Thursday, 9 May 2013

the mystery of the holes

Whenever I see any kind of natural phenomenon, I want to know how it happened, and why. A recent example has been the round holes that occur in mud banks in the local rivers, which I first noticed several months ago. The following photo, taken yesterday in the Shum Chun River on my way back from a journey to the west, shows how extensive this phenomenon is.


So how were these surprisingly regular holes in the mud of the river, each about 30cm in diameter, formed? I originally came up with two hypotheses, neither of which was even remotely convincing.

All the rivers in the northern New Territories have been canalized as a flood prevention measure, which means that the beds of the rivers were constructed using prefabricated concrete sections. And these concrete sections have neat rows of holes, each just slightly smaller than the holes in the mud. These holes remain full of water even when the river level is very low, making them ideal fishing holes for the local egrets, although it does appear, from passing observations, that this is the avian equivalent of shooting fish in a barrel. Unfortunately for my hypothesis, the holes in the mud are anything but regular in their spacing.

The second hypothesis centred on the notion of turbulence when the mud is underwater. The problem here is that it is impossible to imagine the kind of turbulence that would be required to produce so many holes, although one can imagine that the amount of water coming down the river for this to happen exceeds the design specifications of the flood defences.

It may be that the holes had always been filled with water whenever I saw them, so an obvious third hypothesis didn’t present itself: that the holes are the work of a horde of unknown creatures. This idea occurred to me only yesterday, when I saw the sandbank about 200 metres downstream from where the first photo was taken:


The holes at the water’s edge are filled with water, but those slightly higher are dry, and I was surprised to see that they are quite shallow. As can be seen in the photo, there is a small parapet of debris around each of the dry holes, which makes it highly likely that the holes were excavated. There is no sign of a burrow at the bottom of any of the holes, which would confirm the third hypothesis, so this photo is merely evidence, not proof. I suspect that crabs are the culprits, but this is just a guess at this stage.

You might wonder why I bother with such a trivial pursuit. After all, absolutely nothing of note hangs on solving this mystery. However, in an era of mobile communications, I prefer to pay attention to my environment, to listen to the sounds of nature rather than to a disembodied voice emanating from a smartphone, to look at whatever is happening in my immediate vicinity rather than at the screen of a mobile device. Unfortunately, my observations to date may have provided a mechanism for how these holes were formed, but I still don’t have any idea why. My best guess at the moment is that the holes are intended as a trap for potential prey, although I wouldn’t expect this to work if the holes are full of water.

13 comments:

  1. I'd love to know the answer to this one too. Weird.

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    1. I’ll do my best to keep you posted Big D, although the investigation has now been put on hold until the autumn (I’m off to the UK this weekend).

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  2. They are Tilapia breeding nests (http://www.daff.qld.gov.au/documents/Fisheries_PestsAndDisease/Stop-the-spread-Module-4.pdf)

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    1. Thank you for that link Kevin, which I read carefully. I’m bound to say that I’m not completely convinced. Tilapia do occur in Hong Kong, but according to the link, “They usually live in...well-vegetated areas”. However, this description does not tally with conditions where these photos were taken (the sandbank in the second photo is in the middle of the river).

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  3. They quite look similar to the clam holes I am used to seeing in Canada, and also to the crab holes I am used to seeing in Thailand. I would highly suspect they are animal related. To solve it once and for you you may want to get a shovel down there and dig, you may find some tasty crayfish, octopus, or other treat at the bottom. Alternatively I am sure someone living very close to there would have the answer if you are can't be bothered to dig.

    Enjoyed the post, I would also like to know the answer.

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    1. Your advice is sound PBS. I just wish I could follow it, but I’d probably be trespassing if I went digging (never mind the difficulty of access), and, believe it or not for Hong Kong, nobody lives nearby. I think you’re right about the animal origin though.

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  4. Opps fresh water, guess no octopus !

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  5. This reminds me of reminds me of razors clams. They do leave a hole but while I've only seen it smaller, it could be bigger like this.

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    1. Thank you for that suggestion Beetle. A few other ideas have been suggested, and I remain undecided on the reason for these holes.

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  6. It's tilapia breeding nests for sure. My family has fish ponds in Yuen Long and I have seen a lot of these holes. These fishes can survive in dirty water like this and it's very difficult to get rid of them.
    They could eat the mud when there is no food and make these breeding nests.

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    1. Thank you for that Hong Chan. The weight of opinion seems to be firmly on your explanation.

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  7. The holes are the burrows of filter feeding worms. If you are in Penrith I will look you up! I am planning to be there in late August on my way to Edinburgh.

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    1. Despite the weight of opinion assigning these holes to the work of fish, I’m not completely convinced Dave. The second photo above shows holes that have dried out, which I wouldn’t expect of fish. Your explanation does sound plausible. Anyway, I plan to look further into the matter when I return to Hong Kong in October. Meanwhile, I shall be in Penrith until then.

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