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A preliminary assessment of the impact of alien rainbow trout (Oncorhynchus mykiss) on indigenous fishes of the upper Berg River, Western Cape Province, South Africa
Abstract
Impacts of alien rainbow trout (Oncorhynchus
mykiss) on critically endangered Berg River redfin (Pseudobarbus burgi),
Cape kurper (Sandelia capensis) and Cape galaxias (Galaxias zebratus)
in the upper Berg River were investigated in terms of predation and spatial
interactions. Trout stomach contents revealed that invertebrates dominate trout
diet within the study area, whilst only six fish were recovered from 45
stomachs. The apparent low fish predation success of O. mykiss within
the stream suggests a smaller impact compared to that of other alien piscivores
such as bass (Micropterus spp.). Galaxias zebratus was the
only fish species identified as prey, and its conservation status in the river
requires further investigation. Snorkelling surveys revealed that rainbow trout
co-exist with S. capensis and adult P. burgi within pools on this
river. Galaxias zebratus was absent from the pools, while P. burgi
juveniles were segregated from rainbow trout along a depth gradient, possibly
indicating avoidance behaviour. Sandelia capensis juveniles may avoid
predation by hiding under rocks. Rainbow trout probably compete with indigenous
fish for food and space in the pools, though this could not be quantified. The
impacts of O. mykiss on all indigenous fauna within the river are likely
to be density-dependent.
African Journal of Aquatic Science
2004, 29(1): 107–111
mykiss) on critically endangered Berg River redfin (Pseudobarbus burgi),
Cape kurper (Sandelia capensis) and Cape galaxias (Galaxias zebratus)
in the upper Berg River were investigated in terms of predation and spatial
interactions. Trout stomach contents revealed that invertebrates dominate trout
diet within the study area, whilst only six fish were recovered from 45
stomachs. The apparent low fish predation success of O. mykiss within
the stream suggests a smaller impact compared to that of other alien piscivores
such as bass (Micropterus spp.). Galaxias zebratus was the
only fish species identified as prey, and its conservation status in the river
requires further investigation. Snorkelling surveys revealed that rainbow trout
co-exist with S. capensis and adult P. burgi within pools on this
river. Galaxias zebratus was absent from the pools, while P. burgi
juveniles were segregated from rainbow trout along a depth gradient, possibly
indicating avoidance behaviour. Sandelia capensis juveniles may avoid
predation by hiding under rocks. Rainbow trout probably compete with indigenous
fish for food and space in the pools, though this could not be quantified. The
impacts of O. mykiss on all indigenous fauna within the river are likely
to be density-dependent.
African Journal of Aquatic Science
2004, 29(1): 107–111
