WHITEFISH FISHERY OF LAKES HURON AND MICHIGAN 313 



of the effects of the deep trap net on the seasonal trend of production in Lake Huron 

 (p. 332). 



UNITS OF FISHING EFFORT 



The units of fishing effort employed in this paper are: 



Gill nets.— The lift of 10,000 linear feet of net (10,000 foot-lifts). 17 

 Impounding nets (deep trap nets and pound nets). — The lift of one net (1 net- 

 lift). 



Fishing effort may be expressed as total effort and as effective effort (with respect 

 to a given species). In large-mesh gill nets, for example, the total effort for a given 

 area over a certain interval of time is the total number of units of 1,000 feet (see foot- 

 note 17) of net lifted. The effective effort with respect to whitefish is the number of 

 units of gill net lifted that actually took this species. Corresponding definitions of 

 total and effective effort apply to the pound net. A distinction between total and 

 effective effort is necessary because both large-mesh gill nets and pound nets are fished 

 for other species on grounds where whitefish do not occur. In the deep trap net, which 

 was designed and operated primarily for the capture of whitefish, the total fishing 

 effort and the effective effort with respect to this species may be considered identical. 

 All tabulations of catch per lift in this paper are based on effective fishing effort. 



In addition to the above "units of effort," the methods proposed by Hile and Duden 

 defined "units of intensity" which included a consideration of fishing time (nights out). 

 The intensity unit for gill nets was defined as the fishing effort of 1,000 feet of gill net 

 over a period of one day, and for impounding nets as the fishing effort of one net over 

 a period of one day. The basis for these definitions of intensity units was the assump- 

 tion that the amount of fishing done by stationary gear varies directly with the time 

 out. This assumption holds, for example, that a net which i- out three nights may be 

 expected to take three times as many fish as the same net in one night. 



Subsequent detailed analyses of hundreds of fishermen's reports made by Hile and 

 described briefly by him in 1935 18 and by Van Oosten (1935) 19 have proved this pre- 

 liminary assumption to be erroneous. Although the catches of both gill nets and im- 

 pounding nets, on the average, become larger with increase in fishing time, the improve- 

 ment in the catch is far less than might be expected on theoretical grounds. A summary 

 of the data on the actual relationship between fishing time and the average size of the 

 lift in the gears most important in the whitefish fishery appears in table 2. In this 

 table all catches are expressed as percentages of the catch of nets one night out. 

 Although the data for the three gears disagree somewhat as to the relationship between 

 the actual size of the catch and the number of nights out, these small discrepancies lose 

 significance in the face of the large deviations that all the actual catches show with 

 respect to the theoretical catches. For example, the largest increase in nets 2 nights out 

 over nets 1 night out (pound nets) was only 16 percent of the expected increment of 

 100. Similarly, the largest increase in the catch of nets 5 nights out over 1 night out 

 (54 in pound nets) was only 13.5 percent of the expected increment of 400. It is obvi- 

 ous, therefore, that only small increases in the catch can be expected as the time between 

 lifts is increased. Consequently, the use of the catch per net per night as a measure 

 of abundance is not valid. The strictly valid unit for the measure of abundance is 

 neither the catch per lift nor the catch per night, but is rather the catch per lift, cor- 

 rected for fishing time (from empirical data of the type contained in table 2). 



The necessity for considering fishing time in the computation of annual fluctua- 

 tions in abundance depends, of course, on the existence of annual variations in the 

 average number of nights out. Annual variations in fishing time occur in all areas and 

 for all stationary gears, but for a single area and a single type of gear these variations 

 have a limited and characteristic range. The limited range of variation in the average 

 number of nights out, together with the fact that a change in fishing time affects the 



17 The unit of effort was defined originally as the lift of 1,000 feet of gill nets. In the present study, however, the catch of gill nets has been 

 recorded in terms of the vield per 10.000 foot-lifts ("tables 11 and 17) in order to obtain values more nearly comparable with the catch per unit of 

 effort of pound nets and deep trap nets. 



>» The Fisherman, vol. 4, no. 12, pp. 1 and 2, 1935. 



19 Van Oosten, John. Logically Justified Deductions Concerning the Great Lakes Fisheries Exploded by Scientific Research. Trans. Am. 

 Fish. Soc, vol. 6.5. 1935, pp. 71-75. 



