TOWNSEND and GRAHAM: GROWTH AND AGE STRUCTURE OF LARVAL HERRING 



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LARVAL LENGTH mm (S L) 



Figure 2. — Length-frequencies of Atlantic herring larvae cap- 

 tured at night in the buoyed and anchored nets in the Sheepscot 

 River estuary, Maine, 5 October 1978 to 27 February 1979. 



larger larvae from the 19 October population 

 remained. By 16 November there was only one 

 major size mode of larvae in the estuary. During 

 December a second size mode of smaller larvae 

 began to appear and in late December and early 

 January at least two modes were present. By 27 

 February the two modes were no longer distin- 

 guishable. The appearance of the larval group in 

 December was also evidenced by a leveling off of 

 the decline in larval catch rate for that month 

 (Figure 3). The catch rates then increased in Jan- 

 uary as the second group became established in 

 the estuary. 



Assuming the otolith growth increments are 

 deposited with a daily periodicity, increment 

 counts can be used to estimate larval age and 

 growth rates. Figure 4 shows the age structure 

 of the Atlantic herring larvae captured with the 

 towed net in the estuary. The larger and presum- 

 ably older larvae represented in the 19 October 

 sample of the buoyed and anchored nets (Figure 2) 

 were not detected in the towed net samples; there 

 appeared to be only one major age-group of larvae 

 occupying the estuary through November. The 

 addition of the second age-group of larvae was 

 readily apparent by 10 January and resulted in a 

 shift of the age-frequencies to the reader's right. 

 This second and younger group of larvae was pres- 

 ent until the last sampling date (6 March), when 

 the remnant larvae from the first group were 

 detected as a negative skewness in the age-fre- 

 quency histogram. 



The bottom panel of Figure 4 shows the bimodal 



Figure 3. — Catch rates of Atlantic herring larvae for the 

 buoyed and anchored nets in the Sheepscot River estuary, 

 Maine, based on flowmeter readings. 



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