size of the coho population off Oregon prior to setting ocean troll fishing seasons. In this case, the 

 abundance of adult three-year-old coho salmon off the coast of Oregon over a series of years has a 

 statistical relationship to the abundance of two-year-old fish that returned in the previous year from the 

 same generation. By observing the number of two-year-old fish that return in a year, predictions can 

 be made about the expected number of three-year-old fish the next year. Because it is a statistically 

 based method, a measure of the probability and statistical confidence of the prediction can be made. 



Statistical techniques offer a suite of powerful tools that can be used to gain insight regarding the 

 sources of observed variation in returns, for instance, or for examining relationships within a limited set 

 of variables. In most cases, they offer the ability to determine the statistical confidence that can be 

 associated with a statement about the relationship between variables. 



One difficulty with these techniques is that it is often necessary to concentrate on some small 

 segment of the life cycle or a single correlative relationship such as the OPI. It is generally not possible 

 to discern statistical relationships over the whole life cycle because of the many conflicting factors that 

 contribute to the observed population size. These techniques, therefore, may not be applicable to the 

 evaluation of the program as a whole. 



Statistical relationships also do not necessarily imply cause and effect but may only be showing a 

 relationship between two variables that are both responding to a change in another variable. In the 

 case of the OPI, for instance, the abundance of fish that mature at age two may have no direct impact 

 on the number that mature at age three. Instead, it is possible that both age classes have similar 

 survival rates in the estuary and early ocean life stage, and that this survival rate is a critical factor 

 affecting the ultimate abundance of both age classes. Observing a statistical correlation between the 

 two year classes provides a useful management tool but does not provide insights that would lead to 

 improved hatchery practices, for example. 



Life cycle method . In a life cycle approach, the various discrete pieces of information that exist 

 about salmon and steelhead are organized around a model of the life cycle. This can include 

 information obtained from observational methods as well as information on the relationships between 

 variables gleaned from a variety of statistical methods. The life cycle approach thus incorporates 

 information from all the previously discussed methods as well as the results of research aimed at filling 

 in critical data gaps. The model is itself a hypothesis about how the pieces fit together which can be 

 tested by comparing its behavior to that of the real world. 



This approach permits the available information to be examined in the context of the behavior of 

 the total system, and provides a consistent framework for comparison and evaluation of actions. As 

 the various components of the model are refined over time through monitoring and research, the 

 model becomes a better and better expression of reality indicating an improved understanding of the 

 system. In contrast to many statistical techniques, the life cycle technique does not attempt to predict 

 year-to-year changes in fish abundance. Instead, it is used to examine the effect of various actions on 

 the long-term trend in various types of production indices. 



A number of output variables can be obtained from the life-cycle method including measures of 

 stock abundance and stock productivity. Stock abundance is the estimated number of fish present, 

 and is analogous to the observational estimates of abundance discussed above. Stock productivity 

 refers to the ability of the population to produce fish surplus to the number that are required to spawn 

 and maintain a stable population size. This is an important expression of the condition of a population 

 since it reflects the number of fish available for harvest or to buffer the effect of environmental 

 fluctuations. The amount of the population surplus to spawning needs is also an expression of the 

 speed with which a population will respond to changes in the environment including mitigating 

 measures. A population with a large surplus proportion will respond quickly to improvements in the 

 environment, whereas in the reverse, the population may respond to improvements only over a very 



