*U.S. Dept Commerce/NOAA/NMFS/NWFSC/Publications*

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**NOAA F/NWC-187 - Status and Future of Spring Chinook Salmon in the Columbia River Basin—Conservation and Enhancement**
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**AN ASSESSMENT OF ADULT LOSSES, PRODUCTION RATES, ANDESCAPEMENTS FOR WILD SPRING AND SUMMER CHINOOK SALMON IN THESNAKE RIVER**
T. C. Bjornn

U.S. Fish and Wildlife ServiceIdaho Cooperative Fish and Wildlife Research UnitUniversity of IdahoMoscow, Idaho 83843

In recent years, increased numbers of adult spring and summer chinook salmon have returned to the Snake River. This rebound in abundance follows a period around 1980 when adult production had fallen toabout 10% of the numbers 20-30 years earlier. Reported herein are summaries of an assessment ofpotential adult losses related to dams, production rates by wild fish, and present wild fish spawningescapements versus those in earlier years.

**Adult Losses Between Dam**

Counts of upstream migrating salmon entering the Snake River were begun in 1962 with thecompletion of Ice Harbor Dam. Additional upstream dams were completed and counting of migrantsstarted in 1969 at Lower Monumental Dam, 1970 at Little Goose Dam, and 1975 at Lower Granite Dam.The discrepancy in counts between Ice Harbor and Lower Granite Dams during the 1975-89 period hasaveraged 15.1% for spring chinook, 0.01% for summer chinook, and 11.8% for both groups combined. Themas have averaged about 30,000 fish during the period and thus about 3,000 more fish, on average, havebeen counted at Ice Harbor Dam than at Lower Granite Dam. Some of the lost fish entered the TucannonRiver (probably less than 1,000 annually) and some probably died between the dams. The loss per dam forthe three dams and reservoirs upstream from Ice Harbor would average about 3 to 401o, about the samerate as has occurred between Ice Harbor and Lower Monumental Dams for the 1969-89 period.

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**Losses Between Dams and Spawning Grounds**

In addition to adult salmon counted at the dams, the redds of spring and summer chinook salmon havebeen counted in many Snake River tributaries, some since the early 1950s. These redd counts provide anindex of spawner abundance that can be compared with counts of adults destined to spawn naturally at thedams to determine if loss rates during the final portion of their upstream migration have changed since theearly 1960s. The number of fish that were destined to spawn naturally in the Snake River Basin and thatcrossed Ice Harbor Dam was determined by accounting for wild and hatchery fish upstream from IceHarbor Dam, assigning each group its share of the lost fish, and determining the proportion of fish countedat the dam that were wild or hatchery in origin for any given year. This number was then compared withestimates of fish that actually spawned (based on redd counts) to determine the proportion that survived tospawn.

From 1962 to 1968, when Ice Harbor Dam was the only dam in the lower Snake River, I estimatethat about 56% of the wild fish survived to spawn (Fig. 1). Since the completion of the three other dams,the estimated survival rate between Ice Harbor Dam and the spawning grounds has been about 46%.The lower rate in recent yews could be due to losses at the most recent dams rather than increasedmortality between the dams and the spawning ground.

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**Production Rates of Wild Fish**

The separation of the Snake River spring and summer chinook into wild and hatcherycomponents for the 1962-88 period provided an opportunity to evaluate production of the wild segmentusing a parent-recruit model. I assumed that spring and summer chinook salmon had a 4-year cycle andemployed a 5-year running average to evaluate trends in production rate. During the 1960s, theproduction rate for spring and summer chinook salmon combined was about five recruits per parent (Fig. 2). By the late 1970s, the production rate had declined by about 80%. Production rates for recent broodyews have risen to levels equal to that of the 1960s.

The number of adults crossing Ice Harbor Dam that were destined to spawn, naturally has declinedduring the last 27 years from an estimate of about 17,000 (not including losses) in the 1960s to less than4,000 fish for the 1980-84 brood yew 5-year average (Fig. 2). The increase in production rates of recentbrood years coincided with the smallest spawning escapements of wild fish in the last 30 yews. If theSnake River spring and summer chinook salmon populations have dynamics Similar to those of modelssuch as the Beverton-Holt parent-recruit relation, increased production rates would have been expectedwith small numbers of spawners. The questions then become: Haw the production rates increased toexpected levels? and Are the increases due to improvements in conditions that caused the lower rate ormerely a response to small numbers of spawners? To evaluate the recent increases in production rate, Iused a Beverton-Holt curve fit to the parent and recruit numbers for the brood yews of the 1960s, andcompared the expected production rates based on the parent-recruit curve (Fig. 3) with actual rates.Brood yews in the 1960s had actual production rates that were similar to the expected rates (Fig. 4)because the parent-recruit curve was based on data for those brood years. In subsequent years,expected production rates increased as the number of wild spawners declined, and the actual productionrates decreased until after the 1978 brood yew, when they began to increase. The recent increases inproduction rate probably signal an improvement in conditions that caused the earlier declines, but therates are still lower than expected, arn indication that problems still remain.

**Spawning Escapements for Wild Fish**

The number of spawners needed to provide optimum or even full seeding in the natural productionareas of spring and summer chinook salmon in the Snake River Basin has not been well defined becauseof the lack of intensive studies in a variety of types of streams. During the 1970s, we monitored thenumber of juvenile spring chinook salmon produced in Marsh Creek, a tributary of the middle fork ofthe Salmon River, and related the number of migrants to the number of redds from the preceding fall.During the 8 yews of monitoring, numbers of redds ranged from about 20 to 350, and the number ofage-0 fish migrating downstream after rearing in the stream during the summer and fall ranged fromabout 10,000 to nearly 130,000 (Fig. 5). Although there is variability and not as much data as we mightlike, I would infer that there is evidence of an asymptotic relation between redds and age-O migrants, and that about 400 redds wouldcome close to fully seeding the Marsh Creek drainage upstream from Capehorn Creek. If the foregoingis accepted, then we can relate redds in Marsh Creek to the estimated number of wild spawners passingIce Harbor Dam (Fig. 6) to obtain an estimate of the escapement of wild spring chinook salmon neededat Ice Harbor Dam for nearly full seeding. If 400-500 redds provides nearly full seeding in Marsh Creek,and the other spring chinook stocks have a similar relation, then we could suggest from the relation inFigure 6 that we need about 20,000 wild spring chinook salmon spawners (not including losses) over IceHarbor Dam for the remainder of the Snake River Basin.

**QUESTIONS AND ANSWERS**

- Q:
- Have you taken into account the effects of ocean conditions?

- A:
- I have assumed that ocean harvest is negligible.

- Q:
- If ocean harvest rates decreased recently, would this give the results you show?

- A:
- The early segments of the graph would show an increase, and they don't.

- Q:
- In estimating numbers of wild spawners, was this based on observed redds in all basins, or expanded from a few observations?

- A:
- The estimate is based on actual counts, but we did expand the number of spawners to assume that we saw only 90% of the redds.

- Q:
- Do you think you might have underestimated the counts recently? Since you have observed the redds and the returns to hatcheries, it appears that there are a number of fish disappearing because of mortality, or you have not accounted for all of the redds.

- A:
- I believe that our redd counts have been accurate. We included adult counts from Ice Harbor Dam.

- Q:
- How much residual can be explained by saying that all fish am 4-year-olds and invoking the ocean environment?

- A:
- We have looked at age structure by 5-year segments and 5-year averages, and this doesn't change the conclusions. We probably don't have age structure data for all tributaries. Using age structures may sharpen the conclusions, but not dramatically. Ocean conditions we just part of the variability. With flows, for example, not much of the variability was due to this at times. Maybe the same applies at times to ocean conditions.

- Q:
- Did you account for removal of the Lewiston Dam and the addition of the Clearwater River as spawning area?

- A:
- Yes, all of the redds we in this relationship—all of the Clearwater River drainage.

- Q:
- In expanding Marsh Creek to Ice Harbor Dam, you assumed all above Ice Harbor Dam were equal to Marsh Creek-is that a supportable assumption?

- A:
- This would vary from stream to stream. Other examples we not markedly different, 24 vs. 20 for some stream. The carrying capacity varies by stream.

- Q:
- In estimating wild spawners at Ice Harbor Dam, did you look at rack returns and subtract the Ice Harbor Dam numbers and use this?

- A:
- I did that for all hatchery fish I could account for.

- Q:
- In hatcheries, some fish do not come back, i.e., they are strays. The early years show less "residual" (expected vs. actual) numbers when there were few hatcheries. How would this affect your conclusions?

- A:
- We don't know the straying rate—it seems to be low, less than 5%. If it is higher, then it would show up as redds. The same would apply to hatchery fish put into streams to spawn.

- Q:
- There have been millions of salmon coded-wire-tagged in Idaho hatcheries on the Salmon River—have you recovered any of these in the Middle Fork of the Salmon River?

- A:
- No.

Figure 1. The proportion of wild spring and summer chinook salmon passing Ice harbor Dam from 1962-88 that survived to spawn in the Snake River Basin. During the 1962-68 period, Ice Harbor Dam was the only dam present in the lower Snake River. After 1975, all four of the present dams were in operation.

Figure 2. Production rates (recruits per parent) for wild spring and summer chinook salmon in the Snake River Basin based on a 4-year cycle (upper), and estimated number wild spawners from 1962-88. Data are smoothed with a 5-year moving average to observe trends.

Figure 3. Production rate of Snake River spring and summer chinook salmon versus number of spawners for the brood years from 1962-84, and the expected production rate curve with a Beverton-Holt relation with parameters as listed next to the curve.

Figure 4. Expected and acutal production rates for the 1962-84 brood years, spring and summer chinook salmon in the Snake River.

Figure 5. Number of redds counted and number of age-0 spring chinook salmon leaving Marsh Creek in late summer and fall for eight brood years, and my inference of the asymptotic relation.

Figure 6. Estimated number of wild spring chinook salmon passing Ice Harbor Dam that survived to spawn, and number of redds counted in the Marsh Creek drainage from 1962-88. The arrow represents an estimate of escapement needed for nearly full seeding based on Figure 4.

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