Biology Reference
In-Depth Information
FIELDWORK: FISHING FOR MERCURY IN THE BAY
Walking in the bay is a test of your patience, the strength of your quads, and the
quality of your waders. The water is too murky to see your feet. If you lean over too
far or move too fast, a gush of cold water may lap over the tops of your waders
and wet your belly. If you fall, you're soaked. The bottom, meanwhile, seems to
suck and grip your rubber-encased foot as if it were a pacifier and the bay a teeth-
ing baby. Your toes grope along, feeling for solid spots, snarls of wires, snags of
roots. With each step, your heart follows your foot—the farther it sinks, the harder it
will be to pull out.
I walked into the bay with scientists from the San Francisco Estuary Institute to
fish for mercury. The institute regularly tests the bay for contaminants to measure
whether water quality goals are being met. On this day, however, our team, led by
environmental scientist Ben Greenfield, is on a special mission: we must catch
100 Topsmelt, gobies, and silversides, all of roughly the same size and weight, to
be tested for methyl mercury. Where each individual fish hangs out, what it eats,
how far it travels: all of these factors affect the body burden of mercury in their tis-
sues. These species neither live long nor stray far, so their bodies reflect immedi-
ate, localized conditions and can serve as what scientists call “biosentinels” of
ecological health.
Though the bottom of the bay may not be the black mayonnaise of spilled fuel
and industrial effluents found in Boston or New York harbors, it contains its fair
share of contaminants, including mercury washed down from gold mining in the
Sierra foothills more than 150 years ago.
As we unload our gear on the bay shore on this sunny fall day, we are sur-
rounded by signs of human changes to the bay. There are the levees we've driven
over to reach the shore, built in the 1890s to drain and reclaim these marshes for
farming; the wooded cove sheltering the rundown shacks and docks of a circa-
1800s shrimping camp; and a pickleweed field furrowed with unnaturally straight
tidal channels created by the rapid deposition of mining debris on mudflats. Of
the 200 million cubic yards of mine spoils that ended up in San Pablo Bay, the
majority arrived over a period of only 21 years (1856-1887). Some of those
spoils ended up here, on the shallow shores of San Pablo Bay at Hamilton Air
Force base in Marin County.
On this fishing trip, we use a net not dissimilar to the one the Chinese used to
catch shrimp long ago: a sack with a drawstring connected to two poles staked in
the water. Ours too has a sack, with two net wings, each attached to a pole. It has
floats on the top and weights on the bottom. The guy who knows where the fish
are, Andy Jahn, takes one pole and I take the other, and we lumber across some
salt grass into the water.
It's an odd feeling, for the uninitiated, to walk up to your waist in water and not
feel wet. Coolness but not wetness tickles your skin. Andy coaches me as I totter,
struggling to keep my footing. He says to go slow, to place the pole bottom in the
mud ahead of me for support and then catch up to it. My leg muscles are scream-
ing after only a few minutes. When Andy gives the nod, we both walk back the way
