Environmental influences on tuna catch rates in the Tonga EEZ

1. Environmental influences on tuna catch rates in the Tonga EEZ Inputs to the Tonga National Commercial Fisheries Conference, February 2007 Brett Molony Oceanic Fisheries Programme Secretariat of the Pacific Community

2. Recent trends in catch rates of the tuna longline fishery in the Tonga EEZ • Relationships between catch rates and oceanographic variables in the Tonga EEZ • Catch rates and SOI in the Tonga EEZ • Thanks to: • Fishers of Tonga for supplying logsheet data • Adam Langley & Don Bromhead (OFP-SPC) Outline of presentation Secretariat of the Pacific Community

3. Why is oceanography important? • Species preferences (e.g. warm/cool water) • Movement: stay within preferred conditions • Productivity (prey, fodder, food) • Extent of favourable habitat/conditions • Influence of reproductive success and recruitment → Availability of fish to fisheries in an area Secretariat of the Pacific Community

4. Recent longline CPUEs: Albacore • Seasonality of catch rates • Albacore – recent CPUEs low but similar to other eastern EEZs • Regional patterns Secretariat of the Pacific Community

5. Recent longline CPUEs: Yellowfin • Seasonality of catch rates • Yellowfin –catch rates similar to most other sub-tropical EEZs (except western EEZs) • Regional patterns Secretariat of the Pacific Community

6. Oceanographic variables and catch rates • ALB/YFT catch rates and SST from Tonga and region Secretariat of the Pacific Community

7. Latitude Albacore Yellowfin Oceanographic variables and catch rates Latitude of isotherm Catch rates increase as fish move through EEZ in response to changes in temperature Secretariat of the Pacific Community

8. Oceanography and catch rates e.g. Temperature • Each species has a range of temperatures in which it can live and a preferred temperature • With seasons, water temperatures vary • Fish respond by moving • Influences fish distribution and abundance • Leads to seasonality of catch rates Secretariat of the Pacific Community

9. Example-albacore and water temperature ~April • Cooling of water temperatures (northern movement of isotherms) • Albacore (sub-tropical species) can move northward with the cooler water • Catch rates increase in Tonga EEZ Late in year • Warming of water temperatures (southern movement of isotherms) • Albacore forced southward to stay with cooler (preferred) water • Catch rates increase as fish move south (environmental forcing) Secretariat of the Pacific Community

10. Example-albacore and water temperature ~April - cooling Late in year - warming Too warm Too warm Isotherm Isotherm Tonga EEZ Tonga EEZ (Environmental forcing) Preferred temperature Secretariat of the Pacific Community Preferred temperature

11. Oceanographic variables and catch rates Albacore: Amount of preferred habitat important Secretariat of the Pacific Community

12. Oceanographic variables and catch rates • Yellowfin: • Catch rates increase when warm water extends through the Tonga EEZ • 2004: unusual pattern Secretariat of the Pacific Community

13. How much variability in catch rates do oceanographic variables explain ? • Temperature and current variables, and previous catches Oceanographic variables and catch rates Secretariat of the Pacific Community

14. Oceanographic variables and catch rates Albacore Secretariat of the Pacific Community

15. Important Variables: • Previous catch • Depth of 22 C thermocline • Temperature difference (surface and 155 m) • W-E current • ~ 40% Albacore Oceanographic variables and catch rates Secretariat of the Pacific Community

16. Oceanographic variables and catch rates Yellowfin Secretariat of the Pacific Community

17. Important Variables: • Temperature difference (surface and 155 m) • W-E current • ~ 15% • Similar to month-strata only model (~16%) Oceanographic variables and catch rates Yellowfin • But previous catch better predictor • ~ 23% • Local depletion? Secretariat of the Pacific Community

18. Oceanography and catch rates: SOI CPUEs SOI: Broad index Neutral conditions recently Secretariat of the Pacific Community

19. SOI: Recent neutral conditions coinciding with lower than average albacore CPUEs How often are these events likely to occur? And for what duration? Oceanography and catch rates: SOI How often? – 5 similar events since mid-1970s What duration? – 2 to 4 years What to do? Be aware Develop risk management strategies Secretariat of the Pacific Community

20. Secretariat of the Pacific Community