TROPHIC ECOLOGY OF RED ROACH (RUTILUS-ARCASII) IN A SEASONAL STREAM -AN EXAMPLE OF DETRITIVORY AS A FEEDING TACTIC

1. Diel feeding activity and diet of red roach (Rutilus arcasii) were determined on five occasions (February, April, June, August and Novemb er 1985) in a seasonal, fluctuating stream subject to severe summer dr oughts and cold winter floods. 2. Except in June, the two age groups o f the population (less-than-or-equal-to 1 + and > 1 +) fed mainly on d etritus and showed no significant differences either in their feeding intensity or in the relative contribution of the main diet components (detritus, plants and invertebrates). 3. The size, abundance and avail ability of drifting invertebrates influenced red roach feeding. When d rift was scarce or inaccessible, both age groups fed on detritus, plan ts, or a combination of the two. These niche shifts were a trophic tac tic aimed at maintaining feeding when other, more nutritional and ener getically valuable foods were scarce. 4. A comparison with other popul ations suggested that a detritus-based diet had no major cost in the l ife history of the red roach. We hypothesize that the highly opportuni stic trophic strategy of this Iberian endemic cyprinid is an adaptativ e response to seasonal Mediterranean streams.