188 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



first, and in this manner those of each succeeding day throughout the 

 season are added to the total of the day previous. At a station where 

 the temperature units are reckoned daily from the opening of the 

 season the age of any lot of eggs may be at once known by subtract- 

 ing from the reading on the day of examination the reading of the day 

 on which the eggs were taken. In practice it is found simple and 

 convenient, and as the season advances the answers to many questions 

 may be had from this interesting column of figures. 



The result of the tests at Baird shows that the incubation period of 

 the quinnat salmon is about 900 t. u. From irregular data and some 

 observation, it seems that this is also the number required for brook 

 trout {Salvelinusfontinalis), while from similar data it is thought that 

 rainbow trout (Salmo irideus) requiie a somewhat less number. Each 

 species undoubtedly has its peculiar norm. 



This unit seems preeminently to include the factors determining 

 the length of the incubation period. Without going deeply into the 

 merits of the old rule, "in 50 days at a mean water-temperature of 

 50° F. trout eggs will hatch, and for each degree warmer or colder 5 

 days less or more will be required, the difference, however, increasing 

 the farther we recede from 50°," it will be seen at a glance that the 

 law is an empirical one, and while recognizing perhaps the factors of 

 incubation, it is not sufficiently accurate and explicit to be available 

 in determining the entire period when the mean is slightly removed 

 from 50° and is entirelj^ silent as to intermediate stages. There are 

 but two important variable factors that affect this period, namely, 

 time and the temperature of the water. There are many other con- 

 ditions that affect incubation, such as quality, volume, aeration, etc., 

 conditions of such importance that success is not possible if they are 

 not right, and these conditions must in some degree affect the length 

 of the incubation period. The two main factors, however, as before 

 stated, are time and the temperature of the water. 



As water freezes at 32° F., and will, of course, congeal all life within 

 it when frozen, rendering growth, if not life itself, impossible, it is 

 only rational, so far as temperature is concerned, that this be the 

 point of starting. In many stations, it is true, hatching operations are 

 conducted in waters that are very cold, and it would seem that the 

 eggs of SalmonidcB could hardly be subjected to as many as 900 t. u. 

 before hatching. Cod work in winter time is also done at very low 

 temperatures. It must be remembered, however, that the growth of 

 the embryo salmonoid in such stations takes place mainly in the fall 

 before the waters reach extremely low temperatures, and again in the 

 spring when they begin to warm up. It is also conceded that cod 

 work at 32° or 33° is very unsatisfactory. 



It is hardly necessary to say that neither the "temperature-unit 

 system," nor any other " system," will give uniform results in waters 

 which through unsuitable temperatures will not produce healthy fry; 



ju 



