138 



PROBLEMS OF LAKE BIOLOGY 



trout over 19 inches long are mature in all 

 Algonquin Park lakes, and above this size 

 reproduction is no longer a limiting factor. 

 Populations from which the fish captured 

 average 20 inches or more are characterized 

 by a progressive scarcity of the smaller size 

 classes, apparently due to the predatory 

 activities of the larger trout. In these lakes 

 production is limited by the presence of 

 large trout, and the number of fish taken per 

 unit effort can be increased by the removal 

 of these by fishing. It will be realized, of 

 course, that Fig. 2 shows only the standing 

 crop, the annual production being influ- 

 enced by the average age of the population 

 as well as by its average density. 



Owing not only to the complexity of the 

 factors involved, but also to the fact that 

 quite different causal agents can produce 

 the same effect, it would seem that there is 

 no hope of devising a workable index with 

 which to classify lakes according to their 

 usefulness in producing fish desirable to 

 man. Since there is no prospect of prog- 

 ress along such a royal road, it would 

 appear that advance toward the solution of 

 our problems of productivity lies along 

 paths which we are already taking. Judg- 

 ing from the history of limnology, the most 

 profitable attack on problems of fisheries 

 production lies in the following steps: (1) 

 measure production, (2) study the organ- 

 ism in nature in detail, and (3) supple- 

 ment the field observations with laboratory 

 experiments. 



Measurement of production is done by the 

 collection of statistics of capture of the 

 animals sought in commerce and in sport. 

 The collection of commercial records is now 

 taken as a matter of course and the collec- 

 tion of statistics of sporting fisheries is be- 

 coming increasingly more widespread. Also 

 it is realized that something more than 

 records of total capture are necessary, and 

 information is being commonly collected 

 concerning fishing effort and size composi- 

 tion of the catch. There is a growing 

 tendency to formulate theories which, while 

 taking into account all the biological infor- 

 mation available, are nevertheless primarily 

 founded upon empirical relations observed 



between the abundance of the population 

 and the history of the removal of its mem- 

 bers by man. Of these probably the most 

 outstanding achievement to date is that 

 evolved and put into practice by the Inter- 

 national Halibut Commission (Thompson 

 1935). 



Experiments are essential both to supple- 

 ment observations in the field and to test 

 practices proposed for the improvement of 

 production before they are introduced into 

 management on a large scale. It is often 

 impossible to predict with certainty the out- 

 come of steps which might at first glance 

 appear to be certain to lead to improvement. 

 If the lake trout may be referred to again 

 for an example it might be pointed out that 

 measures taken to improve the growth of a 

 slow growing population of this species need 

 not necessarily improve the production of 

 fish, but may conceivably lead to a poorer 

 fishery than before. With the methods of 

 angling that are currently in vogue, lake 

 trout shorter than 12 inches are rarely cap- 

 tured and thus there is a minimum size at 

 which lake trout take the hook which acts 

 as surely for their conservation as does a 

 legal length in other species. In lakes 

 where lake trout mature at a small size 

 their own habits protect them from capture 

 until the season in which they become ma- 

 ture. If the rate of growth increased, such 

 a population would lose this natural pro- 

 tection in large measure, because with more 

 rapid growth the fish would be larger at 

 maturity, and would be liable to capture a 

 year or even two years before the season in 

 which they spawned. Hence if the popula- 

 tion were subjected to a high fishing inten- 

 sity any initial improvement of production 

 resulting from improvement in growth 

 would soon be more than lost because of the 

 greater effect of predation upon reproduc- 

 tion. 



Experiments which are designed to ana- 

 lyze the factors underlying the behavior of 

 the organism observed in nature are of 

 more fundamental importance than those 

 designed to test theories before they are put 

 into general practice. Shelf ord (1934) has 

 pointed out the importance of being tlior- 



