A couple of weeks ago, over Tuscan roast pork and some good wine, I asked a fellow dinner guest who works on the UC campus if there were still three-spined sticklebacks in Strawberry Creek. He wasn’t sure about the sticklebacks, but he said the crayfish were still around.
The crayfish were news to me. As it happens, this particular species—the signal crayfish, Pacifastacus leniusculus—is a protagonist in one of those ecological horror stories about invasive exotics running amuck. Native to the Pacific Northwest, it’s made itself at home not just in Strawberry Creek, but all over California, from the Delta to Lake Tahoe, and in Britain, Scandinavia, and Japan. And here in the Bay Area, it probably drove a close relative to extinction. It’s hell on small fish, too.
Named for the white patch on near the hinge on each of its claws, this beast is about six inches long, bluish-brown to reddish-brown in color in an unboiled state. Signal crayfish originally ranged from British Columbia south to Oregon and east to Idaho, where they inhabited small streams and ate whatever they could get their pincers on: aquatic plants, algae, carrion, insects, snails, small fish.
They mate in the fall, and the females schlep the fertilized eggs around with them for the next seven or eight months. The tiny crayfish, miniatures of their mother, hatch in the spring and stay with her until after their second molt, when they strike out on their own.
What are they doing here? In 1912, someone had the bright idea of using signal crayfish to study crayfish predation on young trout. Batches of crayfish were shipped from the Columbia River to a state Fish and Game hatchery in Santa Cruz County. When the study was completed, the crayfish were released into the San Lorenzo River.
From that beachhead, the exotics spread through most of Northern California. They were popular as fishbait, and anglers would dump the leftover contents of their bait buckets into lakes or streams.
When they got to the Sacramento-San Joaquin Delta, they must have thought they’d reached crayfish heaven. Signal crayfish, along with Louisiana red swamp crayfish (Procambarus clarkii), thrived and multiplied there. They became part of the Delta food chain, comprising 90 percent of the diet of river otters. And they supported a commercial fishery that as of the 1970s shipped almost all its catch to Sweden.
Scandinavians, like Cajuns, take their crayfish seriously (in Sweden, boiled in salted water with dill.) Their native species, the noble crayfish (Astacus astacus), was nearly wiped out by a fungal disease called the crayfish plague that first struck in 1907. In the 1960s, the Swedes decided to eliminate the American middlemen and raise their own signal crayfish, which they considered acceptably close in taste to the noble crayfish. Signals are resistant to the plague, although they can be asymptomatic vectors. So 60,000 signal crayfish traveled from Tahoe to Sweden.
Signal crayfish thrived in Swedish waters. They were also introduced to Britain in 1972 to stock farms for the restaurant trade. Some, inevitably, escaped, and now they’re preying on native British fish and outcompeting the native white-clawed crayfish (Austropotamobius pallipes). Exotic crayfish can devastate streams, eating their way through the aquatic vegetation, then going after fish, frogs, turtles, and snakes. Biologist Philip Fernandez, who fights crayfish infestations in Arizona, says “I used to like eating them, but now I think of them as aquatic cockroaches.”
Other European countries, notably Ireland and Norway, were alarmed enough to ban the import of all non-native crayfish. But similar efforts elsewhere have been stymied by one of those wonderful institutions of economic globalization, the General Agreement on Trade and Tariffs. Because it had a preexisting trade in crayfish, Germany was not allowed under GATT to block the import of exotic crayfish, regardless of any disease risk to its native species. (It’s a good thing the British abolitionists celebrated in Adam Hochschild’s new book Bury the Chains didn’t have GATT to contend with.)
Meanwhile, back in California, the signal crayfish had also reached the Bay Area, which was already home to the sooty crayfish, P. nigrescens, found here and nowhere else in the world. I haven’t been able to identify the tipping point, but before too long there were no more sooty crayfish; it was the first North American crayfish species to meet global extinction. What happened can be inferred from the competition between signal crayfish and the Shasta crayfish (P. fortis), endangered but still hanging on in the Northern California mountains. Compared with Shasta crayfish, signals have a broader diet and tolerate a wider range of water conditions; they’re larger, more prolific, faster-growing, more aggressive. And they’re pushing their Shasta relatives to the wall.
The sooty crayfish may be long gone, but signal crayfish are still impacting stream ecosystems in the Bay Area. A couple of years ago, biologist Frank Yoon implicated the exotic species in the decline of a small fish called the prickly sculpin in Strawberry Creek.
He sank four isolation cages in the creek and stocked them with sculpins (caught elsewhere) and signal crayfish. Control cages contained only fish. After 10 days, only two sculpins remained in the experimental cages, while the control cages had 100 per cent survival rates. Yoon didn’t catch the crayfish in the act, but you could make a prima facie case for predation. He did see crayfish attacking sculpins in a laboratory setting.
Can anything be done about these invaders? The British have reported some success with pheromone traps using slow-release gels. And while I don’t have a population estimate, I suspect even in Strawberry Creek you’d have the makings of a lot of etouffe. Hey, if P. leniusculus is good enough for those picky Swedes, I’m willing to approach it with an open mind.