Population structure, distribution, reproduction, diet ...

Journal of the Royal Society of New Zealand

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Population structure, distribution, reproduction, diet, and relative abundance of koaro (Galaxias brevipinnis) in a New Zealand lake D. K. Rowe , G. Konui & K. D. Christie To cite this article: D. K. Rowe , G. Konui & K. D. Christie (2002) Population structure, distribution, reproduction, diet, and relative abundance of koaro (Galaxias brevipinnis) in a New Zealand lake, Journal of the Royal Society of New Zealand, 32:2, 275-291, DOI: 10.1080/03014223.2002.9517695 To link to this article: http://dx.doi.org/10.1080/03014223.2002.9517695

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Date: 28 January 2016, At: 20:09

© Journal of The Royal Society of New Zealand, Volume 32, Number 2, June 2002, pp 275-291

Population structure, distribution, reproduction, diet, and relative abundance of koaro (Galaxias brevipinnis) in a New Zealand lake

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D. K. Rowe1, G. Konui2, and K. D. Christie3

Abstract The koaro population of Lake Rotoaira was investigated during September 1998 and April 1999. Abundance (mean CPUE) in fyke nets was lower on the eastern side of the lake than on the western side where underground springs occurred and rocky substrate predominated. Adults were 54-135 mm long and the sex ratio varied with size, with 68% of the smaller fish (60-7) mm) being male and 79% of the larger fish (70-135 mm) being female. All fish over (6) mm were sexually mature with 60% having either well developed or ripe gonads by September. Few females (12.5%) had spawned by September whereas most (83%) were spent by April, so spawning occurred over summer months. Abundance in the lake dropped by over 70% between September and April and this seasonal decline is consistent with a summer emigration of many adult koaro into inlet springs and streams to spawn in lake tributaries during summer and autumn. Overall, the diet of lake-dwelling koaro was dominated by purse caddis larvae (Paroxythira sp.), but small fish (9 0 mm) fed on Odonata larvae, snails, and common bullies. Although the introduction of rainbow trout in 1906 reduced the koaro population of Lake Rotoaira, they were still relatively abundant in the lake in the early 1970s. However, by 1999, no koaro were found in the sublittoral zone of Lake Rotoaira, and trapping in the littoral revealed a relatively low abundance relative to other comparable lakes. These data, plus anecdotal information on abundance, suggest that they are now much less abundant than in the 1970s. Keywords Lake Rotoaira; landlocked population; reservoir; hydroelectric power; spawning; size; prey species; migration; rainbow trout; koaro INTRODUCTION Before the introduction of rainbow trout (Oncorhynchus mykiss Richardson) into Lake Rotoaira in 1906 (Whitney 1944), the koaro {Galaxias brevipinnis Giinther) was the only fish present (McDowall et al. 1975). Anecdotal reports indicate that it was abundant at this time. For example, in 1920, Phillipps (1924, p. 190) witnessed the annual harvest of large numbers of koaro from a spring in the lake. He stated: "In November of each year Tokaanu Maoris journey to Lake Rotoaira, where they remain for a few months and secure enormous numbers of this fish. In the summer of 1920I had the opportunity of visiting such an encampment and 1 NIWA, National Institute of Water and Atmospheric Research Limited, P.O. Box 11 115, Hamilton, New Zealand. Email: [email protected] 2 c/o Lake Rotoaira Trust, P.O. Box 36, Turangi, New Zealand. 3 NIWA, National Institute of Water and Atmospheric Research Limited, P.O. Box 329, Turangi, New Zealand. R01012 Received 19 July 2001; accepted 3 January 2002; published 5 June 2002

Rowe etal Population P65

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observing the ingenious manner in which koaro were caught. A large spring is selected, and operations commence at 2:30 in the morning, when a fishing-party armed with a hinaki, or funnel-shaped basket net, visit the spring and place the hinaki with the mouth pointed towards the lake to block the entrance of fish from the lake to the underground channel. The fishing party then return to bed. About 6 a.m. the hinakis are raised, and found to be crammed with koaro about 2 in. to 6 in. in length". Phillipps (1924) indicated that Maori thought that the koaro hibernated in the springs during winter as few were seen in the lake during winter months. Because of its habit of moving into springs and subterranean caves, the Lake Rotoaira koaro was initially thought to be a different species to the Galaxias brevipinnis found in other lakes and was given the name Galaxias koaro (Stokell 1955). Today it is recognised as another landlocked population of Galaxias brevipinnis. Little is known about the ecology of koaro in New Zealand. In rivers, it is an amphidromous species, with adults living and breeding in small streams, and the larvae and juveniles rearing at sea until their annual migration back into rivers during spring months (McDowall 1990). Stokell (1955) outlined the basic life history of koaro in New Zealand lakes. He thought that spawning took place in the rocky or shingly tributary streams of lakes, and that the young existed in these streams until they were about 2.5 cm long. He assumed that, after reaching this size, they migrated down to the lake where they formed large shoals. Therefore, the main difference between lacustrine and riverine life histories was that lakes served as an inland sea for the rearing of juveniles, and that the adults of landlocked lacustrine stocks inhabited the lakes as well as the inlet streams. Kusabs (1989) studied fluviatile populations of adult koaro in several inlet streams of Lake Taupo. He found that juveniles that had been reared from larvae in the lake migrated into the streams from August until February, with a peak in December. Fish grew from 50 mm (when they first entered streams) to 130 mm in 4 years. Ripe female fish were caught in spring and, as gonadosomatic indices (reflecting gonadal size) were highest over summer/ autumn months, spawning was thought to occur during these months. O'Conner & Koehn (1998) reported Galaxias brevipinnis eggs from the cobbly margins of an Australian stream, and Allibone & Caskey (2000) found a koaro nest in the margin of a stream on cobbles 0— 2 cm deep. Therefore, lacustrine populations of koaro can be expected to spawn in the cobbly margins of inlet streams of lakes. However, it is unlikely that the larval stage remains in these streams for long as proposed by Stokell (1955). Larval koaro presumably hatch when rising water from flood flows raises the stream level and inundates the eggs (McDowall & Suren 1995; O'Conner & Koehn 1998). Larval fish are not strong swimmers, so most would be carried downstream to the lake by such flood flows. Here, they rear through to the juvenile migrant stage and then congregate in large shoals near the stream mouths as described by Stokell (1955). Observations on the depth distribution of koaro indicate a limnetic existence for juveniles and a near benthic distribution for adults. Juveniles (36—49 mm long) are believed to be schooling and limnetic as they feed mostly on planktonic prey, particularly Daphnia (Stokell 1955). Adults are generally caught close to the lake bottom down to at least 80 m (Michaelis 1982; Rowe 1993a). Koaro were once abundant in many New Zealand lakes but declined following the introduction of trout. Rainbow trout preyed heavily on the juvenile stage (Fletcher 1919a; Buck 1921; Armstrong 1935). In Lake Rotoaira, the traditional Maori harvest of koaro declined soon after trout were introduced into the lake (Grace 1959; Cudby 1984) but the fishery was still in existence in 1923 (Phillipps 1924), 18 years after the introduction of trout. Koaro also remained abundant enough in lakes containing rainbow trout to constitute an important prey species. For example, although koaro are now rarely consumed by trout in the


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Rowe et al.—Koaro in Lake Rotoaira

277

Fig. 1 Lake Rotoaira showing the main inlet streams, hydro canals, and the water outlet to the Tokaanu Power Station (#,61601110 fishing sites; • , fyke net sites September 1998, O, fyke net sites April 1999; —, transect line for G-minnow traps).

Rotorua lakes, they were still a major forage fish in several lakes in 1918, 20 years after the liberation of trout (Rowe 1990). Similarly, nearly 60 years after the introduction of trout to Lake Rotoaira, koaro were still a major prey species for trout (Rowe et al. 2000). Predation by rainbow trout therefore reduced the abundance of koaro in these lakes, but did not result in them becoming rare or extinct. Today, koaro are still common in lakes where rainbow trout are present, such as Waikare-iti in the North Island (McDowall 1990), and Lakes Pukaki, Tekapo, and Alexandrina in the South Island (D. K. Rowe unpubl. data). Their decline to the point where they are rare only occurred when smelt were introduced to lakes (as a forage fish for trout) and displaced the koaro (Rowe 1993b). As smelt have not been introduced to Lake Rotoaira, koaro could be expected to still be relatively common and to constitute an important prey for the rainbow trout. However, in 1998, the status of the koaro in this lake was unknown. In November 1972, large numbers of juvenile migratory koaro were photographed attempting to climb the vertical side of the concrete blockchute in the Wairehu Canal (McDowall 1990). This canal (Fig. 1) brings water


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from Lake Otamangakau into the northern end of Lake Rotoaira. The presence of large numbers of juvenile migrant koaro at the base of the blockchute indicates that they were attracted into the canal. The high water velocity, steep slope, and height of the blockchute prevents upstream passage by fish. However, juvenile koaro have been observed climbing the wetted concrete margin (McDowall 1990). Many fish would presumably perish while attempting this climb, and those that did succeed (e.g., when flows were reduced or stopped), would find another six velocity barriers further upstream. To date, no koaro have been recorded from Lake Otamangakau (Dedual et al. 1997), so whereas some koaro may climb the first blockchute, none make it past the remaining six barriers and into Lake Otamangakau. A similar situation is likely to have occurred in the Poutu Canal (Fig. 1) which conveys a relatively large flow of water into the southern end of the lake. Juvenile migrants entering this canal would not be able to ascend the dropchute where water from the Tongariro River enters the canal. They would be confined to the canal which, being concrete lined, provides poor habitat for fish. Most juvenile migrant koaro attracted from Lake Rotoaira into these two canals are therefore likely to be lost from the lake and, by attracting large numbers of migrant juveniles out of the lake into poor habitat, these canals could potentially reduce adult abundance in the lake. Because of such concerns for koaro, a study was undertaken in Lake Rotoaira to determine the main features of this species' life history and ecology. In this paper, we present the first data on the population structure, distribution, habitats, reproduction, diet, and the relative abundance of a koaro population in a New Zealand lake. We also provide an assessment of the current population status of koaro in Lake Rotoaira. METHODS Study site Lake Rotoaira (Fig. 1) is located near the centre of the North Island of New Zealand, between Lake Taupo to the north and Mt Tongariro to the south. Its altitude (564 m a.s.l.) is 200 m higher than Lake Taupo, making it one of the highest natural (versus constructed) lakes in the North Island. It is a relatively shallow lake (mean depth 8.9 m, maximum depth 14.0 m) with a surface area of 15.3 km2. It is fed by two large tributary streams which drain the flanks of Mt Tongariro, and by a number of smaller streams, many of which contain springs (Fig. 1). The western side of the lake around Ngapuna and Tautaranui (Fig. 1) is characterised by a relatively steep, rocky, littoral zone and by underground springs, whereas the northern, eastern, and southern shores are less rocky and contain large expanses of shallow beach. Before construction of the Tongariro Power Development scheme, the only outlet for water from the lake was the Poutu Stream which has now been dammed and replaced with the Poutu Canal (Fig. 1). The mean flow in this stream was 6.9 cumecs. Now the only outlet is the power station intake at the northern end of the lake. Sampling To determine the distribution and size of koaro in a range of lake habitats, sampling was undertaken in spring (September 1998) and autumn (April 1999). As adults in lakes are generally benthic (Rowe 1993a,b), and are readily sampled by fyke netting (MeredythYoung & Pullan 1977), the shallow littoral zone of the lake was sampled with 16 fyke nets. The nets (length 2 m, diameter 55 cm, mesh size 6 mm) were set in 1—3 m deep water around the lake shore at locations differing in substrate composition (i.e., sand/rock/weed). Proximity to stream mouths and/or springs was also noted. The nets were unbaited and set overnight at


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right angles to the shoreline. They were lifted the following morning. The catch was sorted, identified to species, and the number and size of fish recorded. The relative abundance of koaro was determined by calculating the mean number offish per net per night, expressed as mean catch per unit effort (CPUE). Mean CPUE was then compared between benthic substrates (rock, sand, weed), between types of water inlet (streams versus springs), between western and eastern sides of the lake, and between spring and autumn. Associations between these factors and koaro abundance were tested using 2-way ANOVA on the log (x + 1) transformed catch data. In the laboratory, koaro were dissected and the gonads inspected to determine sex and reproductive status. The stomach of each fish was removed and weighed both with and without contents to determine the weight of food present. Prey species were identified, and the number and weight of each prey type in each stomach was determined. The occurrence of these prey groups was compared among koaro size classes. The importance of each prey group was assessed by comparing the overall proportion of each in the diet of koaro in terms of both numbers and weight. Sex ratios were determined from the pooled catch data and compared between spring and autumn months. Sex ratios were also compared among size/age groups. Gonad status was determined on a four-point scale (immature, developing, mature/ripe, and spawned) and compared between spring and autumn. Adults on the lake bottom below the littoral zone were sampled with G-minnow traps. This method has been successfully used to sample koaro from 0-30 m in several South Island lakes (D. K. Rowe unpubl. data). In Lake Rotoaira, it was used to examine koaro depth distribution and to see whether koaro occurred below the littoral zone. Traps were set at depths of 0.5, 2.5, 5.0, 7.5, 10.0, and 12.5 m along four transects, one in each quadrant of the lake. Traps in the 0.5—7.5 m depth range were placed in the littoral zone. Traps in the 10— 12.5 m depth range were on the lake bottom 50-200 m from the shoreline. The traps were baited (with trout pellets as in the South Island lakes sampled) and set overnight. The following morning, the traps were lifted and the number and size of fish recorded. Juvenile koaro are shoaling and limnetic (Stokell 1955) so can be expected to occur in the surface waters of the lake. Beach seining (7 m long X 2 m deep X 3 mm mesh) was carried out on beaches in the northern and southern ends of the lake in both September and April. Two to three hauls, each for 10-20 m along the beach, were carried out at each location. Purse seining with a 25 m long, 3 m deep, and 1 mm mesh net was also used to sample the top 3 m of water in the limnetic zone. Five sets were carried out in September before lacustrine juveniles moved into stream mouths. Sets were equally spaced along the main axis of the lake. In addition, a high frequency echosounder (200 kHz), capable of detecting schools of small fish such as smelt and koaro (Rowe 1993a), was used to sample the open, limnetic zone. An acoustic profile was run across the lake, from Ngapuna (Fig. 1) on the western side of the lake to the nearest point on the eastern shoreline. Little is known about larval koaro in lakes, but they can be expected to be planktonic (McDowall 1990). Drop netting was therefore carried out in both September and April to determine whether larval fish were present in the water column. Assuming that spawning occurs in summer/autumn months, newly hatched larvae would be expected to be present in the lake by April, with larger/older specimens still present by September. A Wisconsin, closing drop net (1.7 m long, 57 cm diameter, 420 |lm mesh) was allowed to sink vertically from the water surface to a depth of c.1 m from the lake bottom. It was then throttled and hauled to the surface. The catch was washed into a container and examined. All larval fish were removed and counted. Drop netting was carried out at five sites spaced regularly along


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280



Fig. 2 Size frequency distribution of koaro sampled in littoral fyke nets in Lake Rotoaira in September 1998 ( • ) and April (D) 1999.

60

70

80

90

100

110

120

130

140

Size range (mm)

the main north-south axis of the lake, and at two other sites either side of the main axis. These latter sites were midway between the main axis and Tautaranui and the Onepoto spring, respectively (Fig. 1). Larval fish were also sampled at the intake embayment at the entrance of the Tokaanu Tailrace (Fig. 1). Four plankton nets (57 cm diameter, mesh size 300 |lm) were set just below the water surface, across the mouth of the intake embayment and above a sill where water depth was 2—3 m. These nets were fished for c. 30 min during both the day (10.00 a.m.—3.00 p.m.) and 1 h after dark, in both January and April 1999. The catch, mainly of zooplankton, was preserved and stained. It was later examined and all larval fish removed, identified to species, and counted. Drop netting (as above) was also carried out in January along the main axis of the lake. The main tributary streams of the lake (Fig.1) were electric fished with a backpack machine (EFM300) to determine the presence/absence of koaro and other fish species. Stretches of stream ranging in length from 15 to 50 m were sampled, except in the Wairehu Stream above State Highway 47A (SH 47A), where pools and rapids were fished approximately every 50 m all the way up to the falls. The species and sizes of all fish present were recorded. RESULTS Size of koaro Fyke nets set around the shoreline of Lake Rotoaira caught koaro, common bullies, trout, and crayfish. No smelt were caught. Overall, 64 koaro were caught in September 1998, and 19 in April 1999. The length of the koaro ranged from 50 to 135 mm (Fig. 2), with the largest fish (TL > 100 mm) all being caught in September. The length/weight plot and regression for the log transformed data is shown in Fig. 3. The two largest (TL > 120 mm) fish caught in September were both females that had spawned, so were 4-5 g lighter than expected from the length-weight relationship for other fish at this time.


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Rowe et al.—Koaro in Lake Rotoaira Fig. 3 Length-weight relationship for koaro from Lake Rotoaira (1998-1999).

281

20 18

Log W = 3.38* log L-5.81 R2 = 0.99

16 14

3

12

|> 10

50

90

70

110

130

150

Length (mm) Fig. 4 Dot histograms showing differences in adult koaro catches (CPUE) between western and eastern sides of Lake Rotoaira in September 1998 and April 1999.

ou •

25 D)

20 -

net A


I

15 •

sz CO *—

LJJ D.

O

10 — •

5—

* ••

• • •

n

West

East

Sep

• • ••••

West

• •

East

Apr

Abundance, distribution, and habitat associations Koaro were 70% more abundant in September than in April (Fig. 4) and overall mean abundance was significantly greater on the western shoreline than on the eastern shoreline (ANOVA, F = 4.67, P = 0.04). This geographical difference probably reflects the generally greater amount of rocky habitat, and the presence of springs, on the western rather than the eastern side of the lake. Associations between the abundance of adult koaro (mean CPUE)


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282


30

O)

20

Fig. 5 Dot histograms showing differences in adult koaro catches (CPUE) between substrate types and water inlets (springs versus streams) in Lake Rotoaira (1998-1999).


CL O

10

and habitat type indicated that most koaro were caught over rocky substrates (Fig. 5). The differences in mean abundance between the three different substrate types (i.e., rock, sand, weed beds) were not significant (P > 0.20), but this probably reflects the small sample sizes. Koaro were more abundant near outlet springs than near stream mouths (ANOVA, F = 4.63, P = 0.04). G-minnow traps, set on the lake bottom, caught common bullies (Gobiomorphus cotidianus) and crayfish (Paranephrops planifrons), but no koaro. Purse seining and beach seining in September failed to catch any juvenile koaro in the lake. Furthermore, spike-like echoes made by schools of small forage fish (Rowe 1993a), were absent in echograms from Lake Rotoaira. In April, several juveniles (39—47 mm long) were caught by electric fishing in the Onepoto Stream and Rolfs Stream (Fig. 1), and one was caught by beach seining in the lake near the mouth of the Tahurangi Stream (Fig. 1). Juvenile koaro were therefore present in the lake and its tributary streams, but were not abundant. No larval fish (koaro or bullies) were caught by drop netting in September or April, but some larval bullies (1.25 fish per haul) were present in the southern end of the lake in January 1999. Drop netting only samples small volumes of lake water (i.e., a 0.25 m2 column) so is not particularly effective when densities of larval fish are low. Plankton nets placed across the mouth of the intake embayment to the Tokaanu Tailrace in January and April sampled a much greater volume of water and caught both larval koaro and bullies. Larval koaro were only present in the night samples (Fig. 6). Their size range (Fig. 7) indicated that they were not all recently hatched emigrants. Adult koaro were common in some streams but rare in others. They were relatively abundant (0.6 fish nr 2 ) in the lower reaches of Rolfs Stream, immediately above its entrance into the Poutu Canal. Both juvenile trout and common bullies were also present here. Adult koaro were also present, along with juvenile trout and common bullies, in the lower reaches of the spring-fed stream in Onepoto Bay. However, no koaro were found in the Wairehu Stream between State Highway 47A (SH 47A) and the falls (Fig. 1), although some were present above the falls. In comparison, juvenile trout were abundant at all sites fished below these falls whereas none was present above the falls. These falls therefore limit the upstream penetration of trout but not koaro. Koaro were absent from the lower reaches of the Tatahi


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Rowe et al.—Koaro in Lake Rotoaira Fig. 6 Mean (± 1 s.e.) catches of larval koaro during the day and night at the Tokaanu Power Station water inlet canal in Lake Rotoaira in January and April 1999.

283

7 6 "u- 5 CD

c 3 CO


I 2 1 0 Night

Fig. 7 Size frequency distribution for larval koaro caught in the Tokaanu Power Station inlet canal of Lake Rotoaira in 1999.

5

6

7

9

10 11 12 13 14 15 16 17

Length (mm)

Stream, a major tributary of the Wairehu, where trout were abundant. In contrast, adult koaro were the only fish present in the Mangamutu Stream, a small tributary of the Wairehu, which was too small to support trout. Koaro were also common in the Matapupuhi Stream where trout were absent. These distributional data indicate that adult koaro occur in the tributary streams of Lake Rotoaira as well as in the littoral zone of the lake, so both lacustrine and fluviatile populations are present. However, koaro were scarce in the larger streams where juvenile trout were abundant and were only common in small streams where trout were scarce, or in larger streams where trout were absent. Gonadal status and sex ratios It is apparent that koaro mature at a relatively small size in Lake Rotoaira as both male and female fish between 60—65 mm long had ripe gonads. In September, most fish (65% of males and 56% of females) were either at an advanced stage of maturation (e.g., large well


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Journal of The Royal Society of New Zealand, Volume 32, 2002 1

Fig. 8 Maturation status of male (n = 34) and female (n = 38) koaro in Lake Rotoaira in A, September 1998 and B, April 1999. Numbers above boxes indicate the number of fish in each category.

0.8 .2 o

0.6

Q.

i

0.4


0.2

Immature

Developing Mature/ripe

Spawned

B 0.8 o

• Male • Female

0.6

o 2

5

0.4 0.2

4

2

0 Immature

Developing Mature/ripe

Spawned

developed gonads) or were ripe (loose eggs or milt), and there was little difference in maturation status between the sexes at this time, except that a few females (12.5%) had spawned (Fig. 8A). The maturation status of males had not changed by April (Fig. 8B), but distinct sex differences in gonad state for females were apparent by then. At this time, there were no immature or maturing females present, and most females (83%) were spent (Fig. 8B). Sex ratio varied with fish size (Fig. 9), mainly because of a decline in the number of males with increasing size. The largest fish (TL > 100 mm) present were mostly females. In September, there were more females (57.6%) than males (42.4%) in the catches but by April there were more males (68.4%) than females (31.6%). This seasonal difference in sex ratio


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285

Fig. 9 Variation in the proportions of adult male (O, n = 23) and female ( • , n = 32) koaro by length in Lake Rotoaira in September 1998.

51-60

61-70

71-80

81-90

91-100 101-110 111-120


Length class (mm)

Table 1 Importance of prey species for 82 adult koaro (length 54—130 mm long) in Lake Rotoaira in 1998/99 (19 fish had empty stomachs). Number n

%

Caddisfly larvae (Paroxyethira hendersoni) Caddisfly larvae (Paroxyethira tillyardi) Caddisfly larvae (mainly Triplectides sp.) Water flea (Daphnia sp.) Snails (Potamopyrgus) Snails (Lymnaea) Dragonfly larvae (Procordulia sp.) Damselfly larvae (Austrolestes sp.) Fish (Gobiomorphus cotidianus) Pea mussel (Sphaerium sp.) Fly larvae Midge larvae

297

28.1 16.6

Totals

1057

Species

176 31 501 26 10 6 5 2 1 1 1

Weight

Occurrence

g

%

%

10.6 11.1

37.3 49.0 23.5 15.7

2.5 0.9 0.6 0.5 0.2 0.1 0.1 0.1

0.773 0.813 0.465 0.226 0.117 2.165 1.504 0.139 1.060 0.022 0.008 0.006

100

7.32

2.9

47.4

6.4 3.1 1.6

29.6 20.6 1.9

14.5 0.3 0.2 0.1

7.8 5.9 5.9 3.9 2.0 2.0 2.0 2.0

100

was close to being significant at the 95% level (Fisher's exact test, P = 0.07). The change from female dominance in September to male dominance in April, together with the reduced abundance of all koaro in April, indicated that females were particularly scarce in the lake in April. Main prey species Twelve prey species were identified in the stomach contents of koaro from Lake Rotoaira in 1998/1999 (Table 1). The water flea (Daphnia) and two species of purse caddis larvae (Paroxyethira hendersoni and P. tillyardi) predominated in terms of numbers, but snails, dragonfly larvae (Procordulia and Austrolestes spp.), and small, benthic fish (Gobiomorphus cotidianus) predominated in terms of weight. Purse caddis larvae were a secondary prey in terms of weight, and were the most frequently occurring prey item overall (37% for P. hendersoni, and 49% for P. tillyardi).


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Journal of The Royal Society of New Zealand, Volume 32, 2002 Table 2 Changes in major prey groups (% occurrence) between size classes of koaro in Lake Rotoaira. Size class (TL) Prey group


Water fleas Caddisfly larvae {Paroxyethira sp.) Caddisfly larvae {Triplectides sp.) Snails Dragonfly and damselfly larvae Fish

50-70 mm

71-90 mm

28 72 33 0 0 0

14 77 27 9 9 0

90-120 mm 0 50 0 63 38 13

Size differences in feeding and prey selectivity were apparent for koaro (Table 2). Daphnia were mostly consumed by the smaller fish (50-70 mm TL) and declined in importance with increasing fish size. In comparison, snails, dragonfly larvae, and fish were not eaten by the smaller koaro, but were the main prey for the largest koaro (90-130 mm TL). Purse caddis larvae were eaten by all sizes of fish. Given their high ranking in terms of both numbers and weight, they are the most important prey for koaro between 60 and 120 mm long in the lake and can be regarded as a staple food. Daphnia are the only prey that are not benthic and were mainly taken by the smallest koaro. Their decline in importance with fish size probably represents the transition from a pelagic schooling existence (where Daphnia would be the main food) to a near benthic existence, where browsing on purse caddis larvae predominates. The presence of snails, Odonata larvae, and fish in the diet of the largest (90-120 mm long) koaro indicates that these fish can consume larger prey items than purse caddis larvae. DISCUSSION The maximum size of koaro caught in Lake Rotoaira was well short of the >200-mm-long specimens caught in Lake Chalice (Meredyth-Young & Pullan 1977) and Lake Pukaki (Rowe 1999). However, koaro (both males and females) were mature at a relatively small size (i.e., 60 mm long) in Lake Rotoaira. Changes in gonadal state indicated that spawning occurred mainly over summer and extended into autumn. A long period of spawning during summer and autumn is consistent with Kusabs' (1989) observations on the spawning of koaro in Taupo streams. It is also likely that spawning occurs mainly in the inlet streams of this lake. Spawning in amphidromous stocks of koaro has been recorded in streams by O'Conner & Koehn (1998) and Allibone & Caskey (2000). They both found eggs attached to small rocks in stream margins close to or above the water line indicating that, like landlocked banded kokopu (Galaxias fasciatus) (Mitchell & Penlington 1982), koaro spawn on stream margins during spates. No lake spawning by koaro has been observed and, given that stream spawning occurs in amphidromous stocks, from which lacustrine ones are derived, most adult koaro in lakes would be expected to migrate from the lake into tributary streams to spawn during summer. The historic observation of koaro migration into springs in Lake Rotoaira starting in November (Phillipps 1924) is consistent with a summer emigration of adult fish out of the lake and into inlet streams to spawn. Moreover, observations by Fletcher (1919b) indicate


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that a return migration occurred in Rotoaira. Fletcher (1919b, p. 264) stated that "There are three streams running into Roto-a-Ira which take their rise from springs gushing up out of the earth These three were the best for koaro The fish caught as they came out of the springs from the underground source were light-coloured, and spotted on the back; those caught ascending the stream were dark. The best month for taking the fish was March". We found that koaro caught in the lake were all dark, but within minutes of being exposed to light turned pale. It is therefore reasonable to assume that the light-coloured fish emerging from the springs and returning to the lake were fish that had recently come from a shallow, exposed environment such as a surface stream. Spawning migrations would deplete adult fish from the lake and the decline in both koaro abundance and size in Lake Rotoaira between September and April is consistent with many fish spawning in the tributary streams during summer. It is apparent that the summer migrations of koaro historically observed in Lake Rotoaira were primarily to allow adult fish to spawn in streams and that the access to such streams was obtained through underground springs. Many of the streams draining the western side of this lake have no defined surface access to the lake edge (Fig. 1). However, subsurface springs are common in this vicinity and probably lead to the surface streams draining the slopes of Mt Tongariro. As a subsurface spring drains Lake Otamangakau (Lumley 1994) and connects with the Onepoto Stream (Fig. 1) which drains into Lake Rotoaira, it is likely that some koaro, especially juvenile migrants, migrated annually from Lake Rotoaira up to Lake Rotopounamu and populated this lake. The virtual absence of koaro in streams where juvenile trout were abundant contrasted sharply with their abundance in streams where trout were absent. Such fragmented and disjunct distributions are similar to those for Galaxias vulgaris and brown trout (Salmo trutta) in South Island rivers (Townsend & Crowl 1991), where the scarcity of galaxiids was attributed to the presence of trout. It is therefore very likely that rainbow trout have displaced koaro from streams. However, such an effect is likely to be dependent on the density and size of trout present, and on stream size, as koaro were present in two small streams where low numbers of juvenile trout occurred, and in another where low numbers of adult trout were present. Nevertheless, they were absent from large stretches of the Wairehu Stream and none was found in the Manga te tipua Stream which is affected by warm geothermal water from the Ketetahi Springs (Fig. 1). Their scarcity in the Wairehu Stream below the falls indicates that trout can reduce lacustrine stocks of koaro by two mechanisms: firstly by displacement of adults from stream habitats which can be expected to reduce recruitment of larvae to the lake, and secondly through direct predation on juveniles in the lake. The sex ratio of koaro changed with fish size such that most of the smaller fish (i.e., those 50-70 mm long) were males, and nearly all fish over 100 mm long were females. It is also apparent that males mature at an earlier size than females, with most females not maturing until they are over 70 mm. In Lake Chalice, where the only fish present is the koaro, females grow to 271 mm and, in a sample of 20 fish over 125 mm long, females predominated (83%) (Meredyth-Young & Pullan 1977). The predominance of females in the larger size classes of koaro is consistent with other galaxiid species (Hopkins 1971, 1979; Allibone & Townsend 1997), and implies either a higher mortality rate for males or a size-related change in sex. A similar change in sex ratio with size (favouring females) occurs for lacustrine rainbow trout that spawn in the Ngongotaha Stream, Lake Rotorua (authors' unpubl. data), and is believed to be due to a higher age-related mortality of male fish after spawning in inlet streams. However, one koaro examined by us contained both male and female gonads. Such intersex fish occur rarely, but widely, within the salmonidae (Kinnison et al. 2000). The role of


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environmental factors in this process is unknown, but size-related sex change occurs in a range offish species and is related to size or hierarchical position (Choat & Robertson 1975). Furthermore, environmental factors including pH, temperature, and density can determine sex ratios for a number of fish species (Conover & Kynard 1981; Rubin 1985; Docker & Beamish 1994; Craig et al. 1996). Thus, a skewed sex ratio for koaro fry, or a size-related sex change, while unlikely, cannot be excluded. Koaro adults in Lake Rotoaira fed primarily on benthic prey, particularly purse caddis larvae which are attached by their cases in large numbers to macrophytes and rocks. However, size-related changes in diet were noted, with some of the smallest koaro feeding on Daphnia, whereas the larger koaro (>100 mm long) fed on larger prey such as Odonata larvae, snails, and fish. Naylor (1983) examined the gut contents of 38 koaro from Lake Alexandrina (South Island). These were mostly less than 50 mm long and their diet was dominated by crustacean zooplankton, particularly Daphnia carinata. It can be assumed that juveniles (40-50 mm long) in Lake Rotoaira are also mainly pelagic and feed primarily on Daphnia in open water. However, it is apparent that by the time they are over 60 mm long, they feed more on benthic prey in the littoral zone, and start grazing on purse caddis larvae. Seven out of 15 large koaro (125-255 mm long) from Lake Chalice (South Island) contained juvenile galaxiids (Meredyth-Young & PuUan 1977), indicating that piscivory can be expected to become more prevalent as size increases. These koaro also fed on a range of large aquatic and terrestrial insecta. Purse caddis larvae were not a major prey for rainbow trout in Lake Rotoaira in 1974 (Rowe et al. 2000) and are not a significant prey for rainbow trout in other lakes (Smith 1959; Rowe 1984). Thus, koaro in the size range 60—90 mm long would not compete with trout for food in Lake Rotoaira. However, the larger koaro (90-200 mm long) feed on larger prey items such as snails, dragonfly larvae, and bullies so could potentially compete with trout as these items are a major prey for adult trout in Lake Rotoaira (Rowe et al. 2000). Competition for food would only develop between the larger koaro and trout if feeding times and locations overlapped. Larval koaro have been caught during the day in the limnetic zone of Lake Coleridge at depths ranging from 4-45 m (Taylor et al. 2000). However, none was caught in the limnetic zone (0-8 m) of Lake Rotoaira by day. A few larvae were present in water flowing through the inlet canal to the Tokaanu Power Station at night but not by day. This indicates that larvae are likely to either move towards the surface waters of the lake at night, or to emigrate into lakes from streams at this time. Adult koaro were common in the Matapupuhi Stream (Fig. 1) and it is possible that larvae entering the Tokaanu Tailrace were recent emigrants from this stream and not the lake. However, the range in size (and hence age) of the larvae in the inlet canal indicates that most were not recently hatched, so it is unlikely that all these fish were recent emigrants from the Matapupuhi Stream. Adult koaro were more abundant in the shallow littoral zone (0-5 m deep) on the western side of Lake Rotoaira than on the eastern side. This is probably related to habitat preferences as extensive rocky habitat was observed in the littoral zone on the western shoreline, whereas it was minimal on the eastern side of the lake. Alternatively, if subsurface springs still provide the main access to spawning habitats for koaro, the prevalence of adults on the western shoreline could reflect their aggregation around spring mouths prior to spawning. In April, koaro were scarce on both western and eastern sides of the lake and this may reflect the fact that many adults were still spawning. Koaro were absent in the littoral zone of Lake Rotoaira below about 3 m but they occurred down to at least 30 m in Lakes Pukaki and Tekapo (D. K. Rowe unpubl. data) and were present at depths of 50—60 m in Lake Rotoiti (Rowe 1993a). Moreover, they were common in


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Rowe et al.—Koaro in Lake Rotoaira Fig. 10 Variations in mean abundance (mean CPUE ± 1 s.e.) for koaro caught by Gminnow trapping at 0—30 m in the littoral zones of Lakes Rotoaira, Tekapo, and Pukaki in 1998-1999.

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1.6 1.4

1.2 Q. CD

1

0.8 to CD

0.6


0.4 0.2 0 Rotoaira

Tekapo

Pukaki

the benthic zones near the middle of the lake in both Lake Rotopounamu (Michaelis 1982) and Lake Rotoiti (Rowe 1994). They were therefore expected to occupy the entire sublittoral and benthic zone of Lake Rotoaira. However, none was caught in these zones in Lake Rotoaira and this indicates that the abundance of koaro in this lake is relatively low. Historic observations also indicate that the abundance of koaro is now much lower than expected in Lake Rotoaira. For example, before the introduction of trout in 1906 (Whitney 1944) koaro can be expected to have been very abundant (Grace 1959). Trout predation will have reduced their numbers after about 1910, but they were still relatively abundant in the 1920s (Fletcher 1919b; Phillipps 1924). In the 1960s, when trout had been in the lake for over 50 years, large numbers of adult koaro often washed up on windward beaches following storms and were collected by locals for food (G. Konui pers. comm.). Even in the 1970s, koaro were still a major winter prey species for trout (Rowe et al. 2000). However, today, they are not seen washed up on shores. Furthermore, anglers no longer fish for trout on the southern beaches of Lake Rotoaira where trout used to feed on large shoals of schooling koaro whitebait (G. Konui pers. comm.). Also, the large numbers of migrant juveniles which once entered the Wairehu Canal and, as recently as 1972, were photographed attempting to climb the first blockchute (McDowall 1990) are no longer present. There are no comparable data for koaro abundance in other North Island lakes, but mean catches of koaro in the littoral zone of Lake Rotoaira were expected to be comparable with those in Lakes Tekapo and Pukaki in the South Island. This is because these lakes also contain trout populations but lack smelt which are responsible for the scarcity of lacustrine koaro in many other North Island lakes (Rowe 1990, 1993b). The abundance of koaro in Lake Rotoaira was clearly much lower than in those South Island lakes (Fig. 10), despite the general lack of macrophytes and the relatively unproductive nature of these lakes compared with Rotoaira. Thus, the abundance of koaro in Lake Rotoaira was much lower than expected. In summary, comparative trapping data, the restricted benthic distribution of koaro in Lake Rotoaira, and the anecdotal and historical information on koaro abundance, all indicate that this fish is now not as abundant in Lake Rotoaira as it should be. It is apparent that its population has experienced a further decline, over and above that caused by trout predation in the early 1900s, and that this has occurred during the past 30 years.


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Lacustrine koaro populations are scarce in New Zealand and their remnant populations require protection. For this reason, nearby Lake Rotopounamu (Fig. 1) was listed under the International Biological Programme as a lake for preservation (Luther & Rzoska 1971). In 1981, it still contained an abundant koaro population (Michaelis 1982). However, soon after the illicit introduction of smelt to Lake Rotopounamu, no koaro could be found (Rowe 1993b). By 1991, there were only two North Island lakes (Lake Rotoaira, and Lake Waikareiti near Gisborne) where koaro were still thought to be relatively common. Their abundance in Lake Rotoaira is now lower than expected. Any further decline of koaro in North Island lakes would represent a further loss in biological diversity and strengthen the need to protect remaining populations as well as to restore those that have declined or been lost.


ACKNOWLEDGMENTS This study was sanctioned by the Lake Rotoaira Trust and Genesis Power Ltd with funding provided by the latter. We thank the Lake Rotoaira Trust for their moral and material support including the provision of the Trust's boat and assistance from the lake ranger. We also thank Tracey Hickman of Genesis Power Ltd for her assistance. Joshua Smith, Jacques Boubee, and Stacey Konui helped with various aspects of the fieldwork, and Richard Allibone and Gerry Close helped improve the manuscript. REFERENCES Allibone, R. M.; Caskey, D. 2000: Timing and habitat of koaro (Galaxias brevipinnis) spawning in streams draining Mt Taranaki, New Zealand. New Zealand Journal of Marine and Freshwater Research 34: 593-595. Allibone, R. M.; Townsend, C. R. 1997: Reproductive biology, species status, and taxonomic relationships of four recently discovered galaxiid fishes in a New Zealand river. Journal of Fish Biology 51: 1247-1261. Armstrong, J. S. 1935: Notes on the biology of Lake Taupo. Transactions and Proceedings of the Royal Society of New Zealand 65: 88-94. Buck, P. 1921: Maori food supplies of Lake Rotorua, with methods of obtaining them, and usages and customs appertaining thereto. Transactions and Proceedings of the New Zealand Institute 53: 433451. Choat, J. E.; Robertson, D. R. 1975: Protogynous hermaphroditism in fishes of the family Scaridae. In: Reinboth, V. ed. Intersexuality in the animal kingdom. Heidelberg, Germany, Springer Verlag. Pp. 263-283. Conover, D. O.; Kynard, B. E. 1981: Environmental sex determination: interaction of temperature and genotype in a fish. Science 213: 577-579. Craig, J. K.; Foote, C. J.; Wood, C. C. 1996: Evidence for temperature-dependent sex determination in sockeye salmon (Oncorhynchus nerkd). Canadian Journal of Fisheries and Aquatic Sciences 53: 141-147. Cudby, E. J. 1984: Fishery aspects of the Wairehu Canal hydro-electric scheme. Fisheries Environmental Report 39. 26 p. Dedual, M.; Maclean, G.; Rowe, D. K.; Cudby, E. J. 1997: The trout population and fishery of Lake Otamangakau. Unpublished NIWA Client Report ELE70207. Docker, M. E.; Beamish, F. W. H. 1994: Age, growth, and sex ratio among populations of least brook lampreys, Lampetra aepyptera, larvae: an argument for environmental sex determination. Environmental Biology of Fishes 41: 191-205. Fletcher, H. J. 1919a: Lake Taupo and its trout. New Zealand Journal of Science and Technology 2: 367-370. Fletcher, H. J. 1919b: The edible fish, &c, of Taupo-nui-o-Tia. Transactions of the New Zealand Institute 51: 259-264. Grace, J. Te H. 1959: Tuwharetoa: the history of the Maori people of the Taupo district. Wellington, A. H. Reed. 567 p. Hopkins, C. L. 1971: Life history of Galaxias divergens (Salmonidae: Galaxiidae). New Zealand Journal of Marine and Freshwater Research 5: 41-57.


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Hopkins, C. L. 1979: Reproduction in Galaxias fasciatus Gray (Salmoniformes: Galaxiidae). New Zealand Journal of Marine and Freshwater Research 13: 225-230. Kinnison, M. T.; Unwin, M. J.; Jara, F. 2000: Macroscopic intersexuality in salmonid fishes. New Zealand Journal of Marine and Freshwater Research 34: 125-134. Kusabs, I. A. 1989: The biology and general ecology of the koaro (Galaxias brevipinnis) in some tributary streams of Lake Taupo. Unpublished MSc thesis, University of Waikato, Hamilton, New Zealand. Lumley, D. 1994: The Rotopounamu 'gurgler'. Tongariro 3:1. Luther, H.; Rzoska, J. 1971: Project Aqua. A source book of inland waters proposed for conservation. International Biological Programme Handbook 21. 239 p. McDowall, R. M. 1990: New Zealand freshwater fishes—a natural history and guide. Auckland, Heinemann Reed. McDowall, R. M.; Suren, A. M. 1995: Emigrating larvae of koaro, Galaxias brevipinnis Giinther (Teleostei: Galaxiidae), from the Otira River, New Zealand. New Zealand Journal of Marine and Freshwater Research 29: 271-275. McDowall, R. M.; Hopkins, C. L.; Flain, M. 1975: Fishes. In: Jolly, V. H.; Brown, J. M. A. ed. New Zealand lakes. Auckland, Auckland University Press. Pp. 292-307. Meredyth-Young, J. L.; Pullan, S. G. 1977: Fisheries survey of Lake Chalice, Marlborough Acclimatisation District, South Island. Fisheries Technical Report 150. 21 p. Michaelis, F. B. 1982: The lakes of the Tongariro National Park. Mauri Ora 10: 49-65. Mitchell, C. P.; Penlington, B. P. 1982: Spawning of Galaxias fasciatus Grey (Salmoniformes: Galaxiidae). New Zealand Journal of Marine and Freshwater Research 16: 131-133. Naylor, J. R. 1983: The relative abundance, age structure and diet of Gobiomorphus cotidianus (Pisces: Eleotridae) and Galaxias brevipinnis (Pisces: Galaxiidae) in Lake Alexandrina. Unpublished MSc thesis, University of Canterbury, Christchurch, New Zealand. O'Conner, W. G.; Koehn, F. D. 1998: Spawning of the broad-finned galaxias, Galaxias brevipinnis Gunther (Pisces: Galaxiidae) in coastal streams of southeastern Australia. Ecology of Freshwater Fish 7: 95-100. Phillipps, W. J. 1924: The koaro, New Zealand's subterranean fish. New Zealand Journal of Science and Technology 7: 190-191. Rowe, D. K. 1984: Factors affecting the foods and feeding patterns of lake-dwelling rainbow trout (Salmo gairdnerii) in the North Island of New Zealand. New Zealand Journal of Marine and Freshwater Research 18: 129-141. Rowe, D. K. 1990: Who killed the koaro? Freshwater Catch 43: 15-18. Rowe, D. K. 1993a: Identification of fish responsible for five layers of echoes recorded by high frequency (200 kHz) echosounding in Lake Rotoiti, North Island, New Zealand. New Zealand Journal of Marine and Freshwater Research 27: 87-100. Rowe, D. K. 1993b: Disappearance of koaro, Galaxias brevipinnis, from Lake Rotopounamu, New Zealand following the introduction of smelt, Retropinna retropinna. Environmental Biology of Fishes 36: 329-336. Rowe, D. K. 1994: Vertical segregation and seasonal changes in fish depth distributions between lakes of contrasting trophic status. Journal of Fish Biology 45: 787-800. Rowe, D. K. 1999: Giant koaro in Lake Pukaki. Water & Atmosphere 7(1): 5. Rowe, D. K.; Cudby, E.; Turner, D. 2000: Prey species and the food web for rainbow trout in Lake Rotoaira, 1972-1974. Unpublished NIWA Consultancy Report GPL00233. Rubin, D. A. 1985: Effects of pH on sex ratio in cichlids and a poecilliid (Teleostei). Copeia 1985(1): 233-235. Smith, D. C. W. 1959: The biology of the rainbow trout (Salmo gairdneri) in the lakes of the Rotorua District, North Island. New Zealand Journal of Science 2: 275-312. Stokell, G. 1955: Fresh water fishes of New Zealand. Christchurch, Simpson & Williams. Taylor, M. J.; Graynoth, E.; James, G. D. 2000: Abundance and daytime vertical distribution of planktonic fish larvae in an oligotrophic South Island lake. Hydrobiologia 421: 41-46. Townsend, C. R.; Crowl, T. A. 1991: Fragmented population structure in a native New Zealand fish: an effect of introduced brown trout? Oikos 61: 347-354. Whitney, C. A. 1944: Rainbow trout in New Zealand. New Zealand Fishing and Shooting Gazette, April: 2-5.


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