A fish eye view: Part 1.
  |  First Published: October 2013

This month, I’ll continue my examination of the fundamental basics of successful angling with a detailed look at the senses of the fish we chase, beginning with sight.

Understanding the biology and behaviour of the fish we hunt is an important part of the process of becoming a better angler. The more we know about our targets, the easier it is to fool them into biting or striking our natural baits or artificial offerings. A big part of this understanding involves having some idea of how a fish’s senses work.

Just like humans and most other vertebrate (back-boned) animals, the majority of fish species we pursue have a well-tuned set of senses that allow them to monitor their surroundings, find food, locate spawning partners and react to external stimuli. Like us, most fish can see, hear, smell, taste and touch. However, in addition to those five basic senses, many fish species also have at least one and possibly two extra senses related to the detection of vibration and, in some cases, weak electrical fields. But let’s begin by looking at arguably the most important fish sense of all: sight.

The visual system in humans and many animals allows individuals to obtain information from their surroundings. The sense of sight begins when the lens of the eye focuses an image onto a light-sensitive membrane called the retina. Photoreceptive cells within this retina produce signals, which are processed by the brain, creating a mental picture of the world.

For sight to work, light is needed to illuminate our surroundings and generate the photons that stimulate those cells in the retina. This light bounces off objects and it’s these reflections of visible light that define the shape, size and texture of various objects. Discriminating between colours is also crucial for many animals, whether they’re identifying food and predators, finding their way or seeking out potential mates.

Animals with backbones, including fish, differentiate between colours by using cells in their eye called ‘cones’. Other sets of cells called ‘rods’ are responsible for the detection of black and white images (especially in low light). The majority of humans have three different types of colour-sensitive cones in their eyes. As a result, our visual experience is referred to as ‘trichomic colour vision’.

Interestingly, many types of fish, birds and even insects are well ahead of us in terms of colour vision. Some of these creatures have four different types of cones instead of three, and a few also have a lot more cones than us. For example, you might be surprised to learn that chickens have twice the number of colour-detecting cones as humans! As the number of these cones increases, so does the creature’s ability to make finer and finer discriminations between colours. So, it’s highly likely that a humble chicken can far more easily differentiate between the yellow of a corn kernel and the yellow of a dandelion flower than we can! Perhaps this visual adaptation aids chickens in their constant search for food.

Just as in chickens, the eyes of fish have evolved to better accomplish various tasks. For example, fish that feed predominately near the surface or in shallower, clearer water, especially during daylight hours, often have more cone cells than rods, as colour vision gives these fish a distinct advantage under such well-lit conditions.

By contrast, fish that feed primarily at night, in dirty water, or at great depth will typically have more rods than cones, as rods are more sensitive to low light levels. This adaptation allows these fish to make better use of the limited available light available.

Some species have taken these low-light adaptations a step further by developing a special reflective layer at the back of the eye called a ‘tapetum’. This mirror-like membrane reflects light that has already passed through the eye back into the retina, giving the photons a second chance of being detected and greatly enhancing the fish’s ability to see in very low light or murky water. It’s this reflective tapetum in the back the eyes of both barramundi and mulloway that cause the distinctive red or pink glow so often seen in flash photos of these popular species. Both species are extremely well adapted to finding prey in low light conditions.

Next month we’ll continue our examination of sight in fish and look at how it affects our results as anglers.

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