Guðmundur J. Óskarsson and Christopher T. Taggart

1. Bergen, May 14, 2004 Recruitment variation in Icelandic summer spawning herring:Is it best explained by ocean environment or by the spawning stock? Guðmundur J. Óskarsson and Christopher T. Taggart Oceanography Department Dalhousie University, Canada Marine Research Institute Reykjavik, Iceland

2. Environmental factors might influence recruitment in a fish stock through: 1. Female body condition  egg quantity (fecundity)  egg quality (egg size) 2. Spawning time  when offspring are exposed to environment 3. Spawning site  where offspring are exposed to environment 4. Effects on egg, larval and pre-recruit survival.

3. Schematic:environment  recruitment variation: Spawning Body condition When Egg production (number and size) Survival Recruitment Where Environment (temperature, mixing, advection etc.)

4. Objectives: Determine how much of the recruitment variation in Icelandic summer spawning herring can be explained by environmental factors and how much by stock-related factors by considering: • Variation in spawning time • Egg or larval (pre-recruit) survival

5. Hydrographic stations Hydrographic stations Life history of Icelandic summer spawning herring Feeding grounds Nursery grounds Over- wintering Spawning grounds

6. Spawning Time Analyses: Logistic regression Dependent variable 0 (unspawned) 1 (spawning or recently spawned) Independent variables • day of the year, 1-365 (the basic model) • Fish size, age, K, spawning experience and location • SSB, mean annual K of pre-spawning females • Sea temperature (S-Iceland), NAO (Hurrell 1995) deviance Χ2 test (Ramsey and Schafer 1997).

7. Total egg production estimates: Where: FP potential fecundity where FP ~ L + K NA,Y annual (Y) number (N) at age (A) MatA,Y proportion of females mature at age FemA proportion of females at age

8. Repeat spawner total egg production estimates:

9. Recruitment variation (number-at-age-3; VPA-based) during 1963-1999 analysed using GAM and GLM with various: stock(Erep. sp.,Erec.sp., K of pre-spawning females) ecological (Zooplankton of N-Icel.; Anon.2003) environmental (NAO, TS-Icel., TN-Icel. ; Anon.2003) covariates

10. n = 11,370 Spawning time simple environmental measures have no influence on spawning time

11. GLMtotal egg production of repeat spawners & influence on age-3 recruitment ( R): r2=0.538 p<0.001

12. GLMeffects of NAO on recruitment r2=0.036 p<0.05

13. Observed R vs Fitted R using GLM r2=0.575 n=37 p<0.0001

14. SUMMARY • Spawning in Icelandic summer spawning herring is primarily a function of stock structure and not a function of the local environment • (also consistent with a time series analyses of ovary weight development (In: "Spawning time variation in Icelandic and Scotia-Fundy herring (Clupea harengus)", Oskarsson et al., in preparation). • Together, total egg production of repeat spawners (54%) and NAO (4%) explain 58% of the recruitment variation in the stock

15. Final remark: Positive impacts of sea temperature (off the north coast of Iceland) on R (Jakobsson et al. 1993) should not be quickly dismissed when 42% of the variation in R remains unexplained.

16. Answer to the original question: Recruitment variation in Icelandic summer spawning herring:is best explained by the spawning stock!

17. Acknowledgements: • Marine Research Institute, Iceland • Canada Department of Fisheries and Oceans, St. Andrews Biological Station • Dalhousie University, Faculty of Graduate Studies • Icelandic Research Fund for Graduate Students • The Fishery Ministry, Iceland • NATO Science Fellowships • NSERC “Discovery Grant” to CTT Thank you to All!