FISHERY BULLETIN: VOL. 75, NO. 1 



Because the regression of \n(R/S) onS, as is done 

 for the Ricker equation, will automatically give a 

 significant correlation coefficient, a nonlinear 

 fitting procedure was also applied to the data 

 (Marquardt 1963). A comparison of the residual 

 mean squares of the two procedures yielded anF of 

 1.02, indicating no significant difference in the fit 

 of the Ricker curve to the spawner-recruit data 

 between the standard technique and the nonlinear 

 estimation. 



Few published stock-recruitment curves appear 

 to fit the observed data well, and the one for At- 

 lantic menhaden is no exception. Application of a 

 power function of the form R = aSh to the data 

 resulted in a fit that was not significantly better 

 from that of the Ricker function. The purpose of 

 the study, however, is to examine and explain the 

 deviations from the curve caused by density- 

 independent factors, to see if they can be predicted, 

 and consequently to improve upon a management 

 plan based solely on a long-term, average MSY 

 concept. The survival index (Table 3) represents 

 the ratio of observed recruits (the number of age 

 l's in the population as estimated from the catch of 

 age l's and estimated exploitation rates) to the 

 number calculated from the Ricker spawner- 

 recruit model. This ratio is an index of survival, 

 independent of density, and should reflect those 

 environmental effects which influence survival of 

 menhaden from the time of spawning until the 

 time of recruitment to the fishery at age 1. 



INFLUENCE OF EKMAN TRANSPORT 

 AND OTHER FACTORS 



The influence of transport processes in the 

 southern part of the spawning range is indicated 

 in Figure 3 which depicts the Ekman transport 

 index for the January-March spawning period for 

 1955-70 and the estimated number of menhaden 

 recruits at age 1 from the year class. The re- 

 sponsiveness of survival to transport shows up 

 well in the Figure where years of strong westward 

 transport correspond with large year classes, and 

 weak transport years with smaller year-class size. 

 Also, increases and decreases in recruitment from 

 one year to the next generally coincide with an 

 increase or decrease in westward transport in the 

 year in which the year class was produced. 



The correspondence is weaker in the 1968-70 

 year classes, although it follows the general 

 pattern. Intense fishing pressure over a number of 

 years changed the age structure of the spawning 



TABLE 3.— Estimated number of eggs, observed and expected 

 number of recruits at age 1, and density-independent survival 

 index for Atlantic menhaden, 1955-70. 



stocks to a considerable extent. For example, 

 approximately 40% of the estimated spawning 

 stock in 1958 were 4 yr or older. The number of age 

 4 and older fish in the 1969 spawning population 

 was only about 9%, and the average number of 

 eggs per spawning female was about 50,000 less 

 than in 1958. Thus, fishing pressure brought 

 about an even greater reduction in spawning 

 potential than is apparent when considering the 

 number of spawners alone, because of a reduction 

 in the average age. This reduction in real spawn- 

 ing potential reduced the opportunity for a large- 

 scale response to favorable transport in the 1968- 

 70 year classes. 



Comparison of the density-independent survi- 

 val index with Ekman transport yields a sur- 

 prisingly consistent relationship (Figure 4). A 



FIGURE 3.— Observed number of Atlantic menhaden recruits at 

 age 1 and sum of average monthly zonal Ekman transport at lat. 

 35°N, long. 75°W for January-March of spawning years, 1955-70. 



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