If you observe spawning chum, coloration is the first thing to notice. In addition, several "displays" or patterns can be seen during the spawning process. The most easily observed activities are female digging, male competition for dominance.
Male and female chum salmon develop their spawning "plumage" when they enter the estuary. Upon entering the creek the males and females have what is known as a "barred" pattern, purplish-red marks down each side of the fish. Soon after entering the creek, and before digging redds, the females lose the barred pattern and develop a prominent black lateral strip. In a camelion fashion, the male chum switch between the "plumages" depending upon the degree of dominance or challenge. Sub-dominant males display the black lateral strip and the dominant ones show the reddish barred pattern. One researcher developed a clever test that demonstrates the speed with which the markings change. A dominant male was hit with a broom handle. The barred pattern was immediately dropped for the lateral strip. With no further challenge from the broom handle the dominant pattern returned in a few minutes. In battles both fish often display the dominant pattern until clear dominance is established. In very crowded streams the dominant pattern is apparently rare as the fish are never free of challenges.
After entering the creek and getting to the spawning area, the fish survive for 1 – 2 weeks. Females choose a nest site (also known as a redd), defend the territory, dig the nest, lay eggs, and bury the eggs. Males swim nearby fighting with other males to establish dominance and courting the female.
To choose a site, females "nose" their way along the bottom occasionally stopping for a test dig. In addition to good gravel, they prefer to spawn above turbulent areas in places called riffle crests and away from other females.
Once the redd site is established, the female guards the territory from other females and starts digging. To dig the fish turns on it’s side and flexes repeatedly, driving the tail on to the gravel bottom. In this manner, she digs a cone shaped hollow 10" to 20" deep. Sand and gravel are generally flushed from the redd leaving clean gravel and cobble. Between digs the female turns and swims to the back of the nest. She also weaves over the nest in a circular and figure eight pattern. As the nest develops, she will also "probe" the nest with her belly. When she can drop her belly deep enough into the redd, the digging is done.
Male courtship consists of establishing dominance among competing males and courting the female. In the principal display, the male makes a quick approach along side the female, stops and "quivers" for 1-2 seconds. This can be observed and is best described as the fish is vibrating. The male does the "cross over" display, repeatedly swiming from one side of the female to the other, crossing over her back.
As the nest nears completion the female decreases her turning, circling, and weaving between digs. The male increases his quivering and crossing over. When the female is ready to deposit eggs, she goes to a "crouch" position inside the redd. This is a probe, with the belly deep in the redd, but with the mouth held wide open. The male immediately moves along side. The female releases eggs and the male releases milt. It takes about 10 seconds to deposit the eggs and both fish quiver throughout. The female immediately starts to cover the redd. The first few digs are gentle and drive the eggs down into the gravel. The rest are full digging strokes and cover the eggs deep in gravel.
As soon as the redd is covered, the female starts to dig a redd just up stream, further covering the eggs in the nest below. Often she will dig 4-5 nests one just above the next, depositing eggs in each one. The whole process takes 30-40 hours, after which the female lives her short life guarding the territory. The final redd, or set of nests, generally looks like a 5-10 foot length of disturbed gravel with a depression at the upstream end. The eggs are buried throughout the disturbed gravel, not just where the depression is.
After spawning, the male moves on to find another female. In this way, a dominant male might fertilize several redds. There is such competition among the males that smaller males must resort to other tactics. These so called satellite males dart up to the spawning pair and release milt, sometimes fertilizing up to 25% of the eggs. Typically 2 or 3 different fish succeed in fertilizing a portion of the eggs.
Much of the material here is based upon Pacific Salmon Life Histories by C. Groot and L. Margolis. People interested in reading further about chum and salmon spawning should start there.