of the differences between abundance on the 

 east and west shores in 1930-1932 can be drawn 

 from the apparently similar abundance in 1953 

 after years of exploitation with the same kind of 

 nets in Michigan and Wisconsin . 



Intensity of the chub fishery in Lake Michigan, 

 1929-1953 



State of Michigan, 1929-1953 . - -Statistics 

 on the intensity of the chub fishery in the Michi - 

 gan districts are presented in three forms . 



First is the record of the number of 

 units of 10, 000 linear feet of small-mesh gill 

 nets lifted that took chubs (table 14). This 

 "effective fishing effort" excludes all or prac- 

 tically all small -mesh nets fished in shallow 

 water for such species as lake herring, yellow 

 perch (Perca flavescens ), and round whitefish 

 since chubs rarely enter the area in which those 

 fisheries are carried on. The exclusion of lifts 

 that took no chubs does not, on the other hand, 

 bias the statistics by the omission of nets that 

 were set on chub grounds but failed to make a 

 catch . The distribution of chubs is such that 

 nets fished at the proper depth always take some 

 chubs . The records of table 14 have the further 

 advantage that the nets were set exclusively for 

 chubs --that is, without expectation of taking 

 significant quantities of other species . 



Because comments on fluctuations in fish- 

 ing intensity and comparisons of districts are 

 better made from later tabulations, the data of 

 table 14 need be discussed only briefly. Most 

 noteworthy information in the table, perhaps, is 

 that on the actual magnitude of the fishery. The 

 intensity unit of 10, 000 feet corresponds to the 

 lifting of 1 .894 miles of gill nets. With this 

 factor some more meaningful figures can be com- 

 puted. It is seen, for example, that within 

 individual districts the total lifts for an entire 

 year were sometimes extremely small- -only 70 

 miles in M-2 in 1943 and but 76 miles in M-8 in 

 1929. Other total annual lifts were large- -3, 941 

 miles in M-3 in 1938 and 4, 591 miles in M-8 in 

 1953 . For the combined districts the total length 

 of chub gill nets lifted ranged from 2, 752 miles 

 in 1944 to 16, 196 miles in 1953. The average 

 number of miles of chub gill nets lifted per year 

 in the 7 districts was 6, 905 in 1929-1943 and 



10, 280 in 1944-1953. 



The second set of statistics on fishing 

 intensity (table 15) figs. 3-9) concerns the per- 

 centage fluctuations of fishing pressure in the 

 individual districts about the 1929-1943 mean. 

 The indices of table 15 are closely similar to 

 the figures that would be obtained if percentage 

 fluctuations were computed from the records for 

 small-mesh gill nets in table 14 but are not 

 identical since adjustments were made to take 

 account of the small catches of chubs from gears 

 other than small-mesh gill nets. This adjustment 

 is made easily by expressing fishing intensity as 

 an "expected catch" in pounds of fish. The pro- 

 cedure is illustrated by the following file records 

 for district M-7 in 1947- 



Production (pounds) 

 In small-mesh gill nets . . .418,316 

 In other gear (large-mesh 



gill nets) . . 5,513 



Total 423,829 



Gill -net units lifted (small 



mesh) 1,225.9 



Catch (pounds) per unit in 1947 



(418,316/1,225.9). . . 341.2 



Catch (pounds) per unit, 



1929-1943 average . . 186.7 



Abundance index (341.2/186.7 



x 100) 182.8 



Expected catch (423, 829/ 1.828) 



231,858 



The "expected catch" is merely the pounds of 

 chubs that would have been caught by the amount 

 of fishing done in M-7 in 1947 if the abundance 

 of chubs had been exactly at the 1929-1943 mean 

 of 100.0 rather than 182.8 percent of average. 

 In making this computation it is assumed that 

 chubs captured in chub gill nets and in other gears 

 experience the same fluctuations of availability . 

 The expected catches computed for each year by 

 this procedure were the statistics from which the 

 intensity indices of table 15 were calculated. 



The intensity of the chub fishery varied 

 widely in all statistical districts (table 16). The 

 ratio of the maximum to the minimum intensity 

 ranged from 8.9 in M-7 to 62.9 in M-8. The 

 greatest fishing pressure was exerted in 1935 in 

 3 districts (M-4, M-5, M-6), in 1938 in 1 (M-3), 



34 



