of the Fishery Board for Scotland. 



9 



has an important bearing on several problems connected with sea 

 fisheries. 



One method by which the growth and age of fishes is determined 

 is by the tabulation of the measurements of large numbers taken 

 at the same time and place. From the fact that the spawning 

 season of a species, and, therefore, the rate at which a new genera- 

 tion makes its appearance, is usually limited to a few months of the 

 year, the range of the sizes and the average size of the different 

 generations or annual series differ from one another, liy the 

 tabulation of large numbers of measurements it is thus possible to 

 distinguish different generations and to assign the range of size 

 and the age of the fishes belonging to them. With the earlier 

 generations this method is in most cases quite satisfactory, but 

 owing to the very different rate at which members of the same 

 generation grow, the larger of an earlier generation overtaking and 

 exceeding in size the smaller members of the next older generation 

 — a process which increases with age — it becomes difficult or im- 

 possible to separate the older generations from one another by this 

 method. 



Another method that has of late been largely adopted consists in 

 determining the number of the zones or lines of growth in certain 

 of the hard parts of the body. Fishes do not grow continuously 

 throughout the year, their growth exhibiting a usually well- 

 marked periodicity in relation to the changes of the temperature of 

 the water, being as a rule, and in most places, rapid in summer and 

 slow in winter. This periodicity is indicated by lines or zones on 

 some of the skeletal structures, notably on the ear- bones, or 

 otoliths, the scales, and certain bones of the skeleton, the structure 

 which shows them best varying somewhat in different species. By 

 counting the lines or zones it is thus possible to tell the age of a 

 fish, just as by a similar method, and for a like reason, the age of a 

 tree may be discovered by the number of rings present in a section 

 of the trunk. 



To the present report Mr. J. T. Cunningham contributes a paper 

 on this subject, dealing specially with the plaice and the cod. He 

 describes the structure and formation of the ear-bones and scales, 

 and the mode in which the lines or zones are produced. One of the 

 chief objects of the observations was to test the question how far 

 the lines of growth in the skeletal structures of fishes were trust- 

 worthy indications of age — whether the annual increments of 

 growth or deposit could be definitely distinguished and counted in 

 all cases. He shows that it is often necessary to test the indica- 

 tions of one structure by an examination of the others, though in 

 many instances the age of the fish may be well determined by the 

 examination of one of them alone. 



The result in regard to the two species mentioned is to show 

 generally that they do not grow so fast or reach maturity so soon 

 as is commonly supposed. It was found that cod at two years of 

 age measure from ten to thirteen or fourteen inches in length, at 

 three years from seventeen to nineteen, and at four years about 

 twenty-seven inches, so that they would spawn as a rule in their 

 fifth year. Plaice from two-and-a-half to about four inches were 

 one year old, from about four to six-and-a-half inches they were 

 two years old, while those at three years measured up to 12 inches, 



