66 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND native species on various occasions; and the MARPLES, B. L 1942. A study of the little owl, Alhene noctua, in New Zealand. Tran.~. Rov. Soc. N.Z. 72: food of both native morepork and intro- - duced little owl includes small birds, either 237-252. MASTERS, S. E., HOLLOWAY,J. T. and McKELVEY, P. 1., native or introduced. 1957. The national forest survey of New Zealand, 1955. VoL I. Govt. Printer, Wel1ington. REFERENCES MOON, G. J. H., 1956. White-faced heron nesting in BULLER, W., 1905. Supplement to Birds of Ncw Zealand. North Auckland. Notornis 6: 244. , The Author, London. MYERS,J. G., 1923. The present position of the endemic CUMBERLAND,K. B., 1949. This is New Zealand. Whit~ birds of New Zealand. NZ. Journ. Sci. Tech. 6: 65- 99. combe & Tombs, ChristChurch. THOM'SON,G. M., 1922. The lIafllralisation of animah Gum, J. A., 1961. Ecology of the birds of Kaingaroa and plants in New Zealand. Cambridge University Forest. Proc. N.Z. Eco/. Soc. 8: 29-38. Press. GUTHRIE-SMITH, H., 1921 and later editions. Tlaira. Blackwood, London. TURBon, E. G., 1953. Notes on the occurrence of the bellbird in North Auckland. Notornis 5: 175-17&. LA<:K, D., 1933. Habitat selection in birds. with special reference to the effects of afforestation on the Rreck- TURBOIT,E. G., 1957. Native and introduced birds. In land avifauna. J. Anim. Ecol. 2: 239-262. Science in New Zealand. Reed, Wellington. LACK,D., 1939. Further changes in the Breckland avi~ WODZICKI, K. A., 1950. Introduced mammals of New fauna caused by afforestation. J. Anim. Ecof. 8: Zealand: an ecological and economic survey. Bull. 277-285. Dept. ScL Ind. Re,~. 9&. RELATIONS BETWEEN SALMONIDAE AND THE NA TIVE FRESHWATER FAUNA IN NEW ZEALAND K. RADWAY ALLEN Fisheries Laboratory, N.Z. Marine Department The freshwaters of New Zealand provide dae since in its native Northern Hemisphere an environment into which relatively few this family contains the majority of the most exotic species have penetra ted, partly be- highly regarded game species. Of the species cause the problems of survival during that were introduced, two have established transportation are so difficult for a fresh- themselves successfully over a large part of water animal as to make accidental immi- the country, and a third in a more restricted gration very unlikely, and partly because the area. These are the brown and rainbow motives which may stimulate deliberate trouts (Salina trutta and S. gairdnerii) and importation by man are so few in this case. the quinn at salmon (Oncorhynchus tscha- So much remains to be done in the studv wytcha) respectively. A few other species of our freshwater fauna that a number of exist in small numbers in limited areas. For exotic species of the smaller invertebrates simplicity, I propose to limit consideration may exist unrecorded in this country, but of the introduced freshwater fauna largely the known freshwater fauna of non-native to these few highly successful species of origin is limited to one snail and some dozen Salmonidae; both because of their wide- species of fish all deliberately introduced. spread abundance and because, on account of their popular esteem, they have been Most of the fish were introduced to provide relatively intensively studied. sport, and it is not surprising that a large proportion of them belong to the Salmoni- It should be realised that fresh waters ALLEN: SAI"lvl0NIDAE AND FRESH\VATER FAUNA 67 do not constitute an environment entirely of our lakes contain numerous small native isolated from those which surround it. Manv fish which provide good trout food; the animals of a variety of groups divide theli- large populations of fast-growing trout in time between freshwater and either land many of the lakes of the Rotorua district or air, either intermittently, or at set stages are, for instance, enabled to develop by the of their life history. It is therefore necessarv abundant smelt stocks in these lakes. to refer occasionally to events occurring out- side the limits of the watery environment. While it is thus a simple matter to declare that New Zealand freshwaters now contain Since it is almost axiomatic that game a native fauna of small animals which is fish should be carnivorous, it follows that generally suflicient to support strong popu- one of the principal impacts of the Salmoni- lations of introduced salmonids, it is much dae on the native freshwater fauna is their less simple to determine what effect, if anv, predation on small native animals, both the salmonids have had upon the original vertebrate and invertebrate. The converse fauna before the present position was is, of course, also true; that the native reached. The difficulty arises partly from the fauna may, by variations in its composition almost complete lack of observations com- and abundance, play an important part in. paring the fauna of waters before and aftcr determining the density and structure of the the introduction of salmonids, partly from introduced fish populations. the scarcity at the present time of really The principal food of the Salmonidae in comparable waters, some of which contain almost all waters is derived from the inverte- trout and some of which do not, and partlv brate bottom fauna and from small fish. from the other environmental changes which The degree to which the various species are have taken place simultaneously with the eaten depends upon their abundance and establishment of salmon ids in New Zealand. upon the extent to which their habits and These environmental changes include both structure make them available to and eat- physical changes like the increased instabil- able by the fish (Allen 1941). The latter mav, ity and destruction of pools which often in turn, be considerably affected by the size follow the clearing of catchment areas, and of the fish themselves; the larger the fish, biological changes such as the introduction the larger the animals which it eats, and the of insectivorous birds which may prey on more easily are hard-shelled forms con- the winged stages of insects with aquatic sumed. Among the invertebrates the princi- larvae. pal groups eaten by stream-living Salmoni- dae in almost all countries are Chironomidae, Workers on various native aquatic animals Ephemeroptera, Trichoptera and Mollusca, have realised that the species they werc the fish tending to advance through a sequence interested in were subject to predation by in this order as they increase in size. In lalres trout and have suggested that this might considerably greater diversity occurs. Small threaten their survival. Generally however fish are mainly eaten by the larger salmonids. little evidence has been adduced as to the the particular species consumed varying actual extent of the threat and one cannot according to the composition of the local avoid the feelino; that sentiment has played fauna; in New Zealand the forms mainly an undue part in the formation of some of eaten are bullies (Gobiomorphus and Philyp- the opinions expressed. In 1915 a paper nodon spp.) and smelts (Retropinna spp.). dealing with the freshwater crayfish (Paran- ephrops spP.) declared "It is evident that jn The native invertebrate fauna of most of common with other members of our native our rivers and streams is generally of about freshwater fauna, the crayfish are being the same level of abundance as that of thc destroyed by eels and the introduced fishes. north temperate region; for instance, there Trout - have been caught with partially would commonly be between 100 and 1000 digested crayfish in their stomachs, and it is animals over about 3mm. in length for each probable that the restriction of crayfish to square foot of stream bed. This fauna is such places as cannot be inhabited bv these clearly sufficiently abundant to support good fish is only a matter of time". Now, 45 years trout populations in waters where condi- later. there are still lakes and rivers in which tions are otherwise suitable. Similarlv., man"., crayfish are abundant despite the presence 68 NATIVE ANn INTRODUCED SPECIES IN NEW ZEALAND , of numerous trout, and there is little doubt amount of pools and other shelter, the cxtent that here, as in the northern hemisphere, of possible habitat for this genus has prob- trout and crayfish can exist side by side in a ably been drastically reduced. It is interest- balanced relationship. ing that one of the few localities where it occurs regularly near Wellington is at the Among the mayflies there are some forms, mouth of a tributarv stream where in times particularly the various species of Deleati- of flood quiet conditions due to backing up dium, which can maintain themselves iu from the parent river occur instead of the very large numbers despite the fact that usual scouring. they are freely eaten by trout. In the Horo- kiwi Stream, for instance, in 1940 they An opportunity has arisen for studying generally occurred at a density of 50 to 100 the effects of the introduction of rainbow per sq. ft. despite the fact that they made trout in L. Waingata in Northland. There up 60-90 per cent. of the food of trout in is some evidence, for instance, that the fresh- their first nine months. It has been suggested water crab, Hymenosoma lacustris, which however that other mayflies, and particu- was previously fairly common in the lake, larly Oniscigaster, are more vulnerable to may have diminished considerably in num- trout and that this may ultimately cause bers since the introduction of trout. their extinction (Till yard 1926). Some direct evidence regarding reduction in the Work has recently started on certain tribu- numbers of O.