Did anyone see Saturday’s ‘Country Calendar’ on TVOne? In case you didn’t, it followed a group of long-line tuna fishermen on their fishing trip North of North Cape. They were testing out a method for detering albatross from trying to take the bait (squid) from the hooks – a decision often fatal for the albatross. The method seemed to have two parts to it – first ensuring the hooks submerged themselves very quickly, and secondly dyeing the squid blue.
Now, I can’t comment about the food-colour preferences of ocean-going birds (apparently blue is not an attractive colour) but I did pick up on the point made by the fisherman that blue is a colour that can be seen well underwater – so the tuna still see the squid. Light doesn’t travel particularly well in water. In particular, the red end of the spectrum gets rapidly absorbed. A couple of years ago I went snorkelling on the Great Barrier Reef – among coral that is full of brightly coloured fish – except that they don’t actually look brightly coloured – under the water everything is this dull greeny-blue colour. All those pretty fish pictures you see are cheats – using artificial light. In that sense snorkelling was a touch disappointing.
So this means a red-coloured squid isn’t going to reflect much natural light, and is going to be pretty hard to spot. But at depth there is still a bit of blue light around, which the blue squid will reflect (that is why it is blue – it reflects blue light) and so it will still look like a squid to the tuna. Though I don’t agree with the statement that the blue colour will help the tuna to see it – after all – it can’t boost the light level down there – it just won’t become invisible like a red one well.
Anyway, an intruiging little bit of physics used in a context where one might not have expected it.