NELSON ET AL : LARVAL TRANSPORT OF BREVOORTIA TYR ANNUS 



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WESTWARD TRANSPORT (METRIC TONS X 10/SEC/KM) 



FIGURE 4. — Linear regression of 

 calculated survival index (observed 

 recruits/calculated recruits) for Atlan- 

 tic menhaden on sum of January-March 

 zonal Ekman transport at lat. 35°N, 

 long. 75°W, 1955-70. 



linear regression of survival indices against 

 transport values for the January-March spawning 

 periods at lat. 35°00'N and long. 75°00'W results 

 in an r of 0.789 significant at the 0.001 level with 

 14 df (Figure 4). This accounts for approximately 

 629c (r 2 = 0.622) of the variation between observed 

 and expected recruitment. Since the transport is 

 indicative of conditions over only a portion of the 

 total spawning range of Atlantic menhaden, and 

 since r 2 accounts for such a large share of the total 

 variation in overall recruitment, the actual effect 

 of transport processes in the southern spawning 

 area must be of overriding significance for the 

 survival of spawn south of Cape Hatteras. With 

 the exception of 1966, the index of survival was 

 greater than 1.0 only when the Ekman transport 

 index indicated a strong westward transport for 

 the January- March period of menhaden spawning 

 activities south of Cape Hatteras. 



The transport data fall conveniently into groups 

 of 0-200, 200-500, and 500-1,000 metric tons/s- km 

 of ocean front. Five years of strong westward 

 transport (>500) were found, and in all of these 

 years the survival index was greater then 1.0. The 

 observed recruitment exceeded the expected by an 

 average of 108%, with the 1958 year class showing 

 the largest value. In 6 yr of low westward trans- 

 port (0-200), the survival index was never greater 

 than 1.0. In 5 yr of moderate or "average" west- 

 ward transport, (200-500) high survival occurred 

 in 1 yr, and poor or moderate survival in the other 

 4 yr, indicating the influence of additional factors 

 over the spawning range that are operating to 

 produce variations in year-class strength. The 

 high index for 1966 may partially result from the 

 fact that the estimated spawning stock production 



of 5 x 10 12 eggs was, by far, the lowest of any year 

 on record (Table 2). Under such low stock size, 

 density-dependent survival may have exceeded 

 that indicated by the Ricker curve, creating an 

 artificially high index of survival. A slight un- 

 derestimation in the computation of the number of 

 spawners would also create a very high survival 

 index, since the slope of the Ricker curve is ex- 

 tremely steep as spawning stock size approaches 

 zero (Figure 2). 



Transport values at lat. 33°N, long. 78°W, 

 approximately 200 nautical miles southwest of 

 lat. 35°N, long. 75°W were also considered. The 

 data are from a point offshore of Long Bay, S.C., 

 the southernmost of the cuspate Carolina bays, 

 and serves as an indicator of Ekman transport in 

 the extreme southern part of the Atlantic 

 menhaden spawning range. A significant corre- 

 lation existed between transport for the 

 January-March period and the survival index 

 (Table 4). Due to the correlation between the two 

 transport values south of Cape Hatteras, however, 

 little additional variation is accounted for by the 

 southernmost transport value (Table 5). Since 

 transport is a function of wind stress and Coriolis 

 force, movements of air masses through the 

 southeastern United States would give parallel 

 transport values at the two locations, with inten- 

 sity of transport dependent on variations within 

 the air mass. The large amount of variation ac- 

 counted for by the two transport indices south of 

 Cape Hatteras is sufficient to account for the rela- 

 tive success or failure of a year class, and supports 

 the observation that a significant portion of 

 menhaden spawning takes place south of Cape 

 Hatteras. 



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