Dungeness crabs (Metacarcinus magister) are literally dissolving as the ocean becomes more acidic. A recently published study found that as the waters of the Pacific become more acidic, Dungeness crabs are having a hard time building and maintaining their shells, impacting their ability to survive and function as crabs should. This unwelcome news comes after California closed the Dungeness crab fishery months early in 2019 to protect migrating whales from becoming entangled in crab fishing gear.
The $40 million t0 $95 million (depending on the year) commercial Dungeness crab fishery in California has faced several challenges over the last several years including elevated levels domoic acid from harmful algal blooms and whale entanglements leading to shortened seasons. The fishery is both culturally important and economically substantial. There are 550 Dungeness crab fishing permits in existence in California, with about 450 of those making landings each year, the remaining 100 permits are issued but not necessarily used every year. The fishery is regulated as two zones: North and Central, and the season is open from November through early-to-mid summer. Closures and shortened seasons can cost fishers millions, but they are at the mercy of nature. Ocean acidification is yet another challenge facing the profitable industry. Shellfish such as crabs are disproportionately affected due to their external shells which come in direct contact with the acidifying water.
The ocean is a major carbon sink, meaning it absorbs carbon dioxide (CO2) from the atmosphere. Human activities such as burning fossil fuels and land use changes have increased the amount of carbon dioxide in the atmosphere. Increasing levels of carbon dioxide in the atmosphere causes ocean acidification through a series of chemical reactions that ultimately lowers the pH of ocean water and increases acidity. As the ocean absorbs more and more carbon dioxide, ocean chemistry changes, threatening the organisms that live in the sea.
According to the Smithsonian, over the last 200 years the acidity of the ocean has increased by 30%, which is faster than any known change in ocean chemistry in the last 50 million years. Scientists and fishers are scrambling to determine what these changes mean for the hundreds of thousands of marine species that have been discovered. Scientists estimate that 91% of marine species have yet to be described, meaning the effects of ocean acidification are far-reaching in ways that we aren’t even aware of yet.
Crabs and other organisms that build calcium carbonate shells are at a disadvantage in acidifying waters, because a lower pH means there is less calcium carbonate available to build and strengthen their shells. Studies show that ocean acidification lowers crab survival at every life stage. For an economically and ecologically important species like the Dungeness crab, lower survival could mean the collapse or shrinking of an industry. The Dungeness crab landings for 2018 in California totaled 18,805,626 pounds, more than any other species of shellfish. The industry is also California’s second most valuable fishery, which is why the findings of the recent study are so concerning.
According to a National Oceanic and Atmospheric Administration (NOAA) release, the NOAA-funded study found damage to larval crab carapaces (upper shell) and sensory structures that help crabs orient themselves in the water. The new findings support previous laboratory studies that showed the same outcome.
NOAA’s recommendations moving forward are two-fold: reduce our collective carbon footprint, and be prepared to adapt. Further research is needed to examine whether the same effects apply to mature crabs and how population dynamics may be affected. Much like humans, crabs have a limited amount of energy and resources. Ocean acidification is impairing crabs’ ability to maintain shells and thus, more of their energy will be allotted to building and maintaining their exoskeletons. This could result in smaller crabs and fewer reaching adulthood. This not only affects the crab fishery, but the entire ecosystem as well.