Accuracy of length estimates for chinook salmon and steelhead in compartmented and standard hatchery raceways

We used three sampling methods to estimate mean fork lengths and variances for chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss that were reared in compartmented and standard raceways. Size segregation of fish in compartmented raceways was also evaluated. Accurate estimates of mean fork length from raceway populations of chinook salmon and steelhead were difficult to obtain. In both compartmented and standard raceways, our sampling methods tended to underestimate the mean. For compartmented raceways, the most accurate estimates of mean fork length were obtained by pooling samples collected from each compartment. In standard raceways, the most accurate estimates of mean fork length were obtained when one net sample was taken after fish were crowded in the raceway. We also found that accurate estimates of fork length variance were difficult to obtain. No sampling method produced accurate estimates more than 67% of the time; therefore, no sampling method was recommended for estimating variance. Larger sample sizes may be needed to obtain accurate estimates of the variance than are needed for estimates of the mean. Finally, fish appeared to segregate by size in compartmented raceways. Biologists selecting sampling methods should account for the possible nonrandom distribution of fish.