Kusabs & Swales--Diet of koaro and juvenile rainbow trout

New Zealand Journal of Marine and Freshwater Research abstracts

Diet and food resource partitioning in koaro, Galaxias brevipinnis (Gunther), and juvenile rainbow trout, Oncorhynchus mykiss (Richardson), in two Taupo streams, New Zealand

IANA.KUSABS1

STEPHEN SWALES2

Department of Biological Sciences
University of Waikato
Private Bag, Hamilton, New Zealand

1Present address: Eastern Region Fish and Game Council, P. O. Box 1098, Rotorua, New Zealand
2Present address: Fisheries Research Institute, NSW Agriculture and Fisheries, P. O. Box 21, Cronulla, NSW 2230, Australia

Abstract This study examined the diet of sympatric populations of migratory juvenile rainbow trout and landlocked koaro in the Waipehi and Omori Streams, Lake Taupo, New Zealand. In both species, diet was dominated, both numerically and by weight, by aquatic prey: Ephemeroptera, Trichoptera, and Diptera larvae were the most numerous prey items. Adult koaro and juvenile rainbow trout both fed on small koaro. Terrestrial prey items were present in low numbers in the diets of both koaro and juvenile rainbow trout, but were more important in terms of weight. Resource partitioning was weak although koaro consumed more small benthic invertebrates such as chironomid larvae, whereas the diet of rainbow trout contained more Ephemeroptera larvae and terrestrial insects. In the Waipehi Stream, koaro consumed both rainbow trout ova and koaro ova; in Omori Stream, trout ova were important in the diet of juvenile rainbow trout. Since the diets of koaro and juvenile rainbow trout in some Taupo tributaries are similar, populations may co-exist by temporal and/or spatial partitioning of food resources, whereas trout predation on small koaro may be a limiting factor for koaro populations.

Keywords benthic invertebrates; competition; diet; dietary overlap; Galaxias brevipinnis; rainbow trout; koaro; Omori Stream; Oncorhynchus mykiss; resource partitioning; sympatry; Waipehi Stream

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