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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
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

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

why is oceanography important
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

recent longline cpues albacore
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

recent longline cpues yellowfin
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

oceanographic variables and catch rates
Oceanographic variables and catch rates
  • ALB/YFT catch rates and SST from Tonga and region

Secretariat of the

Pacific Community

oceanographic variables and catch rates7

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

oceanography and catch rates
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

example albacore and water temperature
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

example albacore and water temperature10
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

oceanographic variables and catch rates11
Oceanographic variables and catch rates

Albacore:

Amount of preferred habitat important

Secretariat of the

Pacific Community

oceanographic variables and catch rates12
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

oceanographic variables and catch rates13

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

oceanographic variables and catch rates14
Oceanographic variables and catch rates

Albacore

Secretariat of the

Pacific Community

oceanographic variables and catch rates15

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

oceanographic variables and catch rates16
Oceanographic variables and catch rates

Yellowfin

Secretariat of the

Pacific Community

oceanographic variables and catch rates17

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

oceanography and catch rates soi
Oceanography and catch rates: SOI

CPUEs

SOI:

Broad index

Neutral conditions recently

Secretariat of the

Pacific Community

oceanography and catch rates soi19

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

slide20

Secretariat of the

Pacific Community

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