Fishing Mortality 185 



on interspecific competition as such or on partial loss of competitive 

 ability due to selective cropping, l)ut looked for causes in pollution, 

 change in the habitat, and disease. However, the fact that in Lake 

 Michigan a big increase in the 1945-47 yield of ciscoes followed the 

 almost complete disappearance of smelt in 1943, suggests that changes in 

 the population might be due to interspecific competition, even in waters 

 as large as the Great Lakes. Within the 1947-1960 period the sea lamprey 

 has greatly reduced the yields of certain Great Lakes fishes. 



In small lakes direct competition plays an important role in the size 

 of the standing crop of catchable fishes. However, larger waters are not 

 as easy to evaluate. For example, the spawning site of a species mav be 

 far removed from its feeding grounds; also, competition on the spawning 

 grounds might be intensive with little competition on the feeding grounds. 

 Competition for spawning space might result in a smaller number of 

 individuals of a desirable species and, if food gathering could not be 

 accelerated in proportion to this decrease in the number of individuals, 

 the fish would show no growth compensation. 



Several authors have attempted to predict a sustained vield in relation 

 to the standing crop of fish. Swingle ^^ estimated that 50 per cent of the 

 total weight of fish in a pond could be removed each year by angling. 

 At this point, the number had been reduced sufficiently so that those 

 remaining found plenty of natural foods and consequently did not bite 

 well. This approach is somewhat theoretical, as the hypothesis is not sup- 

 ported by statistics on yields with related statistics on the total popula- 

 tions of fish that produced them. 



Maximum Yields and Length of Growing Season 



Thompson,^^ on the basis of digestive rates of fish at different tempera- 

 tures and on the lengths of warm seasons at various latitudes in North 

 America, calculated the maximum annual yields (based on theoretical 

 replacement of protein) that could be taken at latitudes from 46° N to 

 30° N. These ranged from 21 per cent of the carrying capacity in northern 

 Wisconsin to 118 per cent in southern Louisiana. Thompson's figure for 

 central Illinois was 50 per cent of the carrying capacity. Cropping tests 

 at Fork Lake (central Illinois), a small pond of 1.38 acres, appear to 

 substantiate a 50 per cent yield potential. ^^ Here the 1939 catch of bass 

 and bluegills was equivalent to 934 fishes, or 162 pounds per acre, a yield 

 of "about half of the theoretical carrying capacity of the lake for hook- 

 and-line fish." These fishes were taken in 1-inch mesh wing nets and by 

 hook-and-line, and all were removed from the lake regardless of size. In 

 1940 and 1941 the yield was reduced in spite of more intensive net 

 fishing,"* but the reduction in fish was attributed to the trapping of nutrient 

 materials by dense stands of submersed rooted vegetation. 



