Study Points to Importance
by Bill Rudolph
Small populations of ESA-protected fish in Idaho may not be getting enough to eat, according to a new peer-reviewed paper by NOAA Fisheries scientists, who say that juvenile fish survival may be limited by the amount of nutrients released into streams by dead spawners. They suggest the pace of recovering those stocks could be speeded up by adding more carcasses to streams.
In particular, the paper's hypothesis--that such small numbers of fish are exhibiting density-dependent effects--contradicts the views of other federal researchers who have assumed that juvenile fish survival is not related to the small number of returning adults.
The paper, published in a recent issue of Ecology Letters [(2003) 6:335-342], hypothesizes that "the evidence of density dependence we report stems from a shortage of nutrients derived from decomposing salmon carcasses." In layman's terms, there may not be enough food in Idaho streams to go around, even for the small numbers of fish that live in them.
Authors Steve Achord, Phil Levin and Rich Zabel of the Northwest Fisheries Science Center said the big drop in salmon populations caused by over-harvesting and hydro dams "ostensibly has resulted in a nutrient deficit in the spawning and rearing streams we investigated."
If their hypothesis is correct, the scientists said, the region must change its view of salmon recovery.
While harvesting in Idaho streams has been cut and the impact of dams on fish has been "largely mitigated" over the past 20 years, ocean productivity has boosted survival of Snake River stocks, the scientists point out. With such conditions in place, fish populations could return to historical levels "fairly rapidly," they said.
But if marine-derived nutrients limit the size of fish populations, the latest generations of salmon will die because of density dependence "and recovery would be much slower than the former case" in which salmon are not affected by marine-derived nutrients, they said.
The paper also points out that non-indigenous brook trout may be impeding recovery of some salmon stocks in the Snake River Basin. The authors found density of chinook parr 30 percent lower in streams with brook trout than in streams without them. The scientists said the trout may be preying on chinook eggs or juvenile salmon-reducing fish density sufficiently to reduce effects of density-dependent mortality. "Thus," the authors said, "the brook trout not only reduce survival of chinook, they may also fundamentally alter the mechanisms that determine chinook population size."
The scientists said earlier conclusions, which suggest there is little room to improve juvenile survival in relatively pristine habitats such as the Snake Basin, may be incorrect.
If their hypothesis is correct, the scientists said a program to supplement nutrients in streams could reduce mortality and increase survival rates. They recommend experiments to "more rigorously test the patterns we report here." But they add that, "Nonetheless, our results suggest that recovery of salmon populations may be hindered by decades of historical human impacts."
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