Perch discover nature's junk food

Ian Popple | According to new research by invasive species biologist Anthony Ricciardi, yellow perch — a freshwater fish found across Canada — has developed an appetite for invasive quagga mussels. Stomach content analysis has revealed that at some point in the past decade, yellow perch learned to eat the troublesome invaders, who cause millions of dollars of damage to the Great Lakes region each year. Unfortunately, quagga mussels make dangerous dining — they contain contaminants, most notably a botulism-causing bacteria called Clostridium botulinum. Ricciardi is concerned that the yellow perch's diet shift provides a new way for this toxin to move up the foodweb, placing countless predatory fish and birds in danger.

Ten years ago, Ricciardi conducted a study of fish stomach contents to assess whether yellow perch preyed on invasive quagga and zebra mussels — collectively known as dreissenid mussels. His results confirmed that perch were one of many species that found the hard-shelled aliens unpalatable. "We found no evidence of mussel consumption in yellow perch," stated Ricciardi. In a follow-up study conducted earlier this year, however, Ricciardi and his assistant Colette Ward discovered quagga mussel fragments, and in some cases even whole individuals, in almost every perch examined. "At some point over the past decade, the perch in our St. Lawrence River study sites added quagga mussels to their diet," said Ricciardi.

The quagga mussel arrived from Europe a year after their renowned cousin, the zebra mussel. Both species exploded in number throughout the Great Lakes region during the '90's, pushing local populations of native mussels to the brink of extinction. "Dreissenids have few natural predators and a relatively short life cycle," said Ricciardi. "They easily out-competed Canadian mussels." In what Ricciardi describes as an ecological coup, the quagga has now muscled out the zebra, and is fast becoming the dominant mussel species in many areas.

At first glance, it appears that one of Canada's own is finally regaining ground against this highly successful invader. But according to Ricciardi, this discovery opens a whole new can of worms. "Dreissenid mussels are junk food," he said. "They accumulate an assortment of toxins from the environment as they feed, and this new feeding behaviour provides an additional path for contaminant transfer."

Most worrying among these toxins is the botulism-causing called Clostridium botulinum bacteria — yet another invasive species. The botulism bacteria, which can increase to fatally high levels in organisms higher up the foodweb, was first identified in 2000 by Ward Stone, chief wildlife pathologist at the New York Department of Environ-mental Conservation. The disease has killed tens of thousands of birds and fish in Lake Erie, and Stone, who is also credited with identifying West Nile Virus in North America, has since tracked the bacteria's movement into Lake Ontario.

Many more animals may be at risk now that perch have discovered the gastronomic delights of dreissenid mussels. "Predation will pass contaminants from one level of the foodweb to the next," explained Ricciardi. "Bacteria build up inside mussels, mussels build up inside perch, perch build up inside bass, walleye, loons and other fish-eating animals — it's like a hideous Hieronymus Bosch painting." At present, it is simply too early to predict the effects that this new behaviour will have on wildlife in the Great Lakes region. Much will depend on the level of this toxin in quagga mussel tissue, the amount that consecutive levels of the food chain consume and how widespread this new perch feeding behaviour is.

For Ricciardi, an experienced general in the battle against invasive species, this new discovery represents a call to arms. "We must develop more stringent measures to protect against invasive species," he said. "Once they arrive, they are virtually impossible to remove." Currently, their preferred method of invasion is to stow away in the huge ballast water tanks that are used to stabilize ships. Freshwater ballast is picked up in the port of departure; this organic soup is then emptied upon arrival at the destination port.

In 1993, ocean ballast water exchanges became mandatory for all ships entering a Canadian port. "It was thought that exchanging the freshwater ballast for saltwater while at sea would remove the stowaways," explained Ricciardi. Although intuitively appealing, the strategy was ineffective. "Most ships can only partially empty their ballast tanks," continued Ricciardi. "Considering the average tank is the size of a thousand-person auditorium, this residual water amounts to an Olympic-sized swimming pool." The addition of saltwater kills most of the freshwater species that remain, but the saline-tolerant organisms — the best invaders — survive. Even after the mandatory water exchange, a ship's ballast tanks can contain billions of viruses, millions of bacterial cells and thousands of species in their planktonic stage, upon arrival in a Canadian port.

There is a sense of futility in the air (and water) when it comes to dealing with invasive species. As witnessed with dreissenid mussels and perch, an interaction effect exists between organisms, which can make a bad situation worse — particularly among invasive species. "Take two invaders," explained Ricciardi. "Species A can interact with species B — this represents one interaction." Three invasive species result in three interactions. Four species result in six interactions and five species result in 10 interactions; the number of interactions grow exponentially with the number of species added. The fight against invasive species often seems futile because of this interaction effect, but there is room for optimism: if the interaction effect can grow geometrically, it can also be reduced geometrically. "Preventing just one species from establishing itself in Canada can have disproportionate benefits," stated Ricciardi. "This is why we must never give up."