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Chum Salmon Life History
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Chum salmon

Chum Salmon Ages

The life histories of all Pacific salmon species begin with egg development and hatching in freshwater, and continue with migration, feeding, and growth in the freshwater and marine environments of the Pacific Northwest. After a period of time, the salmon to begin to sexually mature, and they commence spawning runs to their home streams. The timing of maturation is controlled by internal genetic codes in combination with the effects of environmental conditions throughout the life of the fish. The spawning process completes the life cycle (ending in death) of Washington's five species of Pacific salmon. The pattern of ages at which this occurs are specific to each species of salmon, however, the age at which individual fish sexually mature and return to spawn is not fixed, but can vary from fish to fish (termed an "indeterminate semelparous" life history).

These "age at return" patterns vary from simple (virtually all pink salmon return as 2-year old fish) to the very complex (chum salmon can return as 2-, 3-, 4-, 5-, or 6-year old fish). In the case of chum salmon, offspring from a single spawning year (called brood or brood year) can mature and return to their natal streams from 2 to 6 years later, although 2- and 6-year fish are very rare. This creates complex biological assessment and management issues for each of the Pacific salmon species that have variable ages at maturity. To effectively manage salmon, it is important to estimate age compositions of major stocks each year.

Aging of returning salmon is typically accomplished by counting the annual growth bands on scales or otoliths (small bones in the head) collected from fish caught in fisheries operating specifically on the stocks to be aged. When there are low numbers of returning fish, or protective harvest regulations as in recent years, there may not be sufficient numbers of salmon caught in local fisheries to provide the opportunity to sample scales in fisheries at acceptable levels for age determinations. When this happens, scales and/or otoliths can be collected from salmon carcasses sampled in the spawning streams or at salmon hatcheries.

Using Chum Salmon Age Data

Stock Productivity - Probably one of the most important uses for age data in salmon management is to assess the basic productivity of a population, a value that is often expressed in terms of adult fish produced (recruits) per parent spawner. This process is of course more difficult in salmon populations where the offspring will return from a single parent spawning over the course of several subsequent years. As an example, it may be necessary to figure out how many adult chum salmon were produced from a hypothetical parent spawning escapement of 50,000 chum salmon in 1995. Chum salmon commonly return at either age-3, -4 or -5 years old. Scales would be collected from fish returning to spawn in the years 1998 (3 years later), 1999 (4 years later), and 2000 (5 years later) in order to do this analysis. Each year there is a commercial chum fishery that harvests fish from our hypothetical chum salmon stock. Scales taken from a portion of these harvested fish are analyzed to determine the age composition of the catch. These samples will represent the annual age composition of our hypothetical run of chum salmon. In the following simplified approach, the result of scale data from sampling 1,000 fish each year is as follows:


age 3
age 4
age 5
1998 300 (30 %) 600 (60 %) 100 (10 %) 1,000
1999 700 (70 %) 250 (25 %) 50 (5 %) 1,000
2000 400 (40 %) 500 (50 %) 100 (10 %) 1,000

If we consider total fishery catches plus the spawning escapement to represent the total runsize returning from the ocean, then application of the age sampling data in the above table will allow estimation of the total number of adults produced from the 1995 spawning escapement.

age 3

age 3

age 4
age 4
age 5
age 5
1998 100,000 30 30,000 60 60,000 10 10,000
1999 200,000 70 140,000 25 50,000 5 10,000
2000 150,000 40 60,000 50 75,000 10 15,000
Note: The highlighted numbers represent the returns of 1995 brood fish in 1998, 1999, and 2000.

The total number of adult chum salmon produced from the 1995 parent escapement will therefore be the 1998 age-3 return + the 1999 age-4 return + the 2000 age-5 return from the above table.

30,000 + 50,000 + 15,000 = 95,000 fish

Since our hypothetical chum salmon run had 50,000 parent spawners in 1995, the total adult production (recruits) and recruits per spawner (R/S) from the 1995 brood can then be derived by the following calculations:

Total age-3+4+5 fish returning ÷ parent 1995 escapement =
95,000 recruits ÷50,000 spawners =
1.9 recruits / spawner

This type of analysis is conducted every year on many Washington State salmon populations. Over time, a basic understanding of the productivity of each salmon run is developed.

Runsize Forecasts - Another use of salmon age data is to predict future returns of fish so various fisheries can be managed to allow the desired numbers of spawners to reach their spawning streams. There are many approaches to forecasting salmon returns, however, most are variations of two methods, survival rate and sibling forecasts. For chum salmon, both forecasting methods require reliable age data.

A simple survival rate forecast uses the average number of salmon that return in each age category from each spawner in the parent year. Assume a salmon population that had an average total return rate of 3.0 fish (recruits) per spawner: 1.0 recruit/spawner (R/S) as age-3 fish, 1.75 recruits per spawner as age-4 fish, and 0.25 recruits per spawner as age-5 fish. This brood age composition ratio (1.00:1.75:0.25; or 34% age-3, 58% age-4, and 8% age-5) is termed the "maturity schedule" for this specific population of salmon. To predict the return in year 2001 the number of parent spawners in 1998 would be used to predict the 3-year old return (1998 spawners x 1.0 R/S), the 1997 parent spawners would be used to forecast the age-4 return (1997 spawners x 1.75 R/S), and the 1996 parent spawners would be used to forecast the age-5 return (1996 spawners x 0.25 R/S). The estimates of returning age-3, age-4, and age-5 fish would the be added together to predict the year 2001 return for this particular salmon population.

The sibling forecast uses the earliest returning fish from each spawning year (typically 2- or 3-year old fish) to predict the subsequent returns of the older fish (typically age-3 or older). To show how this works, assume a salmon population that has an average age composition of 40% age-3 and 60% age-4 fish (based on ages from the same broods). If the 4-year old fish were returning this season, and 1,000 3-year old fish from the same spawning year had returned the previous year, a sibling forecast for 4-year returns would be:

4-year returns = [(1,000 x 0.6) ÷ 0.4] = 1,500 salmon

Or, a simpler way of looking at the problem is that on average, there will be 1.5 times as many 4-year fish as 3-year fish (0.6 ÷ 0.4) in this hypothetical salmon population. The sibling forecast for 4-year returns could also be calculated:

4-year returns = 1,000 age-3 recruits x 1.5 = 1,500 salmon

Risk Assessments - Based on the age compositions in the above sibling forecast example (40% age-3 and 60% age-4 fish), the average age at return for this hypothetical salmon population would be 3.6 years. The average return age can be used for assessments of the risk of extinction faced by a stock of salmon. A calculation of "total population size" can be made by multiplying the average number of spawners in a population times the average return age. Total population size is an estimate of the total number of spawners that will result from all of the individual fish alive at any given time (representing production from all current brood years). If this number drops too low, the population can be at risk of extinction.

Evaluate Mortality - Another important use of survival estimates derived from age and return data is to measure the impacts of various limiting factors on a salmon population. Factors like flooding, urbanization, and the effects of dams or logging can be related to the survival of salmon populations, and recovery efforts can be developed if negative impacts are identified.

Aging Chum Salmon

Most chum salmon are aged by analyzing scales collected from three sources: 1) fish caught in commercial fisheries, 2) fish sampled at hatchery racks, or 3) from carcasses in spawning streams. Otoliths (ear bones) are rarely used for aging chum salmon; but have occasionally been used in programs designed to monitor and evaluate fish returning to artificial production facilities.

Scale selection area
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Scale Sampling - Specific methodologies are used by the state and tribes to collect salmon scales as a part of annual biological sampling programs. Scales are collected from a "preferred area" of the fish. The preferred area is on the side of the fish from the second to the seventh rows of scales above the lateral line and on a diagonal from the back of the dorsal fin to the front of the anal fin. Two separate scales are collected from this area within 10 scales in front or behind the diagonal line. The scales are removed from the fish with forceps, and are placed on a gummed scale card. Information for each fish is also recorded on the card with the scales including: date, fishing area, gear type, and the species, sex, and length of the fish.

Analyzing the Scales - The scale cards hold the scales samples from 20 individual salmon. In the age analysis lab, a combination of high pressure and heat are used (with a hydraulic press) to create impressions of the scales in strips of clear acetate. Multiple sets of scale impressions can be made. The scale impressions on these plastic strips are an archival record of the scale samples that can be freely handled and will not deteriorate over time.

The process of determining the age of a fish from scales is called "scale reading". The scale impressions are magnified by a factor of 24X and are viewed on a rear projected screen (using a microfiche reader). Salmon ages are determined by examining patterns of rings (called circuli) that are laid down on the scale as the fish grows. The development of these growth rings is similar to tree rings; during periods of rapid growth the rings are widely spaced, and when growth slows the rings are more tightly spaced. Salmon grow rapidly in the summer months when water temperatures and food availability are highest, and they experience slower growth during the colder winter months. Salmon scales typically exhibit alternating bands of widely spaced (summer) and narrowly spaced (winter) growth rings. The winter bands of circuli are called annuli, because the end of each winter season generally corresponds to the completion of another year of life.

Chum salmon are relatively easy to age because they spend nearly their entire lives in marine waters, and do not have the complicated freshwater growth patterns associated with some other salmon species. Reading a chum salmon scale is simply a matter of counting the bands of widely spaced circuli representing summer growth. Using the age-3 chum salmon scale shown below as an example, the first summer is represented by the widely spaced circuli in the center of the scale, the second summer by the band of wide circuli spacing between the first and second annuli, and the third summer by the band of circuli outside of the second annulus. At the time this chum salmon was sampled, it was nearing the end of its third summer of life. If the fish was a maturing adult or was sampled on the spawning grounds, it was completing its life cycle at the end of its third year. Four and five year fish are aged in the same manner (see examples below).


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