Salmon Watersheds Program

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Assessment of Sockeye and Pink Salmon stocks in the northern boundary area using run reconstruction techniques, 1982 – 95

Abstract

A multi-time-period method of stock reconstruction was used to estimate harvest and stock interception rates by area and time period, exploitation rates and total run size by stock. The data required for these reconstructions were catch by time and area, daily escapement by population (stock), the residence time of each population in each harvest area, and the routing of the populations. Migration routes for each stock were defined using information from the 1982-85 north coast tagging studies. The initial set of migration routing parameters were adjusted until the run reconstruction results approximated those from the tagging study years. Two different sets of migration parameters were required to fit the 1982 and 1983 interception rates for sockeye. While the 1983 set provided the best fit to the interception rates for Alaskan fisheries based on scale data for 1984-95, year to year variability can be substantial. Consequently, we incorporated all the available stock composition estimates for Alaskan fisheries into our sockeye reconstruction analysis. Limited sensitivity analysis on sockeye indicated that uncertainty in migration parameters has a greater effect on stock size estimates than interception rates. For pink salmon, one set of migration parameters was defined that approximated the tagging study results for 1982, 1984 and 1985. Substantial changes in the annual abundance of major pink salmon stocks appears to explain most of the variability observed in the tagging study interception rates. Comparison of the Canadian interception balance computed from the run reconstructions with estimates produced by the Joint Interception Committee revealed good agreement for sockeye but some substantial differences for pink salmon. Both methods used the same total escapement estimates; however, the current JIC method does not take into account the relative abundance of each Alaskan pink salmon stock and the sequence of fishery harvests. Further examination of the escapement estimates and JIC procedures for pink salmon is recommended.