Comparison of the Growth Rate of Brown Trout (Salmo trutta) in a New Zealand Stream with Experimental Fish in Britain

Abstract

(1) Comparison of the growth rate of brown trout, Salmo trutta L., fed to satiation under experimental conditions in Britain with that of fish at the same size and temperature under natural conditions in a fast-growing population in the Horokiwi Stream in New Zealand showed that, although there was good general agreement, there were two important differences. (2) At temperatures below 13 ?C, trout in some parts of the Horokiwi grew more rapidly than experimental fish of the same size at the same temperature. Between 13 ?C and 19.5 ?C, growth in the Horokiwi was again more rapid than that of experimental fish, but appeared to decline with rising temperature. It was not possible to determine the temperature at which Horokiwi fish ceased to grow but it seemed likely to be above 19.5 ?C, the limit for the experimental fish. (3) It is possible that there has been genetic selection for more rapid growth and a higher temperature limit in the Horokiwi trout; the population is derived from a very small initial liberation in 1885 and the temperature range is slightly above that in British streams. An alternative explanation for the more rapid growth by the Horokiwi fish is that the experimental conditions did not allow the fish to achieve the maximum growth rate of which they were intrinsically capable, even though they were fed to satiety.