Relations between Salmonidae and the native freshwater fauna in New Zealand

[First paragraph...]
The freshwaters of New Zealand provide an environment into which relatively few exotic species have penetrated, partly because the problems of survival during transportation are so difficult for a freshwater animal as to make accidental immigration very unlikely, and partly because the motives which may stimulate deliberate importation by man are so few in this case. So much remains to be done in the study of our freshwater fauna that a number of exotic species of the smaller invertebrates may exist unrecorded in this country, but the known freshwater fauna of non-native origin is limited to one snail and some dozen species of fish all deliberately introduced.