Larval transport by ocean currents

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Larval transport by ocean currents
7 December 2005

• A short set of examples
• Far away and long-lived larvae
• Southern hemisphere spiny lobsters
• Local and shorter-lived larvae
• Fish larvae in the Gulf Stream
• Consequences and ethics
• The impact of improved marine environmental data
  on fisheries

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West Australian Rock Lobster

• An example (one of very few) of a fishery where
  sustainable management is practiced
• Clear connection between inter-annual climate
  variability (SOI), larval settlement, and
  recruitment to the fishery
• This correlation is a useful fishery management
  tool
• Precise details of mechanism are not understood
• To understand the hypothesis of the role ocean
  circulation plays in the life of a WA rock
  lobster we need to understand
• (a) some regional ocean physics
• (b) the life cycle of the spiny lobsters

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West Australian Rock Lobster

• An example (one of very few) of a fishery where
  sustainable management is practiced
• Clear connection between inter-annual climate
  variability (SOI), larval settlement, and
  recruitment to the fishery
• This correlation is a useful fishery management
  tool
• Precise details of mechanism are not understood
• To understand the hypothesis of the role ocean
  circulation plays in the life of a WA rock
  lobster we need to understand
• (a) some regional ocean physics
• (b) the life cycle of the spiny lobsters

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Australian regional ocean circulation
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Dynamic height climatologyfrom which we can
compute mean geostrophic currents
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Schematic diagram of the life cycle of the
western rock lobster Panulirus cygnus. Time runs
from top to bottom. Main recruitment to the
fishery is from animals in the year 4 class (76
mm carapace length)
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Main recruitment to the fishery is from animals
in the year 4 class (76 mm carapace length)
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Note 4 year lag
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• THE GENERAL CIRCULATION IN THE WA REGION
• large scale geostrophic inflow
• southward Leeuwin Current (LC) at the shelf
  edge
• offshore Ekman transport due to southerlies
• April/May relaxation of winds causes
  acceleration of LC into GAB
• THE LIFE CYCLE OF WESTERN ROCK LOBSTER
• eggs hatch in summer and are carried offshore by
  Ekman drift
• mid to late stage larvae can migrate vertically
• they are influenced predominantly by the
  eastward geostrophic flow
• upon encountering the Leeuwin Current, the
  larvae metamorphose into a free swimming puerulus
  stage that settles on the reefs (Aug-Jan).
• 3-5 years after settlement the lobsters mature
  and migrate from the nearshore reefs to the outer
  continental shelf, at which stage they are
  considered to be recruited to the commercial
  fishery.
• MESOSCALE EDDIES
• Superimposed on the mean circulation is an
  energetic mesoscale eddy field. Passive
  particles, such as weak-swimming larvae, are
  transported by the total velocity field, not just
  the mean, and this can lead to them being widely
  dispersed - much more widely dispersed than if
  they simply headed due east under the influence
  of the steady mean currents.
• ENSO
• The ENSO cycle affects the Leeuwin Current
  strength. In ENSO years the sea level in the
  WestPac region drops, thereby dropping the sea
  level along the WA coast with respect to
  offshore, and weakening the LC. It may be that
  decreased Leeuwin Current strength diminishes the
  occurrence of mesoscale currents that assist the
  puerulus stage of the lobster in migrating across
  the LC to the reefs, and settling. Lobster
  settlement rate is lower during ENSO years.

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Does ocean circulation alone explain inter-annual
variability in puerulus settlement?

• No!
• The model confirms the basic larval transport
  hypothesis, but
• modeled inter-annual variability is less than a
  factor of 2, with large uncertainty
• The skill of the SOI/sea-level predictor of
  lagged recruitment is not explained by the model
• There must be other controlling factors that are
  also correlated with sea level
• Temperature
• Time of hatching, growth rate, survival
• Prey density (chl-a low during El Nino)
• Environmental trigger to metamorphosis

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Use climatology of density (from T and S) to
compute geostrophic currents at the sea surface
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SST
SSH
SSS
Gulf Stream animations from http//www7320.nrlssc.
navy.mil/global_ncom/gfs.html
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Crayfish with Tomato, Fennel and Cognac Crème
Fraîche http//cuisine.co.nz/index.cfm?pageId3300
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Consequences and ethics (?) in revealing
bio-physical interactions in fisheries

• Fishing is the only remaining way that a large
  number of people are fed by hunting
• Tragedy of the Commons
• Benefit accrues to the individual, but the cost
  is shared by all when the resource crashes
• Economics alone seldom fosters sustainable
  utilization
• Modern methods of ocean observation (both physics
  and biology) reduce fishing effort
• This can distort resource assessment based on
  catch per unit effort data
• If its easier to catch fish, there must be more
  fish, right?
• Fishers adopt new technologies rapidly
• How can science help fisheries management, not
  just fishers?

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How can bio-physical knowledge and information
help?

• Smarter fishing
• reducing costs increases profitability for quota
  managed stocks
• reduce by-catch (presently about 25)
• Smarter stock assessment
• predictive skill in natural inter-annual
  variability allows adaptive quotas
• technology change must be immediately factored
  into management
• Better ecology
• larval and juvenile stages are often poorly
  known, so it is difficult to safeguard critical
  nursery habitats, or integrate practice across
  management boundaries
• e.g. WRL larval transport study needs to add
  growth, mortality, predator and prey fields to
  the model
• Long-term societal and ecosystem benefit from
  application of ocean observing technology to
  fisheries will only occur if a deliberate
  commitment is made to foster improved management
  as well as reducing industry costs.