Diel feeding and prey selection of three size classes of shortfinned eel (Anguilla australis) in New Zealand

Abstract

Benthic macroinvertebrates and three size classes of shortfinned eel (Anguilla australis, Anguillidae) were sampled in a lowland New Zealand stream over a 24-h period in summer to investigate diel feeding periodicity and prey selection. Fish of all three size classes fed mostly from post-dusk to dawn, with larger fish tending to consume a greater proportion of their prey during night-time. The number of prey taxa consumed increased with increasing size of fish. Prey selection indices indicated that: (1) the smallest eels (80–100 mm total length, TL) preferentially consumed ostracods (Crustacea) and larvae of Chironomidae (Diptera) and Psilochorema sp. (free-living Trichoptera); (2) medium-sized eels (101–199 mm TL) preferred larvae of Chironomidae, and Costachorema sp., Hydrobiosis sp. and Psilochorema sp.; and (3) large eels (200–300 mm TL) preferred ostracods, Psilochorema sp. and Hudsonema amabilis (cased Trichoptera). Eels of all size classes showed selection for soft-bodied or easily broken prey over those with hard cases which would have been more difficult to digest. There was considerable overlap in the diets of medium and large eels. Differences in the timing of feeding between the three size classes of eel are explained in terms of microhabitat use; differences in diet are explained in terms of prey size, and are also related to eel size.