Effect of incubation temperature on post-embryonic survival and growth of steelhead in a natural stream and a hatchery (Study sites: Dworshak Hatchery and North Fork Palouse River; Stocks: Dworshak hatchery; Year classes: 1994 and 1995)

We tested whether varying incubation temperatures to match development between embryos from different spawning dates affected survival and growth of unfed steelhead Oncorhynchus mykiss fry released in a stream and in hatchery ponds. Hatchery steelhead returning to the Clearwater River, Idaho were artificially spawned on two dates separated by a four week interval. Progeny from the early date (ExE, from early males and early females) were incubated in chilled (7°C) water and those from the late date (LxL) in ambient (12°C) water until developmental stage matched. A third group, created by fertilizing eggs from late females with cryopreserved milt from early males (ExL), was included to control for any genetic differences between early and late returning adults. Survival in the stream to 3 and 15 months after release was similar among crosses. Survival in the hatchery to near the end of the standard one year rearing period was similar among crosses for one of two year - classes but different for the other; however, it was difficult to ascribe the differences (ExL>ExE; LxL intermediate but closer to ExE) to incubation temperature differences. We conclude that there was little if any effect of incubation temperature on survival. Length of juveniles of one year - class differed among crosses in the stream and in the hatchery. Length of the other year - class differed among crosses in one pond at the hatchery but not in the other pond or in the stream. When length differed the pattern was always the same: ExE>LxL; ExL intermediate but closer to LxL. We speculate that incubation temperature may have affected growth of juveniles, and in particular that a longer period of incubation in chilled water may have caused fast juvenile growth relative to a shorter incubation period in ambient water.