Research Document - 2007/020
Assessment of chinook salmon returns to the Fraser River watershed using run reconstruction techniques, 1982-2004
By English, K.K., R.E. Bailey and D. Robichaud
Abstract
The Fraser River watershed is the largest Canadian producer of Chinook salmon. In 2005, technical advisors to several Fraser River First Nations recommended that a run reconstruction analyses similar to that conducted in 1994 be conducted to provide estimates of the total return of Chinook to the Fraser River that could be used to define abundance based Treaty allocations. This document provides a description of data and model parameters used to reconstruct 1982-2004 Chinook salmon returns to the Fraser River. The fundamental building blocks for our run reconstruction analysis are DFO estimates for in-river harvests, tributary specific escapement numbers and timing, and upstream migration rates. Annual escapement estimates are derived from a combination of visual survey, mark-recapture studies and counting fences. Annual escapement summaries for each timing-age aggregate show increasing trend for most aggregates and the substantial difference in the abundance of fall Chinook relative to other aggregates. The only aggregate that has not shown a substantial increase in escapement since the late 1980s is the Spring 5.2 aggregate. On average, 44,046 Chinook were harvested in annual fisheries within the Fraser watershed from 1982-04. Catch estimates by aggregate indicate that harvest of spring and summer timing groups increased substantially since 1999. The largest annual harvest occurred in 2004 when over 73,000 Chinook were harvested in river fisheries. The average total return of Chinook salmon to the Fraser River was 315,233 over the period from 1982-04. The average contribution of timing-age aggregates were: 6% Spring 42, 15% Spring 52, 15% Summer 52, 16% Summer 41 and 48% Fall. The trends in total return are very similar to those observed in the escapement data. Trends for all Fraser stocks combined are largely determined by the escapement estimates for the fall timing aggregate. The similarity between escapement and total returns for fall stocks is not surprising given the relatively low harvest rates for this run timing group. The closure of the Area 29 Chinook gillnet fishery in the mid 1980s resulted in a substantial decline in the annual harvest rates for the spring and summer timing groups. Since 1988, in-river harvest rates for the spring and summer aggregates have varied between 10% and 30% in most years. The six stocks with consistently high in-river harvest rates since 1997 (Chilako, Cottonwood, Upper Chilcotin, Louis, Coldwater and Spius) are believed to represent the majority of the Fraser Chinook that entry the river before the end of May each year. The average annual harvest rates for these stocks (38-60%) from 1997-2004, were notably higher than those for all other Fraser Chinook stocks (19%). Estimates for total mortality in Canadian marine fisheries for early and late Fraser Chinook stocks were used to convert the river entry abundances into estimates of the Total Return to Canada for each timing-age aggregate.
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