JUNE: ATLANTIC MENHADEN GROUPINGS 



viduals within a school was variable. The mean 

 of the length range of fish within samples from 

 275 single-school summer catches was 5.8 cm, 

 with a standard deviation of 2.11 cm; the range 

 was 1.5 to 13.0 cm. The seasonal means ranged 

 from 5.1 to 6.8 cm, with no trend indicated dur- 

 ing the 8 years. These findings support Breder's 

 (1959) generalization that the diff'erence in size 

 of fish that will form an acceptable school does 

 not exceed 50%. 



To determine if variations in the length range 

 within summer schools were associated with the 

 relative lengths of the members, I plotted the 

 variance against the mean length of fish in sam- 

 ples from 275 single-school catches (Figure 5). 

 The variances obviously are heterogeneous, with 

 variation within schools being greater among 

 fish that averaged over about 25 cm in length. 



9.5 



9.0 - 



6.0 



5.5 



5.0 



4.5 H 



40 



c 



.? 3.5 I- 



k. 

 o 

 > 



3.0 - 



2.5 

 2.0 

 1.5 

 10 

 0.5 



*> 



r. •■ .N 



u'l.' ni •"'." . ■**' 



I I I I I I I I I I I I I I I I I I I I I M 



15 20 25 30 



Mean ForK Length (cm) 



35 



Figure 5. — Variance plotted against the mean of the 

 length-frequency distribution of Atlantic menhaden in 

 samples from 275 single-school summer purse-seine 

 catches, 1955-62. 



A plot of variances on time failed to show any 

 trend within or between seasons. Thus, while 

 summer schools of Atlantic menhaden are highly 

 length-selective, schools of mixed lengths do oc- 

 cur, and variation seems to be greater within 

 schools of larger fish than within schools of 

 smaller fish. 



The length range of fish within individual 

 summer schools was less than the length range 

 among schools inhabiting any given area of the 

 coast, or of the population as a whole. In Fig- 

 ure 6, for example, are graphed the length-fre- 

 quency distributions of fish in samples from 52 

 single-school catches made throughout the range 

 of the summer purse-seine fishery in 1959. The 

 least range in length within schools was 3.0 cm 

 and the greatest 12.0 cm; the mean difference 

 was 6.1 cm, with a standard deviation of 2.42 cm. 

 In general, these data show that the lengths of 

 fish found in the different areas of the coast var- 

 ied considerably, and there was a decrease in 



T — I — I — 1 — I — r 



20 29 30 39 10 19 20 23 

 Fork l.«n9lh (cm) 



Figure 6. — Length-frequency distributions of Atlantic 

 menhaden in samples from 52 single-school summer 

 purse-seine catches in 1959. The samples are grouped 

 by major statistical areas and arranged in order of de- 

 creasing latitude. 



705 



