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 - 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?
Earlier this year, I wrote about how the human obsession with size is reshaping the bodies of other species at an incredible pace. Unlike natural predators that cull the sick, weak and unfit, human fishermen prize the biggest catches and throw the smallest ones back in.
As a result, fish and other species harvested by humans are shrinking, often within a few generations, and are becoming sexually mature at an earlier stage. These changes are bad news for populations as a whole, for smaller individuals often have lower odds of survival and produce fewer offspring.
But David Conover from Stony Brook University has found a silver lining in this tale – selectively harvesting fish can lead to dramatic changes in body size, but these changes are reversible. Release them from the pressure of constant hunting, and some of the animals start to rebound to their previous state.
Conover spent ten years raising a commonly harvested species called the Atlantic silverside in six captive populations, each containing about 100 individuals. Every year, the fish produced a new generation and for five years, Conover would remove 90% of the fish, either by taking the largest ones, the smallest ones or randomly selected individuals. In every other way, the fish were all reared under exactly the same conditions. This constant upbringing ensured that any changes to their bodies would be the result of genetic influences rather than environmental ones.
In the Goualougo Triangle of the Republic of Congo, a chimpanzee is hungry for termites. Its prey lives within fortress-like nests, but the chimp knows how to infiltrate these. It plucks the stem from a nearby arrowroot plant and clips any leaves away with its teeth, leaving behind a trimmed, flexible stick that it uses to “fish” for termites.
Many chimps throughout Africa have learned to build these fishing-sticks. They insert them into termite nests as bait, and pull out any soldier termites that bite onto it. But the Goualougo chimps do something special. They deliberately fray the ends of their fishing sticks by running them through their teeth or pulling away separate fibres – just watch the chimp on the right in the video below.
The result is a stick with a brush-like tip, which is far more effective at gathering termites than the standard model. This population of chimps has modified the typical design of the fishing stick to turn it into a better tool. They truly are intelligent designers.
As a species, our unflinching obsession with size is just as apparent in our dealings with other animals as it is in our personal lives. Fishermen prize the biggest catches and they’re are obliged to throw the smallest specimens back in. Hunters also value the biggest kills; they provide the most food and make the flashiest trophies. This fixation isn’t just a harmless one – by acting as a size-obsessed super-predator, humans are reshaping the bodies of the species we hunt, at a remarkable pace and to a dramatic degree.
Predators already put a lot of pressure on their prey to evolve new ways of escaping untimely death. But humans are predators like no other. Not only do we target an uncommonly wide range of species, but we also tend to focus on larger and older specimens, ignoring the sick, weak and unfit individuals that fall prey to most other hunters. In doing so, we have become a leading evolutionary force, setting off some of the most abrupt physical changes ever observed in wild populations.
Chris Darimont from the University of California has exposed the scope of these changes by analysing the results of 34 earlier studies that looked at 29 different species, all harvested by humans. From mighty bighorn sheep to innocuous limpets, Darimont found that almost all of the harvested species had shrunk in some way, becoming about 20% smaller within a few generations. The vast majority also became sexually mature about 25% earlier.