356 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



ments of these two concentration zones are such as to label their existence as a real 

 phenomenon, and not a chance result to be ascribed to inadequate data. However, the 

 mere presence of two distinct groupings of whitefish throughout all or most of the 

 season does not make absolutely necessary the assumption of two permanently separated 

 stocks. It is possible that conditions within the lake at certain seasons may produce 

 an "ecological division" of an otherwise homogeneous population. 



Records of a number of vertical series of temperature readings made in northeastern 

 Lake Michigan 33 failed to give a clue to the cause of two zones of concentration of 

 whitefish. Both the inshore and offshore concentrations of legal fish were below the 

 thermocline 3 * in June, July, and August and hence were in a region with extremely 

 small temperature gradients. Preferences for water of different temperature, there- 

 fore, do not provide a logical explanation for the presence of two concentrations. The 

 illegal whitefish of the inshore concentration were in the region of the thermocline in 

 July and August, hence in substantially warmer water than were the fish of the offshore 

 concentration. However, both groups were below the thermocline in June, and an 

 inshore concentration at the thermocline was lacking in September. 



Important arguments in support of the assumption of the existence of inshore and 

 deep-water populations of whitefish are: 



(1) The separation into two groups involved both large (legal) and small (illegal) 

 fish. Consequently, the two groups are not entirely the result of different reactions of 

 fish of different size to the same or similar environmental factors. This statement 

 holds even though the concentration zones of the legal and illegal fish were not always 

 identical in the same month. 



(2) The fish of both concentration areas have similar seasonal vertical movements. 

 The similarity of vertical movements kept the two zones of concentration distinct in all 

 months but September. The presence of only one peak in the September data may 

 represent the temporary approximation of the two concentrations or may be the result 

 of lack of information on the distribution of whitefish beyond the 110-foot contour. 



(3) There is evidence that some whitefish seldom, if ever, spawn in shallow water. 

 The introduction of the deep trap net on gill-net grounds or in areas beyond the reach 

 of pound nets was marked by the capture of considerable numbers of whitefish of ex- 

 ceptionally large size. These large fish could not be taken on these same grounds by 

 the gill nets commonly employed since their great size prevented their becoming gilled. 

 Pound nets, which are selective only with respect to small fish, are fully capable of 

 taking large individuals of any size. Consequently, their failure to capture many fish 

 as large as those found in the early catches of the deep trap nets may be taken as evi- 

 dence that these giant individuals were seldom, if ever, present on the inshore pound-net 

 grounds, at least during the period of fishing operation. 



It must be remembered, nevertheless, that there is no proof that the smaller mature 

 fish of the offshore group of whitefish do not spawn in shallow water. The separation 

 of the whitefish into two depth groups may represent only a summer and early-autumn 

 condition. Possibly most of the small fish of both groups spawn in shallow water and 

 most of the large fish of both groups spawn in deeper water. However, it also seems 

 logical to hold that the giant fish taken in deep trap nets were members of a deep- 

 water population (that lived beyond the reach of pound nets) that had survived to a 

 size at which they could not be taken in gill nets, and hence had become exempt from 

 capture in the commercial fishery. 



Even if the inshore and offshore groups of whitefish are held to be semi-independent 

 or independent, it must be recognized that both groups exhibit similar fluctuations in the 

 fishery. The records of the catch per lift and of production in M-3 (table 17 and ap- 

 pendix B) demonstrate a close correlation between the annual fluctuations in the 



a Temperature data were uot available from the north channel (region north of the Beaver Island archipelago), the center of the deep-trap-net 

 fishery However, the relatively limited local variation in temperature conditions at stations southeast, south, and northwest of Beaver Island and 

 southeast of Manistique suggests that the data from these localities may be indicative of conditions in the area in which the deep-trap-net turnery 

 was centered. 



« The average positions of the thermocline were: last half of June, 24-33 feet; July, 67-77 feet: August, 69-80 feet; first 10 days of September. 

 72-86 feet. The thermocline had not yet formed in the first half of June; no readings were made in the area after September IU. 



