Fraser River Sockeye Fisheries and Fisheries Management - Cohen ...

This type of model requires accurate estimates of total purse seine catch and gear countby area, and information on the duration of each fishery and area restrictions.Cumulative Passage ModelThese models use the historic relationship between run size of Summer-run stocks andthe daily-reconstructed, cumulative abundances of those stocks in the Fraser River atMission. Daily catches in all Panel and non-Panel Area fisheries and daily Missionhydroacoustic estimates of escapement for individual stocks are adjusted to commondates of migration and summed by day for the period of migration. The daily totals arethen summed to give cumulative totals for each date and year. Regression analysis ofannual run sizes on cumulative abundances to a particular date can then provide a meansof assessing the current run size at any date by using cumulative catch and escapementdata from that year. These models are not used for Late-run stocks because of thepotential migration delay of 3-6 weeks in Georgia Strait before entering the Fraser River.Cumulative-Normal ModelsThese models are based on the assumption that the migration of each returning stockapproximates a normal distribution. In-season catches and escapements are adjusted to acommon migration point and provide an estimate of the daily un-fished abundance at thatpoint. The cumulative daily abundance estimates for each date are then compared (usingregression analysis) to expected values obtained from a set of normal curves with varyingrun size, timing and duration parameters. The abundance and timing parameters of thenormal distribution scenario that give the best fit (highest R 2 value) to the observed databecome the "best" estimates for the run size. The model is updated as additional days ofcatch and escapement data become available. The method relies on high quality catchand escapement data and the assumption that the speed of fish movement between areasis consistent between years. Particularly important is the determination of whetherSummer-run sockeye are delaying off the mouth of the Fraser River.The cumulative-normal model will provide an estimate of run size at any stage of the run,but the estimates stabilize only after the peak of the run passes Mission. Cumulativenormalmodels, in theory, provide a valid method to estimate run sizes beyond the rangeof previous experience as long as the basic assumptions of the model are met. However,simulation studies have shown that the model is sensitive to deviations from normality(e.g., skewness or bimodality of the run).82