Columnist: Our freshwater clams are in peril

Published 8:21 am Thursday, July 16, 2009

What do you think is the most endangered animal group in North America? According to the U. S. Geological Survey it’s freshwater clams.
Freshwater clams inhabit every continent except Antarctica but most – some 270 species – live in North America.  Currently 72 percent of our native clam species are listed as extinct, endangered, threatened or of special concern. I believe that.  When I was a kid clams were everywhere but discounting northern Canada I haven’t seen a clam in years.
Why such a drastic decline? As so often the case it’s a combination of multiple causes. Pollution is at the top of the list. Clams subsist on nutrients they filter from the water. Clams can live 100 years and over time toxins and harmful bacteria they filter from the water build up in their bodies. Our pollutant glut of DDT, PCB and all the other junk of the mid-1900s was almost more than they could withstand.
Of course, clams live on the bottom of lakes and rivers where most pollutants congregate. They are extremely pollution sensitive and even today’s “acceptable” levels of herbicides and pesticides are more than they can tolerate. Vast amounts of agricultural lands are tiled allowing the surface water to drain directly into waterways, carrying the farming chemicals with it. No freshwater clam in the U.S. is considered safe to eat.
Commercial harvesting has also been devastating.  Until the plastic age beginning in the mid-1900s nearly all buttons were made from North American clams. It’s estimated some 600 million clams per year were turned into buttons. Today, clams are still being harvested from some areas in huge quantities for their shells. Cultured pearls are created by inserting an irritant into certain saltwater clam species. The clam then forms a pearl around the irritant. A piece of North American clam shell is deemed to be the best for this.
Virtually every cultured pearl sold world wide has a piece of North American clam shell in the middle.  Along this line, freshwater clams rarely form pearls of value. However, one high grade pearl was found in an Ohio clam. That sparked a nationwide pearl rush where hundreds of thousands were harvested in the futile hope of getting rich.
Man’s mucking around with waterways has also had dire consequences. Dams turn streams into lakes, which wipe out river inhabiting clam species. When wetlands are drained the clams perish. At maximum speed a clam can only move about 10 feet a day so it can’t dodge dredges and cranes or relocate when its habitat becomes unsuitable. There are other less obvious perils. During part of their life cycle clam larvae must mature on a fish’s fins or gills. With most clam species the type of host fish is very specific. If their host fish declines so, too, do the clams and a number of fish species are being displaced by introduction of exotics like the various carp, gobies and even sport fish stocking.
And speaking of exotics, even the clam’s distant relative, the zebra mussel, has joined the attack. The tiny zebra mussels attach to the larger clams in number, making it too heavy to move. They are also filter feeders and get the food before the clam can, causing starvation. Raccoons, muskrats, mink and otter, among others, find clams a special delicacy.  With the demise of the fur trade their numbers have exploded and in many areas predation is significant.  It takes about ten years for clams to sexually mature and they are slow reproducers so they don’t rebound from catastrophic events easily.
The loss of our clams has major environmental impact. Clams were nature’s water purification system. A single clam can filter up to 10 gallons of water a day. Many of their favorite foods are undesirable to us such as E. coli bacteria and blue-green algae. Many watersheds that once held a hundred clams per square yard are now completely void of them and the water quality has significantly deteriorated. Here in Michigan we have 46 clam species. It is illegal to possess any of these clams or even their shells. If only all the other states would realize the importance of clams and follow suit.  Carpe diem.

Larry Lyons writes a weekly outdoor column for Leader Publications. He can be reached at