Sachihiko ITOH and Takashige SUGIMOTO

Effect of eddy transport and blocking on migration

of small pelagic fishes

Subtropical and subarctic gyres

!a" SST #°C$

!b" SSS #psu.$

!c" Dynamic height !2000%db" #m$

!d" Sea surface chlorophyll #µg/l$

!Data from World Ocean Atlas 2001"

Meridional migration of pelagic fishes in the Western North Pacific

Spawning area!Over%wintering" Feeding area

Upper%layer oceanic/fishery condition

Kuroshio region

Oyashio region

Southern coastal area

Northern coastal area

Kuroshio%OyashioTransition region

25N

35N

30N

40N

50N

45N

130E 140E 150E 160E 170E sardine

anchovy

mackerels

Pelagic fishes in the Transition Region

SardineImportance of juvenile mortality !Watanabe et al. 1995" Temperature criticality!Noto and Yasuda, 1999"

Kuroshio Extensio! hypothesis

Anchovy Faster growth more survival!Takahashi, 2001"

Mackerels Largest young fish abundance in frontal areas !Kuroda et al, 1997"

Importance of fronts

Importance of migration phaseSchool concentration at fronts

Eddies and fronts for small pelagic fishes

High productivity and plankton convergence !Uda, 1960"Blocking migration !Uda, 1960"Transport/diffusion/retention

Modeling approach of this studyExamine basic processes of eddies and fronts on the northward migration of fishes with small swimming abilitySimple box%ocean ! <=> realistic simulation"Migratory eulerian tracer

Wind%driven double%gyre ocean circulation model

2000%km x 2000%km closed flat domain

Idealized wind stress and heating/cooling amplitudes

360%days period forcing!begins from spring"

Physical model

Schematic diagram of model domain

2000m

0m

2000km

2000km

logarithmic 9 levels

zonally homogeneouswind stress amplitude

10km x 10km grid

Direction northward

Speed limited by temperature gradient

Migratory eulerian tracer

†

∂N∂t

=

†

-u∂N∂x

- v ∂N∂y

- w ∂N∂z

†

+ AH∂ 2N∂x 2 +

∂ 2N∂y 2

Ê

Ë Á

ˆ

¯ ˜

†

- umig∂N∂x

- vmig∂N∂y

tendency physical advection physical diffusion migration

†

umig = 0, vmig £ 0.1 m/s

†

¢ T y +vmigDt - ¢ T £ aDt

†

a = 0.05 [°C/day]

Migration

†

umig, vmig( )

Tracer input

at the beginningof the seasonal cycle !Spring"in 30m depth, 17°C latitude along the western boundary

Three sets of 100 days calculation

Settings and cases

30m temp. at Day 0 of the 1st year

(a) Standard

(b) Windy winter (c) Calm winter

0km

1000

1000

-1000

-1000

0km

0km 1000 20000km 1000 2000

0

1

2

Day 0, 100

SpringS F W SS S F W

(b)(a)

(c)

Heating/coolingamplitude

Wind stressamplitude

Basic cases

Standard -500

500

0km

0km 500 0km 500 0km 500 0km 500 0km 500

No migration !Passive" -500

500

0km

0km 500 0km 500 0km 500 0km 500 0km 500

No blocking !constantmigration" -500

500

0km

0km 500 0km 500 0km 500 0km 500 0km 500

Day 20 Day 40 Day 60 Day 80 Day 100

Change of wind in winter

1st year 2nd year 3rd year

Windy winter

Constant wind stress amplitude

! Standard "

Calm winter

0 km

-500

500

0 km 1000

Average movement and temperature history

-500km

-250km

0km

250km

0km 250km 500km 750km

BasicWindy winterCalm winter

0km 250km 500km 750km

BasicWindy winterCalm winter

0km 250km 500km 750km

BasicWindy winterCalm winter

Day 0

Day 10Day 50

Day 100

Day 0

Day 10 Day 50

Day 100

Day 10

Day 50

Day 100Average movement

1st year 2nd year 3rd year

-1.5

-1.0

-0.5

0°C

0 days 20 40 60 80 100

Std. 1

Std. 2Std. 3

Wnd. 1Wnd. 2Wnd. 3

Clm. 1Clm. 2

Clm. 3

Average environmentaltemperature history

km

[°C]

Std. 1

Std. 2

Std. 3

Wnd. 1Wnd. 2

Wnd. 3

Clm. 1

Clm. 2Clm. 3

Average temperature filed

Latitudinally different waters and migration strategy

!c" !d"

SSC #µg/l$ climatology !World Ocean Atlas 2001"

SST #°C$ snapshots by a simulation!Earth simulator center"

!a" !b"

Dynamic confluence

Shelf break

Cross-gyre migration

Along-shoremigration

Eddy transport and blocking of migratory tracer

Concentration at the ridge of water intrusions

Influences of warm eddy activity to the migration path and temperature history

Meridional migration in the W.N.Pacific

Required to sustain large population

Utilizing eddy%intruding dynamic confluence

Summary

Thank you

Parameters examination

Cases X ± s.d. !km" Y ± s.d. !km" T ± s.d. !°C"

Standard 580 ± 243 571 ± 292 %1.3 ± 3.6

Vmig ≤ 0.20 m/s 605 ± 225 758 ± 373 %1.8 ± 3.4

Vmig ≤ 0.05 m/s 390 ± 159 378 ± 186 %0.4 ± 3.6

a = 0.10 °C 580 ± 216 768 ± 294 %1.8 ± 3.6

a = 0.025 °C 513 ± 266 436 ± 265 %0.6 ± 3.5

No heating 582 ± 249 545 ± 275 %3.2 ± 2.8

Average distance and increase/decrease of environmental temperature of tracer at Day 100 from the initial