Restoring predator numbers by culling their prey

Helping out a threatened predator by culling their prey seems like a really stupid idea. But Scandinavian scientists have found that it might be the best strategy for helping some of our ailing fish stocks.

Lennart Persson and colleagues from Umeå University came up with this counterintuitive concept by running a 26-year natural experiment with the fish of Lake Takvatn, Norway. At the turn of the 20th century, the top predator in Lake Takvatn was the brown trout. Over-fishing sent its numbers crashing, and it was virtually gone by 1980.

In its place, a smaller fish - the Arctic char Â- was introduced in 1930. Char should make a good meal for trout, so it was surprising that when the trout were reintroduced they failed to flourish despite an abundance of food.

It was only in the 1980s, when the researchers removed over 666,000 char from the lake that the trout started bouncing back. While their prey population fell by 80%, the trout have increased in number by 30 times. The lake's temperature and nutrient levels were mostly constant during this time, so why did the trout do better when they prey was culled?

Persson believes that it's not the numbers of the char, but their size that matters, and that changed irrevocably when the trout first vanished. Paradoxically, predators like trout, can actually increase the numbers of small prey by eating them. It seems like a strange idea, but it happens because the remaining prey face less competition for food. As a result, they grow more rapidly, mature faster and give rise to more young.

This means the population becomes, on average, smaller, since individuals spend less time growing and fill the water with baby fish. And that's good news for predators. But take the predator out and the whole system grinds to a halt.

Conservationists often like to believe that an over-hunted predator will just bounce back into its original niche once hunting is stopped. But things are rarely that simple. In the predator's absence, other species will rush in to fill the gap and the entire system can settle down into a new balance, which the predator can find very hard to slot back into.

In Lake Tyvatn, the absence of the trout meant that the char population faced no threats and competed heavily for resources. They grew and reproduced slowly, reaching sizes too large for the trout to tackle. The proportions of small char fell to a level which could not support reintroduced trout. Essentially, while the predator was away, the prey took it easy and locked the door so it couldn't get back.

When the char were culled, this mimicked the effects of trout predation by removing the largest individuals. As a result, the numbers of smaller, trout-friendly char doubled and began to dominate the lake. And that shift finally allowed the trout to regain a foothold (or finhold) in the lake. The two fish have now established a balance in numbers for over 15 years.

Persson's study clearly shows that removing a predator from its habitat (an all-too common occurrence) doesn't create a predator-shaped hole in the ecosystem, ready to be filled again. Instead, it causes drastic changes to local food webs, that can only be reversed with counter-intuitive and ingenious strategies.

Stocks of predatory fish, including sharks, salmon, cod and trout, are threatened by over-fishing all over the world, and Persson believes that his strategy could help them to recover. For example, the falling cod population in the Baltic sea could potentially be restored by fishing for its prey, like herring or sprat. It's so crazy, it just might work.

Reference: Persson, Amundsen, de Roos, Klemetsen, Knudsen & Primicerio. 2007. Culling prey promotes predator recovery - alternative states in a whole-lake experiment. Science 316: 1743- 1746.