184 Fishing and Natural Mortality 



ponent species of fishes. This type of cropping tends to hold down the 

 numbers of all species. Also, since there are more predators of small 

 fish than of large ones, a severe culling of small fish is continuously taking 

 place. 



Man has yet to devise a cropping system comparable to a natural 

 system of population control. His nearest approach in artificial waters has 

 come from stocking predatory fishes in sufficient numbers to dominate, 

 momentarily, the aquatic environment. 



Yields and Standing Crop 



It is logical to assume that a yield of fish, if it has been sustained over 

 a period of years, must bear some definite relationship to the population 

 of these fish (poundage) that the body of water supports and to the rate 

 of their replacement. As the crop is taken, increased food resources 

 become available for the potential replacement of fish flesh removed, and 

 this replacement must depend upon ( 1 ) the efficiency of food gathering, 

 (2) the replacement time available, and (3) the ability of the number 

 of digestive tracts uncaptured and those added through new recruitment 

 to convert the food into proteins, to replace those taken. 



Usually when one speaks of a sustained yield, he is talking about a yield 

 level repeated in each successive season for a period of years. He probably 

 is not speaking of the maximum poundage of fish that may be replaced 

 in each growing season. Rather, he may be thinking of some poundage 

 below the absolute maximum, that may be taken by a reasonable amount 

 of effort on the part of the fishermen. Thus, there are many levels of 

 so-called sustained yields, none of which is, in reality, the maximum. 



In small lakes and ponds containing a limited number of kinds of fishes, 

 it is sometimes possible to demonstrate competition between species and 

 individuals for food and space. One may suspect that the same factors are 

 active in larger waters with a greater variety of species; but direct observa- 

 tion is difficult or impossible and changes are measured largely on the 

 basis of variations in the commercial catch of species valuable for human 

 consumption. 



In the Great Lakes, for example, yields were higher before 1920 than 

 they have been since. According to Van Oosten ^'^ the annual yield of 

 fish before 1920 varied around 100 million pounds; from 1920 to 1947 it 

 ranged around 78.5 million pounds, a reduction of 22 per cent. Van Oosten 

 believed that factors leading to the decline of this fishery were (1) in- 

 creased fishing pressure, (2 improvement in fishing methods, (3) exten- 

 sion of fishing grounds, (4) replacement of better classes of fishes with 

 poorer types and ( 5 ) (a variation of 4 ) the introduction of the smelt. 



In considering the factors that reduced the standing crop of the more 

 desirable fishes. Van Oosten hesitated to place major importance either 



