Influence of water chemistry and migratory distance on ability to distinguish progeny of sympatric resident and anadromous rainbow trout (Oncorhynchus mykiss)

Otolith Sr:Ca ratios near primordia (Sr:Ca-Core) have been used to distinguish progeny of resident and anadromous Oncorhynchus mykiss and to estimate rates of exchange between the two forms; however, the influences of confounding variables on Sr:Ca-Core have not been quantified. We analyzed Sr:Ca-Core in juvenile O.amykiss produced at 13 California hatcheries that spawn primarily resident or anadromous broodstock. Mean Sr:Ca-Core of progeny of resident females increased with increasing Sr:Ca ratio of the stream (Sr:Ca-Water) in which the mother spawned (r(2) = 0.71). Mean Sr:Ca-Core of progeny of anadromous females averaged 1.0 x 10(-3) higher, also increased with Sr:Ca-Water, and decreased with increasing migratory difficulty (distance x elevation) of the mother (r(2) = 0.96). Model results predict that discrimination of sympatric progeny is equally good among streams where Sr:Ca-Water is less than 5ammol center dot mol(-1), but limited at higher Sr:Ca-Water or when anadromous females return to freshwater 6 months or more before spawning (e.g., summer steelhead). The models also provide an alternative method of interpreting Sr:Ca-Core data that can improve discrimination between sympatric progeny. Analysis of adults from one stream and eight hatchery sites suggested that resident females made little or no contribution to populations of anadromous adults, but anadromous females contributed to populations of resident adults.