AHRENHOLZ: RECRUITMENT AND EXPLOITATION OF GULF MENHADEN 



large indicate that mortality rates were generally 

 similar for all areas and years (Figure 4). Since not 

 all areas and years had 4 yr of recoveries, and two 

 of the three areas did not have strongly linear plots 

 (hence a constant annual Z) for 1970 releases, I 

 selected the more linear 3 yr of recovery data from 

 each area for 1969 and 1971 for further analysis 

 (points joined by solid lines on Figure 4). Based on 

 the plotted data of Figure 4, I assumed that Z 

 (hence survival and rate of fishing) was constant 

 for years 1-3 following release for the western area 

 in 1971 and the central area in 1969 and 1971. I 

 assumed a Type C error condition for the first year 

 after release for the western area in 1969 and the 

 eastern area in 1969 and 1971 and assumed Z to be 



constant for years 2-4 in these three data sets. I 

 also assumed that all the fish tagged as adults, 

 which were released in late spring just after fish- 

 ing had begun, were subjected to a full season of 

 natural mortality. 



I used the slope of a weighted regression of the 

 natural logarithms of recoveries on years-at-large 

 where mortality was assumed constant as an es- 

 timate of Z. Each regression point was weighted 

 by the number of unadjusted recoveries that pro- 

 vided the basis for that point. 



For release groups with an estimate of constant 

 Z for years 1-3, called here Zi , the constant annual 

 exploitation rate ( Ui ) was estimated directly by 

 (Ricker 1975): 



E 



o 



O) 



o 



- O'V 



O 



o 



< 



o 



< 



Western Gulf 

 1969 Releases 



Western Gulf 

 1970 Releases 



Central Gull 

 1969 Releases 



± 



Eastern Gulf 

 1969 Releases 



Central Gulf 

 1970 Releases 



X 



Eastern Gulf 

 1970 Releases 



> 

 O 



CD 



3 

 Z 



Western Gulf 

 1971 Releases 



Central Gulf 

 1971 Releases 



Eastern Gulf 

 1971 Releases 



2 3 



YEARS AT LARGE 



Figure 4. — Natural log frequency of recoveries by years at large for tagged adult Gulf menhaden by fishing area and release year, Gulf 

 of Mexico. Lines connect points used to estimate instantaneous total mortality rate(Z) (assumedconstant for 3 consecutive years), and 

 dashes connect points not used in the estimation procedure for Z. 



331 



