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Predators need prey. So, theoretically, brown trout (Salmo tutta) should thrive in lakes teeming with arctic char (Salvelinus alpinus), a potential meal. Yet, brown trout, depleted by overfishing, remained scarce in northern Norway's Lake Takvatn even though fishing was halted and there were plenty of char for them to eat. Only when scientists removed 660,000 of the char (between 1984 and 1989) did brown trout once again rule the waters.
Ecologist Lennart Persson of Umeå University in Sweden and colleagues tracked the two fish populations through 2005 to determine how culling prey affects predators. At first glance, it would seem odd that the trout population would recover so quickly when its main food source was reduced by 80 percent.
But Persson says that the dwindling number of predators allowed the char to grow too big for the remaining brown trout to swallow—and, in fact, enabled the former prey to become competitors for other food sources such as insects. The brown trout needed small, quickly reproducing char to feed on in order to rebound, so killing off the bigger, more sluggish prey actually benefited the predator. "Before the thinning, the density of char was very high but the population consisted of slowly growing individuals with low fecundity," Persson says. "The predator, brown trout, increased 30 times in numbers following the char thinning."
Further, the brown trout were capable of sustaining this ideal condition for themselves; numbers of smaller char increased along with the numbers of their predator in subsequent years. "The predator causes an increase in its own resource—small prey—availability by eating on it," Persson says. Brown trout and char numbers have reached a new equilibrium that has been maintained for the past 15 years, the researchers report in Science.
Neither temperature nor nutrients in the lake changed during the study period and a nearby lake showed no similar recovery without the culling. Given the collapse of a wide variety of predatory fish worldwide due to similar overfishing, this new strategy might help promote recovery. "It may be more feasible to thin out the prey fish population than to stock with predatory fish," Persson says.
For example, collapsed cod stocks in the Baltic Sea might be saved if its prey, herring and sprat, were more widely fished, he says. And assigning individual quotas on the amount of fish anglers are allowed to catch, credited with fish recovery from New Zealand to Iceland, might prevent depleting populations in the first place, according to another report in Science. By combining such tradable quotas and selective fishing of prey species, there may be hope yet for restoring the bounty of both the ocean and freshwaters.