Fishery bulletin of the Fish and Wildlife Service - NOAA

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370 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ination of the frequencies and averages indicates that probably few legal-sized trout escaped through the meshes of 3*/2 to 3 15/16 inches (about 4 to 4 7/16 inches as manufactured) since the'modal weight of the fish in these meshes (between lYz and 1% pounds) was the same as in the 3- to 3 7/16-inch meshes and the average weight of legal-sized fish increased only 1.6 ounces in nets of the latter sizes of mesh. Individuals of these sizes did escape through meshes larger than 3 15/16 inches. It is doubtful, however, whether many fish of 1% pounds or larger were able to pass through meshes of exactly 4 inches (about 4*4 inches as manufactured). It may be observed from the frequencies of weights that the bulk of the gilled trout shifted to a higher weight-interval with each increase in the mesh between 3 and 4 7/16 inches, but the fish were always concentrated within a relatively small range of weight (% to 1 pound). The majority of the gilled trout weighed between 1 г /4 and 2 pounds in the 3- to 3 7/16-inch, mesh, between iy 2 and 2~У 2 pounds in the З 1 /^- to 3 15/16inch mesh, and between 1% and 2% pounds in the 4- to 4 7/16-inch mesh. The ranges in length of the bulk of the trout (table 42) varied from about 1 to 2 1 /£ inches in these various meshes (18 - 19%; 17У 2 - 20; 20-21 inches). The average weight of the undersized gilled trout, as well as the average length (table 42), increased with an increase in the size of mesh from less than 3 inches to 3 to 3 7/16 inches (indicating release of some small fish). The size of fish did not change, however (slight increase in weight; slight decrease in length), .with ä'further increase of % inch in mesh size suggesting that, though additional undersized fish were released by the larger meshes, the size of mesh was not yet sufficiently large to permit the larger undersized trout to escape. An increase of another У» inch in the size of the mesh apparently did permit this escapement for no undersized trout were gilled in meshes of 4 to 4 7/16 inches. Even though these meshes or larger ones are used, it may not be assumed that no undersized fish would remain in the net. They do not all attempt to escape. :' ' The average weight and length of the legal-sized gilled trout increased slightly with an increase in mesh size from 3 to 3 7/16 to 3/у 2 to 3 15/16 inches (indicating release of only a few fish), but increased to a greater degree with a further %-inch increase of mesh size, suggesting that some of the smaller fish of legal size had escaped. Nearly all of the trout gilled in meshes of 4 to 4 7/16 inches weighed 1% pounds or more. In general, the data on the gilled fish and on the average sizes of fish retained in the impounding nets indicate that Michigan's minimum size of mesh (4У> inches as found in use) prescribed for the pots of impounding nets employed in catching whitefish and Jake trout should not be reduced. This mesh is in fact too small to liberate a large proportion of the undersized whitefish found in the nets, although on the other hand it is too large to hold the smaller individuals of the legal-sized trout. A 4-inch mesh as found in use would probably prove more effective for the capture of trout at the present size limit of iy 2 pounds. A better solution than a reduction in mesh to prevent the escape of legal-sized trout would be a substantial increase in the legal size limit since most lake trout (especially the females) under 3 pounds are sexually immature. It is not practicable to prescribe different meshes for whitefish and trout as both species are usually taken together од the same grounds. Further, a 4yo-inch mesh is also prescribed for gill nets employed for both species. DESTRUCTION OF WHITEFISH THROUGH GILLING IN THE MESHES OF POUND NETS AND DEEP TRAP NETS The gilling of undersized fish in the meshes of impounding nets constitutes a certain source of destruction since death follows soon after the individual is enmeshed. It is, therefore, of importance to know what percentage of the illegal-sized whitefish become gilled in commercial pound nets and deep trap nets, and how this percentage varies with the size of the mesh. The death of legal individuals through gilling 4 is of lesser importance, although the market value of such fish may be impaired and large numbers.of gilled fish of any size add considerably to the fishermen's labor in clearing their nets.
WHITEFISH FISHERY OF LAKES HURON AND MICHIGAN 371 The data of table 43 on the numbers and percentages of gilled whitefish in pound nets and deep trap nets are based on a combination of all nets of similar sizes of mesh irrespective of fishing grounds, depth of water, and the month and year in which the nets were fished. 37 None of these variables was found to affect the percentage of gilled fish. TABLE 43.—Numbers and percentages of legal and illegal whitefish gilled in large-mesh and smau-mesh pound nets and deep trap nets, 1931-1933 data combined for all localities and all depths of water T tal be [The table is based only on the lifts in which gilled fish were counted and separated according to size] Item Legal 48,441 154 0.3 Less than 4 inches Whitefish taken in nets of mesh siie Illegal 44,759 1,077 2.4 Legal 18,024 161 0.9 4 inches and more Illegal 12,613 340 2.7 ' A larger percentage of both the legal and the illegal whitefish became gilled in large-mesh nets than in small-mesh nets. The percentage of the legal fish gilled in large-mesh nets although small was three times that gilled in small-mesh nets, but the percentage of the illegal fish gilled in large meshes was only slightly above that in small meshes. It may be considered probable that the greater ability of large meshes to gill the larger illegal fish is compensated by the numbers of smaller illegal fish that can pass through the meshes. The percentages of gilled illegal whitefish in pound nets and deep trap nets (2.4 percent in small meshes and 2.7 percent in large meshes) do not point to gilling as a very important source of destruction of undersized fish in a single lift. Should the same fish be taken repeatedly the risk of death by gilling would be increased. BLOATING OF LIVE WHITEFISH IN POUND NETS AND DEEP TRAP NETS Another possible source of destruction of illegal-sized whitefish is the bloating (the result of changing pressure) that frequently occurs when nets are lifted. It cannot be stated exactly how serious the effects of bloating may be. It is possible that many fish that are not visibly bloated when a net reaches the surface may have been injured seriously by the change of pressure, particularly if the net was lifted rapidly. On the other hand, visibly bloated fish often appear to make a complete recovery, and swim away vigorously upon return to the water. Table 44 shows the relationship between the depth of water from which nets were lifted and the extent of bloating of whitefish of legal and illegal size. The percentage TABLE 44.—Relationship between the depth of water and the bloating of live whitefish in pound nets and deep trap nets, 1931-1932 data combined for all localities Depth of water (feet)
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UNITED STATES DEPARTMENT OF THE INT
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