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Fishery Bulletin 91(2), 1993 



model is a simple extension of the T-B Y/R model 

 (Gabriel et al. 1989) and, consequently, the relation- 

 ship to the delay models was similar. To avoid redun- 

 dancy, only the relationship between the T-B Y/R model 

 and the delay models was represented. 



The sensitivity of the delay model to changes in the 

 transition rates was examined. The transitional size- 



range was divided into 8 cm length-intervals and the 

 transition rates within each interval were doubled. 

 Spawning stock biomass-per-recruit was estimated for 

 each interval of increased transition rates over a range 

 of fishing mortalities and a constant size-at-first-capture 

 of 16 cm. 



Results 



To confirm that the model accurately represented 

 growth in the absence of any harvest mortality, we 

 calculated the mean lengths of the cohort (by sex) at 

 successive 365d periods as generated by the model, 

 and compared the results with independent back- 

 calculated mean lengths-at-age (Mercer 1978) (Table 

 1). Growth simulated by the delay model was com- 

 parable to growth observed from back-calculations, 

 although sizes-at-age tended to diverge in older ages, 

 with slightly higher lengths-at-age produced by the 

 delay model. Under the influence of natural mortal- 

 ity only, a cohort introduced into the delay model 

 was reduced to 0.1% of its initial number by the 

 time the maximum length was attained. The reduc- 

 tion of the cohort using only natural mortality indi- 

 cated that the model provided an accurate portrayal 

 of the black sea bass growth rate for the stated set 

 of mortality parameters. 



To evaluate the effect of harvest mortality in the 

 absence of sex transformation, we calculated Y/R as 

 generated by the delay model without transition and 

 compared the results with those obtained using the 

 T-B model. Under various L c values, yields-per-recruit 

 from the two models were similar, indicating the basic 



