upstream summer migration of eels

7/24/2019 Upstream Summer Migration of Eels

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Summer upstream

migration of juvenile

freshwater eels in New

ZealandD. J. Jellyman abaMarine Laboratory, Department of Zoology ,

Victoria University of Wellington , Private

Bag, Wellington, New ZealandbFisheries Research Division , Ministry of

Agriculture and Fisheries , P.O. Box 19062,

Wellington, New Zealand

Published online: 30 Mar 2010.

To cite this article:D. J. Jellyman (1977) Summer upstream migration of

juvenile freshwater eels in New Zealand, New Zealand Journal of Marine and

Freshwater Research, 11:1, 61-71, DOI: 10.1080/00288330.1977.9515661

To link to this article: http://dx.doi.org/10.1080/00288330.1977.9515661

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N.Z. Journalof Marine andFreshwater Research11 1) :6 1 - 7 1 . March1977

SUMMER UPSTREAM MIGRATION OF JUVENILE

FRESHWATER EELS

IN NEW

ZEA LA N D

D. J.

JELLYMAN*

Marine Laboratory, Department of Zoology, Victoria University of

Wellington, Private Bag, Wellington, New Zealand

ABSTRACT

During summer, upstream migrations

of

juvenile freshwater eels (elvers)

of

both species (Anguflla australis schm idtii Phillipps,

the

short-finned

eel, and

A. dieffenbachii

Gray,

the

long-finned

eel) are

observed

at

hydro-electric dams

and waterfalls. The migration was sampled at Karapiro Dam on the Waikato

River,

1970, 1971, and 1974, and

samples from

a

further

11

areas throughout

the country were analysed.

Elvers, active

at

night

or

during overcast

and

damp days, surmount steep

obstacles

by

climbing; they adhere

to

damp surfaces

by

friction

and

surface

tension, obtaining maximum resistance

by

undulating their bodies

and

keeping

them closely adpressed

to the

substrate.

The

maximum size

for

vertical climbing

is

12

cm .

As

elvers were sampled

at

sites where their migration

was

interrupted,

no distinct schools were observed,

but the

elvers were strongly social

and

thigmotactic.

Migrations are made up of elvers of similar size of one or both species.In

large river systems, elvers apparently migrate successively further upstream each

year for several, years. North Island samples contained elvers up to 3 y of age,

but

in the

South Island elvers

up to 7 y old

were recorded; aging

was by

burnt

otoliths. Migration

is

perhaps initiated

by

increases

in

water temperature,

day

length, and sociability; increased water flow may also coincide w ith 'runs '.

Electric fishing of streams shows that the two species hav e different ha bita t

preferences,

and

these

are

reflected

in the

proportion

of

elvers caught

at

different

sites: longfins prefer swift stony rivers, shortfins silty backwaters and lakes,but

both

are

often found together. Longfin elvers have thick dermal skin layers,

and experiments in air of constant temperature and hum idity showed that they

cannot survive prolonged exposure requiring cutaneous respiration

as

well

as

shortfins; longfins

may

also have h igher oxygen requirem ents.

INTRODUCTION

With the onset of summer each year, mass upstream migrations of

juvenile freshwater eels of both Anguilla australis schmidtii Phillips,

the short-finned eel, and A. dieffenbachii Gray, the long-finned eel, take

place in rivers throughout the country. Such eels are referred to as

elvers , as distinct from glass-eels , which is the stage at which eels

arrive in fresh water.

Present address:

Fisheries Research Division, Ministry

of

Agriculture

and

Fisheries,

P.O. Box

19-062, Wellington,

New

Zealand.

Received8July1975;revision received21July1976.

Fisheries Research Division Publication

262

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62 N.Z.

J O U R N A L

OF

M A R I N E

&

F R E S H W A T E R R E S E A R C H [ M A R C H

TABLE 1Species

proportions

(%) and

mean lengths L,

cm) of

migratory

elvers

(Anguilla

spp.) sampled 1970, 1971, 1974.

ARE A

Te Kauwhata

Karapi ro

DAT E

Jan.

1971

J a n - Fe b

1970,

1971, 1974

Matahina

Mokau River

Shannon

Makara si te

a

Makara si te

b

D obs on

Lake Coleridge

Aviemore

Waitaki

Roxburgh

Jan

1974

Jan

1971

Feb.

1971

Feb.

1972

Jan. -Feb.

1972

Jan.

1971

Jan.

1971

Feb.

1971

Feb.

1971

Jan-Feb.

1971

n

7 4 3

3646

1 6 8

2 3 2

2 3 3

169

4 4

2 1 6

8 0

2 6

6 5

2 0 7

%

100

6 2

7 9

18

18

9 6

6 1

12

0

0

9

1

A. austral.

L

8. 1

9.0

9.0

9.4

10.2

7.5

7.9

10.1

13'.

5

12 . 0

(1

S.D.)

(0.87)

(0.98)

(0.88)

(0.81)

(0 .66)

(0.75)

(1.22)

(0.88)

( L 8 6 )

(0.30)

is

Range

5 . 9 - 1 1 . 4

6 . 6 - 1 2 . 9

7 . 2 - 1 1 . 5

7 . 6 - 1 1 . 1

8 . 3 - 1 1 . 6

6 . 2 - 1 1 . 3

6 . 3 - 1 1 . 6

8 . 1 - 1 1 .

8

10 8 16 1

11.7-12.3

%

0

38

2 1

8 2

8 2

4

39

88

1 00

1 00

9 1

9 9

A. di effenbac hli

L(l S.D.)

9'.

9 .

1 0 .

11.

8 .

8 .

1 1 .

1 6 .

1 6 .

1 6 .

1 4 .

9 (L36)

2 (0.86)

2 (0.99)

1 (0.80)

1 (0.91)

4 (0.92)

9 (1.30)

2 (1.81)

9 (2.79)

7 (2.41)

9 (4.00)

Range

6 .

8 .

8 .

9 .

7 .

7 .

9 .

1 2 .

1 1 .

1 2 .

1 0 .

8-15.1

1-12.5

3-13.0

4-13.6

4-10.2

2-10.4

7-15.8

9-21.9

6-23.7

6-22-3

8-31.5

As glass-eels

are

generally sedentary during their first year

in

fresh

water, elvers which have been resident

in

streams

and

rivers

for a

year

or more comprise most

of the

m igration. B ecause man y eels spend their

first year

in

fresh water

in the

upper estuarine

or

tidal area,

the

greatest

concentrations

of

elvers

are

recorded from

the

lower reaches

of

water-

ways.As

elvers make their

way

steadily upstream , their num bers become

reduced

by

mortality

and by

diversion into tributaries

or

other suitable

habitat.

The

most easily observed

and

well-known migrations

are

those

whose progress

is

interrupted

by

hydro-electric dams.

Karapiro

Dam, 130 km

upstream

on the

Waikato River,

is the

first

of several power stations

on

this river.

The

ann ual elver m igration the re

is sometimes great enough

to

clog

the

chambers

of

turbines shut down

for maintenance. Smaller migrations

are

known

at

other hydro stations

and waterfalls. Normally elvers

can

surmount such obstacles

by

leaving

the water

and

climbing along splash zones

of

waterfalls

or

discharge

chutes. Some falls

can act as an

effective barrier

to the

passage

of

elvers, particularly

if

there

is a

swift current rushing over bare smooth

rock: Huka Falls

on the

upp er W aika to R iver prevent eels from enter-

ing Lake Taupo (Cairns 1941).

MA T E R I A L S

AND

ME T H O D S

The elver migrations were sampled

by the

author

at

Karapiro

Dam

(February

1.970,

January

1971,

Feb ruary 1974),

Te

Kauwhata

on the

Waikato River (January 1971),

and at two

sites

in the

M akara Stream,

Wellington, during January

and

February

1972. The

downstream site

at

the Makara Stream (Site

a in

Table

1) was 1.2 km

from

the

mouth ,

while Site

b was 2.5 km

from

the

m outh . Elvers from seven ad ditional

localities were obtained during January

and

February

1971

with

the

co-operation

of

hydro station staff

(Fig. 1). The

specimens from

the

Mokau River were collected

by Mr J.

Josephs

of New

Plymouth

in

January

1971.

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J E L L Y M A N M I G R A T I N G E L V E R S

63

FIG. 1Locationsmen-

tioned

in the

text

at

which elvers

(Anguilla

spp.) were sampled,

1970-74.

170 E

Lake Cofetidge

Aviemore*

W a i t a k i *

Roxburgh*

Hydro station

Further elvers from theW aikato R iver w ere collectedby the author

at the E l b o w , 16 kmfrom theriver m ou th, w hile fishingfor glass-eels

during August andSeptember 1970.Theseareincluded here,but, com-

pared with the summer migration, thenumbersof elvers moving during

the winter

are

small.

The elversat the dams were densely concentrated, and samples could

be scooped out with a hand-net or large container. At the E lbow ,

Makara St ream, and M oka u Rive rs, set-nets were use d. Elvers were

preserved with 7% formalin, or frozen, or sent to thelab orato ry alive

in plastic bags containingalittle waterandinflated with oxyg en.

MIGRATORY BEHAVIOUR

PERIODICITY

Most samples were collected during January andFeb ruary (Table1).

Discussions with hydro station staff indicated

a

consistency

in

arrival

times of elvers from year to year; C airns (1 941) recorded the first

appearance

of

elvers

at the

lowest hy dro station

on the

W aikato River,

then slightly above Karapiro, between 18 and 22Jan ua ry during four

consecutive years, 1936-39. My data are similarly consistent: in 1970

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64 N.Z. JOURNAL OF M AR INE & FRESHWATER RESEARCH [MARCH

the run com men ced in m id -J an ua ry (precise date of first arrivals un-

known), in 1971 on 18 January, in 1973 on 17 January, and in 1974 on

15 Jan uary (no data are available for 1 972 ).

Some elvers move upstream in the company of glass-eels during

winter and. spring months. During August 1970 a sample of 206 elvers

was caught am ong several thousan d glass-eels at the E lb ow . These

were all shortfins (m ean length 8.6 cm, rang e 6.9-1 3.2 cm ). In ad dition ,

during Septem ber-D ecem ber 1971, 94 elvers were caught at the mo uth

of the Makara Stream among a total catch of 773 glass-eels: 93 were

shortfins (m ean length 13.9 cm, range 8.2-27.4 c m ) , and some stom achs

contained marine invertebrates. Probably, these elvers had been resident

in the lower estuary or the adjacent sea.

C L I M B I N G

During visits to Ka rap iro Da m in 1970 and 197 1, little daytime activity

of elvers was observed. However, power station operators report that

during the peak of the season, elvers actively ascend the spillway and.

face of the dam during daylight if rain is falling or the weather overcast.

Large numbers of elvers were present at night. Larger eels of both

species were also present. These would immediately move out of a

strong torch beam, but elvers were less disturbed but would eventually

leave an area of strong illumination.

Concentrations of elvers were greatest where running water entered

the main pool, and here the concrete was covered for a width of up

to 3 m by a mass of elve rs. Progress against the spillway cu rre nt w as

either in a series of vigorous darts forward or by crawling along the

damp margin at the edge of the main flow.

The method of vertical climbing consisted of a series of jerky move-

ments, comm encing with the body forming a com pacted W shape.

While the tail remained stationary, the head was extended forward and

swung to one side. The hind part of the body was then pulled upward.

The head alternately swung to the right and left, and so the body

formed a W or an M shape. A fuller acco unt of similar vertical

climbing by South African elvers is given by Skead (1959). Climbing

elvers appeared to cling by both friction and surface tension, as elvers

climbing vertically could not pass over any object which caused a

substantial break in contact between themselves and the substrate.

The lengths of elvers found, climbing a vertical wall were as follows

with the figures in brackets being measurements of a random sample

collected from the base of the spillway:

Shortfins: m ean length 8.8 cm (9. 5), range 7.4-11.8 (7 .4- 1 2.9 ),

n = 174 (227)

Longfins: m ean length 9.6 cm (10 .8), range 7.9-12.0 (7 .7 -1 4.8 ),

n = 50 (257)

T he p ercen tage of eels less tha n 10 cm long in the clim bing sam ples

was 9 1 % and 7 2 % for shortfins an d longfins respectively; equivalent

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figures for the general sample were shortfins 73% and longfins 28%.

Small size is an advantage in climbing, as the surface area to weight

ratio decreases with increasing size. The maximum length for vertical

climbing is prob ably 12 cm, which corresp ond s to a w eight of 2.5 g.

Elvers also showed distinct .thigmotaxis and crammed together in all

crevices and pools on the spillway and elsewhere. These aggregations

were partly enforced by the limited cover and resting areas available,

but laboratory observations confirmed this social behaviour. When

placed in an aquarium containing several sections of pipe as cover, the

elvers did not disperse but congregated under one pipe. Thigmotaxis

was also demonstrated in escape reactions. If the leading members of a

column of climbing elvers were disturbed, their vigorous attempts to

escape were transmitted through the whole column, and most elvers

would rapidly re-enter deeper water.

H A B I T A T PRE FE RE N C E S

A comparison between glass-eel and elver catches for the Waikato

River shows gross differences in the species composition for the two

stages. Th us, the gran d me an for the E lb o w glass-eel samples (N =

3004) recorded to early 1974 is 98% shortfin in contrast to 62% short-

fin, elvers at K arap iro

(N

= 36 46 ). Elvers attempting to enter La ke

Waikare (Te Kauwhata) were all shortfins (Table 1), and local fisher-

men confirmed that this lake is inhabited by shortfins only.

During electric fishing in Wellington streams and rivers, small eels of

both species were frequently observed to show definite habitat prefer-

ences.

F or exam ple, the catch of eels less than 26 cm long o n coarse

gravel and rock su bstrate in, the W aika nae River (Fig. 1) was 94 %

longfin and 6% shortfin (n = 41 2), but samples from the W aimeha

Stream , 2.5 km north, w here the b ottom is sil ty mu d, contained 9 7 %

shortfins a nd 3 % longfins (n = 1016) .

T o exam ine hab itat preferences am on g eels 26 cm and less long, a

scries of five stations, chosen for consistency of bottom type was electric-

fished in the Makara Stream. Although shortfins were predominant in

all areas, longfins preferred stony areas with swift flows to sillier areas

with slower flows. Thus, in two stations where the bottom type was silt

an d the flow rates 0.12-0.18 m .s-

1

, 99% of all eels

(N =

136) were

shortfins, whereas in two upstream stations where bottom types were

line-coarse rubble and. flow rates were 0.41-0.48 m.s~\ only 76% of all

eels

(N

178) we re shortfins. In bot h species (bu t mo re obviously in

shortfins), the smaller size groups predominated in downstream areas;

this observation supports the concept of successive summer migrations

penetrating further up stream.

Sig

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66 N.Z .

JOURNAL

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TABLE 2Species proportions (%) and mean lengths (L, cm) of samples of migratory

elvers

(Anguilla

spp.) from base of spillway of K ara pi ro Dam (samples arranged

by mon ths over the migration season; * = data from W oods (1964); - = not given;

.. = not calculated)

DATE

19 January 1971

26 January 1971

28 January 1971

28 January 1971

5 February 1970

12 February 1970

19 February 1974

*27 February 1962

*5 March 1963

Total

n

276

484

657

333

376

647

254

35 7

314

3698

86

47

63

65

62

48

74

74

76

63

L

8 .4

9 . 5

9 .3

9 .3

8 .9

8 .9

8 .9

9 .3

9 . 7

9 . 1

A.au stralis

(1 S.D.)

(0.83)

(1.03)

(0.97)

(1.15)

(0.73)

(0.81)

(2.34)

(2.22)

Range

6 .7-10.6

7 .4-12.9

7 .1-12.5

6 .9-12.4

7 .1-11 .1

6 .6 -11 .6

6 .8-11 .1

-

-

%

14

53

37

35

38

52

26

26

24

37

A.

L

9 . 0

10.8

10.3

10.3

9 . 3

9 . 4

10.2

9 . 8

10.4

10.0

dieffenbachii

(1 S.D.)

(0.97)

(1.29)

(1.26)

(1.13)

(1.42)

(1.14)

(1.24)

(2.24)

-

Range

7 .3-12.2

7 .7-14.8

7 .8-15.1

7 .3-13.3

7 .0-14.4

6 .8-13.9

7 .7-15.3

_

COMPOSITION

OF

SAMPLES

SPECIES PROPORTIONS

T ab le 1 gives the percentag es of each species for th e various sam ples.

M ost specimens (8 9% ) came from the N orth Island. H ere, 6 5 % of the

catch were shortfins, whereas in the South Island shortfins comprised

only 6%. However, these figures are not representative of the country

as a whole, as habitat has a marked influence on species composition.

The fluctuations which occur in the proportions of both species

arriving at Karapiro Dam are shown in Table 2. No trend is obvious,

but the overall predominance of shortfins is apparent.

LENGTH

T h e m ean lengths given in Table 1 indica te the variable size of

migrating elvers at different localities. In all elver samples containing

both species, the mean length of longfins exceeds that of the shortfins,

as it does in glass-eels (Jellyman, in press). As expected, elvers recorded

at upstream sites are larger than those from down stream on the same

river. This is seen in both the Makara Stream and the Waikato River,

but not from the Waitaki River (Waitaki and Aviemore samples),

where numbers are small and cover a large size range (12cm). General-

ly, the length-frequencies of elvers eorrespond well to a normal distribu-

tion pattern; Fig. 2 shows four typical samples.

In a large river system, several separate migrations may take place

concurrently during summer, with waves of eels, grouped approximately

by size, penetrating further upstream each year; migrations apparently

cease at an app rox im ate length of 30 cm, wh en a m ore sedentary mo de

of life is adopted. Table 2 gives the mean lengths of elver samples

collected from the base of the spillway at Karapiro Dam. During 1971,

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JELLYMANMIGRATING ELVERS

67

A austral is

A dieffenbachi i

Karapiro

Dam

28-1-71

n=415

Dobson

8-1-11

n=27

Roxburgh

25-1-71

6 8 10 12 14 16 18 20

n = 147

10

12 14 16 18 20

l e n g t h

(cm)

L e n g t h

(cm)

2Length-frequency distributions

of

samples

of

elvers

(Anguilla spp.)

from four

localities.

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N.Z.

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TABLE

3Age

distributions and length ranges (cm) of elvers in Waikato and Mokau

River samples (SF=short-finned eel

Anguilla australis,

LF=long-finned eel

A. dieffenbachii;

=number aged; - = nil)

AREA

Waikato

'Elbow'

DATE

August

1970

Waikato January

Te Kauwhata 1971

Waikato

Karapiro

D am

Mokau

January

1971

January

1971

SPECIES

SF n

Range

%

SF

n

Range

%

SF n

Range

m

LF n

Range

/o

SFn

Range

/

0

11

6.9-8.4

57

7

6.5-7.3

21

8

6.6-7.7

8

1

7.8

6

1

7.6

2

8

7 .

7

8,

8 .

A G E

1

15

.2-10.7

39

6

5-9.6

76

10

.9-9.8

77

15

.2-10.4

63

4

.5-9.3

55

GROUPS

2

4

11.0-12.9

4

2

10.4-11.0

3

7

10.2-12.3

15

10

9.9-12.4

31

4

9.8-11..1

43

3

-

_

-

-

-

_

L F

n

Range

8.5-10.1 10.6-11.3 12.2-13.0

57 30 13

seasonal variations in length of 1.1 cm and 1.8 cm occurred for short-

fins and longfins respectively. Also, an increase or decrease in mean

length

for one

species

is

paralleled

by an

equal change

in the

other

species: the two samples collectedon 28 Jan u ary 1971 (Table 2) were

takenan hour apart,and gave almost identical analyses.

A G E G R O U P S

Age groups could not be distinguished from examination of the

length-frequency data,and age determinations were m ade by examina-

tion of burnt otoliths. If an overlap in body length occurred between

successive

age

groups

the

mid-point

of the

overlap

was

taken

as the

limit between groups. Similarly, if successive ranges did not meet, the

mid-point of the difference wasdefinedas thegro up limit. Resu lts from

Waikato and M okau Rivers are presented in Table 3. Unfortunately,

South Island specimens were all preserved in strong formalin, and the

otoliths were generally unreadable. Roxburgh and Aviemore samples

were examined, and both contained eels from age groups 3-7, but

deteriorationof otoliths prevented reliable reading.

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MORTALITY

Numerous dead elvers and others in very poor condition were found

at Karapiro Dam, and predation by larger longfins and catfish Ictalurus

nebulosus

(Le Sueur) was observed. Other predators include brown

trout Salmo trutta L., shags (cormorants), ducks, kingfishers, and

wa ter rats.

In live samples, the proportion of longfins that died in transit far

exceeded that of shortfins. A trial was run to compare survival rates

of elvers of both species exposed to humid air in a constant temperature

cabinet: three trays containing ten elvers of each species were used,

and the deaths recorded over a week. At 15c, 70% of the longfins

died, but no shortfins; at 20c, 90% of the longfins and 40% of the

shortfins died; and at 25c, 100% of both species died, but the longfins

died before the shortfins. Thus, longfins are less able to survive pro-

longed periods of exposure which require cutaneous respiration,

especially under the additional stresses of high temperature and crowd-

ing.

D I SC U SSI O N

Summer migration of juvenile freshwater eels is not confined to New

'Zealand.

Skead (1959) reviewed several years' observations on the

migration of elvers of

A. mosscimbicci

Peters in the B uffalo Rive r, So uth

Africa. Summer migrations of Australian elvers are recorded by Kershaw

(1911) and Whitley (1929). A similar phenomenon was witnessed in

New Guinea by Herre (1930), and summer migrations of juvenile

European eels have long been known.

In New Zealand and apparently elsewhere, the stimuli which initiate

migration are unknown. Although low temperatures (less than 10c)

could inhibit mass movements of elvers, the fact that summer migrations

occur within a few weeks of each other throughout the whole country

indicates that it is not the reaching of any specific temperature which

triggers migration. Temperatures recorded in the Makara Stream over

the migratory period ranged from 14.2-21.5c, bu t the two ru n s

recorded at the downstream site did not correspond with any common

value or fluctuation. The lunar cycle appears to have no direct effect, as

seen by the consistency of arrival times each year at Karapiro Dam.

A small migration was recorded at M aka ra during a fresh in the

stream, but larger movements took place when the stream level was

normal. Hardy (1950) observed that increased water flow did not

induce a larger number of elvers to climb the margins of a small water-

fall in a North Canterbury stream.

Migration may be initiated by the summation of several factors. A

temperature greater than a threshold value, the increased day-length of

summer, and a rapidly increasing tendency to school shown initially by

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70 N.Z. JOURNAL OF M AR INE & FRESHWATER RESEARCH [MARCH

a few elvers, may all contribute to the onset of migration. Increased

water flow producing a strong rheotactic reaction among elvers may

also be a contributing agent, but is not the causative one.

The annual migration enables small eels to populate upstream areas.

If suitable habitat is encountered, many eels probably choose to remain

rather than progress further upstream. In this way, eels are able to

disperse and occupy the available habitats along the complete river

system, including territories vacated by migrant eels.

The differences noted between species proportions of glass-eels from

the E lbo w and elvers from K arap iro Dam , and also the age group

distributions from the E lbo w samp le (see T ab le 3) , indicate that a

considerable proportion of shortfins choose to spend at least the first

2 y of freshw ater life in th e up pe r estua rine a rea s. Age class 0 w hich

dom inates the Au gust E lb ow samples, is pro m ote d to age class 1

in the January Te Kauwhata and Karapiro Dam samples, where i t is

again dominant. In both these latter samples, age class 0 represents the

previous year's glass-eels and is understandably more prevalent in the

downstream (Te Kauwhata) sample than in the Karapiro Dam sample.

Longfins, which are absent from the E lb o w and Te K auw hata

samples, are also most strongly represented at Karapiro Dam by age

class 1. Although longfins prefer swift and stony waters typical of

upstrea m areas, substantial num bers of age gro ups 0 and 1 live down-

stream of Karapiro Dam. Therefore, although a large percentage of

shortfin glass-eels remain resident in the lower reaches of rivers, those

that do migrate upstream do so more rapidly than longfins.

Specific habitat preferences reduce intraspecific interaction and imply

different feeding habits. Unfortunately, few data are available to com-

pare the feeding habits of small eels of each species. It is not certain

whether selective feeding occurs in areas where both species coexist:

Cairns (1942) combined both species for the smallest size group he

examined, and Hopkins (1970) assumed diets of both species were

similar. Burnet (1952) showed that apparent feeding preferences in

longfins largely reflected the abundance of organisms in the fauna.

Thus, the suitability of the habitat rather than the availability of

specific food types may be the primary requirement in habitat prefer-

ence.

Relative mortalities of asphixiated glass-eels and elvers suggest that

longfins have higher oxygen demands than shortfins, but this has not

been tested experimentally. If correct, the habitat preferences of both

species would be plausibly explained. Also, the thicker dermal layer of

longfins may result in less efficient cutaneous respiration than in short-

fins. Under critical conditions, this and their possibly higher respiratory

demands would lead to the increased mortality of longfins relative to

shortfins.

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1977] JELLYMANMIGRATING ELVERS 71

ACKNOWLEDGMENTS

I wish to thank the various personnel of the NewZealand Electricity Depart-

ment

and Mr J.

Josephs

of New

Plymouth

for

providing elver samples.

I

also

wish to thank Mr A. M. R. Burnet and Dr R. M. McDowall for adviceand

criticism

of the

manuscript.

LITERATURE CITED

BURNET,

A. M. R. 1952:

Studies

on the

ecology

of the New

Ze ala nd long-finned

eel, Anguilla dieffenbachii Gray. Australian Journal of Marineand

Freshwater Research

3

(

1

)

: 32-63.

CAIRNS,

D. 1941 :

Life-history

of the two

species

of New

Ze alan d freshwater

eel. Par t 1 -Taxonom y,

age and

growth, migration,

and

distribution.

N.Z. Journal of Scienceand Technology, SeriesB, 23

(

2

) :

53-72.

1942: Life historyof the twospeciesof freshwater eel in NewZealand.

II. Food,

and

inter-relationships with trout. N.Z.

Journal

of

Science

and Technology, Series

B, 23

(

4

)

:

132-48.

HARDY, C. J. 1950: Summary of a report on eel trapping and observations at

Lakes Taylor

and

Shcppard.

North Canterbury Acclimatisation

Society, Eighty-Sixth Annual Report:24-6.

HERRE,

A. W. 1930: A New

Cuinea eel-fair.

Science (

New

York

) 71

: 16.

HOPKINS,

C. L. 1970:

Some aspects

of the

bionomics

of

fish

in a

brown trout

nursery stream.

N.Z.

Marine Department Fisheries Research Bulletin

4.38 pp.

JELLYMAN,

D. J. (in

p re ss ): Freshwater invasion

of a New

Zealand stream

by

glass-eels

of two Anguilla spp.

N.Z.

Journal

of

Marine

and

Fresh-

water Research.

KERSHAW, J. A. 1911: Migration of eels in Victoria. Victorian Naturalist 27:

196-201.

SKEAD,

C. J. 1959: The

climbing

of

juvenile eels.

Piscator

13

(

46

)

:74-86.

WHITLEY,

G. P. 1929: An

eel-fare

at

Parramatta.

Australian Museum Magazine

3

(

10

)

:

348.

WOODS,

C. S. 1964:

Fisheries aspects

of the

Tongariro power development

project. N.Z. Marine Department Fisheries Technical Report 10.

214

pp.

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