A False-Positive for Anadromy in Dolly Varden Otoliths in Alaska Reveals Likely Diet Effect of Pacific Salmon Subsidies on Resident Fishes

Determining anadromy in teleost fishes via otolith microchemistry using Sr/Ca, and to a lesser extent Ba/Ca, has become an established technique in life history studies. One underlying assumption of this concept is that diet is a minimal contributor to Sr precipitation in otoliths and that fluctuating elemental ratios in the water (Sr high in salt water, low in fresh, the inverse for Ba) are the primary driver of elemental variation in otoliths of anadromous fish. However, in cases of high marine subsidy use by resident fishes from spawning Pacific salmon (i.e. drifting eggs), it may be unclear whether fish went to the ocean, or the ocean went to the fish. Initial Sr/Ca microchemistry analysis of otoliths from both a large-bodied Dolly Varden in the Iliamna River and a nearby small-bodied pond Dolly Varden in southwest Alaska indicated anadromy. However, a radio-telemetry study did not detect salt water migration of large river Dolly Varden and migration was unlikely for the pond Dolly Varden (max. length 300 mm). Sr/Ca ratios in Arctic charr from Iliamna Lake showed resident elemental signatures. Diet and stable isotope analyses of these populations revealed that marine subsidy from sockeye salmon eggs contributed up to 90 percent of Dolly Varden diets for both populations but less than 50 percent for Arctic charr. Salmon eggs are formed in salt water and reflect marine Sr/Ca ratios and are likely to be the fresh water source of Sr. To accurately place Sr uptake in a fresh water setting, we again analyzed otoliths from both Dolly Varden populations but for both Sr/Ca and Ba/Ca under the hypothesis that there would be an inverse relationship between Sr/Ca and Ba/Ca if fish had indeed migrated to salt water, but that Ba/Ca would remain within freshwater ranges if they had not. Dual elemental analysis revealed Sr/Ca fluctuations consistent with anadromy but Ba/Ca ratios remained well within freshwater ranges. These findings strongly suggest that marine subsidy in the form of sockeye salmon eggs created a false-positive for anadromy in Dolly Varden otoliths. Despite strong opposition in the literature to this concept, this scenario highlights the potential role of diet and marine subsidy in otolith microchemistry and introduces the concept of opposing elemental analysis (Sr and Ba) to attain more conclusive results in fish populations that experience high marine subsidy use.