Restoring predator numbers by culling their prey

By Ed Yong | May 15, 2009 10:44 am

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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.

The brown trout increases the numbers of its prey by eating it!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 ­- 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.

By growing too large, the Arctic char muscled the trout out of Lake Takvatn.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.



CATEGORIZED UNDER: Animals, Conservation, Ecology, Fish, Fishing
MORE ABOUT: char, Fishing, predator, prey, trout

Comments (6)

  1. My, how provocative! However, we wouldn’t lack for predators if it were just a matter of culling their prey. Prey are already among the most heavily fished species in the world. Fishing for pollock in Alaska hasn’t helped bring back Steller’s sea lions, nor has the sardine fishery helped with salmon recovery. Strategies like this require serious study of a particular species’ natural history, and that’s hard to come by in the open ocean.

  2. NoAstronomer

    One thing I’ve read about the herring-cod relationship is that herring are also carnivores. Amongst other things, they eat cod larvae.
    So fishing cod down to the bare bones means more herring which means fewer cod are able to reach maturity. Which explains the stubborn refusal of the Grand Banks cod population to recover.

  3. Who Cares

    Miriam might one of the things that contributes to the Alaskan problem that they stopped fishing in the lake while the pollock fishing continues?

  4. It is counter-intuitive, but fascinating.

  5. Bob

    Great, now I’m thinking of the phase-space predator-prey graphs and how to add size/growth to the whole mess. Is size an average value or a distribution (model-wise)? It’s not even lunchtime here and I’m having a ‘Get thee to Scilab/Mathcad!’ moment.
    Thanks for a fascinating idea; I’ll probably be gnawing on that for days.

  6. Who Care – Could be. I don’t know enough about either species’ life history to speculate. We do know that fishing selects for specific traits in fish, such as earlier sexual maturity and cautious behavior, and that could be having an effect on whatever it is that the sea lions need.

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