Use of Otolith Chemistry and Radiotelemetry to Determine Age‐Specific Migratory Patterns of Anadromous Bull Trout in the Hoh River, Washington

Abstract

AbstractThe complementary use of otolith chemistry and radiotelemetry demonstrates that bull trout Salvelinus confluentus from the Hoh River, Washington, exhibit considerable life history variability. Adult bull trout lived exclusively in the river, inhabited freshwater for prolonged periods and later became anadromous, or were anadromous and made multiple migrations between freshwater and salt water. Twenty of 40 radio‐tagged juvenile bull trout emigrated to the ocean at lengths ranging from 243 to 360 mm (mean, 287 mm), which is the first published verification of anadromy at this life history stage. Otolith chemistry analyses of 105 bull trout that were incidentally killed in commercial gill‐net fisheries revealed that 85% had migrated from freshwater to the sea at least once and that 75% had migrated multiple times. Anadromous females produced 95% of all individuals examined, but both anadromous and nonanadromous females produced progeny that were anadromous. Age at first seaward migration ranged from 3 to 6 years, 88% first emigrating to sea in their third or fourth growth year. For ages 3 and 4, anadromous individuals were larger than those that remained in freshwater. A wide size range (287–760 mm, 0.2–4.9 kg) of bull trout were killed in commercial fisheries; ages 3–5 composed 88% of the total bycatch. Relocation data from radio‐tagged juvenile and adult bull trout provided important insights on anadromous movements that helped to validate inferences drawn from widely oscillating strontium levels in otolith chemistry. In view of the direct mortality in gill‐net fisheries, an understanding of the age‐specific movements and life history variability of anadromous bull trout will be crucial to future conservation efforts, which should focus on improved monitoring of recreational and commercial bycatch in Pacific salmon fisheries.