Fast-moving marlin depend on vision
Marlin brains are tiny by comparison to the brains of mammals. An average size marlin has only enough brain tissue to fill a dessertspoon — and precious little of this is devoted to the higher brain activities as we understand them. Nonetheless, the brain and nerves of marlin obviously provide the decision-making and communication systems needed to coordinate their minute-by-minute activities.
Because marlin are so large and fast, their nervous system must be capable of very rapid responses to the constantly changing conditions around them. It is not especially useful being able to swim at 50 mph if you can’t sense, make decisions about and take appropriate actions to avoid obstacles that suddenly appear when swimming at such speeds.
The nervous system provides fast communication of outgoing commands and incoming information between the brain and all parts of the body. The brain’s visual and olfactory (sense of smell) is combined with memory and status reports from hunger level and reproductive state so that decisions about the next maneuver can be made. Of course, not all decisions result in overt actions.
Vision is the dominant sense in marlin. The eyes are large and the optic lobes dominate the brain. The nerve from the olfactory (smell) rosette to the brain is the next largest, which indicates that the sense of smell is also important to marlin sensation.
Marlin are colorblind by human criteria. Color vision is not particularly useful to fish that live and hunt in water deeper than about 35 feet, since even in clear waters, below this depth the sea is a world of shades of blue. Color vision is only effective in high light intensities, and intensity is sharply reduced by increased water depth.
Studies have shown that marlin have only one retinal pigment. This pigment responds to light of a wavelength of around 485 nanometers. As might be expected, this is in the blue range into which most of the light of the aquatic world of the marlin falls.
Marlin are known to feed at depths to 1,100 feet and probably deeper. Their stomach contents indicate that they feed at a variety of levels. Not only do they feed at a range of depths, but they also actively forage at night when light intensities are much too low to sustain color vision.
Thus, the indirect evidence suggests that marlin have a high visual acuity and lack color vision. There is some scientific basis to the belief that fishing lures catch more fishermen than fish.