Fishery bulletin of the Fish and Wildlife Service - NOAA

316 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE
The fact that in all districts but H-2 the deep trap net was not fished throughout
the entire "period of reference" (1929-1934) introduced certain difficulties into the
estimation of abundance. For example, the deep trap net was operated in H-l during
only 5 years (1930-1934) of this 6-year period. The average catch of whitefish per
lift of deep trap nets in the years, 1930-1934, was 111.08 pounds. However, the data
for large-mesh gill nets and pound nets indicated that the 1930-1934 abundance averaged
only 99.12 percent of the 1929-1934 mean. Consequently, the average catch per
lift of deep trap nets would have been higher had the gear been fished in 1929 also. It
was necessary, therefore, to base the computations of the expected catch (p. 314) of deep
trap nets on the "corrected" catch per lift, 111.08/0.9912=112.07 pounds.
Although this method of "correcting" the average catch per lift of deep trap nets
(in some districts the data for pound nets had to be treated similarly) is sound logically,
the actual reliability of the results is open to question in some districts in which the rise
of the deep-trap-net fishery was accompanied by the practical extinction of the gill-net
and. pound-net fisheries (for whitefish). The correction was based, for example, on the
data for only 3 years in H-3 and H-5 and for 2 years in H-6. "The difficulties involved
in following annual changes in abundance in areas in which the deep trap net replaced
other types of gears completely or nearly completely will be mentioned again on page "328.
The deep trap net was important also in Green Bay and northern Lake Michigan,
but the disturbance of the fishery was not as severe as in Lake Huron.
Although the greatest need for dependable statistical data existed with respect to
those districts in which the deep trap net became almost the only gear that produced
whitefish, it was for precisely these areas that the original data were least trustworthy.
This lack of dependability had its origin in the extensive inaccuracies and misstatements
of fact known to have occurred in the reports of numerous deep-trap-net fishermen.
This observation is not intended as an indictment of any fisherman or group of fishermen.
Nevertheless, the fact that these inaccuracies existed cannot well be ignored.
To discuss changes in abundance computed from deep-trap-net data without giving some
idea as to their degree of dependability would be misleading. Misstatements were found
in the reports of deep-trap-net fishermen as to the type of gear fished, the numbers
of nets lifted, and the size of the catch.
Numerous deep-trap-net reports were indicated erroneously to be reports of poundnet
operations. Most of the errors of this type were made by operators in the Saginaw
Bay region in 1931 and in both the Saginaw Bay and Harbor Beach regions in 1932. In
other years and in other districts the designation of deep trap nets as pound nets was
much less frequent. Without naming sources of information or explaining the procedure
followed, it may be stated that we are certain that we have detected and corrected
practically all, if not all, of the misstatements as to the type; of gear. Consequently,
this originally serious source of error does not affect materially the data of this
paper.
It has not been possible to correct the inaccuracies of data as to the number of nets
lifted and the size of the catch, nor is there any basis for a good estimate of the extent
of these inaccuracies. Where there was opportunity of comparing actual and reported
data the discrepancies were sometimes appalling. Some fishermen not only reported
incorrectly the number of nets lifted but gave dates of lifting that did not coincide with
the dates on which they actually left port. The reported catches were often understatements.
The extreme in this type of misrepresentation is offered by the report of an
operator who is known to have taken more fish in a single day than he reported for the
entire month. It must be considered highly probable that the actual total production of
whitefish in deep trap nets was far above that recorded in this study.
In calling attention to the defects in the deep-trap-net data it is not intended to
imply that all operators of deep trap nets submitted erroneous and carelessly prepared
reports. There is good evidence that many of them prepared scrupulously accurate accounts
of operation and of catch. 'Although the number of inaccurate reports may be
sufficient to invalidate the deep-trap-net data as descriptive of details, these data still
serve satisfactorily to indicate the trends of the fisheries in the different districts. This
view finds support in the fact that for the whitefish as well as for other species there