plankton-rich water, explaining the intense productivity of the area (EG&G 

 1981). The problem is that an oil spill might be similarly retained, and its 

 impact on the ichthyoplankton thereby increased. 



The oil spill - fishery impact assessment model system has been used to 

 investigate this question for winter (Julian day 32) and spring (Julian day 121) 

 spills. The residual advective field underlying these two test simulations is 

 the summer data set shown in figure 11 derived from charts compiled by Bumpus 

 and Lauzier (1965). This residual current field, which was held constant through 

 the two simulations, was selected because it showed the strongest gyre-like 

 configuration over the banks. Random walk diffusion and the definition of the 

 wind driven hydrodynamics remain as in prior simulations. 



The impacts on cod of the 68 million gallon north blowout spill scenario 

 under this altered current pattern are summarized in table 7. Not surprisingly, 

 the replacement of the winter residual current field, which is essentially 

 unidirectional to the south and southwest, with the strong summer gyre results 

 in greatly increased impacts. The impacts for spills on days 32 and 121 are 

 nearly the same, subject to this constant residual advective field. This is due 

 firstly to the fact that dispersion of oil into the water column is relatively 

 complete after a few days. The effect of wind on the subsurface distribution 

 is therefore limited primarily to this initial period. Secondly, the two spills 

 occur on either side of the cod spawning peak near Julian day 90 (fig. 6). 

 Because the day 32 subsurface oil remains in the general vicinity, it affects 

 the eggs spawned during this peak time. Conversely, in the case of an oil spill 

 occurring on day 121, the eggs stay in the area longer, and therefore remain 

 subject to the toxic action of hydrocarbons released after the time of maximum 

 spawning activity. These observations simply reflect the expected tendency of 

 a gyre-like formation to retain transported constituents. 



Although this model experiment is relatively artificial, it demonstrates 

 the importance of the inclusion of subsurface representations in oil spill 

 impact prediction work, and strongly underscores the importance of determining 

 the correct structure of the advective field for correct estimation of impact 

 magnitudes. 



CONCLUSIONS 



It is clear from the above discussion that spill timing, spatial and temporal 

 spawning distributions, and population dynamics of the species of concern are 

 critical factors in determining the impact of spill events on the Georges Bank 

 herring and cod fisheries. In the case of cod, compensatory mechanisms within 

 the fishery, combined with the existence of several year classes of mature fish, 

 will tend to attenuate or reduce large losses in one year class caused by the 

 proximinity of spawning site to spill location. For herring, diffuse spawning 

 locations protect the population from localized oil spill pollution events, 

 although internal population dynamics result in relatively large catch losses 

 in the long term due to small numbers of year classes. 



Sensitivity studies on percent loss of a year class further display the 

 importance of compensation in determining the impacts of a pollutant event on a 

 fishery. A fishery such as cod, with relatively strong compensatory behavior, 



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