*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**

**EFFECT OF MAINSTEM SURVIVAL RATES ONSPRING CHINOOK SALMON PRODUCTION**

Willis E. McConnaha

Northwest Power Planning Council851 S.W. Broadway, Suite 1100Portland, Oregon 97204

For the last several decades, mortality of juvenile and adult fish passing through the Columbia River hydroelectric system has been the major factor limiting and shaping the production of spring chinook salmon in the Columbia and Snake Rivers. Despite general acceptance of this point, it remains unclear how and to what extent passage losses affect spring chinook populations. Nonetheless, our ability to plan effective mitigation measures and the success of multimillion dollar efforts to rebuild spring chinook populations will ultimately depend on our understanding of the processes by which fish populations are shaped by the hydroelectric system.

The theme of this workshop is "The Status and Future of Spring Chinook Salmon in the Columbia River Basin." To contribute to this topic, I have examined how mainstem passage losses might act to limit production of spring chinook. This was done using a simple computer model. The model allowed manipulation of factors that would be impossible to do in the field. The purpose of the study was to gain insight regarding two questions: 1) How are natural spring chinook populations in the Columbia Basin presently limited by mainstem passage? and 2) What will be the effect of planned efforts to improve mainstem passage on spring chinook populations? It also provided an indication of the degree to which feasible subbasin and passage survival actions might mitigate the effects of hydroelectric development on production.

The study has indicated that natural spring chinook populations in the upper basin are, and will continue to be, severely limited by passage losses. Habitat measures appear to have a very limited ability to mitigate the losses caused by the dams. In this study, natural upriver spring chinook populations continued to be depressed even after improvements in passage survival and in subbasin survival and capacity.

**Overview of the Analysis**

Output for the study was the population size at the point of maximum sustained yield (MSY). Calculation of this point was done using the System Planning Model (MEG 1989). This permitted a variety of passage survival changes to be examined as well as modification of the subbasin production parameters. Density dependance in this model is based on the function described by Beverton and Holt (1957).

The model was used to place an. identical, idealized spring chinook subbasin at four locations in the Columbia Basin. Study locations were as follows:

- 1.
- Below Bonneville Dam, River Mile 146, no dams below,

- 2.
- Above McNary Dam,  River Mile 292, four dams below,

- 3.
- Above Lower Granite Dam, River Mile 400, eight dams below,

- 4.
- Above Wells Dam, River Mile 515, nine dams below.

At each location, four subbasin conditions were examined:

- 1.
- A baseline intended to simulate a moderate condition regarding juvenile survival and smolt carrying capacity,

- 2.
- The baseline condition with egg-smolt survival rate increased by 25%,

- 3.
- The baseline condition with the smolt carrying capacity increased by 50%,

- 4.
- The baseline condition with increases in both the egg-smolt survival and the smolt carrying capacity.

The baseline subbasin condition used data provided to the System Planning process (Columbia Basin Fish and Wildlife Authority (CBFWA) 1989) and represented mid-level values in regard to juvenile survival rates, carrying capacity, and fecundity. The other scenarios simulate feasible actions such as habitat improvement (increase the egg-smolt survival) and habitat expansion (increase the smolt carrying capacity).

The effect of each subbasin condition on the MSY was examined in light of five passage survival conditions:

The baseline conditions simulated existing passage conditions while the other scenarios represent planned or possible changes in passage conditions. TBR refers to the transport benefit ratio which is the ratio in adult return rates between fish that were transported as juveniles and those that were not transported. FGE is the fish guidance efficiency, which indicates the proportion of fish approaching a turbine intake that are diverted into a bypass system.

Throughout the study, adult passage losses at the lower four projects (Bonneville through McNary) were assumed to be 10% per project, while losses at the upper projects in the Snake and mid-Columbia areas were set at 5% per project.

Harvest rates were set to zero for all combinations.

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**Results**

The results from the study are summarized in Table 1. The effect of carrying capacity is linear in the Beverton-Holt relation, so that a 50% increase in capacity will result in a 50% increase in the number of fish. The effect of egg-to-smolt survival rate, however, is nonlinear and will vary with the scenario. In Table 1, the percent change of each scenario relative to the baseline subbasin condition (/l) is provided in the third, fifth, seventh, and ninth columns. Also shown is the percent reduction in the population at the maximum subbasin condition (/4) as a result of moving the subbasin to the locations above Bonneville Dam.

**Discussion**

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The results of this study indicate the degree of reduction in spring chinook natural production in the upper Columbia Basin imposed by the hydroelectric system. They also indicate that extreme reduction in fish productivity will likely continue even after implementation of all presently available options for increasing passage survival rate. This point is reinforced in Table 2. Other results of the study are as follows:

- 1.
- Populations may be limited either by survival rate or capacity.

- 2.
- The limiting effect of survival rate on the population size increased as the subbasin was moved upriver. The population in the lower Columbia River area was largely limited by capacity.

- 3.
- Natural spring chinook populations in the upper Columbia Basin decreased substantially as a result of law passage survival rates.

- 4.
- Natural production of spring chinook in upper basin areas, particularly the upper mid-Columbia. River area, remained low even under the most optimistic passage assumptions and optimum subbasin conditions.

- 5.
- In upper basin areas, the effectiveness of habitat improvement measures to increase the population size decreased because of the effect of main-stem passage survival rates.

- 6.
- At all locations, the effect of decreasing reservoir mortality by 20% was less than anticipated. This was because a seemingly large reduction in reservoir mortality resulted in a relatively small increase in reservoir survival rate.

- 7.
- Spring chinook productivity in the upper Snake River area was low but substantially greater than that in the upper mid-Columbia River area due to smolt transportation in the Snake River and the assumption of a positive transport benefit.

- 8.
- Productivity of natural spring chinook in the above Lower Granite area was increased by measures that increased the benefit of transport or the proportion of the migration affected by transport.

- 9.
- Production in the upper mid-Columbia River area was most benefited by installation of efficient smolt bypass systems at all projects.

**Acknowledgements**

Thanks to Duane Anderson for computer assistance and discussion. John Marsh, Willa Nehlsen, Barry Wagner, and Jack McIntyre provided helpful review of the manuscript.

**References**

Beverton, R. J. H., and S. J. Holt. 1957. On the dynamics of exploited fish populations. Fish. Invest. London HMSO 19:533.

Columbia Basin Fish and Wildlife Authority. 1989. Preliminary system analysis report. Columbia Basin Fish and Wildlife Authority, 2501 SW let Ave., Suite 200, Portland, Oregon 97201.

Monitoring and Evaluation Group. 1989. Documentation for the system planning model. Northwest Power Planning Council, 851 SW Sixth Ave., Suite 1100, Portland, Oregon 97204.

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**QUESTIONS AND ANSWERS**

- Q:
- Regarding your third conclusion, that production can be described by main-stem passage losses—if stocks "a genetically unique, and there are efforts to preserve this genetic diversity, how would you support water management strategy which is based on numbers of fish rather than specific stocks? This has been the approach in the Snake River. Spill management and water budget, for example, have not addressed the temporal separation of certain stocks.

- A:
- That is the beauty of models, which don't have to deal with practical matters. In the Snake River, there am not too many water options. Information is being accumulated that may lead us to customize water use and better protect stocks. Up to now, this has been conjecture. I agree that this should be done, and to incorporate genetic data as well, in the decisions. This could lead to a very different management system.

Comment: Managers are confronted with the problem of whether to protect overall numbers of fish or particular stocks. This hasn't been addressed.

- Q:
- Have you run the model with other races or species with smolts outmigrating later in the spring, especially in regard to predation?

- A:
- A model to relate residence time with flow doesn't exist now. Predation was less than I expected, but it is still significant. Predation would be a greater factor in the summer, with higher temperatures, etc.

- Q:
- Regarding changes in survival, I am optimistic about the increases that you noted-but what about population levels today?

- A:
- Yes, if you look at percentage changes, for example 700%, this is dramatic. But if you look at populations, in some this means that a 300% increase comes from only three fish! Can these co-exist with downriver populations? We need to temper our actions and look at a broader perspective.

- Q:
- In the model, you enhanced particular factors, but what about increased flows? Is this not an option or perhaps not a benefit?

- A:
- The effects of flows may be felt in the predation aspect. How the decrease actually occurred may be due to flow enhancement.

**Table 1. Maximum sustained yield of ran size at four locations when passage and subbasin parameters are modified.**

Note: Coding of scenarios is Passage Condition/Subbasin Condition.

Table 2. Comparison of the change in spring chinook salmon run size for an idealized subbasin at four locations in the Columbia Basin under enhanced subbasin and passage conditions relative to the baseline condition.

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