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WWRC 90-21
Fines in Redds of Large Salmonids

Chapman (1988) noted that the relation between the survival to emergence (STE) of embryonic salmonids and several measures of substrate composition was not quantitatively consistent. He stated that fisheries biologists cannot accurately and precisely predict STE from the existing data on the intragravel environment. He largely attributed these problems to a lack of understanding of the structure of the egg pocket, which is the exact location of the eggs in a salmonid redd. To address these deficiencies, he proposed an intensive study of egg pocket structure, followed by laboratory assessment of STE from simulated egg pockets and field assessment of STE in natural redds.

We agree that current predictions of STE in natural redds are unreliable, but the unreliability is only partly due to the lack of understanding of egg pockets. Although the research proposed by Chapman on the structure of egg pockets can only improve our understanding of the effects of fine sediment on STE, certain problems remain. Specifically, fisheries biologists have overlooked several other sources of variation that may produce inaccurate predictions of STE, such as variation in egg viability and egg deposition, and the inappropriate analysis of STE data.

Researchers tend to assume that egg viability equals 100%, but in response to environmental or genetic causes, egg viability may vary substantially among stocks or individuals of a single stock. Furthermore, the fecundity of a salmonid of a given size varies with location, age, and time. The imprecision of fecundity estimates based on fish size, as well as the difficulty of estimating egg loss and fertilization success during spawning, seriously reduce the accuracy of estimates of egg deposition. The selection and computation of measures of substrate composition lack uniformity and thus hamper comparisons of the effects of substrate on STE. The inappropriate selection and application of independent variables in regressions involving STE have compounded this problem and have produced spurious relations between STE and substrate measures.

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