314 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 2. — Relationship between fishing time and the average size of the lift 



£* [In order that the data for the different gears may be comparable, the catch per lift at one night out is set at 100 and all other catches 

 expressed as percentages of this value. In parentheses, the number of fishermen's reports upon which determination was based] 



■Less than preceding catch. 



size of the catch only slightly, suggested the possibility that abundance curves calculated 

 from the average catch per lift without reference to time might differ only slightly from 

 curves calculated from the average catch per lift, corrected for time. To test this possi- 

 bility a series of abundance curves for the period, 1929-1934, was computed by each of 

 the two methods, covering all types of stationary gear and a variety of species and fish- 

 ing areas. For each gear particular care was taken to select the statistical district with 

 the widest annual variation in the average fishing time. Despite this selection, in each 

 example the two curves resembled each other so closely that the same conclusions con- 

 cerning the annual changes in abundance would have been drawn from either of them. 

 As the neglect of the time element does not affect the results materially, all computa- 

 tions of abundance have been based on the catch per lift, without reference to time. 



ESTIMATION OF ABUNDANCE AND FISHING INTENSITY 



The method employed for the estimation of the abundance of species of fish in the 

 State of Michigan waters of the Great Lakes in different calendar years was outlined 

 by Hile (1937). 20 The definition of fishing intensity was given by Hile and Jobes 

 (1941). 21 The steps in the determination of the general abundance of a species within 

 a fishing area (statistical district) in a particular calendar year are: 



(1) The "expected catch" of each important gear is determined as the product of 

 known fishing intensity (number of impounding nets or thousands of feet of gill nets, 

 that took the species, lifted within the district during the entire 12 months) and the 

 average catch of that species per unit of fishing effort over a period of years. 



(2) The expected catches as determined individually for the important gears are 

 added to obtain the "total expected catch." 



(3) The "general abundance" is the actual catch of the important gears expressed 

 as a percentage of the total expected catch of the same gears. 



As the average catch per unit of effort is constant in all of the computations of 

 expected catch for a single type of gear, annual fluctuations in the expected catch by 

 each gear and hence for all gears combined depend only on the amount of gear lifted. 

 Consequently, the total expected catch of any single year, expressed as a percentage of 

 the average total expected catch over a period of years, provides a measure of the relative 

 intensity of the fishery in that particular year. 



The above procedures make it possible to combine the data for all gears important 

 for the capture of a particular species in such a way as to obtain estimates of the "gen- 

 eral" abundance and of the total fishing intensity. 



Originally all estimates of abundance and fishing intensity were made with reference 

 to average conditions over the 6-year period, 1929-1934. Later, the percentages were 

 adjusted to describe deviations about the mean for the 11 years, 1929-1939. 



w Hile, Ralph. The Increase in the Abundance of the Yellow Pike-Perch, StizoRtedion vitrrum (Mitchill), in Lakes Huron and Michigan, in Rela- 

 tion to the Artificial Propagation of the Species. Trans. Am. Pish. Soc, vol. 66, (1936) 1937, pp. 143-159. 



31 Hile, Ralph and Frank W. .lobes. Age, Growth, and Production of the Yellow Perch, Perca flavescenx (Mitchill), of Saginaw Bay. Trans- 

 Am. Fish. Soc, vol. 70, (1940) 1941, pp. 102-122. 



