It came from the pond
It squats in the mud and shallow water, its huge, protruding eyes and superb hearing attuned to only one purpose: swallowing whole any living thing it can fit in its gaping mouth. This Gollum of the wetlands is a frog, but not just any frog. It is the American bullfrog, Rana catesbeiana, an introduced species that is many times larger than B.C.'s native frogs and has made itself at home in the Lower Mainland, the Fraser Valley, and southern Vancouver Island. It eats insects, fish, and other amphibians (even its own species), but it will also gobble up turtles, snakes, small mammals, and ducklings. A female can lay floating mats of up to 25,000 eggs that hatch in three to five days.
Within the bullfrog's general geographic distribution swims its voracious newcomer counterpart. Nicknamed “bucketmouth” by freshwater anglers, the largemouth bass has an eating style that brings to mind a cannibal vacuum cleaner. Anything swimming the surface of slow or still waters is at risk if Micropterus salmoides can wrap its 10-centimetre-wide mouth around it. Underwater, any smaller species of fish is on this apex predator's menu as well, including other bass.
In Canada, five percent of mammal species and 27 percent of vascular plant species are alien, according to Environment Canada's Canadian Wildlife Service and the Canadian Wildlife Federation. There are hundreds of introduced or alien species in B.C., most of them plants of different types. Some are referred to as “invasive” species if they move into and dominate new habitats, sometimes displacing other species. Common alien vertebrates that have been around B.C. for so long that most people don't remember ever not seeing them include grey squirrels, mule deer, mute swans, European starlings, black rats, and house sparrows.
But the bullfrog and largemouth bass present some unique and relatively recent threats. Both were deliberately introduced by humans, and populations of some native species of frogs and salmonids have recently experienced declines. There are no studies that can point to either alien as being even partly responsible for those local declines, but that's exactly what worries some researchers and biologists: the lack of information. That means there are no studies proving them as not culpable, and both populations appear to be spreading relatively rapidly. As well, the possible effects of global warming on the viability and dispersal of the two heat-loving species locally is unknown.
“IT'S THE UNCERTAINTY,” Scott Hinch says of what bothers him about the largemouth bass's effect on salmon and trout populations. Hinch is a professor in UBC's forestry department and has studied adult-salmon-migration biology. While sampling various sites in the Lower Mainland in 2000 and 2001, he found juvenile largemouth, “just a few” , from the mouth of the Fraser River up to Mission. Then he found some in sloughs near Pitt Lake, and some adults were caught in that lake. The bucketmouths next turned up, ominously, in several unconnected ponds and lakes nearby.
“Bass are very good at exploiting their environment,” Hinch says. “They spread so well on their own in connected waters.” He suspects that sport fishers are responsible for the largemouth populations in Pitt's outlying lakes, but it's their cohabitation with juvenile salmon, trout, and some endangered species””such as the Salish sucker and the Nooksack dace””in connected waterways that really worries him.
“They're voracious; they're highly predatory and they'll eat all sorts of fish species. You're toast if you are a small fish and in their vicinity. They can be the same size as a small salmon, two or three kilograms.” As with bullfrogs, largemouths grow much bigger in the southern U.S., with the record weighing in at 10 kilograms and measuring almost 100 centimetres.
Hinch wanted to study the situation because although there is research showing adverse effects on salmonid populations after Micropterus introductions, it is not B.C. research. “I tried to get some [government] money to study the interaction between bass and other species, but I wasn't successful. These sorts of invading fish species get caught in kind of a [bureaucratic] netherworld.”
The real possibility of warmer temperatures in the future is even more reason to fret, Hinch says. “It's a worrisome thing with the climate changing. They [largemouths] are adapted well to slower and warmer waters. They are known to compete with and prey upon juvenile trout and salmon, and the warmer the water gets, the better position they will be in.”
The Reel Angler's Outdoor Magazine Web site (www.anglingbc.com/reelangler/placesforbass.html/) lists 41 bodies of water in the Lower Mainland and Vancouver Island (including some in the Okanagan Valley and the Kootenays) known to harbour bass. Although it is illegal to stock bass in B.C. waters and a permit is needed to even capture and transport live bass (and it is illegal to use live minnows or other fish as bait in freshwater), the largemouth bass's range still steadily expands in unconnected waters. The finger of suspicion routinely gets pointed at sport fishers, so much so that the West Coast Bass Anglers group posted a statement on a Web site (anglersatlas.com/) noting that its members “do not encourage non legitimate stockings which are illegal and greatly frowned upon” . The WCBA's “conservation director” , Shawn Smith, even issued a news release in April 2004 that promised a $1,000 reward for information leading to the arrest and conviction of anyone illegally stocking bass in B.C. (Justin Horsman, a WCBA ex–vice president who was with the club for a year after the news release and describes himself as “still in the bass scene, so I'd know” , says on the phone: “As far as I know, there haven't been any [payments].” )
MIKE PEARSON CAN tell you what a tough customer the bucketmouth is. Pearson is a Vancouver resident who earned his PhD at UBC, studying local endangered fish species. He is now a private consultant and contracts with various levels of government on stream-restoration and endangered-species matters. He has no doubt how the first bass got to B.C a few decades ago””“There's no way they got here without human help across the Rockies” ””and he says it's a no-brainer that sport fishers are behind some of the recent bass appearances in southern B.C. lakes. “I was at a few meetings with them [bass anglers] a couple of years ago, and it was a kind of 'nudge-nudge, wink-wink' thing,” he relates with a laugh.
Pearson notes that the presence of bass on Vancouver Island shows “there had to be human help” , adding: “There are no native members of the bass family in B.C.”
As far as the paucity of information on the effect of largemouths on juvenile salmon stocks, Pearson says: “We all know that there have been major declines in a number of salmon populations. The lack of research is very troubling.”
In 1999 and 2000, while researching endangered stocks of Salish sucker and Nooksack dace in Pepin Brook, south of Aldergrove, Pearson started coming across bass. Explaining that largemouth bass “are particularly problematic for freshwater fish, which are among the most vulnerable” , he says that freshwater species don't have the vast expanses of the ocean to disperse to in the face of such a threat. “If you think about it, they're on an island of freshwater surrounded by an ocean of land.” (According to a 2005 NatureServe Canada report, more than half of all endangered Canadian vertebrates live in freshwater. The biodiversity branch of the provincial Ministry of Water, Land and Air Protection notes that the extinction rate for freshwater species is predicted to be five times that of all other groups of species, and that invasive species are probably the cause of more than 70 percent of the past century's extinctions of native freshwater species.)
Pearson says he was especially worried because the introduction in the 1990s of brown bullheads to Hadley Lake on Lasqueti Island in the Strait of Georgia resulted in the extinction of two species of stickleback there.
Above Pepin Brook was an abandoned gravel pit that had filled with water and contained three ponds with a tributary-stream outflow. The ponds had been stocked with largemouth bass, and so began Pearson's nightmare bucketmouth battle.
He installed a “two-way trap on a fish fence” to monitor fish movement into Pepin and found that thousands of bass were moving downstream into the endangered fishes' habitat. Along with representatives of a streamkeepers group, provincial and federal officials, and other biologists, Pearson set out to take the bass down.
“We started with trapping in the ponds, then we gillnetted. We destroyed nests [in shallow water during breeding season]. We seined it, dragging nets through.” It was all to no avail.
So they finally decided to get serious””they dynamited the basses' asses.
Pearson isn't really proud of the decision and doesn't like it played up: “It's a pretty extreme measure,” he admits. But they didn't want to use poison, and there didn't seem to be much left in the arsenal.
“So we dynamited it, but in the end there were still bass. The very young bass survived.” He theorizes that the thick mud bottom had something to do with their victory. “There was this enormous pillow, this huge shock absorber.”
Soon after, the newly elected B.C. Liberals cut funding, which meant the fence had to come down. “And now there is a mature population again,” Pearson says of the ponds. “Once these things are out there, they are out there.”
Which brings us back to the American bullfrog, sort of. Back while still tending his fish fence, Pearson started seeing something different heading downstream. “In two days, we caught over 10,000 bullfrog tadpoles coming out of the ponds. It was a horror show. I was hauling bullfrog tadpoles away in wheelbarrows.”
The funny thing is, Pearson says, stomach-content inventories on trapped largemouth bass in the quarry showed lots of bullfrog tadpoles, with some of the bass full of them. He speculates that although a foul taste keeps many predators away from the bullfrog tadpoles, the largemouth's table manners preclude that as a survival technique. “They don't exactly savour their food,” he points out with a chuckle.
And now he wonders: “Will trapping bass remove a prime predator of bullfrogs?”
ON MANY BALMY summer nights, Purnima Govindarajulu can be found lying in the bow of a canoe, flashlight in hand, among shoreline vegetation in lakes around Victoria, on Vancouver Island. She looks for the betraying “eyeshine” , or reflected light, from bullfrogs' bulging eyes before grabbing them around the waist. She has been researching bullfrogs for several years now, earning her doctoral degree in the process at the University of Victoria, and she is now a research fellow at UVic's biology department.
Govindarajulu says bullfrogs came to the Fraser Valley and Vancouver Island in the 1930s and 1940s, when some optimistic entrepreneurs thought they could farm them and corner the restaurant market for frogs' legs. When the dining crowd proved less than responsive, the farms shut down and the frogs were set free. She acknowledges accounts of bullfrogs' legendary appetite (“If they can catch it and they can swallow it, they will eat it” ), but she laughingly draws the line at an account she heard about a bullfrog leaping for a red-winged blackbird: “I think that would be pretty rare.” A widely reported story about a bullfrog attacking “Boots” , a cat, in Langley in 2004 draws a similar reaction. Still, the largest bullfrog Govindarajulu ever caught weighed an impressive 750 grams and was 30 centimetres long, and she says “they get much bigger in the southern states” .
Of her earlier research, she recalls: “We were worried that bullfrogs were outcompeting and displacing native frogs. They will eat native frogs if you have them in the same habitat. Overall, the consensus is that bullfrogs displace native species if humans modify habitat to favour bullfrogs.” Such modifications could mean anything from introducing non-native fish species to development around waterways that could result in lakeshore vegetation clearance, she explains.
Now Govindarajulu is studying another invasive species, a chytrid fungus, called Batrachochytrium dendrobatidis, that is the cause of a deadly amphibian disease responsible for some species extinctions worldwide. The disease was first noticed in Australia and Central America in 1998, according to Govindarajulu, after “catastrophic declines in frog populations” . Last year marked its first discovery in B.C. Because they have found large loads of the fungus on bullfrogs, she and her colleagues think it is a vector, or carrier, of this pathogen, although it doesn't appear to be affected by it in any significant way. She says no one knows exactly how the fungus kills, but it affects certain species of frogs, toads, and salamanders and not others. “And when it happens, it happens fast.”
Govindarajulu now thinks that it's possible something else could be more of a threat to native B.C. amphibians than Rana catesbeiana's famous appetite.
“Could it be not predation but the disease they brought with them? I'm not saying bullfrogs can't cause rapid declines under certain circumstances...but let's pay a little more attention to other circumstances that may be playing a part. In ecology there are no simple answers, and there are usually a few other things going on.”
An illustration of how that plays out in the interaction of alien species in a new habitat is a situation that is the reverse of Mike Pearson's observation of how one alien, the largemouth bass, controlled the spread of another, the bullfrog. Govindarajulu tells of a U.S. Pacific Northwest study showing that when alien American bullfrogs colonized ponds that also had non-native sunfish introduced, the bullfrogs thrived. This was because the sunfish feasted upon ferocious dragonfly nymphs, which are themselves significant predators of tadpoles, even the relatively huge bullfrog tadpoles.
As far as global warming helping the spread of American bullfrogs in B.C., Govindarajulu can only echo Scott Hinch and say: “We don't know. It might.”
It's the uncertainty.
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