(This information is provided with permission of the author by the Native Fish Society. This information will be published by the Oregon Department of Fish and Wildlife in their Information Report series in the spring of 2001. I have not included the scientific evaluation the following statements are based upon. I recommend that the reader consult the full document to review the scientific basis of this report. The following excerpts are direct quotes form the text. Bill Bakke, Director)
For most populations the trigger point is a mortality rate of 30% or higher. Taking a conservative approach, it seems that a mortality rate limit of 20% is a reasonable conservation standard for most steelhead populations in Oregon. (Page 32)
Therefore, in terms of mortality impact, the zone of inference is sufficiently broad to conclude that as long as the mortality rate does not exceed 20%, the probability of extinction is very low (the model results suggest zero). (Page 41)
For many of the populations assessed, hatchery fish are present in the production areas used by wild fish and spawn naturally. In comparing such mixed populations, it appears that the higher the proportion of hatchery fish, the poorer the subsequent recruitment of naturally produced offspring (Chilcote, 1998) (Page 37)
The results presented here lead to the conclusion that overall population productivity can be adversely effected by naturally spawning hatchery fish. Further that this effect is not minor. For nearly all of the time intervals evaluated, it appears that when the proportion of hatchery fish exceeds 60%, the population can no longer replace its self, even at very low densities where the recruitment function would predict that survival would be at its greatest. (Page 38)
It is unclear whether the mechanism for this relationship is genetic or environmental. However, if it is genetic, the use of wild fish from local populations for hatchery broodstock does not appear to be a corrective solution. (Page 38)
These results lead to one of two conclusions. Either that the use of wild fish in hatchery programs does solve the genetic problem that makes hatchery fish genetically maladapted for natural survival. A conclusion that implies rapid and significant genetic change occurs when fish are brought into the hatchery environment. (Page 40)
Alternatively, that the use of wild fish for hatchery broodstock greatly reduces the genetic difference between hatchery and wild fish, but this really doesnāt matter because the mechanism causing the reduced productivity for naturally spawning hatchery is not genetic. The problem is caused by some unknown environmental impact of the hatchery rearing environment that results in hatchery fish being less able to produce viable offspring under natural conditions. (Page 40)
Regardless of the mechanism, when hatchery fish mix with wild fish in natural production areas, the overall productivity of the population declines. In effect the freshwater habitat becomes less efficient in producing steelhead. Not only does this mean that natural production goals are compromised, it means that the populationās vulnerability to extinction is increased. (Page 51)
It was not possible to perform an assessment on every steelhead population in Oregon. This was either due to lack of representative data or data sets that did cover enough years for the analytical approach used here. In particular, specific coverage was not possible for much of the Oregon coast as well as portions of the Grande Ronde and Imnaha basins.
However, in examining the results for populations for which there were data, there was not a great deal of variation with respect their status within each ESU. With certain exceptions, such as the Deschutes, the consistency of the assessment results suggests that the zone of inference concerning the biological health for these populations is probably at the ESU level. Therefore, where specific information on a specific population or sub-population does not exist, it is reasonable to infer that its status is probably similar to that of other populations within the same ESU for which an assessment exists.
In addition, the sensitivity of steelhead populations to mortality (fishery or other sources) appears relatively consistent, again with a few exceptions. Therefore, in terms of mortality impact, the zone of inference is sufficiently broad to conclude that as long as the mortality rate does not exceed 20%, the probability of extinction is very low (the model results suggest zero).
Finally, with respect to hatchery programs, the impact of naturally spawning hatchery fish on the capacity of a population to produce recruits appears universally adverse. Therefore, the zone of inference concerning the impact of naturally spawning hatchery on wild populations is statewide. Without specific data to the contrary, it is a reasonable inference that wild steelhead populations are better off when returning hatchery fish are prevented from escaping into natural spawning areas. (Page 41 and 42).