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

Marine Purse Seine Test FisheriesThe information from purse seine test fisheries became of increasing importance in theestimation of abundance beginning in 1995, with the restructuring and reduced frequencyof the major commercial purse seine fisheries in Johnstone Straits and Area 20. Thesechanges severely compromised the run-size models that utilized these data. As the seinetest fisheries gained in importance, assessments were directed towards understanding theinformation gained from the data (Gazey 2001, 2002). [Seine test fisheries have beenreviewed in 2003, (Cave: Analysis of the 2nd site purse seine test fishery at SheringhamPoint; Cave 2003: Purse seine test fishing expansion factors; Tovey: Investigation of thePrecision of CPUE and Catchability with Increased Purse Seine Test Fishing Effort inArea 20-1 in 2002).]Gillnet test fisheries in Area 12 and 20 demonstrate similar catchability and are addedtogether without adjustment to estimate the daily passage projected to reach Mission.However, the seine test fisheries show quite different efficiencies between Area 12 and20, with catch-per-sets in Areas 12 and 13 being significantly greater for a givenabundance than in Area 20. Because of this observation, historically catchability wasassessed only for the approach for which the migration was dominant: either for thenorthern approach (through Area 12) on years of high diversion through Johnstone Straitor through for the southern approach (through Area 20) for years of low diversionthrough Johnstone Strait. The catchabilities derived in this manner would then be used toestimate the abundances through each approach uniquely. There still needed to be anassumption on what catchability to use for the “non-dominant” approach and circularityin the method is quite clear, as there would be an inherent calculation of annual diversionthrough Johnstone Strait (Diversion rate, D):DyCPUE12 ,yCPUEq1212 ,yq12CPUE20 ,yq20Where q12=0.005 and q20=0.002, as derived from other years when all the migrationpassed through one approach or the other. This method was used to estimate purse seineexpansion lines through 2008 (Table I-2) although, the circularity is an obviousweakness.I-6