Dungeness crabs show indication they may be vulnerable to effects of ocean acidification along Pacific coast

MONTEREY — Ocean acidification along the Pacific coast is
slowly corroding the shells of young Dungeness crabs floating in
the plankton just offshore. Their persistence in the face of
ongoing climate change, in which the ocean will continue to
acidify, could determine the future of one of the state’s most
lucrative fisheries.

A recent study by Dr. Nina Bednaršek, a senior scientist with
the Southern California Coastal Water Research Project, examined
the shells of larval Dungeness crabs no bigger than a dime and
found evidence of pitting and scarring on crabs collected in areas
with more acidic water. The results, published in the journal
Science of the Total Environment, suggest that ocean acidification
is already impacting wildlife.

“It seems like it’s a very easy-to-trigger response, and the
first line of evidence that they respond to these negative
conditions,” Bednaršek said.

While adult Dungeness crabs are widely monitored, the lives of
larval crabs are almost completely unknown. Before settling onto
the seafloor, juvenile Dungeness crabs drift about as plankton,
pulling free-floating minerals from the surrounding seawater to
build a shell like so many microscopic bricks.

Declines
and closures of other fisheries have pushed more people to fish for
Dungeness, a finding confirmed by California Department of Fish and
Wildlife. (Monterey Herald archives) 

Bednaršek collected larval crabs off Oregon, Washington and
British Columbia, using a high-powered microscope to magnify their
shells by as much as 20,000 times. Through computer modeling, she
was able to back-trace their likely path along the coast, linking
them to areas of upwelling that bring acidic water to the
surface.

Ocean water becomes more acidic by absorbing carbon dioxide from
the air. While this happens naturally, human activities are also
increasing the amount of carbon dioxide in the atmosphere. These
gases move throughout the ocean in deep currents, emerging close to
shore in upwelling regions such as the west coast of the United
States.

Saturated with carbon dioxide, the natural balance of water
chemistry shifts to one that eats away at a larval crab’s
delicate shell.

“They end up pooled in the coastal waters with extended
exposure to more acidic conditions,” Bednaršek said, adding that
plankton are generally at the mercy of currents to move them about.
“So it becomes a double whammy. They are living and having to
mature under conditions that are unfavorable.”

This phenomenon has been observed in other types of plankton,
including microscopic animals called foraminifera and swimming
snails known as pteropods. Recent research comparing modern-day
plankton to samples taken during the famed 19th century Challenger
expedition has shown that today’s shells are 76% thinner on
average.

What happens at this microscopic scale also matters because
plankton form the base of the food web. Even the largest whale is
linked through the food it eats to these seemingly invisible
organisms. Their proliferation in the waters of Monterey Bay
sustains the area’s rich ecosystem.

Monterey Bay sits at the southern tip of the range for Dungeness
Crab, and both commercial and recreational fishermen gear up each
winter to deploy their crab pots over the bay’s deep water.

Mike Ricketts, president of the Monterey Commercial
Fishermen’s Association, has been fishing for more than 40 years
and says the fishery has generally been dependable and healthy.

“The Dungeness fishery is one of the most well-managed
fisheries on the west coast,” Ricketts said, adding that as more
volatile fisheries change over time, “more commercial fishermen
are depending on crabs for their livelihood.”

But even as the population seems to fluctuate around a stable
average — Ricketts says it varies over predictable six-year
intervals in a boom and bust-type cycle — Bednaršek’s study is
only one bellwether pointing to a less certain future, even if the
full effects are not yet known.

“What we have observed so far can not yet be used as an
indication that the population will suffer,” Bednaršek said.
“What we have is an early warning signal that there is a
vulnerability.”

In adult Dungeness, that vulnerability manifests in different
ways.

The opening of the Dungeness season has long been dictated by
testing for the presence of domoic acid, a natural byproduct of
some toxic species of plankton that can cause illness in humans.
Ricketts says that fishermen have adapted to this regulation,
“but recently it has become more of an issue.”

The fishery was delayed in 2015 over high levels of domoic acid
attributed to an unprecedented toxic algae bloom. That bloom, in
turn, was linked to a persistent marine heatwave called “The
Blob” that kept water temperatures 2.5 degrees above normal for
226 days.

Ricketts also points to increased fishing pressure on crabs.
While many fishermen will spread risk by targeting two or three
different species each year, Ricketts said the declines and
closures of other fisheries have pushed more people to fish for
Dungeness, a finding confirmed by California Department of Fish and
Wildlife environmental scientist Christy Juhasz.

“There are always a limited number of permits, and only a
certain proportion that are active in any given season, but we have
noticed an uptick in the number of those active permits in recent
years,” Juhasz said, although she added they aren’t able to
attribute a direct cause to the increases.

Similarly, the recreational demand for Dungeness is on the rise.
Fisheries are built around Size, S*x and Season (the “3 Ss”),
meaning commercial fishermen can harvest during open season only
male crabs wider than 6 ¼ inches, which generally corresponds to
an age of about four years. But recreational fishermen can harvest
above 5 ¾ inches, placing a higher burden on the younger age
classes.

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According to Bednaršek, an interesting finding in her work was the
disproportionate effect that ocean acidification had on larval
shell width, the metric by which adult crabs are sized for
harvesting. Her study found that crab larvae found in more acidic
areas were less wide.

“We don’t understand how that’s going to travel through
the different life stages, but it was particularly interesting to
see that they are growing smaller in the parameter that is actually
most decisive in the fishery,” she said.

Because crabs take several years to reach a size at which they
can be fished, any effects happening now will have a time delay of
several years before they become clear. Similarly, carbon dioxide
being put into the ocean now will remain in the water for
centuries.

With Dungeness season currently underway after a delayed start,
time will tell how this story plays out in California’s
waters.

Source: FS – All – Interesting – Lifestyle
Dungeness crabs show indication they may be vulnerable to effects of ocean acidification along Pacific coast