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Mortality of Cod and haddock Eggs on Georges Bank, (…wind-driven mortality…) D. Mountain, J. Green, J. Sibunka and D. Johnson Northeast 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 stage eggs over the whole season. (exponential decrease over period of average development time) Egg Mortality Rate – percent per day CodHaddock

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Determined from difference in number of early and late stage eggs over the whole season. (exponential decrease over period of average development time) Egg Mortality Rate – percent per day CodHaddock – high mortality rate 1998 – 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 Average SE wind stress: mid-Feb to mid-Apr for cod mid-Mar to mid-May for haddock SE wind stress (pascals) Mortality rate (ppd) Cod R 2 = 0.81 Haddock R 2 = 0.58

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SE Wind Stress vs Egg Mortality Rate 1. Winds from Georges Bank Buoy 2. No winds for Average SE wind stress: mid-Feb to mid-Apr for cod mid-Mar to mid-May for haddock SE wind stress (pascals) Mortality rate (ppd) Cod R 2 = 0.81 Haddock R 2 = 0.58 Relationship to SE winds suggests transport 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 approach Considering only near surface drift

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Test of the Ekman current approach Using satellite tracked drifters, drogued at 10m depth Three 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 depth Three 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 of early eggs (#/10m 2 ) for 10 day bins for cod and haddock

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Drifting the eggs: 1. For each 20 eggs/10m 2 at a grid point, assign one egg particle (about 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) Mortality rate (ppd) Modeled vs Observed Mortality Rate No point for 1996 (Buoy 11 winds missing) Conclusions: Relationship between egg mortality and SE wind stress likely is real. ~8 ppd mortality without drift loss Cod R 2 = 0.51 R 2 = 0.23 Haddock

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What was the difference between 1997 & 1998? 1997 Wind driven transport cross isobath (off-bank) 1998 Wind driven transport along isobath Wind-induced movement over 17 day drift period D-45 D-75 D-45

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Drift of early Haddock eggs – 75 day bin Initial After 17 days

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CodHaddock % 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) SSB * Egg survivorship Recruitment Cod R 2 = 0.59 Haddock R 2 = 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) Haddock Mortality rate (ppd) Haddock Year Class

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Cod R vs SSB x Egg Survivorship (1986, 1987, ) SSB * Egg survivorship Recruitment R 2 = 0.81

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SE wind stress (pascals) Mortality rate (ppd) Cod Egg Mortality Rate vs SE Wind Stress (1986, 1987, )

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Cod: ssb*survivorship vs R R2 = 0.59 slope = 2.4

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Haddock: ssb*survivorship vs r R2 = 0.57

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R2 = 0.81, slope = 3.47

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Drift of early Cod eggs – 45 day bin Initial After 17 days

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R2 = 0.24

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R2 = 0.56

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Cod – egg hatching vs recruitment R2 = 0.63 For every 1000 eggs, get 5.5 recruits

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Haddock Egg hatching vs Recruitment R2 = 0.50 For every 1000 eggs get 14.6 recruits

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