FISHERIES IN MICHIGAN WATERS OF GREEN BAY 



23 



be had from the records of the catch per unit 

 effort for the months January to April (table 17) . 

 Particular attention should be given to the figures 

 for January, February, and March, the 3 months 

 of highest production (tables 7 and 8). Annual 

 fluctuations in the catch per net during these 

 months exhibited no pronounced trend prior to 

 1943. In that year the January-February catches 

 of gill nets and the January catches of pound nets 

 gave no inkling of the unusual events to come, but 

 in February the catch of smelt per pound net was 

 the lowest since 1939. This decrease is to be asso- 

 ciated with the mortality which Van Oosten 

 (1947) believed to have started about the middle 

 of February. By March 1943 the fishery was in 

 a state of collapse. Almost no smelt were taken 

 in 1944 and production (table 4) and catch per 

 net (table 17) both were low in 1945 and 1946. 

 In 1947-49 both production and the catch per unit 

 effort exhibited an upward trend that bids fair to 

 carry the fishery soon to the premortality level. 



Examination of table 10 gives strong indication 

 that the annual fluctuations in availability of cer- 

 tain species tended to be similar, whereas with 

 others the trends were distinctly opposite. To 

 bring out these relationships more clearly coeffi- 

 cients of correlation (r) between abundance per- 

 centages have been computed for all pairings of 

 the six species (table 18). In these calculations 

 all coefficients involving lake trout were restricted 

 to the 15-year period, 1929-43, since it is believed 

 that the abundance of that species has not followed 

 "natural" fluctuations in recent years but rather 

 has been controlled by depredations of the sea 

 lamprey. All other coefficients were based on the 

 21-year interval. 



Of the 15 coefficients listed in table 18, 7 were 

 significant at the 5-percent level of probability 

 (p) ; 6 of these 7 were "highly significant" 

 (p<0.01). Thus we have strong evidence that 

 the fluctuations in abundance of several of the 

 species were in fact correlated. It would be futile 

 at this time to speculate how these relationships 

 came about — whether they represent interreactions 

 between species, similar or opposite reactions to 

 changing ecological conditions . . . Before we 

 can hope to improve greatly our understanding of 

 the changes within the fish populations in northern 

 Green Bay we must increase our knowledge of the 

 biology of the various species; nevertheless, data 



of the type given in table 18 can be most helpful 

 by suggesting lines of attack in the general re- 

 search program. 



Table 18. — Correlations between, fluctuations in abun^ 

 dance of lake trout and five other species, 1929-43, and 

 among species other than lake trout, 1929-$ 



[Absolute values of r corresponding to probabilities (p) of 0.1, 0.05, 0.02, 0.01, 

 and 0.001, respectively, are 0.369, 0.433, 0.503, 0.549, and 0.665 for the 21-year 

 interval and 0.441, 0.514, 0.592, 0.641, and 0.760 for the 15-year period] 



Still further useful information can be had 

 through an investigation of the correlations be- 

 tween abundance percentages for intervals shorter 

 than the entire period for which data are at hand 

 or after the establishment of a time lag of one or 

 more years. This latter procedure can be justified 

 logically since fish of different species hatched in 

 the same year commonly do not enter the fishery 

 simultaneously, and an abundance of large fish of 

 a predator species may reduce stocks of prey 

 species, . . . Examples of the results obtained 

 from this type of analysis (table 19) bring out 

 some interesting relationships. We have evidence, 

 for example, that the correlation between the 

 fluctuations in availability of whitefish and wall- 

 eyes was negative in 1929-38, but that this situa- 

 tion was reversed in 1939—47 over which period the 

 correlation was strongly positive. Equally strik- 

 ing are the data for the lake herring. Fluctuations 

 in the abundance of this species were not corre- 

 lated with those of other species when indices for 

 the same calendar year were paired but exhibited 

 significant positive correlations with the abun- 

 dance of walleyes 1, 2, or 3 years later or 1 or 2 years 

 earlier, and significant negative correlation with 

 the abundance of suckers 1 year later. Again, the 

 expansion of the type of analyses illustrated in 

 table 19 and the inquiry into the possible causes 

 underlying the observed relationships must await 

 further investigation of the natural histories of the 

 various species. 



