DECLINE OF LAKE TROUT IN LAKE MICHIGAN 



93 



5.000 F 



a 



z 



o 



a 

 z 

 < 



D 

 O 



X 



o 

 O 



3,000 - 



2.000 — 



1,000 — 



1930 



1935 



1940 



1945 



Figure 6. — Production, abundance inde.x, and fishing-intensity inde.x for lake trout in combined districts of State of 

 Michigan waters, 1929-49. Solid line = production; Iohr dashes = abundance index; short dashes = fishing-intensity 

 index. Scale at left (thousands of pounds) applies to production only; scale at right is in terras of 1929-43 mean for 

 each item. 



offer no evidence for a correlation between the pro- 

 duction and abundance of lake trout in M-1 and 

 M-4. A negative correlation between abundance 

 and fishing intensity in each of the two districts in 

 1929-^1 (see preceding section) imquestionably 

 was a major disturbing influence. 



From the values of the coefficients for 1929-^1 

 and/or 1929-43 it appears that production served 

 as a more or less reliable indicator of at least the 

 more significant fluctuations of abundance in five 

 of eight districts and in the lake as a whole, was 

 of highly limited value in one district and was com- 

 pletely undependable in two (see figs. 3, 4, 5, and 

 6). The failm-e of production and abundance to 

 follow similar courses in M-1 and M-4 (to a con- 

 siderable extent in M-3 also) brings out the impor- 

 tance of being constantly alert to identify and, if 

 possible, evaluate distmbing factors in the use of 

 production figures for detecting changes in abun- 

 dance. It should be stressed also that catch statis- 

 tics should be employed only to detect changes of 

 abundance and not as measures of those changes. 



The coefficients of correlation for 1929-49 had 

 high positive values — far beyond the level accepted 

 as higlily significant — in districts M-2 through 

 M-8 and in the combined districts. District M-1 , 

 where abnormally intensive fishing kept produc- 

 tion high in later years, offered the single excep- 

 tion. These high values for districts M-2 through 

 M-8 can be attributed to the enormous declines 

 in both production and abundance that occiured 

 in the later years of the period. Too much should 

 not be made of the high coefficients for 1929-49 as 



an argmnent for the value of production statistics 

 for following trends of availabihtj'. When a 

 fishery suffers a decline as disastrous as the one 

 that has overtaken the lake-trout fishery of Lake 

 Michigan, statistical analyses are hardly required 

 to prove that fish are too scarce to support com- 

 mercial operations. 



During the years of the decline in the lake-trout 

 fisheries of Lakes Huron and Michigan we heard 

 the opinion expressed both privately and publicly 

 that the sea lamprey had not contributed signifi- 

 cantly to the collapse, that the stocks of lake trout 

 simply had dwindled away under the pressure of 

 overfishing, that the distress of the fishing industry 

 was but just retribution for a wanton despoliation 

 of a valuable public resource. The facts given in 

 an earlier study of the lake-trout fishery of Lake 

 Huron (Hile 1949) demonstrated rather conclu- 

 sively that excessive fishing intensity could not 

 have brought about the collapse of the fisheiy in 

 the United States waters of that lake. Corre- 

 sponding data for the State of Michigan portion 

 of Lake Michigan compel a similar conclusion for 

 the lake-trout fishery of those waters. 



The data of table 16 (see also fig. 6) fail com- 

 pletely to show a level of fishing intensity that 

 would account for the recent decline in the lake- 

 trout fishery of Lake Michigan. On the contrary, 

 the most intensive fishing operations of the 21-year 

 period, 1929^9, occurred m 1930-32 (112 to 122 

 percent of the 1929-43 mean — figures that do not 

 indicate excessive fishing even at that time) 

 whereas in the later years fishing intensity has 



