Nature Notes From Crater Lake
Volume 32-33, 2001/2002
Fish in Crater Lake: Their Size and
Number
By Scott Girdner
Ranger-naturalist Arthur Hasler showing a
rainbow trout to visitors in 1938. NPS photo.
|
Anglers who routinely 'wet a line' in
Crater Lake have learned that fishing success as well as the size of
kokanee salmon and rainbow trout in the lake can fluctuate dramatically
from year to year. Analysis of fish length and fish population size over
the last 15 years provides insight into the patterns of change and may
help anglers appreciate the ups and downs of fishing Crater Lake.
Crater Lake was naturally barren of
fish until park founder William Steel first stocked Crater Lake with
trout fingerlings in 1888 to "improve" recreational opportunities.
Despite altering the lake's natural condition, introductions of
non-native fish continued until 1941, when stocking the lake ended. In
all, five species of salmonids, totaling nearly two million fish, were
introduced to the lake over the intervening 53 years. Brown trout (Salmo
trutta), cutthroat trout
(O. clarki), coho salmon (O. kisutch), kokanee salmon
(O. nerka, a landlocked sockeye salmon), and several stocks of
rainbow trout (O. mykiss) including steelhead were introduced
during this period. Only the self-sustaining populations of rainbow
trout and kokanee salmon persist in the lake today.
Kokanee salmon. NPS photo by C. Warren
Fairbanks, 1954. |
Detailed annual fish population
estimates using high-tech acoustic systems were initiated in Crater Lake
in 1996. The largest fish population observed in Crater Lake since that
time occurred during the summer of 2000 when biologists estimated the
total number of fish in Crater Lake at 633,000—a density of 48 fish per
acre. Lake Billy Chinook near Bend, by contrast, is estimated to contain
between 530 and 5500 kokanee per acre depending on the year. Coeur d'
Alene Lake in Idaho typically fluctuates around 1400 fish per acre. The
lowest number of fish in Crater Lake occurred in 1998 when only 8,400
(less than 1 fish per acre) were observed. Just two years later the fish
population in Crater Lake increased by an astounding 7450 percent!
The relative abundance of kokanee has
been monitored with gill nets since 1986. Population size and the
average length of individual kokanee salmon cycles over a period of
approximately ten years (see the first two graphs in Figure 1).
According to fisheries studies around the Pacific Northwest,
fluctuations in kokanee population size and fish length are fairly
common in unproductive bodies of fresh water such as Crater Lake. Large
increases in kokanee population size that occurred in the lake during
1990-91 and 1999-2000 could therefore be expected.
Biologists studying Crater Lake believe
that at low population size, food is plentiful for the kokanee and a
fewer number of fish eventually reach larger size. These large and
healthy adult fish reproduce successfully leading to an increase in fish
numbers. As the fish population increases, the primary food of kokanee
(microscopic animals in the water column called zooplankton) decreases
to the point that the lake can no longer support such a high population
of fish. The kokanee population size then falls through time, allowing
the zooplankton population to recover so that the cycle starts over
again. In water bodies other than Crater Lake, the patterns in kokanee
size and number are subject to more variables such as fluctuations in
water level (this is especially pronounced in reservoirs), water
temperature, fish stocking density, the timing of stocking, and harvest
pressure from anglers.
Rainbow trout do not feed on
zooplankton like kokanee do, but instead rely mostly on aquatic insects
near the lakeshore as well as those that land on the lake surface. Large
rainbow trout will also eat small kokanee. The length of the largest
rainbow trout caught in Crater Lake over the last 15 years has varied
similarly to that of kokanee, but delayed by 1-2 years (compare the
upper and lower graphs in Figure 1). Large rainbow trout were prevalent
in 1991 (see Table 1) and have been increasing in number the last few
years. Not surprisingly, the presence of exceptionally large rainbow
trout appears to be associated with the presence of large numbers of
small kokanee.
Research has shown the ecology of
Crater Lake to be very dynamic and the fish population is no exception.
Recent studies suggest that the quality of fishing in the lake for the
foreseeable future will fluctuate depending upon the year. The extremely
large increase in kokanee numbers during 1999 and 2000 will probably
result in their population crashing (probably due to over exploitation
of their food resources) in the next few years. This was already
becoming apparent in the summer of 2000, given the dramatic increase in
kokanee numbers over the previous two years.
With numerous small kokanee salmon
present, the summer of 2001 could turn out to be a great time to catch
that big rainbow trout in Crater Lake. Although the density of fish in
Crater Lake will probably never be high like other more productive lakes
of the Pacific Northwest, some days at Crater Lake still promise to be
very good fishing. Other days may not be so good, but if you are going
to experience a bad day of fishing I cannot think of a better place to
go than beautiful Crater Lake.
Note: Fishing access at Crater Lake is fairly
limited because of the steep and dangerous caldera walls encircling the
lake. A quarter mile section of shoreline is accessible at the base of
the Cleetwood Cove Trail for fishing. Anglers can reach Wizard Island in
the tour boat operated by the park concessioner, a service allowing
passengers to disembark and fish on a day use basis. Devices such as
float tubes and rafts are not allowed on the lake due to erratic winds,
jagged rocks, and steep shorelines.
Crater Lake National Park's Long-term
Limnological Monitoring Program assesses many chemical and physical
aspects of Crater Lake's ecology. Part of monitoring the fish population
involves setting nets to collect fish for analysis. Each fish collected
in the nets is measured for length, weight, sex, and maturity level.
Scales are collected from each fish to determine fish age. The stomach
contents of the fish are preserved for later analysis. The monitoring
program also uses a scientific-grade acoustic system (a fancy "fish
finder") to accurately estimate the population size in the lake and
assess fish distribution within the water column.
