Ultraviolet colors in fishing lures
Rapala VMC Corporation manufactures and sells artificial fishing lures with the ultraviolet (UV) finishes that combine fluorescent paints, reflective surfaces and optical brighteners (see http://rapala.fishing/lure-finishes). Lures with theses finishes are marked by signs “UV BRIGHT” or simply “UV”. However, Rapala does not understand the abilities of ultraviolet (with the wavelength below 400 nm) vision in fish and its role in their responses to UV reflected objects in the nature and fishing.
Fig. 1. The sigh used by Rapala (brands Rapala, Storm, Blue Fox and Luhr Jenssen) to mark the lures with the UV finishes.
UV finishes in mafacturing fishing lures are also used by other companies like Lakeland Inc., USA (see http://lakelandinc.com/UFI/UFI_vibrant.html).
Numerous freshwater small-sized fish like three-spined stickleback, Gasterosteus aculeatus, reflect (Rick et al., 2004) radiation in the ultraviolet part of the electromagnetic spectrum and have UV vision. In particular, three-spined stickleback use UV vision in schooling (Modarressie et al., 2006), sexual (Rick & Bakker, 2008) and foraging (Rick et al., 2012) behavioural responses. The similar results are found for guppy, Poecilia reticulata (Smith et al., 2002), sailfin molly, P. latipinna (Palmer & Hankison, 2015), and other freshwater small-sized fish in the adult age.
However, UV reflection by some body does not provide the success per se. For example, during the nest decoration in artificial conditions males of three-spined stickleback choose rather red foil strips which absorb UV radiation than silvery or blue foil strips which reflect UV radiation (Östlund-Nilsson & Holmlund, 2003).
In turn, yearlings of predatory brown trout, Salmo trutta, use UV reflection of three-spined stickleback to hunt these prey (Modarressie et al., 2013). However, only young trout are sensitive to UV (see data by Bowmaker & Kunz, 1987, for Salmo trutta; Hawryshyn et al., 1989, for Salmo gairdneri), while older (over two years) fish lose this ability.
The same ontogeny of UV vision is typical for other freshwater predatory fish like perch and others (see Bowmaker, 1990). With the age, the ocular structures change radically and do not allow the fish to perceive UV radiation.
Great care must be taken in relation to marine fish and invertebrates (like crustaceans) many of which have UV vision (Losey & Cronin, 1997; Siebeck & Marshall, 2001; Losey et al., 2003).
According to Fritsches et al. (2000), marine predatory fish of the younger age groups and medium-sized fish (like slimy mackerel, Scomber australasicus, and others) are sensitive to UV, while marine predatory fish of the older age groups and large-sized fish (like blue marlin, Makaira nigricans, black marlin, Makaira indica, sailfish, Istiophorus platypterus, and others) are UV blind.
In general, UV signals are mainly used by small-sized and juvenile fish (both freshwater and saltwater) to form private communuication channels that are relatively inaccessible for potential predators (Siebeck, 2014).
Thus, UV finishes of Rapala’s lures and lures of other companies are useless for freshwater and saltwater predatory fish of the older age groups which lose UV vision with the age.
In addition to reflective surfaces, Rapala uses optical brighteners. The use of optical brighteners complicates the description of the optical properties of UV fishishes.
It is well known that optical brighteners are fluorescent substances which absorb UV radiation and immediately re-emit it in the visible part of the spectrum with the maximun of re-emission in violet and blue parts of the spectrum. White covers with optical brightners reflect partly the falling sun light which is mixed with the light of fluorescence, so the human’s eye perceives these covers as “more bright” and “more white” (well known as “snow white”) than white covers without optical brighteners.
In the pure form, fluorescent white finishes are used, for example, by Lakeland Inc. to cover its metal spoons and spinners (see http://www.lakelandinc.com/finishes.html).
In general, white and fluorescent white colors are most visible in the freshwater and saltwater environments (Kenney et al., 1967, 1968). But the great visibility of white and fluorescent white colors does not guarantee their attractiveness for fish.
For example, Dooley (1989) has studied using trolling technique the responses of rainbow trout, Salmo gairdneri, to wobblers, spoons and spinners of various colors and found that lures of the solid white color were less effective than lures of blue, green, yellow and red colors. Moraga et al. (2015) have studied using sink-and-retrieving technique the responses of largemouth bass, Micropterus salmoides, to soft plastic worms (of 12.7 cm length) of various colors and found that worms of the “pearl white” color were less effective than worms of natural and dark colors.
The same results were obtained in marine fishing. For example, according to Hsieh et al. (2001), in mackerel longline fishing white lures were slightly more effective than blue, purple and transparent lures (cryptic on the background of marine column) but less effective than black and red lures.
Psychological perception of white objects
It is known that relatively large objects of white color may scare fish. So, Moraga et al. (2015) have found that white soft plastic worms of 12.7 cm length allow to catch largemouth bass of greater sizes than the same worms of darker colors. It means that white lures warn of danger or scare largemouth bass of smaller sizes.
In general, white objects are perceived greater in size than the same dark objects (e.g., Kremkow et al., 2014).
On the other hand, because the natural sun light contains all the chromatic colors, which may be detected with the assistance of Newton’s lens, we perceive the sun light as “white”. In the same manner, we perceive any white surfaces (like white clouds, snow, paper, etc.) as “white” because these surfaces reflect more or less evently all components of the sun light.
However, our perceptions can not be automatically transferred to fish perceptions!
It is known that fresh water absorbs short-wavelength rays and transmits long-wavelength rays, so the maximum of spectral sensitivity of eyes of freshwater fish is shifted to the orange and red parts of the optical spectrum (e.g., Tamura & Niwa, 1967). Therefore, the “white light” for freshwater fish is enriched with the long-wavelength rays (we name this light as “worm light” or “warm white”). In contrast, marine water absorbs long-wavelength rays and transmits short-wavelength rays, so the maximum of spectral sensitivity of eyes of saltwater fish is shifted to the blue and green parts of the optical spectrum (Tamura & Niwa, 1967). Therefore, the “white light” for saltwater fish is enriched with the short-wavelength rays (we name this light as “cool light” or “cool white”).
How fish perceive colors, see Vorobyev et al. (2001).
In addition, for small-sized and juvenile freshwater and salwater fish the “white light” is enriched with UV rays (see above), which are invisible for the human’s eye.
Numerous freshwater and saltwater fish have white or whitish with the different tints belly (or the lower side in flat fish) that masks them on the backgrounds of the bright water surface illuminated with the sun light. Subjected to the conditions of crypsis in the water environment, boldly white fish (like arctic animals in winter) are absent in this environment, excepting white morphs.
In order to estimate roughly the composition of the underwater li
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