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.
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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.
“We find that Nemo is at the mercy of a habitat that is degrading more and more every year”
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
