DECLINE OF LAKE TROUT IN LAKE MICHIGAN 



89 



Table 10. — Catch of lake trout per lift of large-mesh gill nets 

 in Michigan statistical districts, 1929-49 



Table 12. — Catch of lake trout per lift of pound nets in 

 Michigan statistical districts, 1929-49 



(In pounds per lift of 1 pound net. Where no figures are given, few or no lake 

 trout were talcen with this gear] 



Table 11. — Catch of lake trout per lift of set hooks in 

 Michigan statistical districts, 1929-49 



[In pounds per lift of 1 .000 set hooks. Where no figures are given, few or no 

 lake trout were taken with this gear] 



' For each district for which data are not given for 1 or more years, the 

 15-year average is estimated by dividing the mean of the available annual 

 averages by the mean of the abundance percentages for the same years. See 

 Van Oosten, Hile, and Jobes (1946) for comments on the estimation of a 

 normal catch when data are not available for all years. 



" No fishing with set hooks in 1949. 



diflFerences are to be seen between the northern 

 and southern areas of the lake. Of the 12 "high- 

 abundance" years listed for districts M-1 through 

 M-4,* 9 were earlier than 1940 and only 1 was 



» District M-1, assigned to the southern districts in the grouping with 

 respect to production, has been assigned to the northern with respect to years 

 of greatest abundance. 



' For each district for which data are not given for 1 or more years, the 15- 

 year average is estimated by dividing the mean of the available annual aver- 

 ages by the mean of the abundance percentages for the same years. See 

 Van Oosten, Hile. and Jobes (1946) for comments on the estimation of a nor- 

 mal catch when data are not available for all years. 



later than 1941. The corresponding record for 

 districts M-5 through M-8, on the contrary, 

 shows all 12 years within the period 1940-44 

 and 9 within the still-shorter interval 1941—43. 



Although the recent progi-essive decline in 

 abundance appears to have started at much the 

 same time in all districts (1942 in M-7, 1943 in 

 M-8, and 1944 in all other districts) it proceeded 

 much more rapidly in northerly than in southerly 

 waters. The last year with abundance at average 

 or greater was 1941 in M-2, 1943 m M-1 and M-3, 

 1944 in M-4, 1945 m M-5 and M-6, and 1947 in 

 M-7 and M-8. The same north-to-south sequence 

 exists in the first year in which abimdance dropped 

 below the 70-percent level,' 1944 in M-1 and 

 M-2, 1946 in M-3, 1947 in M^, 1948 m M-5, 

 and 1949 in M-6, M-7, and M-8. This north- 

 to-south progression resembles closely that de- 

 scribed for production in the preceding section. 

 The situation invites speculation about the 

 possibility that a southward spread of the sea 

 lamprey was a contributmg factor. 



Despite the differences in timing just described, 

 the districts agreed in that all showed an ex- 

 tremely low level of availability of lake trout in 

 1949 (range of abimdance percentages from 45 in 

 M-1 down to 12 m M-3). Admittedly, the 

 dependability of the estimates of abundance 

 decreases rapidly as production falls to low levels. 

 Nevertheless, the consistently low returns per 



• The 70-percent level Is considered preferable here to the 50-percent flgiu^ 

 employed for analogous items In the data for production (and for fishing 

 hitensity, discussed later). Usually the fishery has all but disappeared by 

 the time the 60-porcent level of abundance is reached. 



