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Headwater Nutrient Loss Idaho Fish & Game Report to the Director 5/1/98

Can a loss of headwater nutrients due to fewer carcasses explain the decline in survival rate of Snake River spring/summer chinook?

Conclusion: Headwater nutrients do not appear to be a dominant factor in accounting for declines in adult-to-adult returns to Idaho.

Loss of headwater nutrients due to fewer adult carcasses could potentially decrease stock productivity at small escapements through a reduction in survival rate or growth rate of juveniles. This mechanism would theoretically operate as depensation, wherein survival rate decreases (rather than increases) as population levels decrease. We have no indication that loss of headwater nutrients, or depensation in general, is currently limiting survival rate of Snake River spring/summer chinook. Several observations suggest that it has not strongly influenced the decline to date. However, maintaining adequate numbers of spawners throughout the species range is an important safeguard against potential depensatory mechanisms.

The Biological Requirements Work Group (BRWG 1994) reviewed available information on depensation, during development of recommended population threshold levels for Endangered Species Act (ESA) jeopardy standards. BRWG (1994) concluded that while the concept of depensation is widely accepted in the fisheries literature and forms the basis for recruitment overfishing, empirical evidence is difficult to obtain. Because of population theory and limited evidence, they assumed that depensation could be important for the Snake River spring/ summer chinook populations.

The estimated numbers of smolts per spawner for aggregate Snake River spring/summer chinook from the 1975-1993 brood years have not shown a strong depensatory response. However, they cautioned that stronger populations tend to dominate recruitment patterns based on aggregate stock data, and those populations most likely experiencing depensation would be underrepresented in the aggregate. Thus the aggregate would be a relatively insensitive measure to detect whether a depensatory response, such as loss of headwater nutrients, was occurring.

If nutrient loss were a strong controlling factor in recent survival patterns of Snake river salmon, one might expect to see a less of a population decline in streams with high natural fertility. This is based on an assumption that nutrient inflow would be less important to parr and smolt survival rate of growth rate in more fertile streams such as the Lemhi River. The Lemhi river is one of the most productive Snake River spawning/rearing tributaries in terms of water chemistry (136 ppm total alkalinity; 8.1 pH and 300 ppm total dissolved solids; Bjornn 1966). Examination of patterns of progeny:parent ratios [ln(spawner/spawner)] for brood years 1974-1992 does not suggest a stronger recruitment pattern for Lemhi River spring chinook relative to other stocks in the Snake River Basin.

If depensation (including limited nutrients from too few carcasses) were the primary mechanism depressing Snake River stocks, a decrease in spawner:spawner ratios would be expected at low spawner abundance. However, the only years in which Snake River index stocks consistently met replacement were the 1980-1983 brood years, which at that time included the lowest spawner numbers on record. The data show synchronous behavior between the populations which suggests factors outside the spawning/rearing tributaries have greater influence.

A plot of spawner:spawner ratios versus spawner numbers did not reveal a dominant depensatory pattern (Figure 2, not shown on web site). In figure 2, the data from Figure 1 (also not shown) were rearranged, and spawner numbers were normalized to the average during brood years 1975-1990. Spawner:spawner ratios tended to increase with decreasing spawner numbers (Figure 2). The patterns in figures 1 and 2 suggest that depensation has not been strongly influencing recruitment patterns in the recent past, relative to other factors. However, data variability is high, and the possibility of depensatory response in the recent past cannot be completely ruled out.

Appendix 3.5 IDAHO's ANADROMOUS FISH STOCKS:
Their Status and Recovery Options
Report to the Director Idaho Fish & Game 5/1/98
Issue Paper: Headwater Nutrient Loss