Overholtz et al.: Assessment advice for Northwest Atlantic mackerel stock 



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Figure 10 



Comparison of standard assessment (STD) model ( ) and 



density-dependent simulation (DDM) model ( ) catch and 



spawning-stock biomass (SSB) for 15-year runs of each model, 

 with fishing mortality set at 0.29 (F , ). 



time for the stock to reach an equilibrium point. At 

 F = 0.05, and after roughly 7-8 years, catch would be 

 higher under the STD model than for the DDM model 

 (Fig. 9). Spawning-stock biomass would also be con- 

 siderably higher, about 1.7 million mt for the STD 

 model versus about 1.1 million mt for the DDM model 

 (Fig. 9). 



Fishing the stock at F .i for a 15-year period would 

 result in an equilibrium catch of roughly 250,000 mt 

 (STD) versus 150,000 mt (DDM) after about 7-8 years 

 (Fig. 10). The spawning stock would appear to be much 

 higher, 1.1 million mt versus 615,000 mt, for the STD 

 and DDM models, respectively (Fig. 10). 



To evaluate the overall differences between the two 

 models, two measures of performance and one measure 

 of risk were produced for fishing mortality values rang- 

 ing from 0.1 to 0.6, in 0.1 increments for a total of 1000 

 runs of each model. Mean catch, coefficient of varia- 

 tion (CV) of catch, and proportion of time the spawn- 

 ing stock fell below 600,000 mt, the management plan 



benchmark, were calculated for year 10 of the simula- 

 tion. Based on the previous model runs, the tenth year 

 of the simulation appeared to be a reasonable choice 

 for a summary year (Figs. 9, 10). 



Mean catch would continue to increase dramatically 

 under the STD model until F reached about 0.40 and 

 would remain relatively constant thereafter. Yields 

 would also steadily increase for the DDM model at F's 

 ranging from 0.1 to 0.3, but would remain constant 

 after that point (Fig. 11 A). A comparison of expected 

 yields under the two models suggests that the STD 

 model is always more optimistic, especially at the 

 higher levels of fishing mortality (Fig. 11). Mean catch 

 at F = 0.5 would be about 125,000 mt higher under the 

 STD model. 



Variability as measured by CV's of the catch in- 

 creased with increases in fishing mortality for the STD 

 model, but remained relatively constant at about 50% 

 for the DDM model. Thus the STD model is more op- 

 timistic at lower levels of fishing mortality and becomes 



