114 NORTHERN FISHES 



Lake trout are cold-water fishes and do not thrive in waters over 65° F. 

 The temperature of the water is probably the most important factor in 

 determining the suita})ility of lakes for this species. To be suital)le for 

 this species a lake should have a depth of 100 feet or more, with rocky 

 bottoms and shores. In such a lake oxygen is always abundant in the 

 bottom water because very little oxygen is consumed by the decay of 

 the scanty vegetation and organic matter. Although the surface water 

 may become too warm for the trout, they can always find cold water 

 with abundant oxygen at the bottom. Rocky lakes of less depth, unless 

 fed by springs or cold bog drainage, often do not have a sufficient volume 

 of cold water to carry many trout. 



Many attempts to introduce lake trout in the deep lakes of central 

 and north central Minnesota have failed because, though the bottom 

 waters are cold enough to carry trout through the summer, the bottom 

 oxygen becomes depleted because of the decomposition of large amounts 

 of vegetation and organic matter. In several lakes of this type a few lake 

 trout have survived, but not in sufficient numbers to warrant continued 

 stocking. The warmth of the shoal waters and the absence of rocky reefs 

 for spawning also prevent natural reproduction. 



SISCOWET (Fat Trout) 

 Cristivomer namaycush siscowet (Agassiz) 



The siscowet (Figure 13) is a form of the lake trout found only in 

 the deeper waters of Lake Superior. In general it resembles the lake 

 trout, but it differs in many details. The length of the siscowet is from 

 3 to 3 1/2 times its depth. The margin of the belly just anterior to 

 the vent is very much swollen, forcing the vent to be directed pos- 

 teriorly instead of ventrally as in the typical lake trout. The body 

 cavity is lined with a very thick layer of fat. The color varies from 

 light to dark, but the spots tend to be smaller than those of the lake 

 trout. The scales of the siscowet are slightly larger, numbering about 

 175 in the lateral fine. 



The siscowet probably hybridizes with the lake trout, for all sorts 

 of gradations in body form occur. Commercial fishermen call these 

 intergrades "half-breeds" and sometimes sell them for typical lake 

 trout. It is improbable that this fish is differentiated from the lake 

 trout by its environment, since typical lake trout occur commonly at 

 the same depths. 



This deep-water trout is seldom taken except by commercial fisher- 

 men, and it is never taken on the comparatively shallow spawning beds 

 of C. namaycush namaycush. Several commercial fishermen have re- 

 cently fished for this trout off Grand Marais, Minnesota, setting their 

 gill nets at depths from 600 to 800 feet. The eggs from four females taken 

 off Grand Marais July 10, 1938 averaged 5000 per fish. Mr. R. G. Gale 



