Genetics of century‐old fish scales reveal population patterns of decline

Conservation scientists rarely have the information required to understand changes in abundance over more than a few decades, even for important species like Pacific salmon. Such lack of historical information can underestimate the magnitude of decline for depressed populations. We applied genetic tools to a unique collection of 100-year-old salmon scales to reveal declines of 56%-99% in wild sockeye populations across Canada's second largest salmon watershed, the Skeena River. These analyses reveal century-long declines that are much greater than those based on modern era abundance data, which suggested that only 7 of 13 populations declined over the last five decades. Populations of larger-bodied fish have declined the most in abundance, likely because of size-selective commercial fisheries. Our findings illustrate how a deep historical perspective can expand our understanding of past abundances to a time before species incurred significant losses from fishing, and help inform conservation for diminished populations.