webs,
rarely make specific mention of marine birds, but the
latter are part of what they refer to as “primary,” “se5ondary,” and “tertiary carnivores” or, in other words, the
“third trophic level” (see Steele 1974), or “other carnivores” (Cushing 1975), or “nekton” (Sverdrup et al.
1942). In their review of trophic relations among marine
birds of five oceanographic domains in the North Pacific,
Ainley and Sanger (1979) found that 77% of seabird
species were predators at the secondary and tertiary
carnivore levels. Most of the remainder (21%) were
scavengers, which still put them in the third trophic level.
Only 2%, in that they feed principally on other species of
seabirds, were in the fourth trophic level. Seabirds thus by
and large occupy the same position in marine food webs
as do the larger fish, mammals, and man.
The crux of the matter is how important are marine
birds as predators relative to other occupants of the third
trophic level in marine food webs. Quite a bit is known
about the species of prey eaten by seabirds (e.g. review by
Ainley and Sanger 1979), but little is known about their
food consumption rates to compare with other upper
trophic level predators. Some estimates though have been
attempted. Wiens and Scott (1 975), by computer simulation, estimated that Common Murres (Uria aalge) consumed 11% of pelagic fishes produced annually in Oregon’s neritic zone. They also estimated that four seabird
species off Oregon alone consumed annually about four
times the tonnage of anchovies (Engraulis mordax)
caught commercially each year in the northern permit area
(Point Conception to Oregon) during the years 1966 to
197 1. They also figured that 22% of the annual pelagic
fish production off Oregon was eaten by seabirds. Furness (1 978), using a different computer simulation, estimated that within 45 km of breeding colonies in the Shetland Islands, seabirds consumed between 20 and 35% of
annual food-fish production. It has been calculated that off
Peru during the height of the commercial anchovy (Engraulis ringens) harvest in the late 1960’s, birds consumed 2.5 X IO6 metric tons of the fish or as much as a
quarter of what was harvested commercially (Idyll 1973).
That commercial harvest, of course,’was by far the largest in the world. Laws (1977) estimated that in the Antarctic pelagic ecosystems birds and seals equal each other
in biomass, an amount for each that is about half that of
whales. Prevost (1 976) figured that all three groups consumed about equal amounts of euphausiids, about 30-40
X 1 O6 tons each. Both authors agreed, however, that more
information was needed. More data are also needed elsewhere, but with seabird food consumption estimates of
such high magnitudes, it is surprising that fishery and
marine biologists, and ornithologists, have not paid more
attention to the seemingly significant impact that marine
birds may have in pelagic ecosystems.
High rates of food consumption and very high metabolic rates in turn mean that seabirds, through production
of excrement, may also play a significant role in the recycling of nutrients and energy in pelagic ecosystems.
This, as pointed out by Wiens and Scott (1 975), may be
especially true in areas where upwelling is not strong but
where some species of seabirds are abundant. There is, in
fact, compared to knowledge on food consumption, even
less known about the role seabirds play in nutrient recycling. Sanger (1972) estimated that seabirds in the
Central Subarctic Domain (see Dodimead et al. 1963)
consumed 278 X lo3 tons of food and voided up to 74 X
lo3 tons of feces per year. Wiens and Scott (1975) estimated that offOregon four seabird species, with numbers
fluctuating seasonally from about 1.2 X lo5 to 4.4 X lo6
birds, consumed about 62,500 metric tons of fish, or 7.56
X 1O’O kcal of food, and returned 2.32 X 1O’O kcal to the
system each year in their feces. They were not able to
equate kcal of guano to nutrients. On an artificial platform in South Africa, less than 240,000 seabirds produced, at a minimum, an average (1941-1965) 777
metric tons of guano per year, the composition of which
included 16% nitrates, 9% phosphates, and 4% potash
(Rand 1963; Berry 1975).
In the types of prey they feed on, most seabirds do not
appear to specialize. In the review by Ainley and Sanger
(1979), it was evident that certain prey over and over
again predominated in the diets of different marine birds
of the eastern North Pacific. These prey included species
of Euphausia, Loligo, Clupea, Engraulis, and Sebastes.
As specific examples, 43% of prey eaten by four Oregon
seabirds (Wiens and Scott 1975) and 80 to 95% of prey
eaten by three Peruvian “guano birds” was Engradis
(Idyll 1973); 23% of prey eaten by seven seabirds in the
central tropical Pacific was exoccetids (Ashmole and
Ashmole 1967); somewhat more than 50% and often
more than 75% of prey eaten by six species nesting at the
Farallon Islands was Sebastes (Ainley unpublished); 26
to 85% of prey eaten by nine species in the Fame Islands
was Ammodytes (Pearson 1968); 19 to 85% of prey
eaten by three South African “guano birds” was Sardinops (Crawford and Shelton 1978); and 50 to 90% of preyeaten by most Antarctic penquins and petrels was Euphausia (Emison 1968; Mougin 1975). There is thus
great overlap in what they eat, and it seems that whatever
prey species is most readily available predominates in
seabird diets. “Readily available” prey, it would seem,
are those species that tend to occur in dense concentrations and within 70 m of the surface. The latter seems to
be a typical maximum feeding depth for diving seabirds
(Kooyman 1974). Offsetting the lack of specialization,
seabird species differ in their feeding by the size of their
prey, which relates to predator bill size (Ashmole and
Ashmole 1967; Bedard 1969), and by the habitat and
method of food capture (Ashmole 1971; Ainley 1977,
unpublished).
SEABIRDS AND FISHERIES
The prey that seabirds prefer, largely because of availability, are often sought in pelagic fisheries for the same
reason, or they are also the prey of pelagic predatory fish
that in turn are fished for by man. Since the time that man
first established pelagic fisheries, he often looked for feeding flocks of birds to tell where the sought-after fish were
located. Direct or indirect “competition” for fish between birds and man is thus theoretically possible, and
the fact that both birds and man are capable of tremendous fish harvest makes an interrelationship likely. Whether or not the fish harvest by birds can affect or has
affected that by man, or vice versa, must be considered
on a case-by-case basis.
There is little doubt that the crash of Peruvian
anchovy populations resulted in the crash of seabird
populations from 30 to 1 million individuals. As summarized by Idyll (1973), overfishing in conjunction with
natural environmental stress was probably responsible
for the reduction in fish. It is also fairly evident that in the
several years before the ultimate crash, intense fishing
pressure resulted in depressed bird populations, or at
least prevented recovery of bird numbers from an earlier
natural reduction.
The story of the Peruvian anchovy remains the outstandiqg, relatively unequivocal example of a human
fishery out-competing birds for fish. Few other examples
are as conclusive. Another example though is given by
Crawford and Shelton (1 978), who equated seabird numbers, guano production, fish availablity, and fishery
catches of pelagic species (mainly Sardinops) off South
Africa from 1940 to the present. Beginning about 1965
the fishery began a sharp decline from which it has not
recovered, and with it the bird populations declined as
well. The authors concluded that the fishery was ultimately responsible for the decline in bird numbers. In a
less conclusive example, Ainley and Lewis (1974) hypothesized that the disappearance of Pacific sardines (Sardinops coerulea) prevented recovery in several California populations of seabirds previously reduced by unrelated factors. In this case, one major question involved the
extent to which overfishing played a role in the disappearance of these fish (Cushing 1975). Other instances
of fishing impact on seabird prey, and ultimately on the
seabirds themselves, are in the realm of discussions over
the potential for such interaction. For example, Furness
(1978) and Bailey and Hislop (1978) recently presented
arguments, pro and con, over whether fisheries in the
northeastern Atlantic could have depressing effects on
seabird populations. The only clear conclusion from this
particular instance was that more information was needed,
particularly on seabirds.
There is also controversy over whether fishery harvest
of predatory fish, by reducing species that naturally compete with seabirds for food, would result in increased
availability of prey for birds and other predators. Few
unequivocal examples are available. Furness (1 978) and
Bailey and Hislop (1978) presented arguments on the
potential for this situation in the North Atlantic, and they
concluded that such an interrelationship is theoretically
possible. On the other hand, several authors (e.g. Sladen
1964; Conroy 1975; Laws 1977) have proposed that
increases in southern fur seals (Arctocephalus gazella)
and Adelie and Chinstrap Penguins (P. adelie and P.
antarctica) in the Scotia Sea area are the result of overfishing on baleen whales, which formerly “competed”
with seals and birds for Antarctic krill (Euphausia superba). The reduction in whales, in fact, has led some
fisheries experts (e.g. Gulland 1970) to propose that a
large “surplus” krill stock now exists and should be
harvested.
There are also potential interrelationships between
fisheries and seabirds that are even more indirect. A
dramatic decline of Thick-Billed Murres ( Uria Zomvia) in
West Greenland waters has been attributed in large part
to heavy mortality due to drowning in drift nets set for
salmon (Salmo salar; Evans and Waterston 1978). Rip
ley (1 976) indicated the potential for such an interaction
in the North Pacific as well. An increase in Northern
Fulmar populations (Fulrnarus glacialis) in the North
Atlantic during this century has been attribued to an
increase in fish offal resulting from fisheries, largely on
demersal species (Fisher 1952, 1966). Others (e.g.
Brown 1970; Bailey and Hislop 1978) argue against such
an explanation for the increase. As in other controversies, it is clear that information on seabird trophic and
energetic relationships in marine ecosystems is inadequate.
SUMMARY AND CONCLUDING REMARKS
Established in this review are the facts that seabirds
are marine organisms and that they can be important
predators on marine vertebrate and invertebrate prey
species. Potentially, they may play another important role
in pelagic ecosystems, that role being in the recycling of
nutrients. In light of these real and potential impacts, the
fact that marine ecologists generally overlook seabirds is
surprising. Because birds are so visible, they should be of
use in helping us to understand marine ecosystem interactions. Indeed, and rather surprisingly, a marine biologist, Green (1 97 l), on the basis of a computer simulation, recently suggested that the study of seabirds may
provide a sensitive and relatively inexpensive means to
monitor ecosystem state in the Antarctic. Using much
more complete data, and an interaction less extensive than
an entire ecosystem, Crawford and Shelton (1 978), fisheries research biologists, proposed that seabirds “have
value in providing an understanding of fish stocks prior to
exploitation and as indicators of the current state of the
resources.”
It was pointed out in this review that overharvest of a
fish stock can depress seabird populations. It would be
unusual if other predator populations were not affected.
The recent history of the Peruvian anchovy made this
clear, at least regarding birds, and the potential exists or
has existed for similar interrelationships elsewhere. There
are certain species of marine fish and invertebrates that,
because of their abundance, availability, and suitability
as food, are important prey for many predators. Of these
predators, only man has the power to “manage” the resource. Especially in the cases of “universal prey” species, management from an ecosystem perspective, rather
than that of the single stock sustainable yield approach,
would seem to be the wisest course of action. In that way
the impact of the fishery elsewhere in the food web may
be lessened before it is too late to do so.
http://calcofi.org/publications/calcofireports/v21/Vol_21_Ainley.pdf
they spend in some nesting activities. As marine organisms their occurrence and distribution are directly affected by properties of their oceanic habitat, such as water temperature, salinity, and turbidity. In their trophic relationships, almost all are secondary or tertiary carnivores. As
a group within specific ecosystems, estimates of their feeding rates range between 20 and 35% of annual prey production. Their usual prey are abundant, schooling organisms such as euphausiids and squid (invertebrates) and clupeids, engraulids, and exoccetids (fish). Their high rates of feeding and metabolism, and the large amounts of nutrients they return to the marine environment, indicate
that seabirds are probably important components in pelagic ecosystems. As such they have been strongly affected by human fisheries; for example, decline in the size of Peruvian anchovy and South African sardine populations have led to tremendous reductions in associated bird numbers. Evidence indicates that seabirds could provide an understanding of fish stock dynamics prior to over exploitation by man.
WHAT ARE SEABIRDS?
Avian taxonomists currently recognize about 156 existing families of birds, and only 9 of these are rather
specialized as seabirds: the Speniscida (penquins), Diomedeida (albatrosses), Procellariidae (petrels), Hydrobatida (storm-petrels), Pelecanoidida (diving petrels),
Phathontida (tropicbirds), Sulida (Boobies), Fregatida
(frigatebirds), Alcida (auks), and a few species (terns) of
Larida (gulls and terns). Members of these families
share the following characteristics: they derive all their
food from the sea, they void virtually all their feces into
the sea, and when individuals die they do so at sea. They
are full-time participants of marine energy cycles. Because of these characteristics, and in spite of the fact that
most birds can travel above the sea’s surface instead of
being confined to the water (as are fish or marine mammals) or that they have to spend some time on land to
raise young, these birds should be recognized as true
marine organisms. The time that they do spend away
from the sea is in fact minimal. Using the Adelie Penguin
(Pygoscelis adeliae) as an example, and the data in
Ainley (1 978) and Ainley and DeMaster (in press), it can be
calculated that a typical seabird spends about 85% of its
time annually at sea during its breeding years; over its
entire lifetime, it spends 90% of its time at sea. This
assumes 6 years of breeding and a life span of 10 years.
All of the time on land is devoted to breeding activities,
mainly the incubation of eggs, and during these activities, seabirds rely entirely on fat reserves built up at sea.
Some species that live much longer than Adelie Penguins and that breed every other year, for instance some
albatrosses, probably spend even less time on land over
the duration of their lives.
Oftentimes avian species that undertake even a small
part of their life cycle at sea are also considered to be
“seabirds.” Included are species from 15 other families,
principally the Gaviidae (loons), Podicipedidae (grebes),
Pelecanidae (pelicans), Phalacrocoracidae (cormorants),
Anatida: (ducks and geese), Scolopacidae (shorebirds),
Stercorariidae (skuas), Rynchopidae (skimmers), and the
majority of the Laridae. It is from these groups that the
general but rather misleading conception of a seabird is
derived, the one shared by most people. The “sea gull” is
the typical example. Such birds, like man, are based on
land and from there undertake trips to sea or to terrestrial
and freshwater habitats for food. They are involved only
part time in marine energy cycles; and because they
spend a third or more of their time on land, they remove
energy from marine ecosystems that is not returned.
THE MARINE DISTRIBUTION OF SEABIRDS
Much has been written about the distribution of seabirds at sea, but the factors that affect occurrence are not
well understood. Murphy (1936) was among the earliest
writers to point out that around South America (since
proved to be true elsewhere) some species were confined
to the “blue” oceanic waters offshore whereas others
occurred in more turbid coastal waters. From there,
through the work of many authors (for example WynneEdwards 1935; Jehl 1973) the idea has arisen that seabirds occur in concentric zones spaced outward from
continents and islands. This view explains the occurrence of some species, especially the coastal ones, most
of which are only part-time marine organisms, but it is
biased towaras the land orientation of man and the sea gull
type of seabird. It is further biased to the breeding season
(10% of a seabird’s life) when birds must return repeatedly to their nests.
We are beginning to understand that the distribution of
seabirds is affected by some of the same oceanographic
factors that affect the distributions of what are thought of
as typical marine organisms, such as fish. One such factor
is water temperature. For instance, when the California
Current weakens, as it periodically does (Sette and Isaacs
1960), California waters become warmer and species of
warmwater birds move farther north than they normally
occur in the eastern North Pacific (Ainley and Lewis
1974; Ainley 1976). Since the California Current moves
parallel to the coast and displaces warmer waters westward, warmwater bird species also move closer to shore
in such circumstances. This concept that birds live where
their preferred water occurs is complicated by their abilities to fly over, but still technically occur in, unsuitable
waters. For instance, Leach’s Storm-Petrels (Oceanodroma leucorhoa) prefer warm oceanic waters, but because the only suitable West Coast breeding sites in the
Pacific are coastal islands they must cross cold waters of
the California Current as they fly between feeding and
breeding areas (Ainley et al. 1975; Wiens and Scott
1975; Ainley 1976).
It is rather amazing, in light of the present land-oriented
zonal conception of seabird distribution, that on two
cruises from North America to the New Zealand sector of
Antarctica, during which seabirds and sea temperatures
were monitored continuously, with a temperature range
of 28” to O’C, 1-2°C changes brought about a consistent
turnover of 30-70% (E about 45%) of bird species (Ainley and co-workers, work in progress). Some species were
very precisely related to certain temperature ranges, i.e.
stenothermal, whereas others occurred over larger ranges,
i.e. eurythermal. This same pattern is evident in other
groups of marine organisms.
Brown et al. (1975) recently tried to correlate seabird
occurrence in Chilean fjords with not just temperature but
also salinity profiles. Pocklington (1979) attempted the
same for seabirds in the Indian Ocean. They found several good correlations. It is rather easy to understand how
seabirds might sense temperature changes, but it is not
easy to visualize how they might sense changes in salinity, even though they do drink seawater. It is just that, as
far as we know, they do not fly about continuously sampling salinity as they would be able to “monitor” temperature. Brown et al. and Pocklington tried to explain the
correlation as an indirect one involving the temperature/
salinity profiles of the preferred prey of different bird species. The opportunistic and unspecialized feeding habits
of most seabirds, as reviewed below, would also argue
against this for all but exceptional species.
Other physical oceanographic factors can also affect
seabird occurrence. For instance, some species that employ certain methods of food capture live only where
conditions favor those methods. The prime example is
given by birds that plunge for food (Ashmole 197 1 ), principally the boobies and tropicbirds. Spotting prey as deep
as 10 m or more below the surface while flying 15 m or
more above it, and thereafter using only momentum from
a “fall” to reach the prey, requires water of high clarity.
For this reason deep plungers occur only in tropical/sub
tropical waters where low phytoplankton standing stocks
(compared to those in cooler waters) result in very clear
water (Ainley 1977). Supporting this is the fact that deepplunging species occur most consistently off California
during the later summer and fall when the annual marine
cycle is in its oceanic period (Ainley 1976). At that time
water temperatures reach the subtropical range and phytoplankton standing stocks are lowest for the year (Bolin
and Abbott 1963).
T RO PH I C R E LATlO NS H I PS
Marine biologists, in their discussions of food webs,
rarely make specific mention of marine birds, but the
latter are part of what they refer to as “primary,” “se5ondary,” and “tertiary carnivores” or, in other words, the
“third trophic level” (see Steele 1974), or “other carnivores” (Cushing 1975), or “nekton” (Sverdrup et al.
1942). In their review of trophic relations among marine
birds of five oceanographic domains in the North Pacific,
Ainley and Sanger (1979) found that 77% of seabird
species were predators at the secondary and tertiary
carnivore levels. Most of the remainder (21%) were
scavengers, which still put them in the third trophic level.
Only 2%, in that they feed principally on other species of
seabirds, were in the fourth trophic level. Seabirds thus by
and large occupy the same position in marine food webs
as do the larger fish, mammals, and man.
The crux of the matter is how important are marine
birds as predators relative to other occupants of the third
trophic level in marine food webs. Quite a bit is known
about the species of prey eaten by seabirds (e.g. review by
Ainley and Sanger 1979), but little is known about their
food consumption rates to compare with other upper
trophic level predators. Some estimates though have been
attempted. Wiens and Scott (1 975), by computer simulation, estimated that Common Murres (Uria aalge) consumed 11% of pelagic fishes produced annually in Oregon’s neritic zone. They also estimated that four seabird
species off Oregon alone consumed annually about four
times the tonnage of anchovies (Engraulis mordax)
caught commercially each year in the northern permit area
(Point Conception to Oregon) during the years 1966 to
197 1. They also figured that 22% of the annual pelagic
fish production off Oregon was eaten by seabirds. Furness (1 978), using a different computer simulation, estimated that within 45 km of breeding colonies in the Shetland Islands, seabirds consumed between 20 and 35% of
annual food-fish production. It has been calculated that off
Peru during the height of the commercial anchovy (Engraulis ringens) harvest in the late 1960’s, birds consumed 2.5 X IO6 metric tons of the fish or as much as a
quarter of what was harvested commercially (Idyll 1973).
That commercial harvest, of course,’was by far the largest in the world. Laws (1977) estimated that in the Antarctic pelagic ecosystems birds and seals equal each other
in biomass, an amount for each that is about half that of
whales. Prevost (1 976) figured that all three groups consumed about equal amounts of euphausiids, about 30-40
X 1 O6 tons each. Both authors agreed, however, that more
information was needed. More data are also needed elsewhere, but with seabird food consumption estimates of
such high magnitudes, it is surprising that fishery and
marine biologists, and ornithologists, have not paid more
attention to the seemingly significant impact that marine
birds may have in pelagic ecosystems.
High rates of food consumption and very high metabolic rates in turn mean that seabirds, through production
of excrement, may also play a significant role in the recycling of nutrients and energy in pelagic ecosystems.
This, as pointed out by Wiens and Scott (1 975), may be
especially true in areas where upwelling is not strong but
where some species of seabirds are abundant. There is, in
fact, compared to knowledge on food consumption, even
less known about the role seabirds play in nutrient recycling. Sanger (1972) estimated that seabirds in the
Central Subarctic Domain (see Dodimead et al. 1963)
consumed 278 X lo3 tons of food and voided up to 74 X
lo3 tons of feces per year. Wiens and Scott (1975) estimated that offOregon four seabird species, with numbers
fluctuating seasonally from about 1.2 X lo5 to 4.4 X lo6
birds, consumed about 62,500 metric tons of fish, or 7.56
X 1O’O kcal of food, and returned 2.32 X 1O’O kcal to the
system each year in their feces. They were not able to
equate kcal of guano to nutrients. On an artificial platform in South Africa, less than 240,000 seabirds produced, at a minimum, an average (1941-1965) 777
metric tons of guano per year, the composition of which
included 16% nitrates, 9% phosphates, and 4% potash
(Rand 1963; Berry 1975).
In the types of prey they feed on, most seabirds do not
appear to specialize. In the review by Ainley and Sanger
(1979), it was evident that certain prey over and over
again predominated in the diets of different marine birds
of the eastern North Pacific. These prey included species
of Euphausia, Loligo, Clupea, Engraulis, and Sebastes.
As specific examples, 43% of prey eaten by four Oregon
seabirds (Wiens and Scott 1975) and 80 to 95% of prey
eaten by three Peruvian “guano birds” was Engradis
(Idyll 1973); 23% of prey eaten by seven seabirds in the
central tropical Pacific was exoccetids (Ashmole and
Ashmole 1967); somewhat more than 50% and often
more than 75% of prey eaten by six species nesting at the
Farallon Islands was Sebastes (Ainley unpublished); 26
to 85% of prey eaten by nine species in the Fame Islands
was Ammodytes (Pearson 1968); 19 to 85% of prey
eaten by three South African “guano birds” was Sardinops (Crawford and Shelton 1978); and 50 to 90% of preyeaten by most Antarctic penquins and petrels was Euphausia (Emison 1968; Mougin 1975). There is thus
great overlap in what they eat, and it seems that whatever
prey species is most readily available predominates in
seabird diets. “Readily available” prey, it would seem,
are those species that tend to occur in dense concentrations and within 70 m of the surface. The latter seems to
be a typical maximum feeding depth for diving seabirds
(Kooyman 1974). Offsetting the lack of specialization,
seabird species differ in their feeding by the size of their
prey, which relates to predator bill size (Ashmole and
Ashmole 1967; Bedard 1969), and by the habitat and
method of food capture (Ashmole 1971; Ainley 1977,
unpublished).
SEABIRDS AND FISHERIES
The prey that seabirds prefer, largely because of availability, are often sought in pelagic fisheries for the same
reason, or they are also the prey of pelagic predatory fish
that in turn are fished for by man. Since the time that man
first established pelagic fisheries, he often looked for feeding flocks of birds to tell where the sought-after fish were
located. Direct or indirect “competition” for fish between birds and man is thus theoretically possible, and
the fact that both birds and man are capable of tremendous fish harvest makes an interrelationship likely. Whether or not the fish harvest by birds can affect or has
affected that by man, or vice versa, must be considered
on a case-by-case basis.
There is little doubt that the crash of Peruvian
anchovy populations resulted in the crash of seabird
populations from 30 to 1 million individuals. As summarized by Idyll (1973), overfishing in conjunction with
natural environmental stress was probably responsible
for the reduction in fish. It is also fairly evident that in the
several years before the ultimate crash, intense fishing
pressure resulted in depressed bird populations, or at
least prevented recovery of bird numbers from an earlier
natural reduction.
The story of the Peruvian anchovy remains the outstandiqg, relatively unequivocal example of a human
fishery out-competing birds for fish. Few other examples
are as conclusive. Another example though is given by
Crawford and Shelton (1 978), who equated seabird numbers, guano production, fish availablity, and fishery
catches of pelagic species (mainly Sardinops) off South
Africa from 1940 to the present. Beginning about 1965
the fishery began a sharp decline from which it has not
recovered, and with it the bird populations declined as
well. The authors concluded that the fishery was ultimately responsible for the decline in bird numbers. In a
less conclusive example, Ainley and Lewis (1974) hypothesized that the disappearance of Pacific sardines (Sardinops coerulea) prevented recovery in several California populations of seabirds previously reduced by unrelated factors. In this case, one major question involved the
extent to which overfishing played a role in the disappearance of these fish (Cushing 1975). Other instances
of fishing impact on seabird prey, and ultimately on the
seabirds themselves, are in the realm of discussions over
the potential for such interaction. For example, Furness
(1978) and Bailey and Hislop (1978) recently presented
arguments, pro and con, over whether fisheries in the
northeastern Atlantic could have depressing effects on
seabird populations. The only clear conclusion from this
particular instance was that more information was needed,
particularly on seabirds.
There is also controversy over whether fishery harvest
of predatory fish, by reducing species that naturally compete with seabirds for food, would result in increased
availability of prey for birds and other predators. Few
unequivocal examples are available. Furness (1 978) and
Bailey and Hislop (1978) presented arguments on the
potential for this situation in the North Atlantic, and they
concluded that such an interrelationship is theoretically
possible. On the other hand, several authors (e.g. Sladen
1964; Conroy 1975; Laws 1977) have proposed that
increases in southern fur seals (Arctocephalus gazella)
and Adelie and Chinstrap Penguins (P. adelie and P.
antarctica) in the Scotia Sea area are the result of overfishing on baleen whales, which formerly “competed”
with seals and birds for Antarctic krill (Euphausia superba). The reduction in whales, in fact, has led some
fisheries experts (e.g. Gulland 1970) to propose that a
large “surplus” krill stock now exists and should be
harvested.
There are also potential interrelationships between
fisheries and seabirds that are even more indirect. A
dramatic decline of Thick-Billed Murres ( Uria Zomvia) in
West Greenland waters has been attributed in large part
to heavy mortality due to drowning in drift nets set for
salmon (Salmo salar; Evans and Waterston 1978). Rip
ley (1 976) indicated the potential for such an interaction
in the North Pacific as well. An increase in Northern
Fulmar populations (Fulrnarus glacialis) in the North
Atlantic during this century has been attribued to an
increase in fish offal resulting from fisheries, largely on
demersal species (Fisher 1952, 1966). Others (e.g.
Brown 1970; Bailey and Hislop 1978) argue against such
an explanation for the increase. As in other controversies, it is clear that information on seabird trophic and
energetic relationships in marine ecosystems is inadequate.
SUMMARY AND CONCLUDING REMARKS
Established in this review are the facts that seabirds
are marine organisms and that they can be important
predators on marine vertebrate and invertebrate prey
species. Potentially, they may play another important role
in pelagic ecosystems, that role being in the recycling of
nutrients. In light of these real and potential impacts, the
fact that marine ecologists generally overlook seabirds is
surprising. Because birds are so visible, they should be of
use in helping us to understand marine ecosystem interactions. Indeed, and rather surprisingly, a marine biologist, Green (1 97 l), on the basis of a computer simulation, recently suggested that the study of seabirds may
provide a sensitive and relatively inexpensive means to
monitor ecosystem state in the Antarctic. Using much
more complete data, and an interaction less extensive than
an entire ecosystem, Crawford and Shelton (1 978), fisheries research biologists, proposed that seabirds “have
value in providing an understanding of fish stocks prior to
exploitation and as indicators of the current state of the
resources.”
It was pointed out in this review that overharvest of a
fish stock can depress seabird populations. It would be
unusual if other predator populations were not affected.
The recent history of the Peruvian anchovy made this
clear, at least regarding birds, and the potential exists or
has existed for similar interrelationships elsewhere. There
are certain species of marine fish and invertebrates that,
because of their abundance, availability, and suitability
as food, are important prey for many predators. Of these
predators, only man has the power to “manage” the resource. Especially in the cases of “universal prey” species, management from an ecosystem perspective, rather
than that of the single stock sustainable yield approach,
would seem to be the wisest course of action. In that way
the impact of the fishery elsewhere in the food web may
be lessened before it is too late to do so.
http://calcofi.org/publications/calcofireports/v21/Vol_21_Ainley.pdf
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