The second major change in chub gill 

 nets was the conversion to nylon twine. Ac- 

 cording to the best records we can obtain, the 

 first nylon nets were fished in Lake Michigan 

 (Wisconsin and Indiana) in 1946 . Some fishing 

 was being done with them from a majority of 

 ports in all areas of the lake by 1948 and 1949, 

 and the changeover was complete or nearly so 

 in most ports by 1952 or 1953 . Despite its 

 current general use nylon twine has met resist- 

 ance . Some fishermen reverted to cotton after 

 trying nylon . At one port 50 percent of the chub 

 nets lifted were still cotton as recently as 1954. 



It is impossible to offer a definite estim- 

 ate of the relative efficiencies of nylon and 

 cotton chub nets . The opinions of fishermen 

 vary widely but a "median" estimate probably 

 would place the nylon nets as between 2 and 3 

 times as efficient as cotton. The actual ratio 

 doubtless depends on such factors as size and 

 flexibility of thread, method of hanging, and 

 kinds of fish on the grounds. A major disadvan- 

 tage of nylon nets is their embarrassing capacity 

 for taking "trash" --bloaters and small chubs of 

 other species . The early difficulties with trash 

 fish were eased somewhat by hanging the nets 

 more open than the one -half basis 11 / that was 

 usual with cotton and linen twine. Benefits from 

 this change in method of hanging seem to be 

 lessened, however, by an increasingly higher 

 percentage of small chubs, especially bloaters, 

 on the grounds . 



In the study of fluctuations in "abundance" 

 it is important not only to keep in mind the 

 sources of bias just outlined but also to remem- 

 ber that the records of catch and the indices of 

 availability computed from them are based on a 

 group of species of changing composition. 



State of Michigan, 1929-1953 . - -The in- 

 dices of abundance (availability) in Michigan 

 districts (table 8) were computed from records 

 of the annual average catch per lift of small - 

 mesh gill nets that took chubs (table 9; the orig- 

 inal file records carried one additional signifi- 

 cant figure) . The fluctuations of abundance are 

 shown graphically in figures 3-9. 



11 / A net being "on the half' has 2 feet of 

 netting, flat mesh, on 1 foot of maitre . 



Fishing success varied widely in indiv- 

 idual districts of State of Michigan waters in 

 the 25 years, 1929-1953. The highest abund- 

 ance index for chubs in individual districts 

 ranged from 2.1 (M-3) to 5.2 (M-8) times the 

 lowest (table 10). For the combined districts 

 the highest abundance (200 in 1952) was 2.6 

 times the lowest (78 in 1940). Minimum avail- 

 ability occurred in all districts within the period 

 1936-1942. Maximum abundance was reached 

 in 1929 in M-4 but in the other districts it occurred 

 in 1944, 1945, 1951, or 1952. The indices of 264 

 in M-7 in 1945 and 299 in M-8 in 1944- -highest 

 values in any district- -are especially significant 

 because these peaks were attained without the 

 use of nylon nets. Nylon of course did not con- 

 tribute to the M-4 maximum (1929) but this twine 

 unquestionably was important in the remaining 

 districts in which the best fishing occurred in 

 1951 or 1952. The lowest as well as the highest 

 abundance indices were in M-7 (56 in 1937) and 

 M-8 (57 in 1940). 



For comparisons between districts of 

 the fluctuations in the abundance index it is be- 

 lieved desirable to divide the 25 years into the 

 sub-periods 1929-1943 and 1944-1953. The 

 generally high level of yield per unit effort in 

 1944-1953 does not reflect the type of "natural 

 fluctuation" in abundance in which we are primarily 

 interested. The statistics for the later years, 

 beginning in some districts as early as 1946, were 

 affected by the gradual introduction and (in most 

 areas) ultimate dominance of the relatively 

 efficient nylon twine . Nylon nets had no effect 

 on the statistics of the earlier years of the 10- 

 year period, but the declining abundance of lake 

 trout probably did since chubs are the principal 

 food of the trout (Van Oosten and Deason, 1938) . 

 Even though the bloater, of limited value com- 

 mercially, benefited most from the disappear- 

 ance of lake trout, the abundance of other species 

 also probably was affected. The data given by 

 Hile, Eschmeyer, and Lunger (1951a) indicate 

 that the decline in the abundance of lake trout that 

 led to the collapse of the trout fishery was under 

 way in M-2, M-3, and M-4 before the middle 1940' s. 

 The abundance of lake trout did not once equal 

 the 1929-1943 average in M-2 after 1941, in M-3 

 after 1943, or in M-4 after 1944. The first in 

 the series of record-low levels of availability of 



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