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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. Recent trends in catch rates of the tuna longline fishery in the Tonga EEZ

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Environmental influences on tuna catch rates in the tonga eez l.jpg
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


Outline of presentation l.jpg

Outline of presentation

Secretariat of the

Pacific Community


Why is oceanography important l.jpg
Why is oceanography important? the Tonga EEZ

  • 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


Recent longline cpues albacore l.jpg
Recent longline CPUEs: Albacore the Tonga EEZ

  • Seasonality of catch rates

  • Albacore – recent CPUEs low but similar to other eastern EEZs

  • Regional patterns

Secretariat of the

Pacific Community


Recent longline cpues yellowfin l.jpg
Recent longline CPUEs: Yellowfin the Tonga EEZ

  • Seasonality of catch rates

  • Yellowfin –catch rates similar to most other sub-tropical EEZs (except western EEZs)

  • Regional patterns

Secretariat of the

Pacific Community


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Oceanographic variables and catch rates the Tonga EEZ

  • ALB/YFT catch rates and SST from Tonga and region

Secretariat of the

Pacific Community


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Latitude the Tonga EEZAlbacore 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


Oceanography and catch rates l.jpg
Oceanography and catch rates the Tonga EEZ

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


Example albacore and water temperature l.jpg
Example-albacore and water temperature the Tonga EEZ

~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


Example albacore and water temperature10 l.jpg
Example-albacore and water temperature the Tonga EEZ

~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


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Oceanographic variables and catch rates the Tonga EEZ

Albacore:

Amount of preferred habitat important

Secretariat of the

Pacific Community


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Oceanographic variables and catch rates the Tonga EEZ

  • Yellowfin:

  • Catch rates increase when warm water extends through the Tonga EEZ

  • 2004: unusual pattern

Secretariat of the

Pacific Community


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Oceanographic variables and catch rates

Secretariat of the

Pacific Community


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Oceanographic variables and catch rates variables explain ?

Albacore

Secretariat of the

Pacific Community


Oceanographic variables and catch rates15 l.jpg

  • Important Variables: variables explain ?

  • 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


Oceanographic variables and catch rates16 l.jpg
Oceanographic variables and catch rates variables explain ?

Yellowfin

Secretariat of the

Pacific Community


Oceanographic variables and catch rates17 l.jpg

  • Important Variables: variables explain ?

  • 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


Oceanography and catch rates soi l.jpg
Oceanography and catch rates: SOI variables explain ?

CPUEs

SOI:

Broad index

Neutral conditions recently

Secretariat of the

Pacific Community


Oceanography and catch rates soi19 l.jpg

SOI: variables explain ?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


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Secretariat of the variables explain ?

Pacific Community


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