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Empirical comparison of historical data and age- structured assessment models for Prince William Sound and Sitka Sound Pacific herring Peter-John F. Hulson, Terrance J. Quinn II, Brenda L. Norcross, Gary D. Marty
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Outline Background into comparison of Prince William Sound (PWS) and Sitka and Age-Structured Assessment (ASA) Compare/contrast data time series Similarities/differences in ASA model structure PWS: Hulson et al (2008), Marty et al (in prep) Sitka: 2007 stock assessment ASA modeling results
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Background: Comparison of PWS and Sitka (Williams and Quinn, 2000a) Williams and Quinn (2000a, 2000b): Strong relationship found between PWS and Sitka Called for detailed comparison
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Background: Age-Structured Assessment PWS and Sitka ASA models constructed in 1990s (Funk and Sandone, 1990). Alaska Department of Fish and Game (ADF&G) currently employs ASA. Main features of ASA model: Integrates a number of datasets Connects observations across years with population dynamics Allows for variability in observations when estimating parameters Provides a statistical means to determine uncertainty in model output
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Data: Total Fishery Yield PWS: Fishery closed in 1989, 1993-1996. Fishery has remained closed since 1998. Sitka: Fishery open in all years from 1980-2007. Recent increase in yield since 2005.
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Sitka 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 PWS Data: Spawning Age Composition Spawning population structures very similar from early 1980s to 2000 After 2000, population structure flattens in Sitka
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Data: Weight-at-age Collected in spring before spawning Large inter-annual variability Decrease in weight-at-age during 1990s
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Data: Weight-at-age Significant linear relationship and nearly 1-1 Large correlation between areas, especially older ages Sitka = 1.0243*PWS R 2 = 0.989
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Data: Egg deposition Measure of female spawning population abundance Dissimilar from 1989-1992. PWS indicates large increase, no increase in Sitka until 1992 Similar in trend from 1995-1997. Sitka deposition larger than PWS after 1994.
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Data: Cumulative Miles of Milt Index for male spawning population abundance Similar from 1980-1992. More variable and larger in Sitka after 1992, except in 1997
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Model: Age-Structured Assessment
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Population dynamics equations are identical Major differences: Natural mortality applied to age groups in PWS Natural mortality directly estimated in Sitka ASA (split in 1998), linear relationship with disease indices in PWS. Maturity-at-age estimated for age-3 and age-4 in PWS, logistic relationship used in Sitka. Split in 1998 in PWS, 2001 in Sitka.
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Results: Maturity-at-age Early maturity-at-age nearly the same PWS: increase at age-3, decrease for age-4 after 1998 Sitka: significant decrease after 2001
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Results: Natural Mortality 1993-1995: larger in PWS than Sitka 1996-1997 nearly the same in PWS and Sitka 1999 to present, natural mortality is estimated to be lower in Sitka than PWS.
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Results: Recruitment Recruitment of age-3 fish to population very similar Since 1994, total age-3 recruitment has been larger in Sitka than PWS.
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Results: Spawning Biomass 1980-1991: PWS population increases to max, Sitka’s trend is variable 1992-1994: Both indicate significant decline in each year. 1995-2007: PWS population remains at low levels. Sitka population increases
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Summary PWS: Low population abundance: High natural mortality Disease Low recruitment Sitka: Increasing abundance: Lower recent natural mortality more fish surviving to older age classes from similar magnitudes of recruitment, flattening age composition Larger recruitment than PWS sustaining population increase
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Acknowledgments Funding sources: Alaska Fisheries Science Center Population Dynamics Fellowship Cooperative Institute for Arctic Research Alaska Department of Fish and Game, Commercial Fisheries Division EVOS Trustee Council Data and ASA model sources: PWS: Mr. Steve Moffitt, Sitka: Dr. Sherri Dressel
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Questions
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