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38" 37" 36° 35° 



DEGREES NORTH LATITUDE 



34° 



32° 



Figure 3.--Distribution of mean seasonal water temperatures along the Atlantic coast of the 



United States , by latitude. 



and with separate spawning times. Further- 

 more the occurrence of two groups of juveniles 

 within the same estuaries in Long Island which 

 could be separated by mean vertebral numbers 

 and identified with either the northern or 

 southern subpopulations suggest genetic origin. 

 A followup study of these same year classes 

 at the time the fish reach sexual maturity 

 should determine whether or not the pattern 

 in vertebral numbers is similar to that exhib- 

 ited when they were juveniles in adjacent 

 estuarine nurseries. 



SUMMARY 



1. Collections of juvenile Atlantic menhaden 

 (Brevoortia tyrannus), comprising 182 samples 

 and 17,024 fish, were made in selected estu- 

 aries from Cape Cod, Mass. to northern 

 Florida during 4 successive years, 1956-59. 



2. Vertebral counts were made from X-ray 

 photographs and stained specimens. 



sex occurred in only 1 of 18 samples tested 

 and was disregarded in subsequent analyses. 

 Positive and negative regressions of vertebral 

 number on fish length were obtained, but the 

 coefficients differed significantly from zero 

 in only two of 18 samples tested. Length was 

 disregarded as a source of variation in sub- 

 sequent analyses. 



4. Analysis of variance was used to ex- 

 amine variation within and between samples, 

 localities, groups, and year classes. There 

 was less variation within and between sam- 

 ples from a given locality and within a geo- 

 graphic group than between two geographic 

 groups. It was concluded that, except for the 

 1956 year class in the southern group, varia- 

 tion between year classes was not significant. 



5. The hypothesis that at least two groups, 

 or subpopulations, of juvenile Atlantic men- 

 haden exist was supported by statistical analy- 

 sis of variation in vertebral numbers. 



3. Preliminary analyses determined the 

 sample size requirements, and variation with 

 sex and fish length. Significant variation with 



6. Variation in vertebral number was asso- 

 ciated with water temperature differences 

 during the spawning seasons. 



