One way of
determining the life forms of a lake is by noting the food of
the fish. During the last three weeks of July and the first week
of August, fifty fish stomachs received from fisherman from the
lake were examined and their contents noted. All but four of
these fish were Silverside trout averaging sixteen and a half
inches in length; the four were small Rainbow trout. In
examining the contents of the stomachs one is impressed by the
fact that the diet of a particular individual is apt to be of
one kind of food rather than from a wide range of kinds. For
example, one fish will have concentrated on snails, one on
midges, one on water fleas, and one on water-stranded land
insects, or perhaps on worms. The largest individual form found
was a seven-inch fish; the smallest individual used for food
(exclusive of forms such as diatoms which are basically food of
the fish food) is the water flea of Daphnia pulex (fig.
1). This minute crustacean was found in 74 percent of the fish
examined and made up 62.8 percent of volume of food eaten by the
fifty trout. These forms are rarely found at the surface of the
lake, being most abundant at depths of 75 to 200 feet. The
closely related Amphipod (fig. 2) locally known as fresh-water
shrimp which is fairly abundant in the marginal waters of the
lake is sixth in importance as fish food; it was found in 24
percent of the stomachs and made up 4.7 percent of the volume of
food used by the fifty fish.
In point of total bulk, the snails
come second in importance as food, as they make up 10.9 percent
of the food eaten. The snail used most by the fish is
Pompholex species? (fig. 3) the middle-sized form of the
three Crater Lake snails. It was found in 22 percent of the
stomachs examined.
The various forms of insects
make up the third largest amount of fish food, 9.6 percent, and
were used by 54 percent of the fishes. However 6.86 of the 9.6
was made up of midges, larvae, pupae and adult stages (fig. 4).
In some cases these made up the entire stomach contents. Land
insects, when stranded by adverse wind currents sometimes make
up the food; one fish had 11 beetles, 1 moth, 1 grasshopper or
cricket, an ant, 1 bee, 2 bumble bees, 2 true bugs, as well as
adult and pupa stages of midges, 3 snails and 1600 Daphnia. Of
the water insects other than midges the Caddis fly, especially
in the larva (penniwinkle stage) (fig. 5) is quite a favorite of
off-shore feeding fish. They made up 1.66 percent of the volume
of fish food though found in but 14 percent of the fish.
Mayflies also form part of the fishes' diet (0.65 percent)
The annelids formed only 2.53
percent of the food mass. Three small leeches (38 inch) were
noted, and one stomach was largely filled with 65 worms, smaller
than the earthworm but closely allied to it. These worms have
been found in muddy pools along the lake margin and also in
bottom tows to a depth of 90 feet.
The two fishes found in the
diet made up 8.7 percent of the total mass though found in only
4 percent of the fish stomachs. The distinguishable plant
material was relatively small, being only 0.8 percent of the
diet and consisted of leathery Nostoc,
a blue-green algae (fig. 6) and wads of filamentous green algae,
probably Zygnema (fig. 7).
