New Zealand Journal of Marine and Freshwater Research abstracts

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Food and energy in the diet of brown and rainbow trout from Lake Benmore, New Zealand

N. H. McCARTER

Fisheries Research Division Ministry of Agriculture and Fisheries P.O. Box 951, Rotorua New Zealand

Abstract The diet of brown trout (Salmo trutta) and rainbow trout (S. gairdnerii) was studied in specimens from Lake Benmore, a deep, oligo-trophic lake in South Island, New Zealand. Between November and July, both species fed mainly on small molluscs (Potamopyrgus antipodarum, Physa sp., and Gyraulus corinna) gleaned from the littoral weed beds. Energy values for the three species of mollusc were determined: P. antipodarum, 6000 Jg-'; G. corinna, 5500 J g¹; Physa sp., 9800 J g"¹. Potamopyrgus antipodarum yielded little energy to the fish, unless its shell broke during passage through the gut. Physa sp. was the most profitable mollusc, irrespective of shell breakage and Potamopyrgus antipodanim the least profitable. The mean energy value per snail for G. corinna and Physa sp. eaten by rainbow trout was 25-30% less than for snails eaten by brown trout, possibly because rainbow trout ingested empty shells from the sediment surface. Rainbow trout extracted about 20% more energy than brown trout from unbroken shells. In July, 84% of the brown trout switched to predation of common bullies, Gobio-morphus cotidianus, probably as a result of bully reproductive behaviour. Rainbow trout did not show the same change, apparently because they were feeding in deeper water where few bullies were available. The low-energy diet and its possible connection with growth rate are discussed.

Keywords Salmo gairdnerii; Salmo trutta; rainbow trout; brown trout; Gobiomorphus cotidianus; molluscs; diet; food energy values; seasonal change; feeding behaviour; growth; lakes; Lake Benmore

New Zealand Journal of Marine and Freshwater Research, 1986, Vol. 20: 551-559 Received 8 January 1986; accepted 16 April 1986

PDF file of entire paper: medium quality (642K); (scanned from paper original: notes about this process)

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