Survival Quiz Part two

[Stuart]

OK my last question was answered much quicker that I thought and I said I'd try to come up with a harder one, But I havent yet so I'll give you a question to tide you over till i can think of one :-D

As many a old poacher will tell you dyeing your gillnet dark red will improve your catch is this true? and why?

I'm looking for a full detailed explanation here :-D

Ed and Gary sorry guys you cant join in on this one I know you know the answer :-)

And please remember that fishing in the UK with a gillnet is highly illegal
as well as being enviromentaly damaging

[steve a]

is it to do with the light spectrum and red light not penetrating the water after a certain depth. Therefore the net is supposed to become invisable.
I'm not sure if this is true having never used a gill net, but I do flyfish and have had many Trout and pike on red lures, no upstream drys for me!!.
so in conclusion I dont think the red dyed nets would work any better.

[MartiniDave]

I think Steve A is close. When I used to be a diver (a class 1 bend put me off a bit) I seem to recall red was one of the first colours you lost. Don't know if its also true for a fish's eye though.

I remember wondering what the green "smoke" was coming from my buddy's hand at about 10 metres once, turned out he'd cut his hand and the blood looked green at hat depth.

Dave

[FGYT]

I think Steve A is close. When I used to be a diver (a class 1 bend put me off a bit) I seem to recall red was one of the first colours you lost. Don't know if its also true for a fish's eye though.

I remember wondering what the green "smoke" was coming from my buddy's hand at about 10 metres once, turned out he'd cut his hand and the blood looked green at hat depth.

Dave

yep thats correct Red is the first colour/wavelength to be absorbed by Water and Blue is the last
this is why i advise all my dive students to carry at least a small back up torch even on day dives its amazing the colours down there you cant see with out it even in British waters

So does this explain why Edible crabs have red shells and if so Why are lobsters Blue black shells

ATB
Duncan

[Stuart]

yep thats correct Red is the first colour/wavelength to be absorbed by Water and Blue is the last

This statement is of course correct, however it has little or no effect on the colour of gill nets used in river water under normal conditions.

the reason for this is that the wave length of red light is not absorbed untill it reaches a depth of 4.5m (so red still looks red untill you are deeper than 4.5m)

you are unlikley to be setting a gillnet deeper than 4.5m in a river

Even if you did set a net deeper than 4.5 meters it would not become invisable it would just appear black/gray (which is why it doesnt really matter what colour your net is if you are deepsea net fishing, as it all looks black/gray below around 23m (by which depth red 4.5m, orange 8m, yellow 11m, green 19m and blue 23m have been absorbed in that order, gray disappears at 31m)

so THIS IS NOT the correct answer, but good thinking :wink:


keep em coming!

[Strider]

What exactly is a gill net...clearly some form of net....is it particularly fine or summink?

Are fish parcial to a nice red? :roll:

[Burnt Ash]

It has to do with the visibility (or otherwise) of a red-coloured net. Most pelagic fish are believed to have vision sensitive to a similar and even wider spectrum of wavelengths cf. humans (most demersal/benthic species have vision geared to the blue end of the spectrum). Under normal, non-turbid, shallow water daylight conditions fish should be able to see all colours more-or-less equally well.

The clue comes from the advice being given by an 'old poacher'. Poaching is mostly done at night. The red end of the spectrum is lost first as the light fails, so a red-coloured net becomes less visible sooner as the light fades and stays less visible for longer at dawn. In the middle of the night, the net could be any colour.

Burnt Ash

[Ed]

A gill net is a large net commonly 12'+ long by about 4' high and is designed to be placed accross a river. Unattended/abandoned gill nets have been responsible for decimating fish stocks on some rivers as they trap everything but the smallest fish.

Ed

[Stew]

Well then, if it is such an effective method of catching fish, is dyeing it dark red just so that it is less noticeable than white to a gamekeeper patrolling at night?

[Keith_Beef]

Well then, if it is such an effective method of catching fish, is dyeing it dark red just so that it is less noticeable than white to a gamekeeper patrolling at night?

That's got to be it.

The poacher knows where his net is, he doesn't have to find it by looking in the water.

The fish doesn't see it so well, because his eyes have been tweaked by evolution for when he's in the deeper sea water (where the red has been filtered out by the height of water above him). It's to beat the gamekeeper's eyes. The answer is in the question: "As many a old poacher will tell you..."

Keith.

[Stuart]

It has to do with the visibility (or otherwise) of a red-coloured net. Most pelagic fish are believed to have vision sensitive to a similar and even wider spectrum of wavelengths cf. humans (most demersal/benthic species have vision geared to the blue end of the spectrum). Under normal, non-turbid, shallow water daylight conditions fish should be able to see all colours more-or-less equally well.

The clue comes from the advice being given by an 'old poacher'. Poaching is mostly done at night. The red end of the spectrum is lost first as the light fails, so a red-coloured net becomes less visible sooner as the light fades and stays less visible for longer at dawn. In the middle of the night, the net could be any colour.

Burnt Ash

Your getting Close and your thinking along the right lines.

You are of course right that fish see colours underwater without difficulty
in fact they can see a wider specturm of light than we can, many being capable of seeing infared (they can actually see your bodyheat) Goldfish are one of the only animals that can see everything from the infared to the ultraviolet spectrum

However dying the net dark red it not solely for the perpose of defeating the Game keeper though this may be a useful side effect

As Burnt Ash said "The red end of the spectrum is lost first as the light fails" however this results in the net simply appearing black

CLUE: the colour of the net is primarly for defeating the fish (who can see colour as well if not better than we can remember) not so much the Game keeper (nets are very difficult to see from the surface anyway)

[Stew]

The fish are attracted to red?

:?:
:-?

[Stuart]

not that i'm aware of...... no

[Stuart]

Hmmmm its gone very quite in here, another clue?

ummm ok its to do with contrast

[Ed]

ooooo what a clue ;-)

Ed

[Burnt Ash]

Your getting Close and your thinking along the right lines.

You are of course right that fish see colours underwater without difficulty
in fact they can see a wider specturm of light than we can, many being capable of seeing infared (they can actually see your bodyheat) Goldfish are one of the only animals that can see everything from the infared to the ultraviolet spectrum

However dying the net dark red it not solely for the perpose of defeating the Game keeper though this may be a useful side effect

As Burnt Ash said "The red end of the spectrum is lost first as the light fails" however this results in the net simply appearing black

CLUE: the colour of the net is primarly for defeating the fish (who can see colour as well if not better than we can remember) not so much the Game keeper (nets are very difficult to see from the surface anyway)

No, I wasn't thinking of visibility/invisibility to gamekeepers, etc. I did mean that the red net would be less visible to the fish under low light conditions.

I'm trying to think. Contrast (or lack) against what?

Burnt Ash

[Tony]

Maybe the net blends into the rest of the river features, when you look at it it gets hidden by, say, the bank because the contrast distorts the depth of vision.

[Stuart]

You guys are getting ever so close, but think more about what a river has most and keep in mind light and how different things can have an effect on it

[Adi007]

say, the bank because the contrast distorts the depth of vision.

Foreshortening effect .. :?:

[Stuart]

Ok if you have an idea that you havent posted yet get it in the answer will be posted tommorrow afternoon

[ChrisKavanaugh]

HINT: Water acts as a lens bending light. Think of the british magician who performed marvels of camouflage and deception in WW2, making entire harbours disappear.

[Stuart]

Ok everyone no one got it this time so heres the answer:

Gillnets dyed a muddy red colour are more effective than an undyed white net. this it due to the light filtering effect of suspended particles of decaying plant matter in river water and contrast as explained below:


Pure water absorbs most red and orange light, less yellow still less green and relatively little blue light and thus appears blue.
Inshore waters however are stained by the yellow products of vegetables decay.
These substances are very persistant and have the property of absorbing
much blue light, less green and very little yellow or red. These yellow substances act as such strong filters as to over-ride the blue filtering properties of pure water this results in the water appearing reddish-brown when viewed from beneath in fresh water lakes and rivers. If the yellow stain is rather more diluted the red and yellow light is absorbed by the water and the blue by the yellow substances and this leaves green as the colour least absorbed. This is the explanation for the green colour of the water around our coast. This colour filter action of a water mass is chiefly responsible for the fact that colours appear different below the surface in river water.

Contrast underwater:
A distant object under water is usually detected because it is either a little darker or little brighter than the background it is seen against. Contrasts are low under water because much the image forming light from an object is either scattered out of the light path or absorbed by the water before it reaches the eye, whilst daylight is scattered out of it's downward path into the eye thus interposing a veil of brightness between the object and the eye. Obviously if the rate of contrast decrease remains the same, the distance that an object can recede before it becomes invisible depends on the original contrast with the water background.

Sorry for the long explanation but in short:

if you open you eyes underwater in a river the water looks muddy red because the water is full of particals of decaying plants whos yellow colour filters out the blue green and yellow light, so a muddy red colour net is camouflaged against the muddy red water and as contrast is reduced by water as explaind above it makes it almost invisible :-D

I'll put another question up on monday

is anyone Enjoying this quiz if not say and i'll stop

[Adi007]

Excellent stuff Stuart! Keep em coming!