Standardizing Spawner Escapement Data: A Case Study of the Nechako River Chinook Salmon

Abstract

Spawner escapements for Pacific salmon Oncorhynchus spp. are frequently estimated by using a variety of methods over time. To evaluate temporal patterns in escapements, the data must be standardized. In our study, we standardized escapement data from 1980 to 1998 for the chinook salmon O. tshawytscha of the Nechako River in British Columbia. This particular stock and period were chosen because (1) escapement had been estimated by several methods that could be standardized and (2) escapement estimation methods and data had previously been summarized in detail and were readily available. The area-under-the-curve method of estimating escapement had been used since 1987, and after minor adjustments in 1989, methods had remained constant. To evaluate whether differences in methods for earlier years affected analysis of trends in escapement over time, we converted the data for years 1980–1988 to area-under-the-curve equivalents. The mean of reported escapements for the 1980–1988 period increased from 18% to 22% after standardization. The increase demonstrates the importance of standardizing data when analyzing trends in Pacific salmon escapement data.