A fish brakes if it tries to breathe when swimming downstream - so Uncle Stanley says

[Deb London]

From "The Old Country" by Jack Hargreaves:

'Now notice,' said Uncle Stanley 'if we show ourselves to those fish lying there they will bunk downstream.' Whoops - downstream they go as fast as they can; and as soon as they're safely away they whip round and face upstream again. He goes very fast, not only to escape danger but also because he can't breathe en route. He's running away from the current of water, and if he tried to open his mouth to force some through it would act all too effectively as a brake.

Is this true, that the fish would brake? If so, how does this work? Uncle Stanley seems to have been an intelligent man who thought a lot about the physics of fishing. He was an electrical engineer who helped build Battersea Power Station.

[nightshade]

I know that fish take water in through their mouths and pass it through their gills but although fish like to face a current, turning and dashing down stream should not make any difference. Fish in a pond will dash in any direction, the limiting factor would more likely be territorial or depending on the type of fish
chemicals that are produced by stress that affect the muscles.

[Deb London]

Thanks Rod.

[Wild-Woman]

Interesting Deb. Used to like Jack Hargreaves and his country knowledge. Probably some truth in what his uncle said.

[Deb London]

Thanks Jules, I would have loved to have had an "Uncle Stanley".

[Dogghound]

I assume he is talking about trout in this? They don't brake in this way, but what he is saying is the fish will lose its streamline profile as its mouth (gape) increases and it's gills open up, this will slow its escape, and in effect act as a brake. Fish are quite poor at braking/ slowing down and often have to circle their body around into the flow, or against the direction in which they swam. They predominantly use the fins as brakes especially the pectoral ones.

[nightshade]

I know that fish take water in through their mouths and pass it through their gills but although fish like to face a current, turning and dashing down stream should not make any difference. Fish in a pond will dash in any direction, the limiting factor would more likely be territorial or depending on the type of fish
chemicals that are produced by stress that affect the muscles.

[Deb London]

Thanks Dan, that sort of makes sense. I was thinking that the gills opening when the fish was going in that direction would act like a sail to carry the fish further in the downstream direction. Maybe I was not considering the way the fish was swimming, it's streamline shape, which would be less so when breathing.

The passage does not state which fish they are, I don't think.

[Dogghound]

I was referring to a fish making a sudden dash then slowing down. A fish swimming down stream does not have to break, it simply turns back round into the flow, and swims against it. Fish can still breath facing downstream, as they draw the water in much like us breathing, but it is much more efficient to face into the flow, especially for species with high oxygen requirements such as trout. They tend to close the gills when moving fast, to keep streamline, which I think is the point he is making.

[Deb London]

Yes, it's something to do with being streamlined I think.