AHRENHOLZ: RECRUITMENT AND EXPLOITATION OF GULF MENHADEN 



between ports and between years. (Landing and 

 effort data from a former plant at Sabine Pass, 

 Tex., were combined with Cameron, La., data for 

 197L) This adjustment, made by multiplying the 

 actual effort by the ratio of actual catch per unit 

 effort in numbers of individuals to the overall 

 port's and year's mean catch per unit effort (in 

 numbers), makes all measures of effort by port 

 equivalent with respect to numbers offish landed. 

 Relative availabilities were calculated for 

 juveniles tagged in nine specific areas from 1970 to 

 1972. No tags were recovered from fish tagged in 

 area 10, probably because so few were tagged. For 

 each of six ports, the estimated number of tags 

 recovered annually from fish tagged in specific 

 areas was divided by the number of tags recovered 

 at all ports over all years. The quotient was di- 

 vided by the amount of standardized fishing effort 

 for the port during the year considered, and a 

 three-dimensional matrix of relative avail- 

 abilities by age of capture, port, and year class was 

 calculated for each release area. The equation 

 used for these calculations is: 



(1) 



where f'jm = standardized effort, 



Rijk = number of recoveries, and 

 RAVijk = relative availability to a unit of ef- 

 fort. 



Here, i refers to age at capture, j refers to port, k 

 refers to year class, and m = k + i (year captured). 

 The relative availability estimates at the dif- 

 ferent ports for each release area were similar 

 between year classes and so were averaged for 

 all years (Figure 2). These results support the 

 hypothesis advanced by Kroger and Pristas (1975) 

 that there is little or no exchange offish between 

 areas east and west of the Mississippi Delta. Large 

 numbers of fish tagged east and west of the delta 

 were recovered at Empire, La., plants; since ves- 

 sels from these plants fish both sides of the delta 

 there is no way of knowing on which side fish 

 bearing these tags were actually captured. The 

 few tags recovered at plants east of the delta offish 

 tagged west of the delta may actually have been 

 taken by vessels from plants east of the delta fish- 

 ing west of the delta, and vice versa. It is note- 

 worthy that of the few tags recovered at plants 

 west of the delta of fish tagged east of the delta 

 nearly all were recovered at Morgan City and 

 Dulac, La. Vessels from these ports were more 



likely to have fished east of the delta than were 

 vessels from the two most western ports, Intra- 

 coastal City and Cameron, La. Almost no fish 

 tagged east of the delta were recovered at these 

 ports. 



As fish age, there appears to be a slow dispersal 

 toward the delta of fish from eastern and western 

 areas. Fish tagged in the two most western areas 

 were captured in greater numbers their second 

 year after release at the two more central ports, 

 Morgan City and Dulac. Although the fish tagged 

 in the other three western areas were captured in 

 greatest numbers their first year after release, 

 they became progressively more available as 

 2-yr-olds to the three central ports than they were 

 to the most western port at Cameron. However, 

 1-yr-old fish from these three western areas (3, 4, 

 and 5) were disproportionately more available to 

 the western extreme of the fishery than to the 

 more central area. Fish tagged in the three most 

 eastern areas were captured in greater numbers 

 the second year after release at the Moss Point, 

 Miss., and Empire plants. Fish tagged in the east- 

 ern area adjacent to Moss Point, although cap- 

 tured in greater numbers the first year after re- 

 lease, appeared to be more available as 2-yr-olds to 

 Empire vessels than to Moss Point vessels. 



As a consequence of the fishery being concen- 

 trated in Mississippi and Louisiana waters, fish 

 reared as juveniles in the extreme eastern 

 (Alabama and Florida) and western portions 

 (Texas to the Mexican border) of the range are 

 recruited at a lesser rate than fish reared in the 

 center of the range. Fish moving toward the center 

 of the range probably are recruited progressively 

 later in the season as age 1, and many may not 

 be recruited until age 2, whereas nearly all fish 

 reared in the center of the range probably are 

 recruited early in the season at age 1. 



The gradual shift toward the center of the fish- 

 ing area as a year class ages, indicated by the 

 pattern of juvenile tag recoveries, is also indicated 

 by the age composition of the catches. Age-1 fish 

 compose a higher percentage of catches at plants 

 or longitudes at the eastern and western ends than 

 at plants or longitudes in the center (Nicholson 

 1978). The observed age composition is not due to 

 greater fishing pressure on fish at either end of the 

 fishing grounds, since attrition rates offish tagged 

 in the more central areas, implied by catch curves 

 of tag recoveries, are equal to or greater than rates 

 observed for fish tagged in either extreme (Figure 

 3). This shifting is apparently superimposed over 



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