Analyzing the chemical tracers in fin rays: a non-lethal approach to infer the migratory patterns of bull trout

Abstract

Effective management and conservation of migratory species requires identifying the habitats that individuals within and among populations use throughout their lives. As a diversity of life history strategies can buffer individuals and populations from changing environmental conditions, it is crucial to identify and conserve the habitats that maintain different life histories that may be present in and among populations. Bull trout (Salvelinus confluentus), native to western North America, were listed under the United States Endangered Species Act (ESA) over 30 years ago due to extensive declines in abundance and distribution. Bull trout can exhibit a variety of different life history strategies both within and among populations. To effectively prioritize bull trout conservation and recovery efforts, it is important to identify the diversity of life history strategies and habitats used by individuals within populations. Certain chemical tracers (e.g., strontium) recorded in fish fin rays reflect the ambient water chemistries a fish encounters throughout its life, therefore offering a non-lethal opportunity to reconstruct life histories and habitat use. In this thesis, I developed a non-lethal technique and chemical baseline using pectoral fin rays, and subsequently applied this technique and baseline to infer migratory patterns of bull trout in the White River, a large tributary to the Puyallup River in Washington state, USA. In Chapter 1, I used the chemical signatures recorded in non-migratory sculpin species and adult bull trout to assess the reliability of fin rays in establishing a geochemical baseline across the Puyallup River basin. I compared the chemical signatures in sculpin fin rays to corresponding water samples from sampling locations and otoliths, bony structures which have widely been used to reconstruct life histories but require lethal sampling. I show that the Sr/Ca, Ba/Ca and 87Sr/86Sr ratios in sculpin fin rays are similar to corresponding otoliths as these tracers incorporated into fin rays linearly with surrounding water chemistries, and the combination of these tracers allows for discrimination of the Puyallup River basin into its three sub-basins: the White River, the Puyallup River, and the Carbon River. Additionally, I found that the laser ablation measurements of 87Sr/86Sr ratios in bull trout fin rays resulted in coarser temporal resolution data and higher measured values compared to corresponding otoliths. In Chapter 2, I used the Sr/Ca and Ba/Ca ratios in White River bull trout fin rays to reconstruct their migratory patterns. I show that White River bull trout use habitats distributed across and outside of the entire Puyallup basin, including the marine environment, with predominant habitat use in the White River sub-basin. Defining the range of migration strategies among individuals within this population using a non-lethal approach will allow resource managers to develop land and fisheries management plans that maintain expression of the diversity of life history strategies that characterize this population.