24 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 19. — Correlations between annual fluctuations in 

 abundance of fishes of northern Green Bap during 

 specified intervals of time 



Abundance indices correlated 



Whitefish: Walleve 



Whitefish: Walleye. 



Whitefish: Yellow perch (1 year later). 

 Lake herring: Whitefish (1 year later). 

 Lake herring: Walleye (1 year later).. 

 Lake herring: Walleye (2 years later).. 

 Lake herring: Walleye '3 years later)_ 

 Lake herring: Suckers (1 year later) . 

 Walleye: Lake herring (1 year later).. 

 Walleye: Lake herring (2 ycarslater). 



0.05>p>0.02 

 0.01>p>0.001 

 0. 02>p>0. 01 

 0.001>p 

 0.05>p>0. 02 

 0. 01>p>0.001 

 0.01>p>0.001 

 0. 05>p>0. 02 

 0.05>p>0.02 

 0.02>p>0.01 



As part of recent statistical studies of the lake 

 trout fisheries of the Great Lakes, inquiries have 

 been made into the dependability of production 

 statistics as indicators (but not as measures) of 

 fluctuations in abundance. Because of its bearing 

 on the use of past data on production for judg- 

 ing changes of abundance that may have taken 

 place, the accumulation of information on the de- 

 pendability of estimation of abundance from pro- 

 duction statistics is desirable. For no other waters 

 of the Great Lakes are statistics on fishing intensity 

 and, hence, on catch per unit effort available for 

 a period as long as that in the State of Michigan 

 (for all Great Lakes waters of the State beginning 

 with 1929) and for certain States the collection 

 of data on intensity of fishing began as recently 

 as 1950. Analyses made to date support the gen- 

 eral view that fluctuations of abundance are re- 

 flected in statistics of production but the exceptions 

 bring out the need for caution in the interpretation 

 of catch data and for a constant alertness to detect 

 disturbing factors that may render those data 

 useless or misleading. 13 



In northern Green Bay, fluctuations in produc- 

 tion and abundance were correlated positively at 

 significant levels (table 20) for four of six species 

 (the value of r for yellow perch fell short of the 

 5-percent level in 1929-43 but was highly signifi- 

 cant in 1929-49). For those fish, production 

 served reasonably well as an indicator of changes 

 in abundance. The lack of significant correlation 

 in the data for lake trout (to be traced to a nega- 

 tive correlation between fishing intensity and the 

 abundance of that species, see p. 25) demonstrates 



once more the need for caution in this use of catch 

 statistics. 



Table 20. — Correlation between production and abun- 

 dance indices for the principal commercial species in 

 Green Bay, 1929-1,3 and 1929-49 



FLUCTUATIONS IN INTENSITY OF THE 



FISHERY FOR THE PRINCIPAL SPECIES 



OF GREEN BAY, 1929-49 



An outstanding feature of the statistics on the 

 1929-49 fluctuations in the intensity of the fishery 

 for the principal species (table 21 and figs. 9 to 14) 

 is the high level attained by most of the species 

 during the later years of the period. For four 

 of the six species (lake trout, lake whitefish, wall- 

 eye," and suckers) , the 21-year maximum intensity 

 was reached in 1948 or 1949, and for a fifth (lake 

 herring) the intensity of the fishery in those 2 

 years was well above the 1929-43 mean. The in- 

 tensity of the fishery for yellow perch was higher 

 in 1948 and 1949 than in the years immediately 

 preceding but was still below the 15-year average. 



There is good evidence that the recent increase 

 of fishing pressure on whitefish, lake herring, and 

 walleye is to be associated with the nearly simul- 

 taneous rise in the abundance of those species (cf. 

 tables 10 and 21). The increases in fishing in- 

 tensity for lake trout and suckers, on the contrary, 

 came about during periods of relatively low avail- 

 ability. These two exceptions indicate that a 

 positive correlation between abundance and fishing 

 intensity may not be the rule; such a view finds 

 support in the data of table 22. 



13 See Hile, Eschmeyer. and Lunger (1951a) for comments on 

 the relation between fluctuations of abundance and production In 

 Great Lakes fisheries. 



" In the preceding section evidence was given that the estimates 

 of abundance of walleyes were probably too high for recent years 

 notably 1947-49. Our method of analysis is such that when 

 abundance Is overestimated, fishing intensity is underestimated 

 correspondingly. Consequently the 1947-49 figures on fishing 

 intensity for walleyes in table 21 are probably too low. It is not 

 believed, however, that these underestimates impair the general 

 validity of remarks in this section based on the intensity indices 

 for walleyes. 



