^66 



DEPTHS OF THE OCEAN 



age-composition of spawning shoals in the two species appears 

 from the following examples : 



Sund found that the majority of sprats spawn when two to four 

 years old, while Dahl found that the herrings spawn from the 

 3rd to the 14th year, the majority between four and eight years 

 old. This difference is fundamental in the life-history of the two 

 species. The life-cycle of the sprat is rapid, indicating a rapid 

 renewal, while the herring lives much longer, spawns for a great 

 number of years, and spawning commences two years later than 

 in the sprat. The herring is a typically boreal fish, its southern 

 limit to the south-west of Britain conforming to that of all 

 the boreal bottom-fishes (see Chapter VH.). Herrings live, 

 at least sometimes, at considerable depths, depositing their 

 eggs on the bottom of the coast banks during winter and 

 spring, now in shallow, now in deeper water. 



The sprat is distributed far south in the Atlantic, occurring, 

 according to Day, round the Iberian Peninsula. It is a 

 surface fish occurring in boreal waters mainly where high 

 summer temperatures prevail ; it spawns during summer, the 

 eggs being pelagic. 



From the study of the age of fishes I was induced to 

 hope that the variations in the magnitude of the fish-stock 

 might be estimated, and my collaborators have made very 

 extensive investigations with most important results. This 

 applies to the cod family as well as to the sprat and the 

 herring. I will here only review some of our results from the 

 herring investigations. 



For a number of years samples for age-analysis have been 

 collected during the various herring fisheries on the coast of 

 Norway, the analysis of which has proved that the age-com- 

 position of immature herrings, as well as the shoals of spawning 

 herrings, vary considerably from year to year. These variations 

 are mainly due to the fact that certain annual classes are 

 exceedingly prolific, while others are very poorly represented. 

 The following table records the results of an analysis of 



