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66 NATIVE AND INTRODUCED IN 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 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 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 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 (Salina trutta and S. gairdnerii) and importation by man are so few in this case. the quinn at ( 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 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 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 food; the 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 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, on small native animals, both the salmonids have had upon the original and . 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 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 , have realised that the species they werc the fish tending to advance through a sequence interested in were subject to predation by in this 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 and the introduced . 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 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 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 , 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 (Till yard 1926). Some direct evidence regarding reduction in the Work has recently started on certain tribu- numbers of O. wakefieldi is given by Hudson taries of the Wangaehu River which trout (1904), who quotes Hutton as saying "In have been unable to reach because of the 1874 the was common in the neigh- permanently toxic conditions in the main bourhood of Christchurch. I have lived therc river. There is considerable variation in the during the last] 9 years without seeing a bottom faunas among the troutless streams single specimen. Whether they have been themselves, apparently due to environmental killed off by the trout or by the sparrows conditions, but a recent comparison of their I cannot say." Speaking of the related O. faunas with that of another tributary with an dislans, Hudson himself seems more sure abundant stock of supports that the insect is destined for extinction the following general conclusions. than of the cause which will bring it about. He says of the , "It is not very activc, and owing to its large size, is easily seen 1. The stream containing trout has gen- and captured by means of the hand alone. erally a greater number of individual Hence its speedy extinction by trout appears animals to the square foot than the inevitable". However, when speaking of the troutless streams. imago, he writes, "Whilst thus resting it is no doubt often devoured by birds and the 2. In the stream with trout the propor- insect's early extinction through this cause tion of the fauna made up of animals alone would appear probable". Whatever freely eaten by trout is at least as their numbers may have been originally the great as in the troutless streams. mayflies of this genus are now far from common in New Zealand, although still 3. The troutless streams apparently con- widely dispersed. If the numbers have indeed tain no species which do not com- diminished severely, introduced fish and monly occur in streams containing birds may have been at least part of the trout. cause, but physical changes may have played a part. The nymphs of this genus are active Thus, here, at any rate, the introduction swimmers and also rest in exposed positions of trout seems to have made no substantial on the tops of stones. They are thus ill- difference to the composition of the bottom adapted to withstand strong water-currents fauna. and normally live in sheltered pools and backwaters. Where, as has occurred in many The general conclusion is therefore that places, man's activities have reduced the the introduction of Salmonidae has probably stability of river beds and diminished the not led to any great change in the fauna ALLEN: SALMONIIJAlj AND FRESflWATER FAUNA 69

of New Zealand freshwaters, particularly as numerous, as in Lakes Taupo and Roto- where the bottom fauna is concerned. It is rua; but where there is abundant spawning however possible that a few species have ground in relation to the food supply the been adversely affected; these are likely to trout may only reach a small size; this be forms which combine habits and appear- appears to occur, for instance, in the upper ance making them attractive to trout, with Waihou River system. Whether the intro- a low biological potential giving them little duction of salmonids has affected the capacity to withstand increased predation. population of New Zealand is hard to deter- This conclusion might have been unexpected mine. Although they have provided an addi- if New Zealand fresh waters had been origin- tional source of food for the larger eels, ally without carnivorous fish, but in most over about 26in. in length, they may be of our waters the bottom fauna was in fact competing to some extent with the smaller already subject to considerable predation eels which feed mainly on the bottom fauna. from a fish population including, to varying Burnet (1959), however, has found that extents in different localities, the smaller fluctuations in numbers of trout sufficient to eels, the larger bullies and galaxiids, and the affect their growth rate through intraspecific grayling (Prototroctes oxyrhynchus). The competition for food, have no measurable virtually complete disappearance of grayling effect on the growth rate of small eels between 1870 and 1925, from previous great inhabiting the same waters. The introduc- abundance, is, incidentally, by far the most tion of trout has also led to some deliberate striking biological change which is known destruction of eels with the object of bene- to have occurred in New Zealand waters. fiting the trout stocks; it is doubtful, how- Since it has happened in some places where ever, whether this has had any significant trout are still scarce or absent, and in others effect on eel populations except occasionally had begun before trout arrived, it does not in very restricted areas. In any case, destruc- seem that, whatever its cause was, it can tion of eels for this purpose may have merely be laid at the door of the salmonids. replaced the taking of eels for food by Maoris in pre-European times. It is more The reverse relationship, predation on likelv that the numbers of eels have been Salmonidae by native animals, may be more reduced by the destruction of cover, since briefly considered. The two principal groups this is a very important factor controlling of predators are shags, particularly the black eel population (Burnet 1952). shag (Phalacrocorax carbo), and eels, espe- cially the long-finned eel (Anguilla dieffen- The relationship between the shags and bachi). Although both species are undoubt- trout is complicated by the fact that, unlike edly predatory on trout to some extent, the eels, shags are able to move freely and existing trout populations were established rapidly between one body of water and an- in the face of this predation, and it seems other, and indeed between inland waters unlikely that the native predators will seri- and the sea. Shags are also to some extent ously menace their continued survival. gregarious, and quite large bands of them sometimes descend on a particular length of river, fish it industriously for a time, and Work in progress on the effect on trout then depart elsewhere. Occurrences of this populations of the removal of eels is still kind may have a considerable effect on the in an early stage, but results suggest that in trout population of the water concerned, some circumstances it may lead to a sub- provided that other prey is not available. stantial increase in the numbers of trout. Where, however, other less active and pre- Areas in New Zealand which are perman- sumably more easily caught fish are present, entlv eel-less, as a result of natural barriers as for instance the goldfish in Lake Rotorua, to ihe upstream migration, tend to have very few trout seem to be eaten (Dickinson abundant trout stocks. The structure of the 1951). Shags probably have their greatest trout population in these circumstances may depend on the balance between food supply effect on trout in areas, such as parts of the and spawning grounds. Where the feeding South Island, where other species of fish are grounds are extensive in relation to the relatively scarce in the rivers. Destruction spawning areas, trout may be large as well for the intended benefit of the trout has 70 NATIVE AND INTRODUCED SPECIES IN NEW ZEALAND probably been more effective against shags BURNET,A. M. R., 1952. Studies on the ecology of the New Zealand long~finned eel, Anguilla diefJmbachii than against eels, and in some areas few Grey. Ailsf, J. Mar. Freshw. Res. 1: 32-63. inland. nesting sites are allowed to remain occupied for long. The coastal colonies, BURNET,A. M. R., 1959. Some observations on natural fluctuations of trout population numbers. N.Z. J. however, which are largely untouched, prob- Sci. 2: 4\0-421. ably provide a reservoir from which the inland populations can be quickly rebuilt DICKINSON, p" 1951. Stomach contents of New Zealand when control measures are relaxed. inland shags. Alut. J, Mar. Freshw. Res. 2: 245-253. HUDSON, G. Y., 1904. New Zealand Neuroptera. West, REFERENCES Newman & Co., London. ALLEN, K. R., 1941. Comparison of bottom faunas as TILLYARO,R. 1., 1926. The insects of and New sources of available fish food. Tram. Amer. Fish. Zealand. Angus & Robertson Ltd., Sydney. Soc. 7\: 275-283.