FISHERY BULLETIN: VOL. 84, NO. 2 



0.5 yr of age, the age at which gulf menhaden first 

 appear in the catch in extremely small numbers and 

 have a very low fishing mortality rata Fishing mor- 

 tality occurs principally during the 2d and 3d 

 quarters of the year (April- September) (Table 8). 

 Various multiples of the average fishing mortality 

 at each age were used to simulate effects of in- 

 creased or decreased fishing mortality (Table 9). 



landings in the fishery of 487,736 1 by only 2.6% for 

 1964-77. 



Yield-per-recruit for the years of higher and lower 

 levels of fishing mortality (Table 8) was estimated 

 to be 18.20 and 15.78 g under F-multiples of 1.00 

 and age of entry at 0.5. Trends were identical to 

 those for average 1964-77 conditions, and thus are 

 not presented in further detail. 



Table 9. — Estimates of gulf menhaden yield-per-recruit (g) under average conditions of growth 

 and as multiples of average fishing mortality rate (F-multiple = 1.00), 1964-77, at varying age 

 of entry. 



Age at 

 entry 



Multiplier of fishing mortality 



0.25 0.33 0.50 0.66 0.75 



1.00 



1.25 



1.50 



1.75 2.00 



The yield-per-recruit increases only slightly with 

 a delayed age-of-entry and then drops rapidly be- 

 cause of the high rate of natural mortality. The 

 model predicts maximum cohort biomass at an age 

 of 1.5, before gulf menhaden are fully recruited in- 

 to the fishery. The high natural mortality rate re- 

 quires that substantial fishing mortality be applied 

 at a young age if gulf menhaden are to be harvested 

 near their peak biomass. 



A three-dimensional representation of yield-per- 

 recruit (Table 9) is helpful in depicting the seasonal 

 nature of the fishery (Fig. 7). Since most of the fish- 

 ing mortality on age-1, -2, and -3 fish is applied dur- 

 ing the 2d and 3d quarters (ages of X.25 and X.50), 

 the impact of delaying recruitment past those 

 quarters results in a sharp decline in yield-per- 

 recruit, due to the high rate of natural mortality. 



Predicted catches based on yield-per-recruit were 

 compared with actual catch during 1964-77. Aver- 

 age recruitment at age 1 (16,030 billion), estimated 

 from the cohort analysis, was back calculated to age 

 0.5, the age of initial entry, and multiplied by the 

 17.09 g/recruit predicted by the model. The resul- 

 tant estimate of 474,829 t differs from the average 



b 



cr 

 o 



HI 



a. 



a. 



hi 

 a. 



Q 

 _l 

 UJ 



> 



4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 



AGE AT ENTRY 



Figure 7.— Yield-per-recruit of gulf menhaden under average con- 

 ditions of growth and with multiples of average fishing mortality 

 by 3-mo interval (F-multiple = 1.0) for the 1964-77 fishing seasons 

 (average conditions indicated by □). 



320 



