A comparison of the amount of oil in the water column with the mean wind 

 speed over the first 30 days of the simulation period, for each spill event, 

 shows a strong correlation. The amount of oil in the water column under a 

 spillet increases approximately as the square of the mean wind speed (Audunson 

 el ai . 1979). The windier winter months show double the oil mass in the water 

 column compared to the quiet summer months. It is also seen that the source 

 for this additional subsurface oil is the sea surface. Water temperature has 

 no effect on the oil mass entrainment as presently formulated, and therefore 

 no correlation between subsurface oil and water or air temperature is noted. 



The slight variations in the amount of oil found in the atmosphere represent 

 the combined effects of temperature and wind with higher temperatures and faster 

 winds giving increased evaporative losses. The summer spills appear to be 

 dominated by the temperature effect, while the winter spill mass balances are 

 largely governed by the wind. 



Fishery model 



Figures 5 and 6 show the temporal distribution of spawning activity as 

 simulated for herring and cod, and the reductions of incoming year classes due 

 to each of the 12 spills investigated. These figures demonstrate the critical 

 importance of spill timing relative to the temporal distributions of spawning. 

 The results shown are based on egg and larval oil-induced mortalities estimated 

 with the ichthyoplankton transport and fate model. The ensuing year class 

 reductions were then incorporated into the adult fish population model, a 

 non-linear, non-spatial matrix formulation (Lorda and Saila 1980), to project 

 variations in catch. 



It is clear also that the size of the spawning grounds and their locations 

 relative to the spill origin (fig. 7 and 8) play a major role in determining 

 the extent of the cohort reduction in each case. The two cod spawning grounds 

 (fig. 7) are well defined, and the spill site is located on the edge of the 

 largest one (on Georges Bank). The result is that the largest cod cohort 

 reduction (77.5%) occurs when the spill starting time (day 60) matches the peak 

 of the combined Georges Bank and Nantucket Shoals spawning. This indicates a 

 concentration of the oil-induced mortality in the earlier life stages (eggs 

 and yolk-sac) before significant larval dispersion occurs. Unlike cod, the 

 herring spawn over a large area of Georges Bank (fig. 8). This initially 

 higher dispersion of the herring eggs results in consistently lower oil-induced 

 mortalities. The largest oil impact for the herring (17.6% cohort reduction) 

 does not match its spawning peak as in the case of cod, but occurs about 45 

 days later. Although a mismatch of 12 days can be explained by the initially 

 demersal herring eggs (yolk-sac is the first pelagic stage), the remaining 30 

 days of difference between the spawning peak and the largest oil-induced mortality 

 seem to indicate that the spreading of the spill must proceed for that number of 

 days before the largest possible number of herring larvae can be oiled at a lethal 

 concentration level. 



The effective reductions of the initial cohort sizes caused by the oil were 

 implemented in the fishery model by specifying equivalent one-time reductions in 

 the probability of survival through year-0. The effects of this reduced cohort 

 survi al on the expected catches over the 50 years following the occurrence of 

 the oil spill are summarized in tables 3 and 4 for the herring and cod fisheries, 



114 



