Fishery bulletin of the Fish and Wildlife Service - NOAA

WHITEFISH FISHERY OF LAKES HURON AND MICHIGAN 379
distinct nevertheless in every month but September in the 5-month period, June to
October. The inshore and offshore concentrations of legal-sized whitefish were separated
by a difference in depth of 20 feet in each of the four months in which both were
present. The offshore concentration of illegal-sized whitefish was 30 feet deeper than
the inshore concentration in June, July, and August, but was only 20 feet deeper in
October.
13. The persistent occurrence of two concentration zones of whitefish in northeastern
Lake Michigan throughout most of the summer and early autumn raises the question
of the possible existence of distinct inshore and offshore populations or races.
Arguments were outlined briefly for and against this interpretation of the two concentrations;
available data do not, however, permit a definite decision.
14. On the basis of the observations on the bathymétrie distribution of whitefish,
it was suggested that young fish would be protected from excessive handling and possible
destruction and legal-sized fish from ruinous exploitation if the operations of deep trap
nets were limited in Lake Huron to depths of 80 feet and less and in Lake Michigan to
depths of 70 feet and less. The proposed restriction has been effective in Lake Huron
since August 1, 1934; the use of deep trap nets was made illegal in Lake Michigan after
1935. 15. A limited amount of information was presented on the bathymétrie distribution
and seasonal movements of the lake trout, yellow pike, burbot, white sucker, and longnosed
or sturgeon sucker.
16. Comparisons of the average numbers of fish per lift of large-mesh (meshes of
4 inches or more, extension measure, in the pot) and small-mesh (less than 4 inches)
pound nets and deep trap nets operated under comparable conditions (on the same
grounds, in the same calendar year and month, and in the same depth of water) revealed
that in general the large-mesh nets took the greater numbers of legal-sized whitefish
and the lesser numbers of illegal-sized individuals. Large-mesh nets took 31.3
percent fewer undersized whitefish than did small-mesh nets that captured an equal
number of legal-sized fish. Further evidence for the escape of undersized whitefish
from the nets with larger mesh sizes was provided by the regular increase, with increase
in the size of mesh, in the average length and weight of illegal-sized whitefish captured
in pound nets and deep trap nets or gilled in the meshes of the lifting pot. On the basis
of the selectivity data a minimum mesh size of 4V2 inches or greater (extension measure
as found in use) in the pots was recommended for pound, nets and deep trap nets
employed for the capture of whitefish and lake trout. (This size of mesh is prescribed
by the present State of Michigan law.) Although the data indicated that meshes of
4% inches or more will permit the escape of the smaller legal-sized lake trout, a smaller
mesh cannot be recommended because lake trout and whitefish ordinarily are taken together.
Furthermore, data on the size .of lake trout at first maturity indicate the need
for an increase in the size limit (now iy2 pounds) rather than a decrease in the minimum
legal mesh size of pound nets and deep trap nets.
17. Observations of the lifting of pound nets and deep trap nets did not indicate
the destruction of illegal-sized whitefish to be excessive even in those areas in which
it was estimated that hundreds of thousands of young fish were captured in a single
season. The "known" destruction of undersized fish (individuals iead from gilling or
other causes at the time of lifting) ranged from 2.40 to 3.80 percent according to the
lake, type of net, and size of mesh. These percentages tended to be higher for deep trap
nets than for pound nets. To the "known" destruction must be added the undetermined
losses from the later death of live bloated fish (only a little more than 1 percent
of the live illegal-sized whitefish were bloated) and of fish killed or injured fatally during
the sorting of the catch. Field observations indicated, however, that most (but not
all) fishermen attempted to avoid rough handling of small whitefish and returned them
to the water as soon as possible.
18. Extensive measurements were obtained of meshes in the pots of pound nets and
deep trap nets in order to determine the amount of shrinkage produced by different types
of preservatives applied to the twine. No significant difference could be found between
the shrinkage brought about by treatment with tar and copper oleate. The mesh size