Terrestrial invertebrates as salmonid prey and nitrogen sources in streams: contrasting old-growth and young-growth riparian forests in southeastern Alaska, U.S.A.

Terrestrial-derived invertebrate (TI) inputs into streams and predation on them by salmonids (40n180 mm fork length) were measured in six coastal Alaska stream reaches from April through October 1993n1994; riparian habitat of three stream reaches contained conifer-dominated old-growth (no timber harvesting) and three were alder-dominated young-growth (31 years postclearcutting). Data from pan-traps placed on stream surfaces showed that TI biomass and nitrogen inputs averaged up to 66 and 6 mg·m n2 ·day n1 , respectively, with no significant difference between habitats. Stomach contents from coho salmon (Oncorhynchus kisutch), cutthroat trout (O. clarki), and Dolly Varden (Salvelinus malma) revealed that TI and aquatic-derived invertebrates (AI) were equally important prey. Additionally, salmonids from young-growth systems ingested a greater TI proportion than those from old-growth systems. There were trends but no significant differences between habitats of TI and AI biomass ingested; however, statistical power was <0.30. These results showed that TI were important juvenile salmonid prey and that a riparian overstory with more alder and denser shrub understory may increase their abundance. Riparian vegetation management will likely have important consequences on trophic levels supporting predators, including but not limited to fishes. Resume : Les apports diinvertObrOs diorigine terrestre (IT) dans les cours dieau et la prOdation exercOe sur eux par les salmonidOs (longueur ‡ la fourche de 40n180 mm) ont OtO mesurOs dans six tronAons de cours dieau cUtiers de liAlaska diavril ‡ octobre 1993n1994; les habitats riverains de trois des tronAons de cours dieau Otaient constituOs de forŒts msres (absence de rOcolte du bois) dominOes par des conifres et les trois autres Otaient constituOes de jeunes forŒts (31 ans depuis la coupe ‡ blanc) dominOes par liaulne. Les donnOes obtenues gr‚ce ‡ des piges ‡ eau placOs ‡ la surface des cours dieau ont montrO que la biomasse diIT et les apports en azote siOlevaient en moyenne ‡ 66 et 6 mg·m n2 ·jour n1 , respectivement, sans quiil y ait de diffOrences statistiquement significatives entre les habitats. Le contenu stomacal de saumons coho (Oncorhynchus kisutch), de truites fardOes (O. clarki) et de Dolly Varden (Salvelinus malma) a rOvOlO que les IT et les invertObrOs diorigine aquatique (IA) Otaient des proies diOgale importance. De plus, les salmonidOs des systmes de jeunes forŒts ingOraient une proportion plus grande diIT que ceux des systmes de forŒts msres. Il y avait des tendances, mais non des diffOrences significatives, entre les habitats pour ce qui est de la biomasse diIT et diIA ingOrOs; cependant, la puissance statistique Otait <0,30. Ces rOsultats ont montrO que les IT Otaient des proies importantes pour les salmonidOs juvOniles et quiun Otage dominant riverain contenant plus diaulnes et un sous-Otage diarbustes plus denses peuvent accroOtre leur abondance. La gestion de la vOgOtation riveraine aura vraisemblablement des consOquences importantes sur les niveaux trophiques qui soutiennent les prOdateurs, y compris mais non exclusivement les poissons. (Traduit par la ROdaction)