December 20th, 2019, by Tim
Radford
Fish catches are falling in
the Gulf of Maine, Baltic cod are getting smaller. Sharks suffer acid waters’
effects as marine climate impacts grow.
LONDON, 20 December, 2019 – Marine
climate impacts are starting to make their mark on marine life at almost every
level, according to a range of entirely unrelated scientific studies published
in the last month.
Baltic
codfish – a valuable commercial catch – have steadily become smaller,
scrawnier and less valuable because of the loss of oxygen in ocean waters as a
consequence of an increasingly warmer world.
Changes in climate over the
last two decades have
cost the fishermen of New England their jobs: their numbers have fallen by
16% since 1996 as the total catch has fallen, along with fishermen’s incomes.
The change may be linked to a
natural ocean climate cycle, but nobody can be sure the decline will not
continue as waters warm in response to ever higher atmospheric levels of carbon
dioxide, driven by ever greater use of fossil fuels to power modern economic
growth.
That steady rise in carbon
dioxide means that marine waters are also becoming steadily more acidic, and
this could be bad news for the sharks. Laboratory experiments suggest they can
respond to short-term changes in water chemistry, but in the long term increasingly
acidic waters can begin to dissolve not just the characteristic skin scales of
the shark family, but the teeth as well.
And if environmental change
goes on hitting tropical corals and the anemones that co-exist with them,
then one
of the world’s most iconic and culturally popular species could also
disappear: the clownfish sub-family Amphiprioninae may not survive
the continued bleaching of the coral reefs. Amphiprion ocellaris swam
into the world’s hearts as the much sought-after cartoon character in the 2003 film Finding Nemo.
Scientists based in the US and
Sweden report in the journal Biology
Letters that the average weight of specimens of Gadus morhua or
the cod fish 40 cms long had dropped from 900 to 600 grams in the last 30
years.
They examined the otoliths or
ear stones of 134 individuals trawled in the last months of the Baltic winter
to read the evidence from trace elements such as magnesium and manganese and
identify the cause: the continued fall in sea water oxygen levels as a
consequence of global warming and pollution.
“The cod themselves are
telling us through their internal logbooks that they’re affected by hypoxia
[reduced oxygen availability], which we know is driven by climate change and
nutrient loading,” said Karin Limburg, an
ecologist at the State University of New York, who led the study. “Our
findings suggest fish are in a worse condition because of hypoxia.”
In the Gulf of Maine, off the
US Atlantic coast, catches of fish and shellfish have been falling, and with
them the number of people employed in the fishery. Kimberly
Oremus of the University of Delaware reports in the Proceedings of
the National Academy of Sciences that successive warm winters have hit
the catch, and incomes.
Pattern found
She matched decades of climate
data, landing figures and sales data to identify a pattern of decline linked
principally to a hot-and-cold pattern of change known as the
North Atlantic Oscillation.
“New England waters are among
the fastest-warming in the world,” she said. “Warmer than average sea surface
temperatures have been shown to impact the productivity of lobsters, sea
scallops, groundfish and other fisheries important to the region, especially
when they are most vulnerable, from spawning through their first year of life.”
The region has 34,000
commercial fishermen, a significant proportion of the 166,000 or so throughout
the whole of the US. The oscillation is a shift in ocean temperatures over
decades, and catches could improve in decades to come – but marine waters
worldwide are warming.
“This is an important signal
to incorporate into the fisheries management process,” she said. “We need to
figure out what climate is doing to fisheries in order to cope with it.”
Acid hazard
One important part of the
marine ecosystem might not in the long run be able to cope: short episodes of
hypercapnia, or a dramatic rise in dissolved carbon dioxide, are a feature
linked to seasonal oceanic upwellings, and can last for days in some waters
before normal ocean chemistry is restored.
In the journal Scientific Reports,
European and South Africa researchers offer evidence that though cartilaginous
fishes – the huge and varied family to which sharks belong – have evolved to
cope with such spells, ever more acidic oceans offer a new hazard.
They caught a number of
puffadder shysharks, known to scientists as Haploblepharus edwardsii and
a species small enough for laboratory tanks, from shallow waters off South
Africa and exposed them to acidic conditions predicted by the year 2300.
The increasingly acid
environment was, literally, corrosive. Their specimens lost a quarter of their
skin denticles – the shark equivalent of scales. Sharks’ teeth are made of the
same biological fabric as the skin, and the implication is that such losses
could, in their words “compromise hydrodynamics and skin protection.” In other
words, some of the ocean’s most feared predators might have trouble both
swimming and feeding.
Poor adapters
Australian and US scientists
have more
bad news for Nemo, the film star from the clownfish family. Rather than
experiment in a laboratory tank, they monitored the numbers and the DNA of real
life specimens for decades in Kimbe Bay off Papua-New Guinea. As waters warmed
and began to bleach the coral reefs, the anemones that live in the reefs were
put at risk.
They report in Ecology Letters that
the tiny clownfish that live in the anemone tentacles proved bad at adapting to
environmental change. The implication is that, as the coral reefs are lost,
many species could be homeless and helpless.
“We find that Nemo is at the
mercy of a habitat that is degrading more and more every year,” said Serge
Planes of the French National Centre of Scientific Research, and one of the
authors.
“To expect a clownfish to
genetically adapt at a pace that would allow it to persist is unreasonable.”
And Simon Thorrold of
the Woods Hole Oceanographic Institution in the US added: “It seems
Nemo won’t be able to save himself.” – Climate News Network
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