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What do billfish see?

By Staff | Apr 21, 2016

Have you ever wondered what marlin or other billfish see when cruising through the water attacking prey or a lure? Living in a clear water environment, vision is a key sense for marlin in detecting and catching food, and their large eyes indicate awesome visual abilities.

As pelagic predators, billfish appear to rely strongly on their visual sense. Irrespective of the time of day, marlin appear to spend the most time within the first 32 feet of the water column interrupted by short, deep dives.

The marlin eye is specifically adapted to cope with the low light levels encountered during diving. Since the marlin is likely to use its vision at depth, it is suggested that this line of research could help estimate the limits of vertical distribution based on light level.

Indeed, the obvious characteristics of billfish eyes tell us quite a lot about what these creatures might see. “Big” is the first word that comes to mind.

Broadbills have the largest eyes of all billfish in relation to their size. An animal with a jaw-to-fork length (body length) of eight feet has an eye diameter of three-and-a-half inches, or something like a good-sized orange. It is interesting to note that a fish’s eyes grow throughout the lifetime of the animal.

There are other consequences of large eye size that are of interest for anglers. When a fish and its eyes grow, its sharpness of vision improves, as has been demonstrated in other fish.

If this is also true for marlin, it means that a big blue marlin will be able to detect bait at greater distances than a little blue, giving the larger fish the edge when it comes to finding food. A large marlin will also see more fine detail when close up to a baitfish or a lure – something that may play a role in whether the fish will actually attack or not.

Another special adaptation of the billfish eye is the presence of a bony layer that encapsulates most of the eye. Although many blue water fish have “hardened” eyes, large billfish top the scale again, protecting their eye with a cup made from sturdy bone several millimeters thick. This armored eye is probably designed to prevent deformation from the eyeball caused by the water rushing past the animal’s eye at swimming speeds of up to 60 mph, thus reducing sharpness of vision.

Focused by the lens, images reach the retina, the delicate nervous tissue lining the back of the eye. This amazing example of nature’s engineering works as a highly sophisticated light detector and image analysis device in one. When a billfish approaches a lure, the retina’s main job is to translate the image of the lure into nerve impulses, which are transmitted to the brain.

The retina also does most of the initial interpretation of what the animal sees, such as determining the direction of the moving lure, its brightness and the speed at which it is moving in relation to the marlin.

In the retinas of marlin, scientists found three different visual pigments. The double cones mainly contain a middle-wavelength pigment sensitive to blue-green, while the single cones reveal a shorter pigment sensitive to violet-blue, concluding that these animals are monochromats.

The part of the eye looking down into the darker water below the fish contains mainly the middle wavelength, blue-green sensitive pigment. This pigment is perfectly matched to the color of the prevailing light in deeper water, allowing the fish to “catch” as much light as possible in deeper, darker waters.

A marlin’s eyes are set on the side of its head, providing the fish with excellent panoramic vision in all directions. However, what is directly in front of the fish is what is most important for it, and this is very neatly reflected in the design of its retina.

The best area of vision, the one that provides the sharpest image, points forward along the bill. It’s a tiny area in the retina, only .20 millimeters in diameter, but the resolution here is up to 30 times higher than in the rest of the retina.

Just as the number of pixels defines the quality of a picture on a television screen, the number of cells per unit area of retina determines how much of the fine detail of the image is transmitted into the brain.

From these cell counts, scientists estimate that a blue marlin of about 150 pounds, swimming close to the surface in clear water, will be able to detect a fish just three inches long at a distance of 95 feet.