A FUNCTIONAL CLASSIFICATION OF THE DRIFT - TRAITS THAT INFLUENCE INVERTEBRATE AVAILABILITY TO SALMONIDS

Twelve categories/traits were used to classify and rank aquatic invert ebrates based on their propensity to drift and importance as a food re source for salmonids. Invertebrate availability was based on their (i) propensity to intentionally drift, (ii) likelihood of being accidenta lly dislodged by the current, (iii) drift distance, (iv) adult drift, (v) benthic exposure, (vi) body size, and (vii) abundance. This study represents the first attempt to characterize the intentional drift pro pensity of stream invertebrates. A ranking procedure separated inverte brates into Baetis and three groups decreasing in availability. Predic ted ranks were significantly correlated with the actual rank of invert ebrates in trout guts taken in three separate studies conducted in the central Rocky Mountains, suggesting that this procedure can effective ly rank invertebrates based on their availability as a food resource f or salmonids. A cluster analysis separated the 95 taxa into four drift guilds and six availability groups. This study provides criteria for determining when alterations in invertebrate community composition wil l affect food resources for higher trophic levels by causing a decline in the most available taxa. This research also supports previous find ings that floods are important in maintaining invertebrates that repre sent an important food resource for salmonids.