Can marlin see colour?

An ongoing topic of discussion amongst those interested in game fish involves the visual abilities of billfish: how well do they see; can they see different colours? Sam Mossman investigates…

Research done in the 1970s and 1980s (Munz and McFarland 1977; Tamura et al., 1972; Kawamura et al., 1981) using several different approaches failed to find any evidence of colour vision in marlin, so for many years, billfish were thought to be monochromats: essentially colour-blind fish that saw the world in shades of black and white.

New findings

But to a great many anglers (myself included), this did not sit right. Many had experienced instances of marlin showing a marked preference for a certain colour of lure above all others, sometimes transcending size, shape or action, and it was certainly a great subject for discussion in game fishing clubs and anchorages around the country.

For researchers, this subject was a tough nut to crack as large predatory deep-water fish are very hard to study: they seldom survive in captivity and are very hard to observe in their natural habitat. But ultimately angler’s suspicions were proved correct when new discoveries were made by scientists.

Australian Dr Kirsten Fritsches conducted a project re-examining the subject (along with Professor Jack Pettigrew and Dr Justin Marshall), and the team has continued to study this subject. They located tell-tale pigments housed in cone structures in marlins’ eyes using microspectrophotometry (MSP) techniques.

Three different visual pigments in cone cells were found in the retinas of striped and blue marlin: a middle-wavelength pigment sensitive to blue-green, a shorter wavelength sensitive to violet-blue; and a third visual pigment which peaks at a longer wavelength also sensitive to green, but also extending through yellow, orange and a little into the red part of the visible spectrum.

These cone cells are located in the bottom of the eye and work when the fish is looking up, suggesting that marlin can see a range of different colours between themselves and the surface. Of course, decent illumination is required to see colours well. This is good news to lure manufacturers who manufacture lures in a wide range of colours.

Red is probably not a very important colour to marlin, as it is filtered out quickly in the surface layer (appearing black in deeper water). But the blue-green area of the visual spectrum, which is where marlin have a lot of optical firepower, is much more useful to them for detecting camouflaged baitfish and other prey items against the blue-green depths.

As well as upward-looking colour vision provided by the cone cells in the bottoms of the marlin’s eyes, the rods in the tops of marlins’ eyes, which look down into the deep blue water, allow them to perceive prey items beneath them in grey-scale and black, and are specialised to work particularly well in low-light conditions.

No UV

But while this research shows that marlin CAN see colour – if the object they are looking at is above them in the water column (as your lures usually are) and is reasonably well-lit, it has also been proven that marlin CANNOT see ultra-violet (UV) light. In fact, there are filters in their eyes specifically to exclude it.

Interestingly, when billfish are ‘lit up’ they display purple-blue bars which include UV light. This process is controlled by elevated levels of adrenaline in the fish, as discovered by NZ researcher Dr Peter Davie of Massey University, author of Pacific Marlins: Anatomy and Physiology. I remember Peter demonstrating his discovery to me in the labs of the Pacific Gamefish Research Foundation in Kona back in 1987 on a sample of fresh marlin skin. By adding synthetic adrenaline to the skin sample, he induced it to ‘light up’, explaining the mechanism as “like mirrors (iridophores) covered by umbrellas (melanophores)”. Adrenaline causes the ‘umbrellas’ to retract and expose the ‘mirrors’ which reflect light at blue-purple and UV wavelengths.

But if marlin, like humans, can’t see UV, what is the point of being able to generate it (by reflection)? Well, it has been proven that some baitfish, including slimy (blue) mackerel, can see UV, so it is postulated that the UV flashes marlin emit (when feeding excitement causes a release of adrenaline) might have some shock/confusion effect on baitfish, increasing the marlin’s feeding success.

The marlin themselves, of course, would not want to be dazzled by their own colours, perhaps explaining their own UV blockers. And as marlin have very large, sensitive eyes, protection from damage caused by excess UV exposure may be another advantage, as well as possibly filtering out the scattering of UV light in surface waters, improving their vision – a little like polarised sunglasses do for humans.

Hot stuff

At the same time (1987) and in the same place (Kona, Hawaii) that Peter Davie was demonstrating to me the ‘lighting up’ mechanism of marlin, one of his colleagues, Dr Barbara Block, was studying their eye structure. (This research relied upon the Hawaiian International Billfish Tournament providing a stream of fresh biological samples for various studies.)

Barbara’s research established that marlin eyes are very large for the size of the fish, especially those of blue marlin, and to a lesser extent, striped marlin. Also, quite a high percentage (30%) of their brains are devoted to interpreting visual signals. This basically means that marlin have very good eyesight and so are largely visual hunters. And while their eyes have a similar muscle structure to human eyes (which allows them to swivel and focus multi-directionally), their eyes are located on the sides of their head, so their vision is less binocular than ours.

There is another difference, too: some of the muscle structure in a marlin’s skull, close to the brain, is modified to produce heat which is transferred to the brain and eyes by counter-current exchange through a large blood vessel leading to the back of the eye.

It is thought that by keeping the eye and brain warmer than the surrounding water the marlin will benefit from enhanced vision and so, improved feeding function. The warmer the eye temperature, the faster the ‘Flicker Fusion Frequency’ (FFF). In film parlance, this means marlin see more ‘frames per second’ when its eyes are warmer, allowing them to see better when swimming at high speed and react faster and more efficiently to movements and visual clues given by baitfish, which do not have this heating advantage, and so have less efficient vision.

This visual advantage the marlin has over its prey is especially marked if it dives into deeper, cooler water, as marlin are proven to do by tracking tag data. Other top-line predators like broadbill, tuna, and some shark species have also developed this ‘brain heater’ function.

More recently, Kirsten Fritsches has measured the density of retinal cells in the marlin’s eyes and found that marlin can see best looking forward along their bills, and also well, back towards their tails. This makes sense, as it is important for them to line up their next meal accurately; and to keep an eye out for any predators (such as mako sharks) sneaking up from behind.

Night moves

Interestingly, the large eyes of marlin (with their superior light-gathering ability), added to the ‘brain and eye-heater’ functions might mean that marlin can hunt effectively after dark, as tracking-tag data suggests. This data shows that from dusk through to about midnight, marlin spend much of their time close to the surface, moving actively as though they may be feeding. We all know about broadbill feeding after dark, but this is not yet proven as a regular thing for marlin.

I have experimented a bit with night-time lure trolling, but only ever managed to catch makos, albacore and yellowfin tuna while trolling lures after dark, although we have had a couple of stripies just after sundown while there was still a little light left in the sky. So, don’t be in too much of a hurry to pull in the lures when travelling after dark – you never know your luck!

Lure Colours

It is established that marlin can see colours when they are looking upward toward the surface. This is good news for anglers and lure manufacturers, as this is just where most skirted marlin lures are run. Skirted lures are mostly reliant on interaction with the surface to give them ‘action’ and help hide the leader in the wake. Baits are another thing altogether, but that is a topic for another time.

UV

And what about UV reflective materials? Just going by fish reactions, these seem to be attractive to some species, such as snapper and trout, but, sorry… as mentioned earlier, marlin have filters in their eyes that prevent them from seeing UV. A lures’ profile, action and position in the lure spread are probably more important.

Match the Hatch

Trout fishermen use this term to describe using artificial flies that match the natural insects the fish are feeding on. In the case of any marlin in a given area, the prevalent feed items may change over time, so a lure that is very successful one week may fail the next and, in fact, may never catch a fish again! So, it pays to keep your ear to the ground to find out what lure models and colours are currently doing the business. Kiwi lure manufacturers like Bonze and Red Gill are constantly coming out with new colour combinations.

Countershading

In the arms race between predators and prey, most baitfish have developed countershading (white or silver bellies and black, brown, blue or green backs, often with disruption camouflage). This helps hide them from predators looking up towards a silvery surface, and also down towards deeper, darker water. It makes sense if lures are dressed in a similar fashion, although some fishermen go by the theory of reversing this (dark back down, light belly up) to make the lure more obvious to the predator.

Luminescence, Phosphorescence and Fluorescence

All of these are various types of light emission, and with different ingredients added to the skirts and head inserts, lures can be made to give off light. The first of these to really ‘make its bones’ was Peter Pakula’s famous Lumo Sprocket, which has been a favourite and reliable striped marlin lure for decades. And pretty much every lure range has a ‘lumo’ model these days. I have also done very well over the years with Black Magic’s ‘Freedom Grand Slammer Lumo’ and Broadbill’s ‘Translucent Squid’.

Favourite Colours

There is a whole slew of lure colours that have produced well at times over the years. Some colour schemes even earned their own names. (Who remembers ‘Mrs Palmer’ or ‘Ghost’, for example?) But to build your own collection of favourites, you must start somewhere. Here is a range of ‘classic’ colour schemes that regularly produce the goods in NZ waters. You will not go too far wrong if you start with a spread that includes some of these colours (see image, R-L):

Black/purple over silver/black: a classic skipjack imitation. The Moldcraft Wide Range and Zuker 5.5 in these colours are particularly great producers.

Pink and purple over pink and mauve: another imitation of small tuna. A Super Plunger in this scheme is a great blue marlin lure.

‘Evil’ (blue and silver over green and gold): associated with Hawaiian lure maker Joe Yee, his ‘Apollo’ in these colours is a great general baitfish imitation.

Pink and sliver: although marlin eyes are not strong in the pink wavelength, they can see it. Pink is an ‘excitement’ colour in the sea and many successful lures have a pink component, while silver adds the flash. Many small tuna species have a pink flush on their sides and belly.

Brown and gold: particularly popular on the west coast and the tropical Pacific, this combination is a good imitation of squid or jack mackerel and often includes a flash of pink or orange.

Fruit salad: (green and gold over blue and pink): this is another classic mix, made popular on the Pacific ‘Baitfish’ lure (now manufactured by Kilwell). It probably imitates mahimahi, yellowfin or mackerel.

Green over black: Initially known as the ‘green machine’, an American tuna lure. Marlin see green particularly well, and black makes for a good contrast.

Blue and white/pink: again, good baitfish imitation colours, particularly pilchards, saury and flying fish, all staple feed for marlin.

Add a lumo lure (a must-have) to this collection and you have a spread of ‘tried and true’ colours, which is a good start. If you aren’t getting any action, change one out from time-to-time until you get ‘bit’!