NELSON and AHRENHOLZ: CHARACTERISTICS OF GULF MENHADEN 



1 (28.34 billion at age 0.5) and an F-multiple of 2.00, 

 the yield-per-recruit model predicts a total yield of 

 546,395 t; the population simulation model predicts 

 a gradual decline from current levels and stabiliza- 

 tion at about 409,304 t. Thus, when using average 

 recruitment levels and yield-per-recruit results, 

 estimates of yield at F-multiple levels higher than 

 about 1.75 times the average fishing mortality for 

 the 1964-77 period will be unrealistic. 



The impact of increasing levels of fishing mortality 

 on the stock is also reflected in estimates of popula- 

 tion biomass under an array of F-multiples (Table 

 10). Biomass estimates were based on predicted 

 population size as of 1 January (i.e., after recruit- 

 ment and before application of fishing mortality). 

 These estimates show a pre-exploitation population 

 biomass exceeding 1.268 million t, followed by an 

 accelerating decline as increased fishing mortality 

 takes progressively larger fractions of the popula- 

 tion and disproportionately larger fractions of older 

 and heavier fish. 



STATUS AND OUTLOOK FOR 

 THE GULF MENHADEN FISHERY 



The gulf menhaden population appears to be 

 healthy, highly productive, and capable of supporting 

 yearly harvests exceeding 500,000 t, although con- 



siderable variation can be expected. It has shown a 

 general increase in abundance through the period 

 covered in this report, although this increase 

 may be a portion of a general cycle of this clupeid 

 stock. 



The high natural mortality rate indicates that fish- 

 ing mortality has to be applied at a fairly high rate 

 and on young fish to avoid loss of surplus biomass. 

 Peak cohort biomass is reached at an age of 1.5 yr. 

 It is not all available to the fishery, because age-1 

 fish are only partially recruited. Partial recruitment 

 appears to have some benefit in that it affords some 

 protection for the spawning stock. 



Recruitment fluctuation appears to be greater at 

 low spawning stock sizes. Initial spawning before full 

 recruitment would assure moderate to high levels of 

 recruitment and reduce chances for large recruit- 

 ment fluctuation. Therefore, if recruitment failure 

 were to occur, it would likely arise from biotic or en- 

 vironmental factors rather than from excessive 

 fishing mortality. 



Significant increases in fishing mortality are 

 unlikely to occur, given the present distribution and 

 operating procedure of the fishery, unless there is 

 a series of recruitment failures. The current fleet of 

 about 80 purse seine vessels appears to be more than 

 adequate to harvest the recruited gulf menhaden 

 stock during years of low to moderate stock size, and 



700- 



600- 



100- 



025 O50 



"0775 1^0 T725 175( 

 F-MULTIPLE 



F00 05 T50 



Figure 9— Sustainable yield predicted by a deterministic population simulation model of the 

 gulf menhaden fishery at multiples of the average fishing mortality (F-multiple = 1.00) for the 

 1964-77 fishing season (see Table 10 for scaling values). 



323 



