THE TOWERING, frost-shattered skyline of South Georgia rises dramatically
above the tempestuous waters of the Southern Ocean. Glaciers cascade into the
sea, occasionally adding to the icebergs that already punctuate the surrounding
seascape. The 160-kilometre-long island is a British territory and lies on a
similar (though antipodean) latitude to Britain itself. But there any
resemblance ends. While Britain鈥檚 shores are cosseted by the warming influence
of the Gulf Stream, South Georgia lies south of the Antarctic Polar Front, a
natural barrier which defines the limits of the cold Southern Ocean. Icy
currents sweep up from the Antarctic, keeping the sea temperature around South
Georgia down to within a couple of degrees of freezing.
For a century or more, the wildlife inhabiting the seas around South Georgia
has been vigorously exploited. First fur seals, then various species of whale,
then the Antarctic cod were hunted almost to extinction in these perishing
waters. Today, the target species include ice fish, squid and a small,
shrimplike creature, the Antarctic krill Euphausia superba.
This undistinguished-looking translucent crustacean, no more than about six
centimetres long, is vital to the health of the entire Southern Ocean marine
ecosystem. As part of an international programme of research, our group at the
British Antarctic Survey has been monitoring the abundance of krill around South
Georgia and studying the ecology of those species that feed on it. This research
has uncovered an intimate link between the seas around the island and the sea
ice off the Antarctic Peninsula, which lies 2000 kilometres away to the
southwest. It has even revealed hints of the possibly devastating effect that
global warming could have on fisheries in the region.
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In the early decades of the 20th century, whalers were prepared to risk much
in these hostile waters for the chance to harpoon a fortune. The slaughter began
in December 1904 and in the first season alone over 200 whales were caught.
Whaling stations, rendering plants and ship repair yards soon dotted the
island鈥檚 northern coastline. By 1965 the industry had processed around 175 000
blue, humpback, fin, sei, sperm and right whales, and stocks had fallen to
commercial extinction. Blue whales were most highly prized, but even before
hunting began to affect stocks, harpoon men and flensing teams realised that in
some years blues did not feature as prominently in the catch as they would have
liked. Instead, they were replaced by the less profitable fin whale.
Profit was everything, and in the hope of maximising returns from whaling
around South Georgia, the British government instigated a series of
investigations into the biology and migratory habits of whales. These Discovery
Committee surveys began in 1925, at first using the same ship鈥� the
Discovery鈥攖hat had been built for Robert Falcon Scott鈥檚 first expedition
to Antarctica more than 20 years earlier. The research continued throughout the
Southern Ocean for 26 years, and many of the Discovery reports remain to this
day the authoritative works on aspects of Antarctic science.
Stanley Kemp, leader of the first Discovery expedition, documented the
variation between 鈥渂lue-whale years鈥�, when this species was abundant, and
鈥渇in-whale years鈥�, when the industry was forced to take the less profitable
species. In a presidential address to the Linnean Society of London in 1930,
Sidney Harmer, director of the British Museum of Natural History, suggested that
these whale years were linked to food availability and oceanographic conditions.
He noted that the fin whales taken during the fin-whale year 1925-26 were
exceptionally thin, and that this was typical of such years. Krill are the
animals鈥� main food, and Harmer suggested that in fin-whale years the krill were
in short supply. Blue whales, with their enormous appetites, would be absent
because they had gone elsewhere in search of the large quantities of krill they
needed to sustain them. Harmer鈥檚 idea was supported by the fact that in
fin-whale years, scientists found fewer krill in these waters.
The end of the Discovery investigations in 1951 was followed by a quarter of
a century in which field research in the seas around South Georgia was only
intermittent. Meanwhile, marine resources were still being plundered. In the
late 1960s fishing fleets went to South Georgia to catch the Antarctic cod that
abounded there. Within less than a decade these cod stocks had crashed,
following a pattern typical of fisheries the world over. Growing international
concern over the fate of Antarctic fisheries led in 1980 to the Convention on
the Conservation of Antarctic Marine Living Resources, to which 29 countries are
party, including the US, Russia, Japan, Australia and Britain. The convention
covers all elements of the Antarctic ecosystem that may be linked with fishing.
But before scientist could determine the possible effects of fishing, they
needed to understand the natural variability of the ecosystem. Our work on krill
forms part of this international effort.
From samples we have netted over the past two decades from the seas around
South Georgia, and from echo-sounder surveys, it is clear that the abundance of
krill is continuing to change from year to year, just as it did in the heyday of
whaling.
But ship time is expensive. Additional and invaluable data have come from
observations of the diet and behaviour of krill-eating animals breeding at Bird
Island鈥攁t the western tip of South Georgia. As well as providing a nesting
site for myriad albatrosses and penguins, it is also home to the Antarctic fur
seal, which has undergone a phenomenal recovery since hunters rendered it almost
extinct a century ago. Today there are some 3 million fur seals. Almost 25 years
of observation by British Antarctic Survey biologists on Bird Island has
revealed a clear pattern. Years of normal breeding activity are interspersed
with years of catastrophic failure in breeding success. Gentoo penguin chicks
die before fledging, and fur seal pups perish through neglect, as their parents
are forced to spend too long away at sea, hunting for food.
The years of poor breeding success correspond directly to those when acoustic
surveys at sea suggest krill abundance is low. These bad years happen two or
three times in a decade鈥攁 strikingly similar frequency to the fin-whale
years noted earlier this century. Now studies of krill predators are starting to
give us a way to predict when crashes in krill numbers will occur.
The vital evidence comes from the size of the krill in the diets of
predators. By examining faecal remains and regurgitated stomach contents over
the past nine years, our group has shown that in years when krill are scarce
around South Georgia, there is a shift during the year from large to small
krill. This important finding suggests that the shortage of krill is linked not
just to a simple reduction in the overall numbers of krill present, but also to
changes in age structure of the krill population.
Long krill
The clue to understanding what is happening comes with our additional
observation that in the year before a season of low abundance, the average
length of krill in predator diets is unusually high. The implication of this
observation is that almost no juvenile krill have entered the population. In the
following year, it is the gap in the krill population left by the failure of
last year鈥檚 juveniles that accounts for low krill abundance. By the end of the
year, all the large krill have been consumed leaving only the new juveniles for
predators to feed on (see Diagram).
In March 1997, for example, small krill were almost absent from predator
diet. We predicted that 1998 would be a year of low krill abundance and that
predators would suffer as a consequence. We were right. In 1998 the abundance of
krill was low, particularly in the early part of the breeding season, and far
fewer Antarctic fur seals and gentoo penguins returned to breed in the colonies
at Bird Island. Those penguins that did return failed to fledge any chicks at
all, and an extremely high proportion of the fur seal pups also perished.
Now that the krill are being fished commercially, this ability to predict
population slumps one year in advance gives us important information we can use
for the future management of this ecosystem. It should help the CCAMLR to
balance the needs of commercial krill fishing against those of the many predator
species which depend so heavily on the krill.
Predicting the pattern of boom and bust is useful, but we also want to know
why it happens. There is no evidence that the krill population at South Georgia
is sustained by adults breeding there, so marine biologists widened the net in
their search for the origin of krill arriving at the island. The logical place
to look was downstream to the southwest of South Georgia, the direction from
where the Antarctic Circumpolar Current travels on its clockwise circulation
around the continent. Oceanographic models and satellite observations of
drifting icebergs suggest that waters from the Antarctic Peninsula, 2000
kilometres away, reach South Georgia. As the Peninsula is known to be a major
breeding area for krill, it seemed likely that krill hatched there might end up
off South Georgia.
Last year, a group of CCAMLR scientists met in San Diego to investigate the
variability of krill populations throughout the Scotia Sea. This is the region
to the south and west of South Georgia, bounded by a mainly submerged ridge that
runs eastwards from the tip of the Antarctic Peninsula before looping back on
itself to meet the tip of South America. From data collected over the past two
decades, they noted a very close relationship between abundances of krill at
South Georgia and at Elephant Island on the tip of the Antarctica Peninsula:
slumps in krill abundance at the Peninsula occur in exactly the same years as
slumps at South Georgia. This finding, together with the evidence for an ocean
supply route linking the Peninsula and South Georgia, suggested that by
understanding what was causing the krill population at the Peninsula to vary, we
might in turn unlock the mystery of variability at South Georgia.
Scuba divers working under the sea ice that flanks the Peninsula鈥檚 western
shores have observed large numbers of juvenile krill grazing on the
under-surface of the ice. Biologically speaking sea ice is a highly productive
environment鈥攁lgae thrive at the interface between ice and water and
communities of herbivorous grazers capitalise on this valuable food source. The
icy ceiling also provides protection from predators diving from above. It is
hardly surprising then that the abundance of zooplankton such as krill is often
greater under ice than in the open ocean. What鈥檚 more, Volker Siegel from the
Institute for Sea Fisheries in Hamburg, and colleagues, have observed that
production of krill is highest following seasons when ice extent has been
great.
But why might ice cover vary? In 1996 Warren White and Ray Petersen, from the
Scripps Institution of Oceanography in San Diego, California, reported the
phenomenon of the Antarctic Circumpolar Wave (ACW). The wave is a complex,
multifaceted climatic feature that brings changes in atmospheric pressure, sea
surface temperature and the extent of sea ice. It appears to rotate periodically
around Antarctica, bringing with it highs in sea ice extent. As a result, sea
ice distribution moves like an oval record rotating slowly on a turntable,
taking about seven years to complete a single revolution. This results in bulges
of ice passing any given location, interspersed with troughs in cover once every
three or four years. To biologists studying krill at South Georgia, this time
scale was uncannily familiar. It looks as though in years when the ACW reduces
ice cover, krill reproduction slumps, and as a consequence there are fewer
juvenile passengers for the ocean conveyor belt to South Georgia.
David Demer, at the US National Marine Fisheries Service鈥檚 South West
Fisheries Science Center in La Jolla, California, has combined data on the
extent of sea ice at the Antarctic Peninsula with the past 20 years of
echo-sounder estimates of krill abundance from South Georgia and Elephant Island
to develop a model of krill abundance in these waters. Assuming these cycles
continue鈥攁nd we know of no reason why they shouldn鈥檛鈥攈is model makes
it possible to make medium-term predictions of the timings of future slumps in
krill abundance. Both it and our own most recent predator diet data predict that
the next slump will be this coming austral summer, 1999-2000. For the
krill-dependent species around South Georgia, there will be no millennium
party.
A history of commercial exploitation combined with many decades of scientific
research has given us a special insight into the fluctuating ecosystem of the
seas around South Georgia. But it may not be unique. Patterns seen here seem to
be mirrored throughout the Scotia Sea and perhaps even across the whole Southern
Ocean. The threat of global warming makes it especially important to know how
fluctuations in the abundance of krill may be linked to changes in sea ice
cover. If warming makes the ice shrink, then prospects look bleak for both
predators and the fishing industry.
The task now for scientists and governments is to use our new understanding
of the dynamics of the Southern Ocean ecosystem to find ways of managing the
environment there through a period of change. There are challenging times ahead
in the kingdom of the krill.
