respectively. These tables show the values of three selected model output 

 variables for each of the 12 spills simulated. The interpretation of these 

 output variables, largest annual catch reduction A, largest cumulative reduction 

 B, and final long-term loss C, should be clear from the headings, and from 

 figures 9 and 10. Variable C, the sum of the catch losses, or ultimate loss 

 to the fishery for each spill, is discussed below. The January (Julian day 1) 

 oil spill for herring and the March (Julian day 60) spill for cod are used for 

 this discussion since they resulted in the largest percent cohort reductions in 

 each of the two species. 



The behavior of the three fishery model variables for these two cases over 

 the 50 years following the spill is shown in figure 9 for the herring and figure 

 10 for the cod. It is clear from these plots that the cod compensatory mechanism 

 over compensates for the added oil-induced mortality, while the herring population, 

 with a weaker compensatory mechanism, is only able to return to its initial 

 equilibrium size. Although A, the largest one-year catch loss, would be similar 

 for comparable oil-induced mortalities in both species, B and C, the largest 

 cumulative catch loss over 50 years and the ultimate loss, respectively, are 

 quite different. In the case of the herring, both B and C are virtually the 

 same variable because the largest cumulative loss always corresponds to the last 

 year in the series of catch projections. For the cod fishery, B is maximum in 

 the 7th year after the spill, decreasing thereafter and converging to an ultimate 

 loss C, which is less than half of the maximum value of B (fig. 10). It should 

 be noted that C, the ultimate loss to the fishery, of roughly 21 percent of a 

 single year's equilibrium catch in the case of cod for the March spill, cannot 

 be shown in figure 10 except as the value to which B ultimately converges. The 

 difference in the dynamics of the impacted herring and cod fisheries results 

 from the different formulations of compensatory mortality used to model these 

 two species. It is interesting to note that for the most damaging time of the 

 blowout, the ultimate catch loss for both fisheries is roughly 21 percent of 

 the one-year equilibrium catch, despite the fact that the actual oil-induced 

 ichthyoplankton mortality for the herring was only 17.6 percent versus 77.5 

 percent for the cod. 



MODEL SYSTEM SENSITIVITY STUDIES 



When applying a system of models as complex as that outlined here, the 

 interactions between the submodels and the sensitivity of model system predictions 

 to various assumptions and parameterizations are critical to determining model 

 validity, and in understanding what processes are most important in controlling 

 system behavior. Sensitivity studies to percent loss of a year class and the 

 effect of the Georges Bank gyre on impact predictions are discussed below. These 

 two topics have been selected for presentation since they are the key to answering 

 some common questions related to the impact of oil spills on commercial fisheries 

 for the Georges Bank Region: What if a spill eliminates an entire year class? 

 Does the Georges Bank gyre trap pollutants and cause increased impacts? 



Additional sensitivity studies on toxicity threshold levels, compensatory 

 mortality regimes, and all major fishery model parameters are documented in Reed 

 et al. (1979, 1981), and Spaulding, Saila et al. (1981, 1982) and are currently 

 an area of active investigation. 



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