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Comparative Stomach Analysis of Crater Lake Fishes

On account of the great interest in fishing in Crater Lake, many studies have been, and are being, carried out to try to find out more about the fish in Crater Lake. In any program such as this, one of the first projects to be carried out is a stomach analysis to determine what types of food present are being utilized by the fish.

Crater Lake, having no known inlet or outlet, could have no native fish. All of the fish in the lake are either planted fish, or offspring of planted fish. The first planting was carried out in 1888 by William G. Steel. At that time he planted 37 rainbow trout fingerlings into the lake waters. Since that time there have been many other plantings. The first recorded plantings of the rainbow trout, (Salmo gairdnerii gairdnerii) and the sockeye salmon, (Oncorhynchus nerka), were carried out in 1909 by park personnel. The last plantings were in 1937 for the sockeye salmon, and 1941 for the rainbow trout.

For the last few years fishing in Crater Lake has not been exceptional, in fact many people would refer to it as poor. The long duration since the last stocking is undoubtedly one of the factors affecting the condition of the fishing, but by no means the only one.

On the basis of analyzing six rainbow trout stomachs, and four sockeye salmon stomachs, it was found that insect forms, both adult and larval, are the most used source of food for the fishes. Every stomach examined contained some sort of insect material.

“Crater Lake fish. Two large rainbow trout flanked by Sockeye salmon. N.P.S. photo.”

There is a great deal of variety among the types of insects used by the fish, as they represented nine orders or groups, and many more families. These orders listed according to frequency of occurrence are; the flies (Diptera), the moths and butterflies (Lepidoptera), the beetles (Coleoptera), the bees, ants, and wasps (Hymenoptera), the caddie flies (Trichoptera), the mayflies (Ephemerida), the stone flies (Plecoptera) the snake flies (Phaphidiodea) and the biting and sucking bugs (Hemiptera).

Along with the insect forms were found the fresh water shrimp (Hyalella), a snail, and a spider.

The breakdown as to the type of insects per species of fish is a follows:

Rainbow Trout (six stomachs)

Sockeye Salmon (four stomachs)

It can be seen from the above that many groups of insects are used by both types of fish fairly equally, while other orders are used more by one type of fish than by the other. The Diptera, Trichoptera, Coleoptera, and Hymenoptera were used by both types of fish. The rainbow trout also made use of the Lepidoptera, Hemiptera, andPhaphidiodea, while the sockeye salmon made use of the Plecoptera and theEphemerida.

Some of these differences in food habits can be explained without too much trouble. The abundance of the Lepidoptera in the rainbow trout, and the lack of the same in the sockeye salmon, is due to the fact that all of the rainbow trout were taken in 1952, which happened to be a year of great abundance for the California tortoise-shell butterfly in this area also. The sockeye salmon were taken in 1953 when the California tortoise- shell butterfly was no longer in abundance, therefore they do not occur in the sockeye salmon stomachs.

The Plecoptera and the Ephemerida in the sockeye salmon stomachs and not in the rainbow trout stomachs might be explained as a matter of chance. Although the individuals in the stomachs were in abundance, all of the Plecoptera found were from the one stomach, while all of the Ephemerida found were also taken from but one stomach.

The presence of the Phaphidiodea and the Hemiptera in the rainbow trout can probably be explained much the same way, merely the chance happening across some concentrated specialized food supply, and not a matter of different food preference between the two types of fish.

All of the fresh water shrimp, along with the snail and the spider, were taken from merely isolated stomachs also, which could lead one to apply the same explanation there as used above.

As far as can be determined by this study, the food preferences in the two types of fish in the waters of Crater Lake do not differ. Both species of fish would probably use the same types of food if they were available to the fish. This is born out not only by this study, but by the angling information also, as both species of fish inhabit the same areas, and can be taken on the same kinds of fishing lures.

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