8100

• UCREFRP Catalog Number: 8100
• Author: American Fisheries Society.
• Title: Proceedings of the 22nd Annual Meeting, Colorado - Wyoming Chapter, American Fisheries Society.
• USFW Year: 1987.
• USFW - Doc Type: March 11-12, 1987.
• Copyright Material: NO

to September. The number of prey items/stomach declined from July to August in
fi sh > 150 mm. At the same time, the abundance of organ isms in the drift
declined over the same time interval. A similar trend was observed by Allan
(1981) in Cement Creek, Colorado. Our results differed for fish ~ 150 mm where
the peak number of prey consumed/fi sh was in August when a moderate 1 evel of
drift density was observed.

With progression from July to September, the proportion of terrestrial
insects in the diet of brook trout> 150 mm increased from 2.9% to 36.2%. At the
same time, the proportion of the drift comprised of terrestrial insects tended to
increase. The terrestri a 1 insect component of brook trout di et was in Deer
Creek, Idaho (Griffith 1974), but a seasonal trend, as in Telephone Creek, was
not described. However, Reed and Bear (1966) found terrestrial insects to
comprise 50% of the diet of brook trout in Archuleta Creek, Colorado, but this
site was at a lower elevation (2,900 m) in a valley that could not be described
as subalpine.

Differences in diet composition were observed between the two size classes
of brook trout. The smaller size class (~ 150 mm) tended to consume smaller
aquatic insect prey, especially chironomid larvae and the early instars of
Ephemeroptera and Trichoptera. The larger fish (> 150 mm TL) tended to consume
larger insect prey, particularly Trichoptera larvae, elmid beetle larvae and
adults, and terrestri a 1 insects. Sel ect i on for 1 arge invertebrates has been
demonstrated among brook trout> 150 mm by Allan (1978).

Selection for Trichoptera larvae, Coleoptera larvae and adults (primarily
e 1 mi d beetles), as well as for terrestri a 1 insects, was observed by Griffith
(1974) and Allan (1981). Allan (1981) also observed selection for Coleoptera by
brook trout in Cement Creek, Colorado. Griffith (1974) observed brook trout
feeding near the substrate, thereby making encounters with large, heavy aquatic
insects more likely. This may explain the observed selectivity for caddis fly
larvae and elmid beetles in Telephone Creek. However, it is also possible that
brook trout were not selecting Coleoptera and Trichoptera in the drift, but .were
feeding on these relatively large, conspicuous organisms while the prey were on
the substrate.

Allan (1981) suggested that three factors may govern the diet compositions
of stream-dwelling brook trout: prey availability, prey size and individual
specialization among fish. Many of our observations tend to corroborate that
prey avail abil ity and prey size i nfl uence di et compos it ion, but our data also
suggest that the manner in which these factors govern the diet may vary with fish
size and time of year. Our observations suggest that organisms on the substrate,
as well as in the macroinvertebrate drift, should be evaluated as available prey
items, especially for brook trout> 150 mm.

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LITERATURE CITED

Allan, J.D. 1978. Trout preda.tion and the size composition of stream drift.
Limnology and Oceanography 23:1231-1237.

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