Earth Watch

The incredible shrinking fish

• Richard Black
• 13 Jan 09, 02:32 PM GMT

The 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.

Different 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?

Here, 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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Ice-cool analysis

• Richard Black
• 13 Jan 09, 10:17 AM GMT

A previous thread here has generated highly divergent views about data on sea ice cover from the University of Illinois, and what it might mean.

I wondered whether the scientists involved could help clear up any confusion on how their data should be interpreted; so I emailed to ask them.

Here's the response from William Chapman of the Arctic Climate Research programme.

"On January 1, 2009, an article by Michael Asher entitled "Sea Ice Ends Year at Same Level as 1979" appeared on the Daily Tech website. We have received many requests for confirmation and clarification on this article from media outlets and interested individuals regarding the current state of the cryosphere as it relates to climate change and/or global warming.

"One important detail about the article in the Daily Tech is that the author is comparing the global sea ice area from December 31, 2008 to same variable for December 31, 1979. In the context of climate change, global sea ice area may not be the most relevant indicator. Almost all global climate models project a decrease in the Northern Hemisphere sea ice area over the next several decades under increasing greenhouse gas scenarios. But, the same model responses of the Southern Hemisphere sea ice are less certain. In fact, there have been some recent studies suggesting the amount of sea ice in the Southern Hemisphere may initially increase as a response to atmospheric warming through increased evaporation and subsequent snowfall onto the sea ice.

"Observed global sea ice area, defined here as a sum of N. Hemisphere and S. Hemisphere sea ice areas, is near or slightly lower than those observed in late 1979, as noted in the Daily Tech article. However, observed N. Hemisphere sea ice area is almost one million sq. km below values seen in late 1979 and S. Hemisphere sea ice area is about 0.5 million sq. km above that seen in late 1979, partly offsetting the N. Hemisphere reduction.

"Global climate model projections suggest that the most significant response of the cryosphere to increasing atmospheric greenhouse gas concentrations will be seen in Northern Hemisphere summer sea ice extent. Recent decreases of N. Hemisphere summer sea ice extent (green line) are consistent with such projections.

"Arctic summer sea ice is only one potential indicator of climate change, however, and we urge interested parties to consider the many variables and resources available when considering observed and model-projected climate change. For example, the ice that is presently in the Arctic Ocean is younger and thinner than the ice of the 1980s and 1990s. So Arctic ice volume is now below its long-term average by an even greater amount than is ice extent or area."

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