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08 Jun 2017
by Simon Blanford
The Carp Part 4: Colour and the feeding fish
Following on from last month’s piece on carp intelligence, biologist and carp fanatic, Simon Blanford takes a closer look at the carp’s eyesight

Last month I wrote about the intelligence of carp, how they and other fish are adept at deciphering what is good for them and, conversely, what is bad. It’s quite clear that carp view being caught as something bad and that they actively try to learn how to distinguish between those food items that carry a hook and those that don’t. How carp track down their food, identify that an item actually might be worth testing and then deciding whether the food is good to eat involves not only the workings of their memory but also the deployment of an array of senses, some similar to our own, some patently not.

Carp can obviously smell food (the olfactory system), sense the movement of live food items through their lateral line (though this is more often used to help monitor other fish in their environment) and closer too, they can taste their food (the gustatory system), feel their food through touch and of course see their food as long as the water is clear enough. James Anderson has done a nice little series on these senses on here and I’d suggest you get an overview from these pieces (search for ‘Inside a carp’). One of the more interesting areas – more interesting possibly because we are visual animals and so give greater weight to how other animals see than we perhaps should, is how carp see and react to what they see.

Rather than laboriously go through all the senses I’m just going to cut to the chase a bit by looking at some aspects of vision, particularly colour vision, and how that relates to foraging decisions.

In vision, simply put, colour perception is determined by two things. What wavelengths of light actually get through the initial barriers of the eye (the lens and the fluid that fills the eyeball) to fall on the retina, and then what range of colour-interpreting cells the retina contains. On the retina, cone cells do the daytime job of colour vision and they are classified by the pigment they contain (a protein called opsin), and the wavelength of light to which these pigments maximally respond. For carp (and us), their Red-Green-Blue colour vision is created by a set of cone cells containing opsin, maximally responsive to long wavelengths in the red part of the spectrum; a set sensitive to medium wavelengths in the green part and those sensitive to short wavelengths in the blue part of the spectrum. Importantly it is the combined information from cones containing different visual pigments, responsive to different wavelengths, which allows carp to perceive colour.

The colour vision of a grown carp is very similar to ours

There have been some claims that carp can see in the ultraviolet portion of the spectrum and at the other end, in infrared. There is little evidence that carp see in the infrared or use this portion of the spectrum for any sort of feeding directed behaviour. On the other hand there is good evidence that carp can see in ultraviolet. But there’s also a catch. The ability to see these very short wavelengths only occurs when the carp are young. Many fish (roach, goldfish, trout, rudd, perch for example) can see ultraviolet when they are young but lose this ability as they age. Ultraviolet vision is especially useful when feeding on zooplankton like daphnia because these small crustacea either absorb UV light or stand out clearly against a UV background. Some fish that remain small and feed on zooplankton through their lives (sticklebacks are a good example), retain their UV vision. Fish that eventually grow to a larger size like our carp, lose UV vision partly because their diet switches and they become less reliant on zooplankton as a food source and partly because UV cone cells cause problems for accurate vision as the eye gets larger. Either way, carp of the size we catch are not thought to be able to see in the UV part of the spectrum.

Now all that said, it’s good to remember that a carp’s colour vision is limited to the daylight hours. Those cones cells that interpret the colours of the daylight world need high light levels to work. As the light decreases towards dusk their efficiency declines until they stop working entirely. Now a second kind of retinal cell takes over, the rod cells. These cells are adapted to providing vision in low light conditions. Rod cells have their own pigment tuned to the green part of the spectrum. But that doesn’t mean that carp see green or any other colour at night. They don’t and they don’t because to see colour, an animal needs two different colour interpreting cells (say red and green or blue and red) working together to resolve colour. Since rods are the only ones working at night carp see in shades of black and white, just as we do. The benefits coloured baits may provide during the day are therefore lost after dark when the carp’s world becomes a monochrome place.

Andy Bell and a diminutive carp. Even at this size the fish will have lost its youthful ability to see in ultraviolet

Contrast is likely to be much more important than colour – a light bait on a dark background or a dark bait on a light background. Carp also have an extra bit of visual kit to help them see in low light conditions – a ‘tapetum lucidum’. If you don’t know the name, all you have to think of is the way a cat’s eyes looks like two glowing disks when a torch is shone on them after dark. The effect is caused by a layer of reflective tissue behind the retina – the tapetum lucidum – reflecting light back through the retina so increasing the light available to the rod cells. Many nocturnal animals have one as do carp and although it’s unlikely to cause sufficient light to stimulate colour vision it will help them see after dark.

Returning to the daylight and colour, it’s certainly fair to say that carp do have good colour vision and in a range very similar to the colours we can see. It seems to follow then that the trend of using brightly coloured baits to make them more visible to the fish, to make them unique against a background of duller, less vividly coloured baits, is a good idea. But such an idea runs counter to a fairly common phenomenon to do with bright colours in nature. Take wasps or any of the myriad of highly colourful poison dart frogs as examples. Their bright colours act as a signal to potential predators that the wasp (or dart frog) wearing the bright colours is well defended. That is they have stings, or bite back, or contain poison. Some predators don’t know or don’t understand the message initially and so try to eat these prey and in the process get stung or get a mouthful of poison. Subsequently the bright colours the predator initially ignored helps the foolish predator learn the error of its ways and leads it to avoid anything similarly coloured in future.

Hi-viz pop-ups. Do bright colours make it easier for carp to learn they might be dangerous?

So successful is this strategy of wearing warning colours to show you are defended, many other animals mimic the colours without being harmful in any way themselves. The classic example is the similarity between wasps and hoverflies. Hoverflies have copied the wasps warning colouration and gain protection from predators because of it even though they have no sting themselves.

All this would suggest that even if a carp gets caught on a brightly coloured boilie once, they should very quickly learn to avoid them in the future and this might particularly be the case when a hi-viz pop-up is used singly against a background of dull coloured bottom baits. Here an educated carp doesn’t have to sort out which one of the more neutral coloured bottom baits might contain a hook. They just look out for the bright pink singleton. Over time then we might expect a bright pop-up’s effectiveness, however attractive they are on first encounter, to rapidly wane.

After dark all the world turns to shades of grey. Now which colour would you choose?

While all this is perfectly reasonable there are a few problems applying the behaviour to the watery world. It’s possible to find numerous examples of brightly coloured insects, frogs and snakes on land all advertising the fact that their bright colours send the message that they are poisonous or can sting. Underwater there are hardly any similar examples. In the sea there are a few brilliant slugs and worms (and even here it’s been debated whether these are true warning colours) but in freshwater there are none. Well, almost none. There are those tiny, bright red mites you can often see when peering into the margins of our lakes and ponds. But that is pretty much it. That means carp and our other freshwater fish haven’t evolved in an environment where vivid yellow or red or any other bright colour is associated with danger. In fact, carp actually show slight preferences when feeding for the very colours terrestrial animals use to warn predators – red and yellow.

This rather ambivalent response to colour is well demonstrated in an experiment designed to look specifically at this question. Researchers fed goldfish (I know, not carp, but the goldfish’s visual system is pretty much the same as a carp’s and their feeding behaviour and food handling is very similar) coloured food pellets until they had got used to them and then introduced pellets of a different colour. The idea was that having got used to say, red pellets, the fish would take time to accept the green pellets simply because they were different and so should at first be treated with caution.

Not a great nighttime picture but it shows the light reflected off the carp’s tapetum lucidum creating a glowing eye. A bit eerie but the reflective disk helps them see in low light conditions

It turned out that the fish didn’t really care. They fed equally on pellets of a colour they had got used to and of a colour they hadn’t seen before. The researchers then made some of the pellets distasteful, all the yellow ones say, to see if the fish would discriminate between colours that tasted bad and colours that did not. Again, the fish didn’t really care. They were perfectly capable of telling that the pellets were red or yellow or green but they didn’t use these colours to discriminate between the pellets containing the bad taste and the ones that did not. They sucked in all the pellets, tasted them, swallowed the nice ones and spat the bad ones back out.

It would seem odd for a carp to carry on testing a bait even though they may know, from previous experience, that the bait might lead to an unpleasant experience. Why didn’t those goldfish use the colour of the pellets to discriminate between the nice tasting and nasty tasting ones? If they had done they could just have eaten the good food and left the bad alone.
If we ignore the behaviour of carp in hungry, over-stocked waters (because such waters make a travesty of any natural behaviour), what determines whether a carp will feed on a carpet of bait or accept something novel to them like a hi-viz pop-up may well be bound up not only in the carp’s previous experience with that bait (good or bad) but also with whether the carp tends more to being AC or to being DC.

One of many kinds of poison dart frogs. The vivid red skin colour says “eat me at your peril”

Now that’s not a comment on the carp’s sexual proclivity but on their feeding behaviour. AC stands for ‘Adventurous Consumer’ and DC stands for ‘Dietary Conservative’. Adventurous consumers rapidly accept novel food items and incorporate them into their diet. Dietary conservatives on the other hand can stay wary of new foods or foods that might be dangerous, for long periods of time even though they may regularly come into contact with that food in their environment. The goldfish in the experiment above were adventurous consumers. There was food in abundance and irrespective whether they had seen it before or whether it tasted bad, they were going to try it. And carp mostly seem to be adventurous consumers rather than dietary conservatives. Seeing a bed of bait their initial caution is rapidly overcome by their desire to see whether the bait is edible. Colour may be much less important for carp than actually getting their noses in the trough and making the most of this abundant, easily accessible food supply. Of course they do get caught but rather than avoiding bait altogether they learn how best to handle the hardware we attach to it – they learn to get rid of the hook.

I’ve mentioned that most carp seem to be adventurous consumers. It’s often said that some of the hard to catch fish, the ones that can go for years before gracing someone’s landing net, are those fish that exist on only natural food diets. There may be some truth to this because across the whole population of carp in any particular lake, there will be a small percentage of fish that in fact are dietary conservatives. These fish are much less likely to accept our baits simply because they are novel and different to the food naturally available. The more exotic the baits we use, the less likely these few DC fish are to eat it.

This mix of character types, the fact that AC and DC fish can live together in the same water but forage in very different styles, leads onto a much bigger area of carp behaviour that affects our fishing. That is the question of carp personalities. Deciphering whether wild animals really do have consistent, individual personality types has been one of the big areas in behavioural research over the last few years. Next month we’ll have a look at how this may affect carp and how they respond to our attempts to catch them.

A chunky common taken on a pink pop-up. It is almost certain that the fish had never seen this kind of bait before. Yet it still ate it.
Andy with a considerably bigger fish. Many carp are AC rather than DC and being adventurous are eager to test novel food sources
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