Identification of life history variation in salmonids using otolith microchemistry and scale patterns : Implictions for illegal introductions and for whirling disease in Missouri River Rainbow trout

Abstract

Proper conservation and management of wild salmonid populations requires correct identification of the array of life histories present and their contribution to adult recruitment. I used otolith microchemistry to identify natal origin and scale patterns to determine outmigration age in Missouri River rainbow trout where whirling disease could potentially cause a population collapse. First, I reviewed the otolith microchemistry literature and concluded that there have been a limited number of freshwater studies, and there has been limited rigorous testing of the many instruments used. Second, I tested the efficacy of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and found that Sr:Ca could be precisely measured in an otolith 'standard', there is a strong relation between qualitative ToF-SIMS results and concentrations measured with an electron microprobe, and the ambient water strongly influences otolith composition. Third, I analyzed the chemical composition of lake trout otoliths to identify the likely source of illegally transplanted lake trout in Yellowstone Lake. Changes in otolith microchemistry of suspected transplants accurately (>90%) determined the likely source of the introduction, and transect analyses indicated that lake trout have possibly been in Yellowstone Lake since as early as the mid-1980s. Finally, I used otolith microchemistry to identify natal origin and scale patterns to identify outmigration age in Missouri River rainbow trout. Otolith microchemistry was not an effective technique for this particular system because of similarities between the two major rainbow trout producing tributaries. However, I demonstrated a technique that uses the relation between otolith and water Sr:Ca to estimate sample sizes required to detect differences between streams. This can be used in future studies as a preliminary test to determine if otolith microchemistry is feasible. Scale patterns revealed that the yearling outmigration life history contributed most (88%) to adult recruitment in the mainstem fishery and the proportion of young-of-the-year to yearling outmigrants did not change post-whirling disease. Rainbow trout in the Missouri River have two obstacles to overcome: 1) severe whirling disease infection if the natal stream has Myxobolus cerebralis; and 2) low probability of surival if they manage to avoid severe infection but leave the stream too early.