AGE AND GROWTH^OF THE CISCO 263 
AGE COMPOSITION OF THE SAMPLES AND THE RELATIVE ABUNDANCE 
OF YEAR CLASSES 
Because of its importance in the study of commercial fisheries the question of 
age composition of the stock and of the existence of dominant year classes has in 
recent years received a great amount of attention. It is now well known that the 
success of different years’ hatches as measured in terms of the number of young 
produced is subject to a wide range of variation from one year to another. The 
hatches of some years may be so successful that that particular year class may domi- 
nate in the fishery over a period of 1 or several years. Yet other hatches may be so 
unsuccessful as to make only minor contributions to the stock. The common occur- 
rence of relatively successful and unsuccessful year classes has been demonstrated in 
many species of fish of commercial importance. The study of the age composition of 
the commercial catch over a period of years has served as a most valuable tool in the 
investigation of fluctuations in abundance. In both Europe and America the known 
age composition of the commercial stock, together with the observed rate of falling 
off of a year group from year to year, has been used by fisheries investigators as a 
basis of prediction of the probable yield of the fishery for the approaching season. 
In spite of the extensive researches that have been made on the subject of fluctua- 
tions little is known as to the causes that make a year class good or bad. Hjort 
(1914) stated that while the basic causes of fluctuations in abundance are unknown, 
it appears that fluctuations “have their origin in certain conditions prevailing at a 
very early period in the life of the fish.” He repeated this opinion in 1926. Storrow 
(1932) pointed out that not only are the factors that determine the success or failure 
of a year class complex, but also that the difficulty of observing the fish in its natural 
environment is great. He called attention further to the failure of attempts to simu- 
late natural conditions in the laboratory, a convincing proof of inadequate knowledge 
of optimum conditions. 
The most thorough-going studies on the question of fluctuations in abundance 
in coregonids and their causes are those of Jarvi (1920, 1924, 1930). He was able to 
show not only a great variation in the abundance of different year classes of the 
“kleine Marane” ( Coregonus albula ) but also a distinct connection between such 
variations in abundance and weather conditions in the spring just after the time of 
hatching. If there are strong winds at this critical period many of the delicate newly 
hatched young are destroyed by the wave action. Jarvi found, however, that strong 
winds at the time of spawning are less harmful. 
Huitfeldt-Kaas (1917) found that in some years in Lake Mj0sen almost all the 
spawning run individuals of Coregonus albula are taken in the commercial fishery 
before they have had an opportunity to spawn, and that poor year classes may result. 
Because of the manner (collecting with gill nets) in which the samples were 
obtained in the present investigation the study of the relative abundance of the various 
age groups in the four cisco populations and along with it the consideration of the 
relative abundance of the different year classes must be approached with great 
caution. (The limitations of gill net samples in the study of relative abundance of 
age groups and year classes are discussed in the section on the selective action of gill 
nets.) The age composition of a gill-net sample must in general be considered as 
descriptive of the sample rather than of the population as a whole. Exceptionally, 
however, the representatives of some particular year class may appear so promi- 
nently or be so scarce at all of the ages at which that year class appears in different 
