1. Modelling population dynamics given age-based and seasonal movement in south Pacific albacore Simon Hoyle Secretariat of the Pacific Community

2. Introduction Outline of fishery and model structure Two alternative approaches: – 4 region model with movement – 1 region model Comparison of results

3. Spatial structure

4. Fishery stratification 30 fisheries defined by region and flag JP, KR, TW fisheries represent main (standardized) CPUE indices in model. Troll fisheries provide information on recruitment. Key PI domestic fisheries defined.

5. Albacore move Movements of tagged albacore in south Pacific (Labelle & Hampton 2003)

6. Seasonal movements Strong seasonal patterns in catch, catch rate, and fish size. Longline fisheries divided seasonally

7. Age-based movement

8. Modelling approaches 1 Separate regions – Estimate relative recruitment, biomass Equal catchability of longline fisheries – Estimate age-based movement Equal selectivity of longline fisheries, by region and season

9. Modelling approaches 2 Single region – Assume common pool of fish Same biomass trend in all sub-regions – Use selectivity to account for local and seasonal size variation – Catchability may vary between regions and seasonally – Selectivity and catchability may vary seasonally – Longline selectivity not asymptotic

10. CPUE by region

11. Results – regional approach

12. Movement estimates Model unable to converge with age-based movement estimates Age-constant movement estimates seem to go in wrong direction Results highly sensitive to assumptions about catchability – More going on than immediately obvious

13. Results – single region approach Catchability estimates consistent with assumptions about availability – High in south in season 2 – High in north in seasons 3 and 4 – Higher northern catch in season 2 due to small fish

14. Selectivity by region, north & season

15. Selectivity by region, south & season

16. Conclusions Movement estimates problematic without tag releases across regions and age classes – Confounding with selectivity, catchability, and other model components – Size variation within regions can cause problems Single region model feasible if biomass trends are consistent throughout – Seasonal selectivity and catchability can be used to account for size variation in catch – Spatial fishery definitions more flexible than regions – Avoids problems with regional scaling and migration – Ignores possible regional variation Choose approach based on sensitivity and needs for management

17. END