Home & Garden Columns

Snakes in the Reservoir, and Other Booms and Busts

Wild Neighbors: By Joe Eaton
Tuesday November 06, 2007

Sometimes I miss out on interesting natural phenomena. It wasn’t until last month, while cruising the posters at the biennial State of the Estuary Conference, that I learned about the water snake invasion of Lafayette Reservoir. I’d go check it out, but it’s too late; they’re all gone. Another exotic-species boom gone bust. 

These were diamondback water snakes (Nerodia rhombifer), to be exact, a reptile I know from Arkansas. I’ve seen them swimming lazily in a sluggish creek in a Little Rock park. They’re good-sized snakes (about three-and-a-half feet long), heavy-bodied, with a chainlike dorsal pattern, red eyes, and black tongues. Unlike the venomous—and equally aquatic—cottonmouth moccasin, their eyes have round pupils rather than catlike slits. Males can be identified by the projecting tubercles on their chins, although I was disappointed to learn that they do not tickle the females with them. 

As their name suggests, you’d find these guys in ditches, creeks, lakes, ponds, bayous, or swamps, from Alabama west to Texas and north into Iowa. They also get up into branches overhanging the water; I have to wonder if the cottonmouth that dropped into the boat in one my father’s fishing stories, prompting its immediate evacuation, wasn’t a diamondback water snake. 

They feed on fish and frogs for the most part, with the occasional cotton rat or small bird, hunting by smell: a few drops of fish extract in the water puts them in attack mode. Some have been seen trapping fish in their coils. Older individuals hang out with their mouths open and their tails anchored to a rock, facing into the current, waiting for something interesting to swim by. 

Like most water snakes, diamondbacks are foul-tempered critters. “If handled, they bite viciously and spray musk,” write Carl and Evelyn Ernst in Snakes of the United States and Canada. Which brings us to the question of what these non-California natives were doing in the reservoir. I don’t think I’ve ever seen a water snake for sale at the East Bay Vivarium; you’d need a special aquatic setup, and the snake would probably bite you when you tried to clean the tank. 

Lacking other explanations, though, let’s assume some dissatisfied snake owner dumped his (I think “his” is a given) erstwhile pets in the reservoir. This would have been some time before 1990, when two snakes found their way from Lafayette to the UC Museum of Vertebrate Zoology. Soon thereafter parties of 5 to 10 snakes were observed basking on tules, reservoir banks, even piers. There were incidents. Some anglers were unhappy to discover a snake on the end of their fishing line. 

Responding to recreationists’ complains, EBMUD hired Wildlife Control Technology in 1996 to evaluate control methods. The contractor concluded that an “original estimate of 200 snakes made by park staff is realistic, and probably conservative.” Although the species is known to hibernate in the northerly parts of its native range, the reservoir snakes were seen sunning in January.  

WCT made its recommendations (favoring trapping over shooting or the more extreme step of draining the reservoir), but EBMUD never had to implement them. On a subsequent survey in 1999, a new outfit, ECORP Consulting, found large numbers of dead snakes, along with dead red-eared slider turtles, throughout the reservoir. Some of the casualties were reported to have a fungus-like growth in their respiratory tracts, although no further analysis was done and no specimens were preserved. El Niño was blamed, or credited, for facilitating the disease outbreak, if it was in fact a disease outbreak. Snake flu? 

And that may have been it. ECORP says there have been no confirmed water snake sightings at the reservoir since late 1999, despite occasional rumors. Oddly, there are other pockets of alien water snakes elsewhere in the state—southern water snakes near Folsom and Long Beach, northern water snakes near Roseville. But it appears the diamondbacks have died out at Lafayette. 

Which is how it often goes with exotic plant and animal species. They flourish for a while, and then something—predator, pathogen, weather—knocks them back, and sometimes out. Case in point: Vancouver, B.C., used to be overrun with crested mynahs, an East Asian relative of the talking variety. They were the dominant bird of the fast-food parking lot ecological niche. Then they dwindled to a remnant, and a couple of years ago the last of them expired. 

If you’re wondering why reservoir snakes would have been a bad thing, recall that diamondback water snakes eat frogs as well as fish. California’s frogs—red-legged, yellow-legged, Cascade—are on the ropes already. The last thing they need is a new predator. The water snakes would also likely have eaten or otherwise displaced our native aquatic garter snakes. Best to leave well enough alone.