A Fish Eye View: Part 2
  |  First Published: November 2013

Last month, in my ongoing examination of the way the senses of fish affect our ability to catch these creatures, I looked at the basics of sight in fish. This time, I’d like to explore that subject a little more deeply…

So, what do fish actually see? According to most fisheries biologists, science can’t yet answer this question with very much certainty. The majority of research in this area involves monitoring the responses of fish to particular stimuli, such as colours, diagrams and pictures. Applying the test results from such experiments in the lab’ to practical fishing scenarios can be tricky.

However, we can definitely assume that most of the fish we chase in coastal waters, bays, estuaries and freshwater environments, as well as all those pelagic fish that hunt in the surface layers further from shore, rely heavily on sight to detect and track their prey, and that almost all of them can differentiate between colours when the water is reasonably clear and light levels are high. As an example, sight in marlin has specifically adapted to a clear water environment with lots of light. A marlin’s, eyes are effectively split into two sections: The part of the eye that faces upwards contains more colour photoreceptors (cones), while the part that faces down contains more photoreceptors that are sensitive to light (rods). Furthermore, as much as 30 per cent of a marlin’s brain is dedicated to processing and interpreting vision. This fact alone indicates just how important sight is to these billfish (and to many other pelagic predators, which also devote a lot of brain space to the sense of sight).

We also know that lots of fish, including species as diverse as carp, slimy mackerel, brown trout and damselfish (to name just a few), can see light reflected off objects at the ultra-violet end of the spectrum: wavelengths which are invisible to our human eyes. This allows them to see patterns and shapes we could never hope to identify, and also to detect UV-reflective objects at greater depth or distance through the water.

It’s also believed that other fish (including bait species such as anchovies) can see “polarised” light, which is mostly encountered early and late in the day, or in the reflections given off by the very silvery scales of these fish themselves (and possibly by some of the predators that hunt them).

In dirty water or low light, fish rely much more heavily on their sensitive rod cells than their colour-detecting cones. As a result, they’re most likely seeing their world in shapes and silhouettes consisting of various shades of grey rather than in colour when light levels are low or the water is discoloured. Obviously, in such low-light environments, the shape, size, silhouette and action of your lure will be far more important than it’s actual colour.

As light levels and water clarity change, many popular angling targets have the ability to switch their reliance from cones to rods, and vice versa. However, this changeover process is thought to take some time (possibly as long as an hour), and this may help to explain why many species are more susceptible to being fooled by our lures during the so-called “change of light” periods around dawn and dusk. It’s also likely that some predators have adapted to change their reliance on cones or rods faster than their prey, giving them a distinct hunting edge at this time of day.

We still have an awful lot to learn about the way fish see and interpret their watery domain, but we can be reasonably certain that most of the species we hunt with our lures have good to excellent eyesight, can perceive a broad range of colours and tones under the right conditions, and that many can see light reflected at wavelengths that are invisible to us (such as ultra-violet). However, as we’ll learn next month, the colour palette in the underwater world is quite different to the spectrum of hues we see above the waterline…

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