FISHERIES IN MICHIGAN WATERS OF GREEN BAY 



21 



Table 17. — Catch, in pounds, of smelt per unit of effort 

 in Qreen Bay, by gear, in each of the months, January 

 to April, 1938-49 



1 Mesh sizes mostly l^i to 1H inches, extension measure. 



Comparison of the annual fluctuations in the 

 catch of a particular species per unit effort of dif- 

 ferent gears reveals that major improvements or 

 declines in the success of fishing commonly were 

 shared by the principal types of nets 12 but that 

 certain discrepancies occurred. Numerous ex- 

 amples can be found in which the catch per lift of 

 one gear increased over that of the previous year 

 while the catch of another gear exhibited a decline. 

 Furthermore, the relative increases or decreases of 

 different gears were often dissimilar even when 

 there was agreement as to direction of change. 

 Some of these discrepancies doubtless reflect inad- 

 equacies (and to some extent inaccuracies) of the 

 original data. On the other hand, extremely close 

 agreement between the annual fluctuations in the 

 catch per unit of effort of a species in different 

 gears was not to be expected. 



One important source of discrepancies between 

 trends in the catch of different gears most probably 

 lies in annual differences in the distribution of lisli 

 as related to hydrographic and other ecological 

 conditions. One year these conditions may tend to 

 concentrate the fish on grounds fished principally 

 by one gear and cause them to be scarce on the 

 major grounds of another, whereas the next season 

 the situation may be reversed. AVe are not in posi- 

 tion to offer quantitative information on this point, 

 but we do know from general observations on the 

 fishery and from statements of fishermen that dif- 



12 The terms "gear" and "nets" are used interchangeably in this 

 section since all fishing apparatus with which we are concerned 

 fall under the category of "nets" except the set hooks formerly 

 fished for lake trout. 



ferent gears fished on different grounds do not 

 share equally increases or decreases in the abun- 

 dance of fish. 



A second factor contributing to the discrepan- 

 cies between trends in the catch of different gears 

 is that some gears operate most effectively over 

 certain size ranges. Thus a progressive change in 

 the size composition of the stock may operate to the 

 advantage of first one and then another type of 

 net. Records for the lake herring (table 13) pro- 

 vide a good example of this type of disagreement. 

 From 1029 through the middle 1930's, fluctuations 

 in the catch of herring per unit effort of small-mesh 

 gill nets and pound nets, although by no means 

 identical, were generally similar. This situation 

 changed during the late 1930's and early 1940's 

 with the development and widespread use of pound 

 nets with extremely small meshes (about 1% inches 

 extension measure, as manufactured, and still 

 smaller after treatment with preservative). This 

 new type of pound net was designed for smelt but 

 proved so efficient at taking small or "pin" herring, 

 that with its general use changes in the abundance 

 of that species became detectable in the pound-net 

 catches before they were noticed by gill netters 

 who take larger fish. The relatively poor pound- 

 net lifts of 1940-42, for example, were followed by 

 poor gill-net fishing in 1943-46. Again, the rich 

 1943 year class led to large catches of "pins" as 

 early as 1945 whereas the catch per unit effort of 

 gill nets did not rise sharply until 1947. In this 

 situation both gears probably offered fair indica- 

 tion of the abundance of fish of the size they took 

 but they fished different size groups within the 

 general population. 



From the considerations of the preceding two 

 paragraphs it appears first, that discrepancies be- 

 tween gears in the annual fluctuations in the catch 

 per lift of a particular species do not necessarily 

 mean weakness of the data; and second, that the 

 procedure followed in our statistical studies of 

 pooling the data from different gears to obtain 

 our abundance index probably gives the best esti- 

 mate of abundance of fish of commercial size to be 

 had at the present time. 



The abundance percentages for all six species 

 listed in table 10 exhibited rather wide fluctua- 

 tions and in some species these fluctuations tended 

 to be periodic. The following brief statements 

 concerning trends for the different species can be 



