New Study Shows Estuaries are Key to Ensuring the Survival of Dungeness Crabs
By Debbie Kay
Little is known about the lifestyles of Dungeness crabs. Their movement, their range, their predicted populations and their DNA diversity are all still relatively unknown. A paper published in May
of this year by the University of South Denmark has solved at least one mystery, however. The presence of parasitic worms, which has been believed to be the cause of crashing crab populations in central California, can be treated if the crabs have healthy estuaries to escape to.
What is an Estuary?
Estuaries are a form of wetland that contains a mix of freshwater and saltwater, called brackish water. The plants in these wetlands are often specialized, and can only grow in this low-salinity environment. Coastal estuaries are common along both the East and West coasts of the US.
What the Study Showed
This study looked at the parasitic nematodes that live among the eggs in a female crab’s abdomen. The worms are passed from crab to crab by mating, and each one can eat eighty of the two million eggs that the female holds, though they don’t affect the meat of the adult crab at all. A large population of these worms can threaten an entire year’s worth of eggs. For a species where only a small percentage of larvae survive to adulthood, even a loss of some of those eggs can have implications for future years of crab catch.
For a long time, people have been trying to figure out why the Oregon crab populations had a smaller issue with these parasitic worms than the California population. There was a belief that it was temperature related, and this led to a large concern about global warming and the future populations of crab. However, all temperature studies done on these worms showed no change to the worm population. This study showed that a drop from the salinity of 33 (this is 33 grams of salt in a kg of water) to 20 was enough to start killing worms. Crabs can survive for weeks in a salinity as low as 8. This means that crabs who have access to these estuaries, which often have salinities in the teens or lower, have the means to kill off any parasite populations on their body.
This demonstrates a pretty direct link between the health of crab stocks and the ability to find healthy local estuaries. These estuaries have other benefits to people as well. They serve as breakwaters for large tidal events, and prevent beach erosion. They are flood control during extreme rain conditions. They are often the nursery grounds for many different game fish. They are a repository for sediment or other harmful things before they are flushed into the ocean. There has been a lot of pressure to build over or remove the temperature controlling shade buffers around these wetlands. However, with the important role they play in keeping many river and ocean species at optimal health, maybe they deserve a second look.