and juvenile rainbow trout, Oncorhynchus mykiss

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Diet and food resource partitioning in koaro, Galaxias brevipinnis (Günther), and juvenile rainbow trout, Oncorhynchus mykiss (Richardson), in two Taupo streams, New Zealand Ian A. Kusabs

a b

& Stephen Swales

a c

a

Department of Biological Sciences, University of Waikato, Private Bag, Hamilton, New Zealand b

Eastern Region Fish and Game Council, P. O. Box 1098, Rotorua, New Zealand c

Fisheries Research Institute, NSW Agriculture and Fisheries, P. O. Box 21, Cronulla, NSW, 2230, Australia Published online: 30 Mar 2010.

To cite this article: Ian A. Kusabs & Stephen Swales (1991) Diet and food resource partitioning in koaro, Galaxias brevipinnis (Günther), and juvenile rainbow trout, Oncorhynchus mykiss (Richardson), in two Taupo streams, New Zealand, New Zealand Journal of Marine and Freshwater Research, 25:3, 317-325 To link to this article: http://dx.doi.org/10.1080/00288330.1991.9516485

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New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25: 317-325 0028-8330/2503-0317 $2.50/0 © Crown copyright 1991

317

Diet and food resource partitioning in koaro, Galaxias brevipinnis (GCinther), and juvenile rainbow trout, Oncorhynchus mykiss (Richardson), in two Taupo streams, New Zealand

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IANA.KUSABS 1 STEPHEN SWALES2 Department of Biological Sciences University of Waikato Private Bag, Hamilton, New Zealand

Keywords benthic invertebrates; competition; diet; dietary overlap; Galaxias brevipinnis; rainbow trout; koaro; Omori Stream; Oncorhynchus mykiss; resource partitioning; sympatry; Waipehi Stream INTRODUCTION

1

Present address: Eastern Region Fish and Game Council, P. O. Box 1098, Rotorua, New Zealand

2

Present address: Fisheries Research Institute, NSW Agriculture and Fisheries, P. O. Box 21, Cronulla, NSW 2230, Australia

Abstract This study examined the diet of sympatric populations of migratory juvenile rainbow trout and landlocked koaro in the Waipehi and Omori Streams, Lake Taupo, New Zealand. In both species, diet was dominated, both numerically and by weight, by aquatic prey: Ephemeroptera, Trichoptera, and Diptera larvae were the most numerous prey items. Adult koaro and juvenile rainbow trout both fed on small koaro. Terrestrial prey items were present in low numbers in the diets of both koaro and juvenile rainbow trout, but were more important in terms of weight. Resource partitioning was weak although koaro consumed more small benthic invertebrates such as chironomid larvae, whereas the diet of rainbow trout contained more Ephemeroptera larvae and terrestrial insects. In the Waipehi Stream, koaro consumed both rainbow trout ova and koaro ova; in Omori Stream, trout ova were important in the diet of juvenile rainbow trout. Since the diets of koaro and juvenile rainbow trout in some Taupo tributaries are similar, populations may co-exist by temporal and/or spatial partitioning of food resources, whereas trout predation on small koaro may be a limiting factor for koaro populations.

M90055 Received 17 October 1990; accepted 22 May 1991

The koaro, Galaxias brevipinnis (Giinther, 1866), is a small native galaxiid fish, widely distributed throughout New Zealand and south-eastern Australia, and is found mostly in coastal streams and rivers (McDowall 1990). Landlocked populations are also present in the alpine lakes of the South Island and in lakes of the central North Island. Koaro were once plentiful in Lake Taupo and were a significant food source for the Ngati Tuwharetoa people living around the lakeshore (Fletcher 1919). Following the introduction into the lake of brown trout, Salmo trutta Linnaeus, 1758, and rainbow trout, Oncorhynchus mykiss (Richardson, 1836), in the late 1800s the koaro population declined dramatically (Fletcher 1919). Today, Lake Taupo supports an internationally renowned trout fishery but only remnant populations of koaro remain in the lake and its tributaries (Stephens 1983). The reasons for the demise of the koaro have not been critically examined. However, the association between trout establishment in the lake and koaro decline, as well as the importance of koaro in the diet of trout at the time (Phillips 1924), suggests that the decline resulted largely from predation by trout (Stephens 1983). Koaro are one of the better known native freshwater fishes of New Zealand, largely because of the importance of koaro juveniles in the "whitebait" fishery (McDowall 1990). Information has been compiled on koaro migrations (McDowall & Eldon 1980), habitat preferences (Taylor 1988), and swimming performance (Moffat & Davison 1986). Several researchers have examined the feeding of koaro in New Zealand and have shown the diet to be dominated by aquatic insect larvae and other invertebrates (McDowall & Eldon 1980; Naylor 1983; Sagar & Eldon 1983; Rounick & Hicks 1985; Main & Winterboum 1987; Glova & Sagar 1989).

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New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25

The diet of adult rainbow trout has been well studied in New Zealand (Phillips 1924; McLennan & MacMillan 1984; Rowe 1984; Graynoth et al. 1986), but little information exists on the feeding habits of juvenile rainbow trout in New Zealand streams and rivers. The summer diets of rainbow trout adults and juveniles in Dalgety Stream, South Canterbury, were found to consist almost entirely of aquatic prey, reflecting the composition and abundance of the benthos (Fechney 1988). Stephens (1989) found that juvenile rainbow trout in the Tongariro River ate mainly aquatic insect larvae. There have been few investigations of the diets of sympatric populations of koaro and other native or introduced species. Sagar & Eldon (1983) investigated dietary overlap between koaro, bluegilled bully, upland bully, longfinned eels, and juvenile brown trout; they concluded their diets were similar and differences in habitat utilisation and feeding behaviour would reduce interspecific competition. McDowall (1990) suggested that competition for food resources between native fish and trout is an important factor in the decline of some native fish populations. The present study tests the hypothesis that juvenile rainbow trout and koaro in Taupo tributaries compete for the same food resources, by examining seasonal changes and inter-stream differences in the diet of sympatric populations of koaro and juvenile rainbow trout in two tributary streams. STUDY AREA Waipehi Stream (Fig. 1) has a mean annual discharge of 0.35 m3 s"1 (Schouten et al. 1981) and a catchment area of 26.16 km2. Its headwaters rise in the native beech/podocarp forest of the Kaimanawa Ranges at a height of 700 m a.s.l. Shallow runs dominate the upper reaches whereas the middle and lower reaches are characterised by a well-established meandering pattern with a pool-riffle structure. Plantation Pinus radiata dominates the catchment vegetation although riparian vegetation at the study sites consists primarily of kamahi (Weinmannia racetnosa), grasses, and blackberry (Rubus spp.). Omori Stream (Fig. 1) has a mean annual discharge of 0.56 m3 s r1 (Schouten et al. 1981) and a catchment area of 27 km2. Its headwaters reach to 1067 m a.s.l in southern beech (Nothofagus spp.) forest. For much of its length there is a meandering pattern and a welldeveloped pool-riffle structure. Although the dominant land use in the catchment is pastoral farming, the stream is bordered by buffer strips of Pinus radiata and Eucalyptus spp. Riparian vegetation at the study

N

-

v

176°00'E

•V

176*E

V , «North'

^

Lake

3 ^

Taupo

i

Waipehi Stream /

fOmon Stream

Fig. 1 Map of Lake Taupo, North Island, New Zealand, showing location of study streams.

sites consisted of manuka {Leptospermum scoparium), kanuka (Leptospermum ericoides), blackberry, and grasses. During the period of this study, water temperatures ranged from 7 to 15°C in the Waipehi and from 9 to 14°C in the Omori Stream. The most common fish species in both streams were koaro and juvenile rainbow trout. A few juvenile brown trout were found in Omori Stream. Spawning adult rainbow trout were numerous in both streams from autumn until spring and brown trout were common in the Omori Stream in autumn. Common bullies (Gobiomorphus cotidianus) were common in the lower reaches of both streams. METHODS Fish were captured by electro-fishing at two sites in each stream between 0800 h and 1700 h NZST. After capture, fish were anaesthetised using benzocaine or MS222 and preserved in 10% formalin or 40% isopropanol for subsequent examination. Fish were measured to the nearest mm. Stomach contents were examined using a binocular microscope. Benthic invertebrate samples were collected using a 0.1 m2

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Kusabs & Swales—Diet of koaro and juvenile rainbow trout Surber sampler with a 0.5 mm mesh net, with five replicate samples being taken from runs or riffle areas in each of the two sampling sites on the Waipehi and Omori Streams. Aquatic invertebrates were killed and preserved in 10% formalin and identified using the keys of Winterboum & Gregson (1981). Both numerical and gravimetric methods (Hyslop 1980) were used to enumerate koaro and trout stomach contents. Percentage composition was used to provide an indication of the proportion of each food type in the diet. To obtain gravimetric values, the abundance of each item of interest was; multiplied by the mean wet weight of an equivalent undigested item (from benthos samples). If taxa were not present in benthos samples, wet weights were determined using only undigested prey found in stomachs (thereby avoiding inaccuracies owing to digestion). When estimating the weights of cased trichopteran larvae and molluscs, shells were excluded. Dietary overlap between large koaro, small koaro, and juvenile rainbow trout was determined using the method of Schoener (1970): PJU

319

The index provides a comparative rather than a statistical measure of dietary overlap and is used to facilitate comparison and discussion (Townsend & Hildew 1979). RESULTS Waipehi Stream The diets of 151 large koaro (fork length (FL) > 55 mm), 119 small koaro (FL < 55 mm), and 76 juvenile rainbow trout from the Waipehi Stream were analysed (Table 1). The stomach contents of large and small koaro combined were dominated by aquatic prey, measured both by number (94.8%) and by weight (84.1%). Larval Deleatidium spp. (30%), Chironomidae (60.5%), and Trichoptera (3%) together made up 93.5% of the total number of food items consumed by small koaro over the entire sampling period (Table 2). Larval Deleatidium spp. (21.0%), Pycnocentrodes spp. (17%), and Chironomidae (16.5%) were the most abundant prey items in the diet of large koaro (Table 2). However, large prey items such as small koaro ("whitebait" stage: 59.5%), Annelida (4.5%), and Gastropoda (7.6%) constituted by weight the bulk of the diet of large koaro (Table 2). The calculated dietary overlap (0.443 by number) indicates no significant overlap in the diets of small koaro and large koaro. Juvenile rainbow trout consumed mainly Ephemeroptera larvae (Deleatidium spp. 34.3%,

~ Pyi

where a is the proportionate dietary overlap between two predators, P^ is the proportion of food category i in the diet of species x, Pyi is the proportion of food category i in the diet of species y, and n is the number of food categories. Dietary overlap values are assumed to be significant when a > 0.60 (Zaret & Rand 1971).

Table 1 Sample sizes and mean fork lengths (FL) of koaro and rainbow trout from the Waipehi and Omori Streams, 1988-89. -, no fish obtained. Rainbow trout

Koaro Waipehi

Omori

Waipehi

Omori

Winter (Jun- Aug 88) Number of fish Mean FL (mm) Range (mm)

23 65 41-118

-

-

-

Spring (Sep-Nov 88) Number of fish Mean FL (mm) Range (mm)

40 79 49-136

-

-

-

Summer (Dec 88-Feb 89) Number of fish Mean FL (mm) Range (mm)

111 65 40-175

74 33-173

Autumn (Mar-May 89) Number of fish Mean FL (mm) Range (mm)

96 83 41-150

20 87 48-135

38

34 83 32-138

-

21 98 32-205

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New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25

Coloburiscus kumeralis 15.3%), adult Chironomidae (25.7%), and small koaro (11.3%). However, small koaro (79.4%) dominated the diet of rainbow trout by weight. Chironomid larvae, which were abundant in the diet of koaro, were comparatively scarce in the diet of juvenile rainbow trout. Rainbow trout consumed more terrestrial organisms than koaro (Table 2); however, terrestrial prey comprised only 5.8% of the total weight of prey items consumed by

rainbow trout compared to 15.9% in the diet in koaro (Table 2). Calculated dietary overlap indices (0.478) indicate that the diets of large koaro and juvenile rainbow trout did not overlap significantly. Omori Stream The stomach contents of 20 koaro and 21 juvenile rainbow trout from the Omori Stream were analysed. Larval Chironomidae (63.1%), Deleatidium spp.

Table 2 Percentage abundance and biomass of prey in large and small koaro and juvenile rainbow trout from the Waipehi Stream, June 1988-May 1989.

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Koaro Small Aquatic prey Ephemeroptera Deleatidium spp. Coloburiscus humeralis Trichoptera Olingaferedayi Oxyethira albiceps Hydrobiosis spp. Hydropsychidae Pycnocentrodes spp. Plecoptera Stenoperla prasina Megaloptera Archichauliodes diversus Diptera Chironomidae Tipulidae Pisces Small koaro Rainbow trout fry Koaro ova Rainbow trout ova Retropinna retropirma Gobiomorphus cotidianus Total % aquatic organisms Terrestrial prey Annelida Arachnida Gastropoda Myriapoda Diptera Chironomidae adults Hymenoptera Lepidoptera Coleoptera Orthoptera Total % terrestrial organisms

30.0

Abundance Large Combined

Rainbow trout Abundance J J XL/111 d^

Biomass

ij

combined

2.5

21.0 5.8

26.8 4.4

4.3 2.8

34.3 15.3

2.9 1.2

0.5 0.5 0.5

0.4 0.6 4.5 17.0

0.4 0.2 0.4 3.1 13.3

0.3 2.4 1.2

0.2 0.2 0.4 0.6 4.0

0.1 0.2 0.2

1.5

3.1

2.1

5.3

1.0

1.3

_

0.4

0.4

1.3

_

_

60.5 0.5

16.5 0.4

33.3 0.4

0.4 _

1.4 —

_ _

_ _ -

9.4 _ 0.9 0.9 0.4 0.4

4.4 _ 4.8 0.4 0.2 0.2

59.5 9.4 0.7 0.9 2.5

96.5

90.2

94.8

84.1

11.3 0.8 _ 0.4 0.8 70.7

79.4 3.7 _ 0.4 4.8 94.2

_ 1.0 2.5 3.5

1.9 0.4 0.4 1.3 1.9 _ 0.4 0.9 2.2 0.4 9.8

0.8 0.2 0.4 0.6 0.6 1.0 0.2 0.4 0.8 0.2 5.2

4.5 _ 7.6 0.3 — 1.0 1.4 1.1 15.9

0.2 _ 0.2 0.2 1.2 25.7 0.6 0.4 0.6 0.2 29.3

0.6 _ 3.8 0.3 0.4 0.5 0.2 5.8

2.5

Kusabs & Swales—Diet of koaro and juvenile rainbow trout

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(8.2%), Olingaferedayi (9.0%), Pycnocentrodes spp. (6.6%), and Hydrobiosis spp. (6.6%) were the most numerous prey items consumed by koaro (Table 3). However, whereas Chironomid larvae formed more than 60% of the diet numerically, their contribution by weight (8.7%) was less than that of Hydrobiosis spp. (44.8%), Deleatidium spp. (8.2%), and terrestrial Lepidoptera (13.3%). Terrestrial Myriapoda and Lepidoptera comprised 4.1% of the koaro diet numerically but 17.0% by weight. In comparison, koaro sampled from the Waipehi Stream in autumn consumed mainly larval Deleatidium spp., Chironomidae, Pycnocentrodes spp., as well as koaro ova and koaro whitebait (Fig. 2). The diet of juvenile rainbow trout was dominated numerically by larval Chironomidae (34.7%), Deleatidium spp. (15.3%), Hydrobiosis spp. (9.3%), and trout ova (9.3%). Trout ova comprised 38.9% of the diet by weight but were not eaten by koaro. Terrestrial prey were more represented, both

321

numerically and by weight, in the diet of rainbow trout than in koaro (Table 3). The calculated diet overlap index (0.636) indicates a significant overlap in the diets of juvenile trout and koaro. Seasonal variations in the diet of koaro The diet of large koaro in Waipehi Stream in spring contained larval Pycnocentrodes spp., Deleatidium spp., and Coloburiscus humeralis which were numerically dominant, whereas Deleatidium spp. comprised 63.3% of the total number of food items consumed by small koaro. In the summer months, larval Chironomidae and Deleatidium spp. were the most numerous prey items in the diets of both large and small koaro, although koaro whitebait, koaro ova, and terrestrial Coleoptera were important in the diet of large koaro (Fig. 3). In autumn, small koaro consumed mainly larval Chironomidae (77.2%) and Deleatidium spp. (22.8%), whereas koaro adults consumed mainly larval

Table 3 Percentage abundance and biomass of prey in large koaro and juvenile rainbow trout from the Omori Stream, 17 April 1989. Koaro Aquatic prey Ephemeroptera Deleatidium spp. Coloburiscus humeralis Trichoptera Olingaferedayi Hydrobiosis spp. Hydropsychidae Pycnocentrodes spp. Plecoptera Stenoperla prasina Diptera Chironomidae Tipulidae Pisces Rainbow trout ova Mollusca Total % aquatic organisms Terrestrial prey Myriapoda Diptera Chironomidae adults Lepidoptera Coleoptera Tipulidae Total % terrestrial organisms

Rainbow trout

Abundance

Biomass

Abundance

Biomass

8.2 -

14.0 -

15.3 0.8

5.6 1.0

9.0 6.6

8.7 44.8

3.4 9.3

0.6 11.7

_ 6.6

_ 6.4

_ 8.5

_

-

-

-

-

5.1

63.1 0.8

8.7 0.3

34.7 -

0.8 -

_ 1.6

0.1

95.9

83.0

9.3 81.3

38.9 63.7

3.3 0.8 4.1

3.7 13.3 17.0

7.6 5.9 0.9 0.9 2.5 0.9 18.7

1.8 0.1 25.2 7.6 1.6 36.3

New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25

322 30

Taxa: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Waipehi Stream n=70

20

g 10 CD O

Ic 0 => n CO

I 60-

Omori Stream n=20

co

1

40 20

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0

0

2

4

10

6

12

14 16 18 20 22 24

T ax a

Fig. 2 Percentage abundance of prey items in autumn in koaro from Waipehi and Omori Streams.

80T

Adults

Deleatidium spp. Coloburiscus spp. Chironomidae Tipulidae Hydropsychidae spp. Olinga feredayi Pycnocentrodes spp. Hydrobiosis spp. Archichauliodes diversus Stenoperla prasina small koaro koaro ova Retropinna retropinna Gobiomorphus cotidianus Lepidoptera Annelida Arachnida Coleoptera Myriapoda Terrestrial Diptera Chironomidae adults Hymenoptera Gastropoda Orthoptera

Juveniles

Spring n=29

Spring n=11

6040200i 0 Summer n=71

Summer n=40 ~

60-

60-•

4020 pg

0 Autumn n=26

Autumn n=70 6040 20 .

. -r-r

Winter n=12

i 0

Winter n=11

604020-

0

Fig. 3

2

4

6

8 10 12 14 16 18 20 22 24 Taxa

0

2

4

6

8

10 12 14 16 18 20 22 24 Taxa

Seasonal changes in composition of the diet of large and small koaro in the Waipchi Stream, 1988-89.

323

Kusabs & Swales—Diet of koaro and juvenile rainbow trout Deleatidium spp. (21.2%), Chironomidae (15.9%), Pycnocentrodes spp. (11.5%), whitebait (8.8%), and koaro ova (19.5%) (Fig. 3). In winter, small koaro consumed mainly larval Deleatidium spp. (61.5%), Coloburiscus humeralis (23.1%), and Chironomidae (15.4%), whereas the diet of large koaro consisted mainly of larval Deleatidium spp. (37.5%), Chironomidae (12.5%), whitebait (37.5%), and koaro ova (12.5%).

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DISCUSSION Dietary composition As in previous studies we found that koaro feed mainly on aquatic prey (Table 4). In both the Waipehi Stream and Omori Stream, the diet of koaro and juvenile rainbow trout was dominated numerically by benthic prey. However, koaro whitebait, fish ova, and terrestrial prey were more important in terms of biomass. Koaro in the two Taupo streams we studied consumed a greater proportion of Chironomidae larvae than did juvenile rainbow trout. In the Ryton River, Canterbury, Glova & Sagar (1989) found that koaro were active mainly at night, feeding primarily on small larval Ephemeroptera, Trichoptera, and Diptera. They suggested that the over-representation of small prey seen in the diet of koaro (compared to that available in the stream), is apparent in many nocturnally active native fish. The diet of juvenile rainbow trout in the Waipehi and Omori Streams was similar to that recorded in other studies (Fechney 1988; Stephens 1989: Table 4). Juvenile rainbow trout are mainly daytime feeders

(Angradi & Griffith 1990) preferring larger prey items than koaro (Table 4). Seasonal changes in diet Large and small koaro in the Waipehi Stream showed similar seasonal changes in diet. In summer and autumn, larval Chironomidae were most numerous in the diets of large and small koaro, whereas in winter and spring, larval Deleatidium spp. were abundant in the diets of both large and small koaro. Throughout the study period terrestrial prey were less numerous than aquatic prey in the diets of both koaro and juvenile rainbow trout, although terrestrial prey was important by weight in summer and autumn. In contrast, a summer study of koaro in several South Westland streams showed that terrestrial prey comprised the bulk of the diet by weight (Main & Winterbourn 1987). Small koaro were important components of the diet of large koaro throughout the year in the Waipehi Stream. This is consistent with the finding that small koaro were present in the lower reaches of both study streams throughout the year (Kusabs 1989). The ova of both trout and koaro were important in the diet of large koaro in autumn and winter, when both species spawn. Food resource partitioning Fish species with similar diets co-exist in many New Zealand streams (Scrimgeour & Winterbourn 1987), and it is thought that inter-specific competition for food does occur between several species (Cadwallader 1975). However, direct competition is often reduced by different spatial or temporal feeding habits, by

Table 4 Most abundant (by number) food items consumed by koaro and juvenile rainbow trout in some New Zealand streams. Food

Reference

Koaro: Ephemeroptera, Trichoptera, Diptera Ephemeroptera, Trichoptera, terrestrial Trichoptera, Plecoptera Ephemeroptera Small koaro, Ephemeroptera, Diptera Diptera, Ephemeroptera, Trichoptera

Glova & Sagar (1989) Main & Winterbourn (1987) Rounick & Hicks (1985) Sagar & Eldon (1983) Waipehi Stream (this study) Omori Stream (this study)

Juvenile rainbow trout: Diptera, Ephemeroptera Diptera, Trichoptera Ephemeroptera, small koaro, Diptera adults Ephemeroptera, Trichoptera, Diptera, fish ova

Fechney (1988) Stephens (1989) Waipehi Stream (this study) Omori Stream (this study)

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New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25

consuming conspecific but different-sized prey items, or by feeding in different micro-habitats (Sagar & Eldon 1983; Glova & Sagar 1989). Cadwallader (1975) investigated the feeding relationships between brown trout and various native fishes in the Glentui River, Canterbury, New Zealand, and found direct competition for food between brown trout and Galaxias vulgaris. However, in Lake Taupo tributaries it appears that resource partitioning through time and different feeding modes may be occurring between rainbow trout and koaro, thus reducing the amount of direct competition for food. Juvenile rainbow trout are mainly diurnal drift feeders (Angradi & Griffith 1990) occupying the middle zone of the water column, whereas koaro are nocturnally active predators feeding mainly on benthic invertebrates (Glova & Sagar 1989). CONCLUSION Koaro in Lake Taupo tributaries may now only be common in habitats where they can avoid direct competition with juvenile trout through food resource partitioning. The success of koaro may depend on the availability of a very heterogenous habitat or a habitat which is unavailable or unusable to rainbow trout. Koaro are common in the Waipehi Stream, where the diet overlap between koaro and juvenile rainbow trout is low, whereas in the Omori Stream—where diet overlap between the two species is higher— koaro densities are lower than in the Waipehi Stream (Kusabs 1989). Trout may have out-competed koaro from certain habitat types, and in some areas preyed upon them to the point of near extinction. Koaro densities are extremely low in the Waiotaka and Waimarino Streams, the larger Taupo tributaries which provide important trout spawning and juvenile rearing habitats (Kusabs 1989). The decline in the koaro population in Lake Taupo has been associated with the establishment of trout at the end of last century (Fletcher 1919; Armstrong 1935; Burstall 1983; Stephens 1983). Our study indicates that juvenile rainbow trout in two Lake Taupo tributaries feed extensively on large prey items, including small koaro. However, most diet studies of adult trout in New Zealand and Australia have suggested that galaxiids are not an important source of food (McLennan & MacMillan 1984; Rowe 1984; Stephens 1984). This may reflect the comparative scarcity of galaxiids in the environment where the studies were conducted, as many of the early diet studies of trout in New Zealand and Australia were undertaken well after the initial introduction of trout.

In Lake Taupo, koaro are virtually absent from the diets of adult rainbow trout taken from the lake, where smelt are the main food source (Stephens 1984; M. Cryer pers. comm.). Before trout were introduced to the lake, adult koaro were abundant in the lake benthos and pelagic zone (Fletcher 1919), but few now appear to be present in these areas, where smelt are now the most abundant forage fish (T. Stephens pers. comm.). Other factors which could have contributed to the decline in koaro numbers include competition between smelt and juvenile koaro in the pelagic zone and the introduction of diseases at or about the time of trout establishment. The results of this study indicate that sympatric populations of koaro and juvenile rainbow trout in two tributaries of Lake Taupo show dietary overlap and may co-exist by partitioning food resources, through different feeding times and feeding modes. The high incidence of small koaro in the diet of rainbow trout fingerlings indicates that small koaro are preyed on by rainbow trout. This is consistent with the hypothesis that trout predation was a major factor in the decline of Taupo koaro populations following the introduction of trout at the end of the last century.

ACKNOWLEDGMENTS We thank M. A. Chapman and M. L. Rosenau for assistance during the study. We also thank D. Bell, L. Laboyrie, D. Tully, D. W. West, and R. J. Pitkethley for assistance in the field, and R. T. T. Stephens, M. A. Chapman, and J. A. T. Boubee for constructive criticism of the manuscript. We gratefully acknowledge the financial support provided by the Tuwharetoa Maori Trust Board, Maori Education Foundation, and the Department of Conservation.

REFERENCES Angradi, T. R.; Griffith, J. S. 1990: Diel feeding chronology and diet selection of rainbow trout (Oncorhynchus mykiss) in the Henry's fork of the Snake River, Idaho. Canadian journal of fisheries and aquatic sciences 47: 199-209. Armstrong, J. S. 1935: Notes on the biology of Lake Taupo. Transactions and proceedings of Royal Society of New Zealand 65: 88-94. Burstall, P. J. 1983: Trout fishery-history and management In: Forsyth, D. J.; Howard-Williams, C. co-ord. Lake Taupo. Ecology of a New Zealand lake. New Zealand Department of Scientific and Industrial Research. DSIR information series 158:

119-131.

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