Notes on Macroinvertebrates and Diet of Rainbow Trout in Guadalupe River Canyon Tailrace

GRTU conducted a study in from Aug 2006 to July 2007 on the diet of stocked rainbow trout in the Guadalupe River downstream of Canyon Dam (Canyon Tailrace). The study found that macroinvertebrates were abundant in the river, but that overall, the fish fed primarily on algae, detritus, and gastropoda. The further downstream the fish were captured, the higher the percentage of their diet that consisted of macroinvertebrates. At site #1 (river bend roughly 1/2 mile upstream from Rio Raft), the diet consisted of roughly 25% gastropoda, and 30% LEAVES. At site 2, (deep pool in bend near island downstream of GRTU access site old #5), the diets consisted of roughly 35% isopods (sowbugs). At site 3, (Rocky Beach), the diet consisted of roughly 15% detritus, 10% ephemeropterans, 20% gastropoda, and 20% unidentified insect parts. At site 4 (near Riverbank Outfitters), the trouts' diets consisted of 40% ephemeropterans and 20% unidentified insect parts. Algae made up roughly 20-25% of the stomach contents of the fish at three of four localities, but only 15% of the diet at site 4.

Macroinvertebrate samples were collected from the river by various methods. Predominant taxa collected in drift nets at sites 1-3 included: Chironomidae, Baetidae, Simuliidae, Isonychia, Stenonema, with the former 3 being the most prominent. Chironomidae were the most abundant taxa at all three sites. Site 3 produced significantly more macroinvertebrates than sites 1 and 2. This could be related to favorable stream conditions further downstream of the dam?

Temporally, the macroinvertebrate community varied. In summer, diptera and hemiptera dominated. In fall, ephemeroptera and diptera dominated. In winter, ephemeroptera comprised almost 50% of the community, with diptera representing roughly 25%. In the spring, diptera comprised over 50% of the community.

Publicado por mattgeo1990 mattgeo1990, 19 de enero de 2018

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