mortality of cod and haddock eggs on georges bank, 1995-1999 (…wind-driven mortality…) d....
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Mortality of Cod and haddock Eggs on Georges Bank, 1995-1999
(…wind-driven mortality…)
D. Mountain, J. Green, J. Sibunka and D. JohnsonNortheast Fisheries Science Center
NOAA/NMFS
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1. Vertically integrated Sampling for eggs
2. Cod and haddock egg abundance by stage for each survey
3. Peak abundance Cod: mid-Feb to mid-Apr Haddock: mid-Mar to mid-May
Cod early stage eggs, February 1997
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Determined from difference in number of early and late stageeggs over the whole season.
(exponential decrease over period of average development time)
Egg Mortality Rate – percent per day
Cod Haddock
1995 13.7 12.0
1996 12.2 11.3
1997 20.4 13.4
1998 9.9 7.8
1999 15.4 9.9
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Determined from difference in number of early and late stageeggs over the whole season.
(exponential decrease over period of average development time)
Egg Mortality Rate – percent per day
Cod Haddock
1995 13.7 12.0
1996 12.2 11.3
1997 20.4 13.4
1998 9.9 7.8
1999 15.4 9.9
1997 – high mortality rate1998 – low mortality rate
With a 17 day incubation time, egg survival rate 3 to 8 times higher in 1998
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Variation in Egg Mortality
1. What caused it?
2. What are it implications for recruitment?
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What caused the interannual variation in mortality rate?
1. Egg viability? (i.e., maternal factors)
2. Predation?
3. Wind Driven Transport off the Bank?
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SE Wind Stress vs Egg Mortality Rate
1. Winds from Georges Bank Buoy
2. No winds for 1996
3. Average SE wind stress: mid-Feb to mid-Apr for cod mid-Mar to mid-May for haddock
SE wind stress (pascals)
Mor
tali
ty r
ate
(pp
d)
0.00 0.02 0.04 0.06 0.08 0.10
8
10
12
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22
Cod
R2 = 0.81
0.00 0.01 0.02 0.03 0.04 0.05 0.06
7
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14
Haddock
R2 = 0.58
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SE Wind Stress vs Egg Mortality Rate
1. Winds from Georges Bank Buoy
2. No winds for 1996
3. Average SE wind stress: mid-Feb to mid-Apr for cod mid-Mar to mid-May for haddock
SE wind stress (pascals)
Mor
tali
ty r
ate
(pp
d)
0.00 0.02 0.04 0.06 0.08 0.10
8
10
12
14
16
18
20
22
Cod
R2 = 0.81
0.00 0.01 0.02 0.03 0.04 0.05 0.06
7
8
9
10
11
12
13
14
Haddock
R2 = 0.58
Relationship to SE winds suggeststransport is associated with time- dependent winds (i.e., episodic forcing)
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Could wind driven transport actually have caused the interannual variability in egg mortality?
(Was the temporal/spatial variability in the egg locations and in the wind forcing likely to have resulted in the observed mortality?)
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Could wind driven transport actually have caused the interannual variability in egg mortality?
(Was the temporal/spatial variability in the egg locations and in the wind forcing likely to have resulted in the observed mortality?)
Use particle tracking model to test this.
Two issues:
1. Estimating the currents
2. The egg distributions to be used
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Currents:
1. Climatological flow fields from US GLOBEC models (3-D finite element model with mean winds; bi-monthly)
2. Time-dependent Ekman current, using observed winds (48 hour wind history)
3. Random displacement – for dispersion
4. Particle tracking by ‘Drogue-3D’ by B. Blanton – hourly time step
Caveats:
Adding climatology and Ekman not a fully rigorous approachConsidering only near surface drift
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Test of the Ekman current approach
Using satellite tracked drifters, drogued at 10m depthThree examples where drift track changed direction with a major wind event.
Red (‘D’) is drifter; Green (‘C’) is climatology; Blue (‘W’) is climatology + Ekman
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Test of the Ekman current approach
Using satellite tracked drifters, drogued at 10m depthThree examples where drift track changed direction with a major wind event.
Red (‘D’) is drifter; Green (‘C’) is climatology; Blue (‘W’) is climatology + Ekman
Captures the cross-isobath movement
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Early Egg distributions:
1. Interpolate each cruise to fine grid
2. Interpolate (in time) to daily values
3. Sum into 10 day bins (e.g., days 40-49, …)
Have distributions ofearly eggs (#/10m2)for 10 day bins forcod and haddock
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Drifting the eggs:
1. For each 20 eggs/10m2 at a grid point, assign one egg particle (about 500-1000 particles for each 10 day bin; up to 50 at a grid point)
2. Drift the particles for 17 days (average development time from the early stage to hatching)
3. If a particle moves across the 200m isobath, it has left the bank and is lost
4. After 17 days, determine how many particles have left the bank
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Drift induced mortality rate (ppd)
Mor
tali
ty r
ate
(pp
d)
Modeled vs Observed Mortality Rate
No point for 1996 (Buoy 11 winds missing)
Conclusions:
Relationship between egg mortalityand SE wind stress likely is real.
~8 ppd mortality without drift loss
0 2 4 6 8
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Cod
R2 = 0.51
0 2 4 6 8 10
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R2 = 0.23
Haddock
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What was the difference between 1997 & 1998?
1997
Wind driven transportcross isobath (off-bank)
1998
Wind driven transportalong isobath 1997
1998
Wind-induced movement over 17 day drift period
D-45
D-75
D-75
D-45
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a) b)
c) d)
Drift of early Haddock eggs – 75 day bin
1997 1998
Initial
After 17 days
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Cod Haddock
1995 11.9 3.8
1996 8.8 18.5
1997 15.8 7.6
1998 18.6 29.8
1999 16.5 9.2
% of Early Stage Eggs on Western George Bank
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Implications For Recruitment
Compare:
R vs SSB x Egg survivorship
(i.e., R vs index of number of hatched eggs)0 1000 2000 3000 4000
0
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14000
0 2000 4000 6000 8000 10000 12000
5000
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SSB * Egg survivorship
Rec
ruit
men
t
Cod
R2 = 0.59
Haddock
R2 = 0.57
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Conclusions:
1. Variability in egg mortality rates due (in large part) to variability in wind-driven loss from the bank.
2. Variability in egg surviorship a significant contributor to variation in recruitment.
3. Future modeling of the egg/larval period should address time-dependent wind forcing.
P.S.
Joseph Chase concluded much the same a long time ago
2003 haddock - ‘boomer’ year class; SE Wind was ‘NW’
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SE wind stress (pascals)
0.00 0.01 0.02 0.03 0.04 0.05 0.06
7
8
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HaddockM
orta
lity
rat
e (p
pd
)
2003
2003 Haddock Year Class
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Cod (dropped 1985)R vs SSB*Egg SurvivorshipRevised R & SSB
SSB*Egg Survivorship
0 1000 2000 3000 4000 5000
R
0
5000
10000
15000
20000
25000
CodR vs SSB x Egg Survivorship
(1986, 1987, 1995-1999)
SSB * Egg survivorship
Rec
ruit
men
t
R2 = 0.81
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SE wind stress (pascals)
Mor
tali
ty r
ate
(pp
d)
Cod
Egg Mortality Rate vs SE Wind Stress (1986, 1987, 1995-1999)
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Cod: ssb*survivorship vs RR2 = 0.59 slope = 2.4
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Haddock: ssb*survivorship vs rR2 = 0.57
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Cod (dropped 1985)R vs SSB*Egg SurvivorshipRevised R & SSB
SSB*Egg Survivorship
0 1000 2000 3000 4000 5000
R
0
5000
10000
15000
20000
25000
R2 = 0.81, slope = 3.47
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a) b)
c) d)
Drift of early Cod eggs – 45 day bin
1997 1998
Initial
After 17 days
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0.00 0.02 0.04 0.06 0.08
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R2 = 0.24
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0.00 0.02 0.04 0.06 0.08 0.10
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R2 = 0.56
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2D Graph 3
Egg Hatching
0.0 0.5 1.0 1.5 2.0
Re
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itme
nt
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4
5
6
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10
11
Cod – egg hatching vs recruitmentR2 = 0.63For every 1000 eggs, get 5.5 recruits
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Egg Hatching
0.0 0.5 1.0 1.5 2.0
Re
cru
itme
nt
0
10
20
30
40
50
HaddockEgg hatching vs RecruitmentR2 = 0.50For every 1000 eggs get 14.6 recruits