may be due to variations associated with fish 

 length, sex, or simply to chance. 



Variation between year classes 



The mean vertebral counts of samples within 

 each major group were further tested for 

 homogeneity between years by analysis of 

 variance, using coded values (table 4). Only 

 the means of the southern group differed sig- 

 nificantly between year classes. The un- 

 weighted means of individual year classes 

 were as follows: 1956—47.09; 1957—47.02; 

 1958—47.02; and 1959—47.00. The hetero- 

 geneity within the southern group may be 

 attributed to the slightly greater mean of the 

 1956 year class, for analysis indicated that 

 the means of the other year classes were 

 homogeneous, 



DISCUSSION 



The number of meristic parts in fishes is 

 known to vary with temperature (Jordan, 1892; 

 McHugh, 1951; Seymour, 1959; Tester, 1938), 

 and variations in vertebral numbers of Atlantic 

 menhaden might be explained on the basis of 

 temperature at the time of spawning. In the 

 North Atlantic area, eggs and larvae have been 

 collected from May to October; from June to 

 October in Long Island Sound (Perlmutter, 

 1939; Wheatland, 1956), and from May to 

 October in Narragansett Bay (Herman, 1959). 

 Fish in near-spawning condition are caught by 

 gill nets as early as March in the vicinity of 

 Point Judith, R.I. Pound net catches inRaritan 

 and Lower New York Bays include some near- 

 ripe fish in March and April (Higham and 



Nicholson, unpublished MS.* ) Thus, it is likely 

 that some spawning takes place in northern 

 waters from April to October. Based on the 

 occurrence of near-ripe fish in purse seine 

 catches and the occurrence of eggs and larvae, 

 spawning in the Middle and South Atlantic 

 areas appears to take place from October to 

 April, although it may begin in September and 

 continue through May, Larvae apparently enter 

 estuaries soon after hatching (June andCham- 

 berlin, 1959; McHugh, Oglesby, and Pacheco, 

 1959; Perlmutter, 1939). 



Mean monthly surface water temperatures 

 at nine locations along the Atlantic coast 

 where spawning is presumed to occur are 

 shown in figure 3 (Bumpus, 1957; Day, 1959a, 

 1959b, 1960). The figure shows a sharp in- 

 crease in the mean April to October tempera- 

 tures sou^h of Ambrose Lightship to approxi- 

 mately Chesapeake Bay. Mean temperatures 

 south of Chesapeake Bay exceed those north 

 of Ambrose Lightship during the spawning 

 period and vary from approximately 45° F. in 

 January to approximately 70° F, in May. 



Although there may be some selection by 

 spawning fish of a temperature stratum en- 

 tirely different from that shown in the graph, 

 it would appear from these data that the varia- 

 tion in mean vertebral numbers of juvenile 

 Atlantic menhaden may be associated with 

 water temperature at spawning time. There 

 is relative stability in vertebral numbers in 5 

 successive year classes despite wide varia- 

 tions in temperature within and between years 



* Sexual maturation anci spawning of Atlantic men- 

 haden, Unpublisheci manuscript. Bureau of Commercial 

 Fisheries Biological Laboratory, Beaufort, N.C. 



Table 4, — Analysis of variance of mean vertebral numbers of juvenile Atlantic menhaden 

 to test for differences between year classes within groups, 1956-59 



^ P < 0.01. 



