GKOWTH OF SALMON AND TROUT. 77 



Merely to mention one form of pisciculture which is of the utmost 

 importance over here, namely, the better utilisation of all the possibilities 

 of our fresh water lakes, the knowledge we have acquired should be of great 

 assistance. 



Hitherto the greatest obstacle has been that it was impossible to 

 control the stock. 



The methods, however, whose results we have here discussed, should 

 cause this difficulty to disappear. 



We can now examine age and growth, and thus obtain the means of 

 estimating what results are derived from the different measures adopted, 

 such as turning out young fish, preserving the waters, and regulating the 

 fishery. We can also keep a better check on our experiments by tracing 

 the age and growth of the fish in general, as well as also each individual's 

 growth-history. 



Just as the forester estimates his forests by borings and calculations of 

 growth, so, too, we should be able to estimate the stock of fish in the water, 

 and endeavour to set up a plan of operations based on assessments of age 

 and growth. No doubt it has not yet been proved that this is possible, but 

 our endeavours should be made in this direction. 



Bound up with the problem of rational utilisation of our waters is 

 the question of recruiting the stock. 



Some of my results have led me to believe that it may be an advantage 

 in many waters to turn out fairly large fish in place of small fry. 



This question arises in all waters where the fish in their young stages 

 have a poor development, followed subsequently by splendid growth. 



Lake Mj0sen may be taken as an instance of this. If we look at the 

 table on page 76 we find that the trout which began their vigorous growth 

 at the age of six winters, grew far better than those which commenced at 

 an earlier age, so much so, indeed, that we should almost gain one kilo, 

 by employing the oldest fish. The difference in weight which we finally 

 get is far greater than the slight difference existing between the weights of 

 the fish when they first commenced their vigorous growth in Mj0sen. 



It might be worth trying whether it would pay to turn out such trout 

 at the most favourable age, and we could mark them and watch the results. 



Another example is also worth mentioning. At Trondenes Manse 

 there is a little shut-in tarn called Laugen, where there used to be no fish, 

 but which swarmed with water-fleas (Gammarus). Some trout-fry were 

 turned into it, and grew capitally, but they did not increase in number, and 

 were gradually all captured. 



Close by the manse there is a little burn, where there are quantities of 

 the ordinary poorly-developed burn-trout. The priest's sons used to 

 amuse themselves by catching these small trout, and turning hundreds of 



