Fishery bulletin of the Fish and Wildlife Service - NOAA

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 88 failed to give a clue to the causé of two zones of concentration of
whitefish. Both the inshore and offshore concentrations of legal fish were below the
thermocline 8 * in June, July, and August and hence were in a region with extremely
small temperature gradients. Preferences for water of different temperature, therefore,
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 wa'ter.
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 exceptionally
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 evidence
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 deepwater
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 appendix
B) demonstrate a close correlation between the annual fluctuations in the
» Temperature data were not available trom the north channel (region north of the Beaver bland 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 bland and
southeast of Manietique suggests that the data from these localities may'be indicative of conditions in the area in which the deep-trap-net fishery
was centered.
» The average positions of the tbermoelina were: last half of June, 21-33 feet; July, 67-77 feet; August, 69-80 feet; first 10 days of September,
7Z-8Í feet. The thenuoolme h&d not yet formed in the first half of June; no readings were made in the area after September 10.