Science - USA (2020-09-04)

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1154 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 sciencemag.org SCIENCE

CREDITS: (PHOTO) KIM TAYLOR/MINDEN PICTURES; (MAPS) N. DESAI/

SCIENCE

;

(DATA) EKLÕF

ET AL

., COMMUNICATIONS BIOLOGY

, 3, 459, (2020)

N


o bigger than a minnow, the three-
spine stickleback may seem a puny
player in the underwater world. But
along the European coastline of the
Baltic Sea, it has edged out its own
predators—toothy pike and perch,
fish that grow longer than your forearm.
Records dating back 40 years show how the
flamboyant little stickleback has shifted
the ecosystem, thwarting efforts to restore
the larger species favored by human fish-
ers. “A little pelagic fish that many people
ignore is having a dramatic ecological

impact,” says Brad deYoung, an oceano-
grapher at Memorial University who was
not involved with the work.
Ecologists say what has happened in the
Baltic is a dramatic example of a predator-
prey reversal, in which two species trade
places on the food chain, drastically alter-
ing the rest of the ecosystem. “It shows you
really need to understand not just who
eats who, but who might eat who to prop-
erly manage [fish stocks],” deYoung says.
Johan Eklöf grew up along Sweden’s Bal-
tic coast and fondly remembers catching
plentiful Eurasian perch (Perca fluviatilis).
Later, as an ecologist at Stockholm Univer-

sity, Eklöf and his colleagues noted that the
three-spine stickleback (Gasterosteus acu-
leatus) seemed more and more common in
coastal waters. To find out what was going
on, the researchers unearthed 13,000 sur-
veys of fish done between 1979 and 2017
by scientists and fisheries managers along
1200 kilometers of the western coast of the
Baltic Sea. “This paper is a good example of
using past data, which can sometimes seem
dull, to explore a problem that cannot be
addressed any other way,” deYoung says.
In the 1980s, Eklöf and colleagues found,
sticklebacks outnumbered not just perch,
but also Northern pike (Esox Lucius), at the
seaward edges of the many islands and shal-
low bays along the Baltic coast. That’s not
surprising—pike and perch are freshwater
fish, able to survive in the ocean only where
river outflows lower salinity. Those fish pre-
vailed in the fresher waters 8 kilometers
closer to shore. But in the 1990s, stickle-
backs began to outnumber their predators
closer to land, their dominance spreading
toward more protected bays and inland
waters. By 2014, sticklebacks reigned a full
21 kilometers landward from the archipela-
go’s edge, Eklöf and his colleagues reported
last week in Communications Biology.
The sticklebacks themselves probably
didn’t initiate their predators’ decline. In-
stead, complex ecological factors appear to
have first touched off the shift. Beginning
in the 1990s, gray seals became more com-
mon, thanks to better water quality and an
end to bounty hunting. The seals, along
with cormorants, began to eat more pike
and perch. Meanwhile, sticklebacks were
thriving in the rapidly warming seas. And
overfishing of cod and large herring meant
stickleback had fewer predators.

2017

SWEDEN SWEDEN SWEDEN

Baltic
Sea

Predatory 2sh
dominance

Stickleback
dominance

Norrtälje

Södertälje

Stockholm

1980 2000

Stockholm

Baltic
Sea

Baltic
Sea

Norrtälje

Norrtälje

Södertälje Södertälje

Stockholm

Archipelago Archipelago Archipelago

By Elizabeth Pennisi

An ecosystem goes topsy-turvy


as a tiny fish takes over


Predator-prey reversal has dramatically altered the


underwater ecosystem along the Baltic Coast archipelago


With a mouth big enough to eat baby pike and perch,
this stickleback species has turned on its predators.

SWEDEN


Stickleback surge
Predatory fish (orange) such as pike and perch once dominated the waters around the archipelago along the Baltic coast. But sticklebacks (red) have now taken over.

ECOLOGY

Published by AAAS
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