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

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

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

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

Data: Total Fishery Yield  PWS: Fishery closed in 1989, Fishery has remained closed since  Sitka: Fishery open in all years from Recent increase in yield since 2005.

Sitka PWS Data: Spawning Age Composition  Spawning population structures very similar from early 1980s to 2000  After 2000, population structure flattens in Sitka

Data: Weight-at-age  Collected in spring before spawning  Large inter-annual variability  Decrease in weight-at-age during 1990s

Data: Weight-at-age  Significant linear relationship and nearly 1-1  Large correlation between areas, especially older ages Sitka = *PWS R 2 = 0.989

Data: Egg deposition  Measure of female spawning population abundance  Dissimilar from PWS indicates large increase, no increase in Sitka until 1992  Similar in trend from Sitka deposition larger than PWS after 1994.

Data: Cumulative Miles of Milt  Index for male spawning population abundance  Similar from More variable and larger in Sitka after 1992, except in 1997

Model: Age-Structured Assessment

 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.

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

Results: Natural Mortality  : larger in PWS than Sitka  nearly the same in PWS and Sitka  1999 to present, natural mortality is estimated to be lower in Sitka than PWS.

Results: Recruitment  Recruitment of age-3 fish to population very similar  Since 1994, total age-3 recruitment has been larger in Sitka than PWS.

Results: Spawning Biomass  : PWS population increases to max, Sitka’s trend is variable  : Both indicate significant decline in each year.  : PWS population remains at low levels. Sitka population increases

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

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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