Fishery bulletin of the Fish and Wildlife Service - NOAA

366 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE
each mesh size when the commercial yields are equal. This determination (third ro^v
in body of table) shows that large-mesh nets that take an average of 81.8 legal fish
may be expected to contain an average of 57.6 illegal individuals as compared with
77.7 undersized, whitefish in small-mesh nets with equal commercial lifts.
From the averages of 77.7 and 57.6 illegal whitefish per lift it may be estimated
77.7 — 57.6
that large-mesh nets released 100 X or 25.9 percent of the undersized
individuals. For every 100 illegal whitefish taken in small-mesh nets, 74.1 should
be taken by large-mesh nets with the same commercial catch.
The data of table 36 and the computations based upon them are open to the very
serious objection that the actual numbers and the percentages of legal and illegal fish
taken in nets of any size of mesh vary according to the nature of the stock at the place
and time the nets are fished. Truly discriminating data on selectivity must be founded
on the lifts of nets that are identical except for the size of mesh and that are fished
under strictly comparable conditions, that is, on the same grounds, at the same depth,
in the same year, and at the same time within the season.
Table 37 contains comparisons of the catch of large-mesh and small-mesh pound
nets and deep trap nets, based on lifts made in the same year (1932), in the same
month, on the same grounds, and at the same depth. The data are confined to comparisons
in which nets of .both sizes of mesh are represented : by at least, 5 lifts. The
necessary restrictions reduced the number of possible comparisons. However, the
averages of the 10 independent sets of observations are reasonably reliable.
TABLE 37.—Comparison of the numbers of legal and illegal whitefish per lift in smatt-mesh and large-mesh
pound nets and deep trap nets fished in the same year (1938) and month, on the same grounds, and at
comparable depths
Fisbing grounds
Northeastern Lake Michigan
Alpena-Oesineke
Corrected for equal commercial
Month
October....
August
June
July
August
August..-..
September..
May
June
July
Depth
(feet)
110
>110
>110
77.7
[Number of lifts in parentheses]
Number of whiteBsh per lift
in nets of mesh sue
Less than 4 inches
Legal Illegal
16.8 4.8
(6)
62.8 80.1
(П)
176.0 150.1
(15)
158.6 97.5
(10)
121.9 64.8
(W)
39.4 25.1
(8)
81.5 79.3
(6)
41.2 288.1
(20)
14.2 73.2
(18)
26.5 54.3
(14)
74.2 91.7
91.8 113.4
4 inches and more
Legal Illegal
22.3 2.9
(22)
150.4 148.2
(5): .
142.5 57.1
(8)
130.7 78.8
(в)
233.0 108.1
(8)
82.3 57.4
76.8 41.0
(8)
24.9 154.0
(7)
6.0 30.6
(5)
49.5 97.9
(8)
91.8 77.9
91.8 77.9
Percentage legal in nets of
mesh size
4 inches
77.8
43.9
54.0
61.9
65.3
61.1
50.7
12.5
19.0
32.8
44.7
4 inches and
more
The averages of t>he Ю comparisons show that the large-mesh pound nets and
deep trap nets took more legal whitefish and fewer illegal fish than the small-mesh
nets fished under comparable conditions. In round numbers, small-mesh nets took an
average of 92 undersized individuals in producing 74 fish of marketable size, whereas
large-mesh nets took only 78 illegal whitefish for a commercial production of. 92 fish.
The correction for equal commercial production shows that small-mesh pound nets
and deep trap nets with a commercial catch equal to that of large-mesh nets (92 fish)
88.5
50.4
71.4
62.4
68.3
58. В
63. в
13.9
16.4
33.6
54.1