Detecting Anadromy in Bull Trout by Non Lethal Sampling Methods

The determination of anadromy in salmonids has primarily been tested using otolith microchemistry to detect elevated levels of the element Strontium (Sr) in marine water versus low levels of Sr in fresh water.  Unfortunately, removing otoliths is lethal, and therefore undesirable for low or at risk populations like bull trout (Salvelinus confluentus).   An alternative structure for microchemical analysis such as scales or fin rays is preferable to lethal collection techniques.  Unknown is whether the Sr signal used to detect ocean/estuarine and freshwater entrance are recorded and stable through time in these non lethal samples. The objectives of this study were to 1) determine if scales or fin rays can be used to assign anadromy in salmonids with emphasis on bull trout, and 2) explore the microchemical incorporation and stability of Sr in salmonid fin rays and scales.  Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to analyze chemical patterns in otoliths, scales, and fin rays.  The overall Sr levels in bull trout scales were elevated from hypothesized freshwater values in anadromous specimens (determined from otolith microchemistry). Scale samples from adult bull trout showed little difference between Sr levels in the early portions of the scale (freshwater rearing) and later regions (marine rearing).  Scale chemistry was an accurate indicator of anadromy, but we were unable to detect evidence of early freshwater residency on returning anadromous bull trout.  In addition to scale chemistry we examined the rays of the pectoral fin for their suitability in chemical analysis and life history interpretations.  The location of sectioning within the fin ray bone (dorsal) was correlated with the chemical output and life history profile such that early chemical signals representing freshwater residency were lost in more distal sections of the structure.  Our results indicate that similar to otoliths, Sr remained stable within the fin ray structures as a fish transitioned between freshwater and saline environments. Both bull trout scales and fin rays appeared to be suitable indicators of anadromous migration, with the latter producing a more stable chemical signal and higher resolution of early growth.