Ocean acidification expected to accompany climate change may
slow development and reduce survival of the larval stages of Dungeness crab, a
key component of the Northwest marine ecosystem and the largest fishery by
revenue on the West Coast, a new study has found.
The research by NOAA Fisheries' Northwest Fisheries Science
Center in Seattle indicates that the declining pH anticipated in Puget Sound
could jeopardize populations of Dungeness crab and put the fishery at risk. The
study was recently published in the journal Marine Biology http://link.springer.com/journal/227.
Ocean acidification occurs as the ocean absorbs carbon
dioxide from the combustion of fossil fuels. Average ocean surface pH is
expected to drop to about 7.8 off the West Coast by 2050, and could drop
further during coastal upwelling periods.
Dungeness crab is the highest revenue fishery in Washington
and Oregon, and the second most valuable in California, although the fishery
was recently closed in some areas because of a harmful algal bloom. The
Dungeness crab harvest in 2014 was worth more than $80 million in Washington,
$48 million in Oregon and nearly $67 million in California
"I have great faith in the resiliency of nature, but I
am concerned," said Jason Miller, lead author of the research, which was
part of his dissertation. "Crab larvae in our research were three times
more likely to die when exposed to a pH that can already be found in Puget
Sound, our own back yard, today."
Scientists collected eggs from Dungeness crabs in Puget
Sound and placed them in tanks at the NWFSC's Montlake Research Laboratory. The
tanks held seawater with a range of pH levels reflecting current conditions as
well as the lower pH occasionally encountered in Puget Sound when deep water
wells up near the surface. Larvae also went into tanks with the even lower-pH
conditions expected with ocean acidification.
"The question was whether the lower pH we can expect to
see in Puget Sound interferes with development of the next generation of
Dungeness crab," said Paul McElhany, a NOAA Fisheries research scientist
and senior author of the paper. "Clearly the answer is yes. Now the
question is, how does that play out in terms of affecting their life cycle and
Larvae hatched at the same rate regardless of pH, but those
at lower pH took longer to hatch and progressed through their larval stages
more slowly. Scientists suggested that the lower pH may reduce the metabolic
rate of embryos. That could extend their vulnerable larval period, or could
jeopardize the timing of their development in relation to key food sources,
Larval survival also dropped by more than half at lower pH.
At pH 8.0, roughly equivalent to seawater today, 58 percent of the crab larvae
- called zoeae - survived for 45 days. At pH 7.5, which sometimes occurs in
Puget Sound now, survival was 14 percent. At pH 7.1, which is expected to
roughly approximate the pH of water upwelling on the West Coast with ocean
acidification, zoeae survival remained low at 21 percent.
"Areas of greatest vulnerability will likely be where
deep waters, naturally low in pH, meet acidified surface waters," such as
areas of coastal upwelling along the West Coast and in estuary environments
such as Hood Canal, the new study predicts.