Can unique amino acid profiles guide adult salmon to natal streams? A comparison of streams sampled prior to and after the arrival of adult Pacific salmon

Homing is an essential aspect of the evolutionary ecology of salmon, and the final stages are guided by odors learned prior to migration from natal sites. Amino acids (AAs) have been hypothesized to provide olfactory information sufficient for homing. We sampled water from five tributaries to Lake Aleknagik, Alaska prior to and after the arrival of adult sockeye salmon to determine whether the AA composition or concentration differed among streams, or changed after adult salmon entered the streams and began spawning. The dominant AAs were generally similar before and after salmon entered, although some shifts in relative contribution were noted. However, there were dramatic increases in concentration of all AAs after the arrival of salmon. Analysis of AA concentrations and relative proportions indicated that clustering of samples in multivariate trait space depended on timing (pre- or post-arrival of salmon) but not among streams. The shift in AA composition in post-arrival samples was consistent with odors from the salmon (e.g., eggs). Collectively, these data are inconsistent with the idea that AAs alone differ sufficiently among streams throughout the migration period to guide homing, though they may contribute to more complex combinations of odorants.