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 sepa- 

 rated 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 northeast- 

 ern Lake Michigan throughout most of the summer and early autumn raises the ques- 

 tion of the possible existence of distinct inshore and offshore populations or races. 

 Arguments were outlined briefly for and against this interpretation of the two concen- 

 trations; available data do not, however, permit a definite decision. 



14. On the basis of the observations on the bathymetric 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 bathymetric distribution 

 and seasonal movements of the lake trout,- yellow pike, burbot, white sucker, and long- 

 nosed 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 I 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) re- 

 vealed that in general the large-mesh nets took the greater numbers of legal-sized white- 

 fish 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 4 1 -; inches or greater (extension meas- 

 ure 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 

 4^2 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 to- 

 gether. Furthermore, data on the size of lake trout at first maturity indicate the need 

 for an increase in the size limit (now 1 ' ■_■ 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 dead 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 undeter- 

 mined 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 I 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 



