FISHERY BULLETIN: VOL. 70, NO. 3 



to represent the annual mortality 



coefficient) , 



fc-i 



L^ = R exp \ —^ Zh 1 = number sur- 



h=l 



Zh 



R 

 M 



vivors to begin interval k, 

 = M during months closed to fishing. 

 = number of recruits = 1,000,000. 

 = 0.12. 



The yields at various levels of fishing intensity 

 were predicted by multiplying all Fa by a con- 

 stant equal to the intensity change desired and 

 recalculating catches for all months. Estimates 

 of expected yield in numbers and expected av- 

 erage weight per shrimp were also provided by 

 this procedure. By setting appropriate values 

 of F = 0, yields for various seasons and entry 

 ages were computed. 



It M — 0.12 is the monthly instantaneous 

 mortality coefficient and if growth in weight at 

 age is taken from the empirical growth curve 

 (Table 3), a year class of shrimp that is not 

 fished will reach its maximum biomass during 

 the period July to August as age I. The bio- 

 mass will then decline rapidly and by July to 

 August as age II it will be about one-half the 

 maximum. 



The estimated yield per recruit for the period 

 1961 to 1967 was 0.00165 lb. per shrimp. Since 

 the average annual catch during that 7-year pe- 

 riod was 1.918 million pounds, it would have 

 required an average of 1.162 billion recruits on 

 April 1 to support the catch. The Murphy meth- 

 od estimates an average recruitment on May 1 

 of about 1.155 billion. Thus, the analysis of 

 yield per recruit is in good agreement with the 

 Murphy method results. 



Given 1.155 billion recruits on April 1, it 

 would require a yield per recruit of 0.00216 lb. 

 per shrimp to obtain a total harvest approxi- 

 mately equal to the maximum sustainable yield 

 estimated from the Schaefer model. To have 

 obtained a yield-per-recruit of 0.00216 during 

 those 7 years would have required an increase 

 in fishing mortality of about 75% (Figure 9). 

 This additional yield could not have been ob- 

 tained by shortening the season or changing age 

 at recruitment unless a substantial increase in 



fishing mortality accompanied the changes (Fig- 

 ures 9 and 10) . With the distribution of fishing 

 effort observed during 1961-1967, the average 

 total monthly instantaneous fishing mortality 

 (^ 2 Fij) operating against a year class during 

 3 seasons was estimated to be 2.0176. While 

 maintaining total fishing mortality at 2.0176, 



1,5 2.0 2.5 3.0 



MULTIPLIER OF F 



Figure 9. — Yield in pounds per million recruits as a 

 function of age at entry into the fishery and fishing 

 mortality. Fixed population parameters used were 1961- 

 1967 means. 



< 



IS 



z 



— MAY 15 



o 



o 

 to 



< APRIL 15 



~i 1 1 1 1 1—1 1 1 1 1 1 1 1 1 r 



500 1200 1500 1700 1900 2100 2300 2400 2500 2600 2700 



J I I L 



J I I I I I I I I 



05 10 15 20 25 30 35 40 



MULTIPLIER OF F 



Figure 10. — Yield in pounds per million recruits as a 

 function of season opening date and monthly fishing 

 mortality coefficient. October 31 season closing date 

 assumed and fixed population parameters used were 

 1961-1967 means. 



1038 



