Baby clown and damselfish, evaluated in both carbon dioxide-rich and conventional environments, were particularly affected by the high CO2 levels. Photo courtesy of Dr Simon Foale, ARC Centre of Excellence for Coral Reef Studies |
Rising
human carbon dioxide emissions may be affecting the brains and central
nervous system of sea fishes with serious consequences for their
survival, an international scientific team has found.
Carbon
dioxide concentrations predicted to occur in the ocean by the end of
this century will interfere with fishes’ ability to hear, smell, turn
and evade predators, says Professor Philip Munday of the ARC Centre of
Excellence for Coral Reef Studies and James Cook University.
“For
several years our team have been testing the performance of baby coral
fishes in sea water containing higher levels of dissolved CO2—and it is
now pretty clear that they sustain significant disruption to their
central nervous system, which is likely to impair their chances of
survival,” Prof. Munday says.
In
their latest paper, published in the journal Nature Climate Change,
Prof. Munday and colleagues report world-first evidence that high CO2
levels in sea water disrupts a key brain receptor in fish, causing
marked changes in their behaviour and sensory ability.
“We’ve
found that elevated CO2 in the oceans can directly interfere with fish
neurotransmitter functions, which poses a direct and previously unknown
threat to sea life,” Prof. Munday says.
Prof.
Munday and his colleagues began by studying how baby clown and damsel
fishes performed alongside their predators in CO2-enriched water. They
found that, while the predators were somewhat affected, the baby fish
suffered much higher rates of attrition.
“Our
early work showed that the sense of smell of baby fish was harmed by
higher CO2 in the water—meaning they found it harder to locate a reef to
settle on or detect the warning smell of a predator fish. But we
suspected there was much more to it than the loss of ability to smell.”
The
team then examined whether fishes’ sense of hearing—used to locate and
home in on reefs at night, and avoid them during the day—was affected.
“The answer is, yes it was. They were confused and no longer avoided
reef sounds during the day. Being attracted to reefs during daylight
would make them easy meat for predators.”
Other
work showed the fish also tended to lose their natural instinct to turn
left or right—an important factor in schooling behaviour which also
makes them more vulnerable, as lone fish are easily eaten by predators.
“All
this led us to suspect it wasn’t simply damage to their individual
senses that was going on—but rather, that higher levels of carbon
dioxide were affecting their whole central nervous system.”
The
team’s latest research shows that high CO2 directly stimulates a
receptor in the fish brain called GABA-A, leading to a reversal in its
normal function and over-excitement of certain nerve signals.
While
most animals with brains have GABA-A receptors, the team considers the
effects of elevated CO2 are likely to be most felt by those living in
water, as they have lower blood CO2 levels normally. The main impact is
likely to be felt by some crustaceans and by most fishes, especially
those which use a lot of oxygen.
Prof.
Munday said that around 2.3 billion tons of human CO2 emissions
dissolve into the world’s oceans every year, causing changes in the
chemical environment of the water in which fish and other species live.
“We’ve
now established it isn’t simply the acidification of the oceans that is
causing disruption—as is the case with shellfish and plankton with
chalky skeletons—but the actual dissolved CO2 itself is damaging the
fishes’ nervous systems.”
The
work shows that fish with high oxygen consumption are likely to be most
affected, suggesting the effects of high CO2 may impair some species
worse than others—possibly including important species targeted by the
world’s fishing industries.
Near-future CO2 levels alter fish behaviour by interfering with neurotransmitter function
