Humans are impossible to live with. Just ask the 820-odd species humans have driven to extinction since 1500 AD. In addition to extinguishing the dodo and the passenger pigeon, humans have fished 20 per cent of the world’s freshwater fish species to extinction. Today, humans harvest over 100 million tons of fish annually. To prevent harvesting fish to extinction, scientists must make careful decisions about the frequency and intensity of harvesting, a field called fisheries management.
One such scientist is Dr. Peter Abrams, a mathematical ecologist in the department of zoology. Abrams develops mathematical models that predict the growth and size of a species based on its interactions with other species. For example, consider a fishery where fish are mass-harvested for human consumption. The size of a fish population depends on its interactions with the tiny plants and insects the fish eat, as well as humans’ harvesting efforts. A fish species’ ability to recover after a harvest depends on its ability to capture and consume food, as well as its prey’s ability to avoid these fish.
After completing his graduate studies at the University of British Columbia in 1979, Abrams taught in the U.S. before coming to U of T in 1999. Here, Abrams began applying his mathematical models to manage fisheries. “I have only gotten interested in fisheries since coming to Toronto. This is in part due to the excellent fisheries biologists who are here,” said Abrams. “I felt there was a need to incorporate what we know about adaptive responses within species and food web interactions to an applied area that has an obvious need for new approaches.”
In a recent study published in the Canadian Journal of Fisheries and Aquatic Science, Abrams describes an odd situation where harvesting fish can trigger a population explosion in those fish. The result is that with every harvest, the number of fish goes up instead of down. Abrams calls this situation the ‘hydra effect,’ after the mythological monster that sprouted two heads for every one chopped off.
“The possibility of a species eventually increasing in numbers in response to greater mortality or worsening conditions is quite counterintuitive,” said Abrams. “This can come about because of adaptive changes in the fish itself.” Unfortunately, the hydra effect has a limit. “This limit tends to occur abruptly without the warning provided by a diminishing population,” he warned. “This is probably why many fish species have been overexploited. It also argues that conservationists should not rely on large or increasing population sizes as indicators that a species is safe from extinction.”
