Aniphipoda ( Themisto ). 



3. The quantities of individual plankton organisms in the food of the herring vary with the 

 month. During the winter-spring months (February-April), Euphausiacea are the characteristic 

 group in the food of the herring, in summer Calanus finmarchicus , Calanu s hyperboreus , and 

 Themisto , and in the autumn months Calanus , Themisto , and Euphausiacea. 



4. During the winter months (February -March), the stomach fullness grade is low, a fact 

 which is due to the absence of food organisms during this period. After spawning, however, the 

 herring- -owing to its exhausted condition- -is in great need of food, and individuals with very full 

 stomachs have been found where food organisms have been available. 



The average fullness grade increases during the spring months (April -May) and reaches its 

 maximum in June-July, at which season we observe the greatest quantity of feeding individuals. 

 The feeding rate declines towards the end of the summer, because a high nutritional condition has 

 been attained. 



During the autumn-winter months, the number of feeding individuals declines sharply and 

 the feeding rate of the herring during this period is extremely low. 



5. The number of species of plankton organisms is much larger in the samples than in the 

 stomachs of herring. 



Of all the forms of plankton which populate the water, the herring selects two or three. The 

 following most frequently serve as food for the herring; Meganyctiphanes norvegica, the genus 

 Thysano^ssa , Calanus finmarchicus , Calanus hyperboreus (Cop)ep)oda) and the genus Themisto 

 (Amphipoda). 



The food conditions are of great importance in the biological rhythm of the Atlantic -Scandina- 

 vian herring, since they determme the time at which fat is accumulated and the gonads develop. 



A noticeable quantity of fat is observed on the internal organs of the herring in May. The 

 herrmg attains its highest nutritional condition and maximum fatness at the end of July and in Au- 

 gust, after fattenmg ceases. At this time intensive development of the gonads begins, and this sub- 

 sequently results in reduced fatness. An abrupt reduction in fatness occurs in early spring, during 

 the spawning and jxjst- spawning periods. 



163 



