WDFW -- Chum Salmon Colors

Author's Note
The following discussion of the color patterns of chum salmon has been adapted from an article titled "Color Variations in Spawning Pacific Salmon" (by Jim Ames and Steve Schroder), published in Breakthrough Magazine in 1995. We wish to thank Larry Blomquist, publisher and editor of Breakthrough Magazine, for allowing the use of this copyrighted material on the WDFW web site.

One of the most overlooked of the many fascinating life history attributes of Pacific salmon is the use of varying color patterns throughout the lives of this group of fish. Few people understand the amount of variation that exists in the color patterns of salmon, particularly during spawning, and that these variations reflect the physical condition, degree of maturation, stock or racial origin, and the social positions of individual fish. For most people, information about salmon color patterns usually comes from photographs of dead fish, or exhausted live fish that have just been caught on hook and line (like many of the photographs presented throughout the Chum Salmon Web Page). Salmon in either of these conditions do not display the colors of living fish happily occupied with the goals of feeding, growing, and ultimately producing as many offspring as possible.

In this article, we will primarily discuss the colors of spawning chum salmon and include photographs showing various color patterns displayed by this species. These salmon color observations are the result of several decades spent studying and observing these fish in streams from Oregon to Alaska. Our goal is to explain some of the things you should look for if you get an opportunity to observe live spawning salmon.

Mature chum salmon entering freshwater to spawn. As discussed in the text below, the color patterns displayed by these fish are very dynamic, being affected by behavior factors and natural aging processes.

FISH COLOR
The variety of colors displayed by fishes is unrivaled in the animal kingdom. Not only do fish exhibit just about every conceivable variation of color, stripes and spots, but they also have the ability to change colors and patterns depending on their environments, individual moods, or life history phases. The physical basis of fish color and the mechanisms controlling color changes provide insight into why fish look the way they do.

Fish color is determined by chromatophores, specialized cells that are present in the skin of nearly all fish species. Chromatophores are irregularly shaped cells that take many forms but usually are roughly circular with multiple radiating branches. There are two basic types of chromatophores; those that provide pigmented color, and those responsible for metallic, iridescent colors. The pigmented chromatophores include melanophores (black and brown), erythrophores (red) and xanthophores (yellow).

These specialized cells absorb light, specifically, the complement of the color being expressed by a particular chromatophore. Color intensity is controlled within these cells by concentrating or dispersing pigment granules. When pigments are concentrated, the skin will be lighter and the chromatophore's color will be suppressed. Conversely, dispersion of pigment granules within a chromatophore will darken the skin and enhance color.

The metallic chromatophores include leucophores (white) and iridophores (silvery), and they contain organic crystals that reflect light to display various metallic or iridescent colors. Fish and other vertebrates have not evolved pigments for colors in the green-blue-violet spectral range. These colors are produced by iridophores reflecting light of specific wave lengths, or by the combined effects of iridophores overlaid with other chromatophores. Pigment chromatophores of differing colors can also overlay each other, and some can carry more than one pigment, processes which add greatly to the overall palette of fish colors.

There are various ways fish produce or change their color patterns. The most fundamental color schemes are the genetically determined (morphological) colors and patterns that are specific for each life history stage. Such colors can be thought of as the generic patterns that occur throughout the range of the species. These general color schemes do show some variations in individual fish, particularly in patterns of spots, stripes and bars. The generic color pattern of the spawning chum salmon (our focus species) is dominated by irregular vertical bars along the sides that vary in color in a set pattern from black just posterior of the opercle, to various shades of red or maroon in the middle of the body, and again to black along the caudal peduncle. Other notable colors include dark, often black ventral surface, and black ventral and anal fins with snow white tips.

Other types of color patterns can be controlled and changed by the fish itself; and are the primary subject of this article. These physiological changes are a response to a variety of environmental and behavioral factors, and are under both neural and hormonal control. Neural, or nervous system, control of color can involve all types of chromatophores except iridophores. Spawning salmon display a range of neurally controlled colors that often reflect their sex and social status. The most obvious example of neural control (or lack of it) is the change in color that occurs when nervous system control ends - when a fish dies. Pigmented colors often fade, but silvery or otherwise metallic areas (iridophores) maintain their normal intensity. Hormonal control of color can take a variety of forms, but usually acts primarily on the melanophores and can cause either a lightening or darkening of the skin. Adrenaline, in particular, is known to concentrate pigment, resulting in lightening of melanophores, which causes a blanching of skin color.

SALMON COLORS
Five species of salmon (genus Oncorhynchus) occur along the Pacific coast of North America, and two other species are found only in Asian waters. Of the North American species, the chinook (king) and coho (silver) salmon are the best known because of their wide distribution and renowned sporting qualities. The sockeye (red), pink (humpback) and chum (dog) salmon have long been thought of as commercial species, but these fish are increasingly valued by sport anglers. Each species of Pacific salmon has unique color patterns that vary throughout their lives and are the most vivid and colorful during the spawning period.

Prior to spawning, Pacific salmon undergo a series of remarkable transformations. They physiologically change from a marine organism to one that can live in freshwater, their jaws become elongated, large teeth may erupt through the gums, the skin and fins thicken and scales are partially resorbed. Accompanying these morphological changes are often spectacular species-specific changes in body coloration. Naturalists have frequently pondered why spawning salmon develop such vivid color patterns and at various times have proposed that they allow the fish to recognize members of their own species, aid in camouflage, increase oxygen absorption through the skin, and serve as important intraspecific signals in spawning communities.

Variations
Mature salmon are particularly interesting because their general morphology can vary regionally and their body colors are temporally dynamic. In this article six factors are described that can influence the color patterns of individual spawning fish. For living spawners in their natural habitat, the most important factors are discussed in the Social and Sexual Differences section, which covers the complex color patterns used by the salmon to communicate social standing and to compete for mates.

The color variations discussed here are general descriptions, and it should be noted that individual fish can vary greatly from the normal species pattern because of factors like genetic mutation, disease, injury, or hybridization with another species. Also, with the degree of color variability expressed by Pacific salmon, it would be expected that some populations could vary substantially from the patterns described below.

1. Maturation Differences
As Pacific salmon return from the ocean (or large lake systems) in the last year of their lives, they exhibit typical pelagic fish coloration: dark blue or green backs with silvery sides and bellies. This counter-shading pattern can make a fish nearly invisible in an open-water environment as illumination from above lightens dorsal surfaces and shading darkens ventral surfaces. The silvery sides and belly reflect the general water or background color, resulting in near-perfect camouflage. At this stage females and males are almost indistinguishable, and it takes experience and a sharp eye to differentiate sexes.

Sexual dimorphism increases as th e maturation process proceeds and the fish approach and enter their natal spawning grounds. The silvery ocean-type coloration gradually changes to full spawning dress, with the males of some species displaying more vivid colors than the females. Each salmon species has a characteristic spawning coloration, making identification relatively simple during this life stage. Sockeye, coho, and northern stocks of chinook adopt various shades of red or maroon as their basic spawning colors. Pink salmon and southern chinook stocks vary in coloration from light to dark brown. The chum is unique in color, with a greenish ground color overlaid with patterns of black and maroon.

Chum salmon showing progressive development of the bar pattern as the fish mature. The first fish is ocean bright, displaying only the slightest indications of bars. The second individual exhibits increased definition of the bar pattern, and the third fish is beginning to change color from the silver-blue ocean coloration to the basic green/yellow spawner color. The last fish shows typical coloration when entering freshwater, and the light color in the center of individual scales shows that it has not yet reached full spawning colors.

As mentioned earlier, marked changes in body form also take place as salmon mature. Males develop characteristic hooked snouts and large spawning teeth, and the body becomes laterally compressed. In a few species, males may also develop pronounced humps. An often overlooked feature is that the adipose fins of mature male salmon are substantially larger than their female counterparts. Female body form does not change as markedly as the male; the skin thickens to a lesser degree, jaws elongate slightly, teeth become somewhat more prominent and developing ovaries fill out the abdominal area. All of these changes are controlled by hormones and occur throughout the range of each species.

Once sexual maturation has occurred, Pacific salmon rapidly age, with death the ultimate result. Males engage in ritualized battles and simple aggression to gain access to potential mates, and females use their caudal fins to prepare two to seven nests which they guard until death. These behaviors cause physical changes in their color patterns. Because of their repeated fighting activities, the skin adjacent to the dorsal fin and caudal peduncle of males is often abraded and turns a light tan or white. The caudal fin and peduncle of females not only becomes abraded but skin, muscle tissue and fin rays are eroded. In general, the vivid color patterns on both sexes become occluded because of injuries and fungal infestations as the fish approach death. The duration of the spawning process is variable, but all five of the Pacific salmon species die within several weeks of the initiation of spawning.

A dead male chum salmon that has completed its life cycle and displays pronounced scale loss in the caudal peduncle area, probably caused by the bites of competing males. All fins exhibit fraying, and the lower jaw shows some skin loss that may also have been caused by fighting behavior. The light colored patches on the body surface are sites of secondary fungal infestation.

2. Latitudinal Differences
Within a salmon species, spawning colors are usually brighter and more vivid the farther north the spawning distribution occurs. Whether there is some form of genetic selection for brighter colors in more northern areas, or if this phenomenon is simply a response to local environmental conditions (water temperature or amount of daylight) is not clear. For most species, the more northern stocks express this color variation as a simple intensification of the generic pattern, with most areas of the body showing brighter hues of the characteristic species colors. For spawning chum salmon in the northern portion of their range, the result is often brighter maroons or reds in the banding pattern, darker and more extensive blacks on ventral surfaces and on the ventral and anal fins, and generally more colorful heads and body ground colors.

The chinook salmon is an exception to the above tendency; there is actually a shift in the basic spawner colors in stocks along the coast. In California and coastal Oregon, chinook spawners display a dark brown or blackish color. In Washington, the typical spawning chinook will be a lighter olive brown, and in the Puget Sound region, an occasional dark red spawner will be seen. Proceeding north of Washington State, chinook spawners become progressively redder, until bright red spawners are the rule, as in Alaska..

3. Stock Differences
Individual populations, or stocks, of salmon can display color patterns distinctly different from adjacent populations. A stock of fish is generally defined as a discrete group that is reproductively isolated from other populations; in other words, no significant interchange of spawners occurs among populations. This reproductive isolation allows a stock to adapt to local conditions, which can result in changes in run riming, survivals, body forms, color patterns, etc. In the case of color patterns, stock differences are often related to latitudinal differences, because different stocks are encountered along the Pacific coast.

There are, however, examples of color differences in stocks of salmon that occupy the same region. Puget Sound chum salmon have developed three distinct times for spawning: summer, fall, and winter stocks. A general observation is that the summer stocks, those spawning in September and October, often exhibit brighter spawning colors than their fall and winter spawning counterparts. A similar example of brighter spawning colors in early run stocks can be seen in steelhead in the giant west coast river systems like the Columbia and Skeena rivers. Early run, upriver steelhead stocks generally display distinctly brighter (redder) spawning colors than the later run winter steelhead spawning in lower river tributaries. Whether these examples of color variations between differently timed stocks represent genetically determined traits, or are just a response to local environmental conditions (different water temperatures or ambient light levels) is unclear. It is certain, however, that heritable traits, like color patterns, can vary in reproductively isolated populations.

4. Social & Sexual Differences
To throughly appreciate the spawning life history of Pacific salmon, an understanding of how and when male and female fish express various color patterns is necessary. All species of Pacific salmon display a similar set of color patterns which represents a non-verbal language that contributes to the individual's success in attracting potential mates and in repelling competing rivals.

The general color patterns evident in spawning Pacific salmon are vertical bars in males, and a single dark horizontal stripe in females. While spawning females of all species display some variation of the stripe pattern, the dominant males of different species express the barred pattern to varying degrees. The barred pattern in males varies from the bold bars of the chum spawner to a more subtle, vertical pattern of mottling in species with a darker body color (e.g. chinook and coho), and some individual fish show no obvious bars. Subdominant males, those individuals that have not successfully competed for and attracted mates, tend to reverse the pattern by displaying a stripe. Spawning color patterns are displayed most distinctly by chum salmon, which makes that species an excellent example for describing the characteristics of spawning colorations. The following descriptions of the color patterns of spawning chum salmon are from a study by Steve Schroder, who observed fall-run fish in Big Beef Creek, a stream entering Puget Sound in Washington.

In chum salmon the bar color pattern consists of black and red streaks that run perpendicular to the length of the fish. From the posterior edge of the operculum to the tip of the pectoral fins the bars are black. From this point to the posterior edge of the ventral fins (the widest part of the fish), the bars are a reddish-purple after which they become black again. They overlie a mottled green-yellow background which becomes tan on the dorsal surface of the peduncle. The stripe pattern consist of a single black purple band that runs parallel to the length of the fish. Sometimes a fine white band borders the top and bottom of the stripe which tends to further enhance it. This stripe covers approximately thirty percent of the lateral surface area of the fish. Both sexes are able to quickly change (within seconds) from one pattern to the other, although females usually maintain the stripe pattern.

There are various intermediate color stages between the bars of a dominant male and the stripe pattern of a spawning female. These intermediate patterns will often be expressed by subdominant and stressed fish (see photo of frightened chum below). The pattern is usually a partial stripe overlying a subdued banding pattern. The result is a broken stripe with the black color displayed only over areas that would normally be a part of the band pattern. Females often totally suppress the banding pattern, leaving only the broken stripe.

Female chum salmon above displaying the horizontal stripe that reduces the amount of aggression that she is subjected to and communicates her femininity to prospective mates. The stripe pattern usually starts with a dark blotch on the operculum, and may extend through the center portion of the caudal fin.

An anesthetized female chum spawner above showing the bar color pattern, which is totally different from the lateral striped pattern of a live, spawning female. If you compare the photo of this female churn with the underwater photograph of the striped female spawner above, it would be easy to incorrectly assume that the two fish were members of different species.

The ability of chum salmon to rapidly shift from one basic color pattern to another is apparently the result of several selection pressures. In spawning populations of chum salmon it pays females to advertise their femininity since that will simultaneously reduce the amount of aggression they are subjected to and enhance their attractiveness to males. Males are also expected to develop masculine traits that repulse or threaten rival and attract females. To the human eye, and most likely to all vertebrate eyes, bars that run perpendicular to the long axis of an object tend to magnify its apparent width while those running parallel to the long axis appear to reduce its bulk. The bar color pattern thus enhances the presumed size of a male to his rivals and prospective mates.

Kees Groot, a Canadian scientist, discovered that a male chum salmon's social position relative to other adjacent rivals is reflected by his color pattern. Groot placed adult chum salmon into an aquarium and allowed the fish to form pairs, complete with satellite males (subdominant males that lie downstream from a pair). He observed that the satellite males and females had the stripe color pattern while the dominant males all possessed the bar pattern. Moreover, Groot was able to quickly (within seconds) alter the color pattern of the dominant male by striking its side with a broom handle. Invariably, the struck male lost its vertical bars and developed a dark lateral stripe. Once the broom handle was removed, the dominant male would usually nip at adjacent satellite males and in the process resume the bar color pattern. Hundreds of observations made in an experimental stream by Schroder corroborated Groot's finding that dominant chum salmon males typically possess the bar color pattern. He found for instance, that dominant males usually had the bar color pattern and that satellite males almost always had a single dark stripe.

Subdominant male chum salmon above displaying the broken stripe pattern. The black color is present in those portions of the stripe that overlay the suppressed vertical bar pattern. The dorsal mottling on this fish is in large part caused by sunlight penetrating the riffled water surface.

Anesthetized male chum spawner above showing the bar color pattern. The reddish-purple bars are places in the widest part of the fish to enhance the apparent size.

In species such as chum salmon where competition for females is often intense, males are expected to develop alternative reproductive strategies. Because satellite males almost always have the stripe pattern and closely resemble females, the use of this pattern may reflect a type of female mimicry. Such mimicry commonly occurs in breeding populations of many species when portions of the male population are denied direct access to females. In chum salmon, males having the stripe color pattern avoid intense male aggression and can get close enough to spawning pairs to successfully steal fertilizations. Females often attack such males and slow down their reproductive behavior when courted by them. However, the capacity to rapidly shift from the bar to stripe pattern clearly enhances a male's ability to fertilize eggs.

A visit to a spawning stream is necessary to observe this important nonverbal form of communication. A typical grouping of spawning salmon that would likely be observed would include a striped female on a spawning nest, flanked by a large heavily barred dominant male. Downstream of this pair might be one or two subdominant satellite males displaying variations of the stripe pattern. These satellite males would most likely be on the side of the female opposite of the dominant male. Further downstream might be another spawning pair, or a number of subdominant males and females that have yet to select spawning sites or attract mates.

5. Stressed Fish
Photographs of live sport caught salmon would seem to represent ideal representations of the color patterns of living salmon. However, there are several problems with photographs of this type. A disturbed or recently captured live fish will certainly not display dominant or subdominant color patterns. As explained earlier, these patterns only develop when the fish is competing for or spawning with potential mates. A frightened fish, or one that has been removed from the security of the stream environment, will default to the generic color pattern for the species, which is usually subdued in color and may be quite different from the colors displayed by a free swimming salmon. Another substantial problem is that extra production of adrenaline and other hormones caused by stress can result in a distinct lightening of the skin (see photograph below).

Frightened chum salmon. This male chum salmon panicked when exposed to human scent in the water. In its rush to escape, the fish beached itself and was thoroughly frightened. The light body color is probably the result of an adrenaline surge, producing a general blanching of skin color.

The opposite effect is observed when a fish is stressed to the point of exhaustion, like the typical sport-caught fish. In this case, the adrenaline blanching effect is overridden by other hormones, which disperse pigments in the chromatophores, resulting in a darkening of ground color on dorsal and other surfaces. In fact, photographs of live fish commonly show just such a dark skin color, which can be very different than the color normally exhibited by the fish in the stream.

The above photograph of the frightened chum salmon shows another interesting feature; a down-turned eye. A living fish lying on its side will rotate its top eye downward in the eye socket, and this down-turned eye in a photograph shows that the fish was still alive. In fact, the fish displaying a down-turned eye is simply demonstrating an equilibrium reflex by reacting to the unnatural position of lying on its side. If you hold a live fish on its side and look at the bottom eye, you will see that it is turned upwards in its socket because the fish is trying to orient itself to the normal upright position.

The same photograph also shows equilibrium responses in fin positioning. The pectoral fin is clamped tightly against the side, the right ventral fin is curled upwards, and the dorsal and anal fins are cupped. These are the fin positions that will return the fish to an upright orientation as soon as it begins moving forward in the water.

6. Dead Fish
The dead fish photograph is the most common source of information on the colors of salmon and other fishes. If one wants to truly understand the color patterns of living salmon as they appear in their natural habitat, dead fish photographs must be used with caution. When a fish dies, neutral control of color ends and the fish will typically display subdued generic colors and often an unnaturally dark body ground color. Dominant and subdominant colors and patterns typical of spawning fish will be missing. Such photographs will show the general generic colors, and can be used to identify patterns of spots and stripes, and to see other fine points like scale detail.

Dead male and female chum spawners showing the subdued colors and dark dorsal surfaces typical of dead fish. The male (top) has the bar pattern of a dominant male but lacks the intensity of color of a live fish. The female also exhibits the generic bar pattern which is entirely different than the characteristic stripe of a live spawner.

IN CLOSING
Wild chum salmon are abundant in many western Washington streams, providing an excellent opportunity to learn about their freshwater life history through first-hand observation. One of the most intriguing characteristics of this species is the non-verbal language of changing color patterns used by chum salmon during competition for mates and the subsequent spawning process. To gain insight into the significance of the fascinating variations that occur in salmon colors and patterns, there are few good substitutes for sitting on the bank of a stream, observing living fish actually engaged in the spawning process. It is also a pretty nice way to spend your time on a sunny fall day.

ADDITIONAL READING
Ames, J. and S. Schroder. 1995. Color variations in spawning Pacific salmon. Breakthrough Magazine. No 41. p. 76-83.

Schroder, S. L. 1981. The role of sexual selection in determining overall mating patterns and mate-choice in chum salmon. Ph.D. thesis, Univ. Wash., Seattle, 274 p.