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The incredible shrinking fish

Richard Black | 14:32 UK time, Tuesday, 13 January 2009

Fish in dishThe 1957 sci-fi movie The Incredible Shrinking Man suggests that humans can have a pretty startling impact on the world.

As a result of exposure to a cloud of pesticide and something else probably involving "radiation", our hero Scott Carey begins to shrink until he's just a few inches high and is seriously threatened by the family cat (OK, the film's title didn't win any awards for lateral thinking but the special effects were pretty good for the 1950s).

Sci-fi directors usually care more about the fi than the sci, and as with many other films in the genre the precise causes of the incredible shrinking aren't detailed too carefully, nor whether all the factors are entirely of human origin.

Out in the natural world, the picture is clearer. Some animals and plants are shrinking. Human activities are responsible, and we know the reason why.

The first time I came across the phenomenon of the incredible shrinking fish was in Australia a few years ago, when I met researchers who'd noted that the world's biggest fish, the whale shark, is getting smaller - and at a startling rate, with the average length falling from 7m to 5m in a decade.

The most likely explanation is that fishermen are pulling the biggest whale sharks they can find out of the ocean, either because they're the easiest to spot or because they're the most lucrative catches.

Individuals that are naturally smaller are more likely to survive and reproduce - and so over time sets of genes producing fish of smaller size become more common in the population.

Over time, the fish shrink.

Whale sharkDifferent groups of researchers have studied the shrinking phenomenon in lots of other fish - cod, flounder, salmon, pilchard - and, to a lesser extent, in land animals and even plants. Now a group of US and Canadian researchers has pulled all of this data together for a paper in this week's Proceedings of the National Academy of Sciences (PNAS).

Chris Darimont and his colleagues assembled a list of studies tracking changes in 29 different species which are hunted or fished or plucked for human consumption.

Some studies had looked at the overall size of the animals (or plants), while others followed changes in the size of various body parts.

The height at flowering of the Himalayan snow lotus (used in traditional medicine) has fallen, the weight of Norwegian caribou has reduced, the horns of bighorn sheep are not as long as they used to be, the volcano keyhole limpet is shrinking.

And commercial fish species after commercial fish species is also getting smaller.

Some researchers had also found that the average age at which species reproduce has changed. On the eastern coast of Canada, for example, cod now reproduce a year earlier than they did two decades ago. Fishing has removed so many of the bigger, later-reproducing fish that the genetic mix has again changed; and this has implications for the long-term health of the stock, as bigger female fish carry more eggs.

The overall picture is startling - across these 29 species traits such as body length are changing about three times faster than in species unnaffected by human hunting.

I called up Chris Darimont (who, holding posts at the Universities of Victoria and California, must be a busy chap) for a chat about what this might mean.

Does it give us a comprehensive view of how human hunting is changing animals and plants? No, because by no means all the species involved have been studied.

Would the organisms grow longer again if hunting stopped? We don't know.

What can we do about it? Is it just a question of reducing the amount of hunting we do?

Big horn sheepHere, things get intriguing. Chris pointed out that human hunting generally targets the biggest, whereas in nature predators generally target the young, the old and the sick. The "genetic winnowing" is very different.

So in fisheries, for example, regulators (and sometimes fishermen) often set mesh sizes delberately designed to let the younger and smaller fish escape.

Could this be entirely the wrong thing to do from an evolutionary perspective? Here I thought back to a radio feature my ex-colleague Tim Hirsch made years ago on the collapse of the Grand Banks cod fishery.

An old-timer who'd lived through the collapse told Tim in his distinctive Newfoundland brogue that fishermen had targeted the old fish, "the mother fish which had been out there spawning over the years", knowing that it was the wrong thing to do. They'd done it anyway because it was the most profitable approach (and also probably because it was what convention dictated).

Economics would almost always push hunters, fishers and gatherers to take the largest of a kind, Chris suggested. It would be challenging too to think of technology that could target the sick, the old and the young as nature does.

The obvious technological shift would be to go back to pre-industrial catching methods, which do not appear to have had the same shrinking impact, as Chris Darimont's colleague Stephanie Carlson found a few years ago when analysing the bones of fish caught in prehistoric hunts and preserved in middens.

Old-fashioned hunters could not target the biggest and strongest in the way we can today. The technology didn't allow it, and presumably getting close-up and personal with a vigorous rampaging caribou in the prime of health could have led to the demise of the hunter rather than the hunted.

Turning the technological clock back is unlikely to be an attractive option. So unless fine minds can come up with another way of doing it, which also adds up economically, it looks as though we will have to live with the fact that our hunting is re-shaping species at an unnatural rate.

It turns out that we don't need clouds of pesticides or mysterious "radiation" experiences to make living things shrink. The modern way of consuming nature is quite powerful enough.

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