Laura Erickson's For the Birds

When I think of blue crabs, I think of whooping cranes. Winter survival for these magnificent birds, all of which winter in coastal areas of the Gulf of Mexico, is directly related to the number of blue crabs they feed on. And the number of blue crabs they feed on in winter is also directly related to the whooping cranes breeding success the following summer.

When physical oceanographer Paul McKay thinks of blue crabs, he thinks of yummy meals. The two of us ate at some wonderful restaurants when I was visiting him in New Orleans, and I could see and taste his point.

Dr. McKay told me about another role blue crabs have in the Gulf’s ecosystem as well:

PAUL MCKAY: One concern is that while oil is not going to cause a lot of damage to certain mature organisms such as mature blue crabs, it does cause a lot of damage to larval stages. And this really came at exactly the wrong time. Happening in late April, in the middle of the spring, it came right during the spawning season for a number of commercially important species including the blue crab. And oil droplets have been detected on blue crab larvae. These larvae are swimming through the oil and trying to get into the estuaries like we discussed earlier, where they will settle and molt and evolve and grow into mature blue crabs.

We don’t know the effect the oil and the dispersant in the oil will have on the larvae, but there’s a very good chance that it will increase mortality and cause a greatly reduced population of adult blue crabs.

Partly this is important because blue crabs are delicious and we all like to eat them, but they play an important ecological role in the health of the marsh. As I said earlier, the marsh is held together through the root system of the marsh grass. And the marsh grass, like anything, it grows and it dies and it reproduces and grows again.

In most of the saltwater marshes, there is a species known as the darter snail. Darter snail likes to feed on dead marsh grass. And this source valuable purpose that consumes the dead marsh grass turns this particulate organic carbon into dissolved organic carbon and particulate inorganic carbon. It just cleans up the marsh.

Blue crabs like to eat darter snails and they keep the darter snail population in check. If you remove blue crabs from the system, or greatly reduce the amount of blue crabs in there, the darter snail population can reproduce out of control. And while they prefer to eat dead marsh grass, they are quite satisfied to eat live marsh grass as well and turn it into dead marsh grass.

So when blue crabs are removed from a saltwater marsh, the darter snail population goes out of control, they start to eat live marsh grass and they kill this grass back. And once this grass dies, the root system dies and it stops holding together the marsh and the marsh begins eroding back out into the ocean.

This has been a major problem in the past couple of decades in Louisiana and especially up the East Coast, where for some reason or other, no one’s entirely sure whether it was fishing pressure or ecological problems or environmental problems, blue crab populations have crashed. Darter snail populations have gone out of control and marsh grass has suffered as a result.

Losing marsh grass is even more critical than it sounds in Louisiana coastal areas, which have in the past 50 or 70 years already lost land greater in area than the entire state of Delaware. Marsh grasses are essential for holding islands together. When grasses die, the soil literally washes away.

We’ll hear more from my conversation with Paul McKay as he describes more of the processes wiping out the coastal marshes of Louisiana in our next program.
I’m Laura Erickson speaking for the Birds.