FISHERY BULLETIN: VOL. 75. NO. 1 



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EASTWARD TRANSPORT 



EASTWARD TRANSPORT 



WESTWARD TRANSPORT 



1969 , 1970 1971 1972 1973 . 



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FIGURE 1. — Monthly average zonal Ekman transport at lat. 35°N. long. 75°W, 1955-73. January-March spawning period is shaded. 



TABLE 1. — Average westward wind-driven Ekman transport 

 computed for November-March 1957-58 and 1963-64 at lat. 

 39°N, long. 72°W and lat. 39°N, long. 75°W. Transports expressed 

 in metric tons per second per kilometer of front. 



Year 



Lat. 39°N, 

 long. 72W 



Lat. 39°N, 

 long. 75°W 



1957-58 

 1963-64 



480 

 250 



520 

 260 



SPAWNER-RECRUIT RELATION 



Over the 16 yr from 1955 to 1970, there was a 

 sharp decline in the size of the Atlantic menhaden 

 spawning stock and the size of resultant year 

 classes. From 1964 to 1970, the annual catch of 

 spawning age fish averaged only 14% of the 

 previous 9 yr. Resultant per-year recruitment 

 from 1964-70 averaged 42% of that for the previ- 

 ous 9 yr (Schaaf 1972). A description of the aver- 

 age relationship between spawning stock size and 

 recruitment is useful for examining this coinci- 

 dent reduction and for predicting the expected fate 

 of the fishery under different exploitation regimes. 

 A stock-recruit function is also the necessary 

 starting point for developing an index of survival 

 (observed recruitment to that expected from 



number of spawners) against which one may 

 assess the impact of density-independent en- 

 vironmental effects of recruitment. The approach 

 in this study has been to determine if selected 

 density-independent environmental factors could 

 explain deviations from a postulated spawner- 

 recruit model. 



Ricker's (1954) comprehensive study of stock- 

 recruitment formulated a dome-shaped model, 

 with strong compensation, resulting in decreased 

 recruitment at stock sizes beyond some maximum 

 value. It is described by the equation: 



R = Se (S r~ S),Sm 



where R = recruitment 



S = spawning stock 

 e = base of natural logarithm 

 S r = maximum equilibrium stock 

 S m = stock size yielding maximum absolute 

 recruitment. 



Ricker's model states that some stock size (S m ) 

 produces maximum recruitment, and that, be- 

 cause of density-dependent mortality and growth, 



28 



