female dominance

Bull. Fr. Pêche
Piscic.
(1998)
349:141-152
— 141 —
WHY ARE MIGRATING M A L E SHORTFINNED EELS
(ANGUILLA
AUSTRALIS)
IN L A K E E L L E S M E R E , N E W Z E A L A N D ,
GETTING SMALLER BUT NOT YOUNGER ?
D.J.
J E L L Y M A N ( 1 ) a n d P . R . T O D D (2)
(1)
N a t i o n a l Institute of W a t e r a n d A t m o s p h e r i c R e s e a r c h L t d , R O . B o x 8 6 0 2 , C h r i s t c h u r c h ,
(2)
M i n i s t r y of F i s h e r i e s , Private B a g 14, N e l s o n , N e w Z e a l a n d .
New Zealand.
ABSTRACT
L a k e E l l e s m e r e is a l a r g e c o a s t a l l a k e in t h e S o u t h I s l a n d of N e w Z e a l a n d w h i c h
s u p p o r t s i m p o r t a n t c o m m e r c i a l f i s h e r i e s for b o t h f e e d i n g a n d m i g r a t o r y s h o r t f i n n e d e e l s
Anguilla australis. O v e r t h e p a s t 5 0 y e a r s , t h e s e x d i s t r i b u t i o n of m i g r a t o r y e e l s in the l a k e
h a s c h a n g e d f r o m a d o m i n a n c e b y f e m a l e s t o a d o m i n a n c e by m a l e s . A g e a n d s i z e data of
m a l e s h a v e b e e n c o l l e c t e d i n t e r m i t t e n t l y s i n c e 1 9 7 4 . D u r i n g this t i m e , t h e m e a n a g e h a s
r e m a i n e d relatively constant while the a v e r a g e length has d e c l i n e d by a l m o s t 2 0 % .
D e s p i t e t h i s r e d u c t i o n , l e n g t h d i s t r i b u t i o n s a r e l e s s v a r i a b l e t h a n w e i g h t , c o n d i t i o n , or
g r o w t h rate d i s t r i b u t i o n s , a n d l e n g t h is c o n s i d e r e d to b e t h e p r i m a r y factor a s s o c i a t e d w i t h
t h e o n s e t of m i g r a t i o n . T h e r e d u c t i o n in a v e r a g e s i z e d o e s not a p p e a r t o be a result of
s e l e c t i v e h a r v e s t of l a r g e r i n d i v i d u a l s , but m u s t reflect c h a n g i n g e n v i r o n m e n t a l c o n d i t i o n s
in t h e l a k e itself - s u c h c o n d i t i o n s c o u l d i n c l u d e loss of m a c r o p h y t e s , a n d a r e d u c t i o n in t h e
p r o p o r t i o n of l a r g e r e e l s of b o t h s p e c i e s w i t h r e s u l t a n t p r o l i f e r a t i o n of t h e s m a l l benthic
bully Gobiomorphus
cotidianus.
T h e s u g g e s t e d m a n a g e m e n t s t r a t e g y is t o m a x i m i s e
h a r v e s t of m i g r a t o r y m a l e e e l s a n d t h u s r e d u c e t h e c a p t u r e of f e m a l e s w h i c h a r e large
e n o u g h t o prey o n bullies.
K e y - w o r d s : Anguilla
australis,
shortfinned eels, length, age, condition, migration.
POURQUOI LES MÂLES MIGRANTS DES ANGUILLES À COURTES NAGEOIRES
(ANGUILLA
AUSTRALIS)
DU LAC E L L E S M E R E , N O U V E L L E - Z É L A N D E ,
SONT-ILS DE PLUS EN P L U S P L U S PETITS M A I S PAS P L U S J E U N E S ?
RÉSUMÉ
L e lac E l l e s m e r e e s t u n g r a n d lac c ô t i e r d e l'Ile S u d d e N o u v e l l e - Z é l a n d e , q u i est le
s i è g e d ' u n e p ê c h e r i e c o m m e r c i a l e i m p o r t a n t e p o u r les a n g u i l l e s à c o u r t e s n a g e o i r e s
(Anguilla
australis)
à la fois a u s t a d e d ' a l i m e n t a t i o n et a u s t a d e m i g r a t o i r e . P e n d a n t les
5 0 d e r n i è r e s a n n é e s , la r é p a r t i t i o n d e s s e x e s d e s a n g u i l l e s m i g r a t r i c e s d u lac a c h a n g é ,
p a s s a n t d ' u n e m a j o r i t é d e f e m e l l e s à u n e m a j o r i t é d e m â l e s . D e s d o n n é e s sur l ' â g e et la
taille o n t é t é r e l e v é e s d e f a ç o n i n t e r m i t t e n t e d e p u i s 1 9 7 4 . P e n d a n t c e t t e p é r i o d e , l'âge
m o y e n est r e s t é r e l a t i v e m e n t c o n s t a n t t a n d i s q u e la l o n g u e u r m o y e n n e a b a i s s é d e
p r e s q u e 2 0 % . M a l g r é c e t t e r é d u c t i o n , la r é p a r t i t i o n d e s l o n g u e u r s est m o i n s v a r i a b l e q u e
les r é p a r t i t i o n s d e s p o i d s , d e s c o n d i t i o n s , o u d e s t a u x d e c r o i s s a n c e , et la l o n g u e u r e s t
c o n s i d é r é e c o m m e le f a c t e u r p r i n c i p a l a s s o c i é a u d é c l e n c h e m e n t d e la m i g r a t i o n . L a
r é d u c t i o n d a n s la taille m o y e n n e n e s e m b l e p a s r é s u l t e r d ' u n p r é l è v e m e n t s é l e c t i f d e s
i n d i v i d u s les p l u s g r a n d s , m a i s doit refléter d e s c h a n g e m e n t s d a n s les c o n d i t i o n s
Article available at http://www.kmae-journal.org or http://dx.doi.org/10.1051/kmae:1998039
Bull. Fr. Pêche
Piscic.
(1998) 349 :141-152
— 142 —
e n v i r o n n e m e n t a l e s d u lac l u i - m ê m e ( c e s c o n d i t i o n s p o u v a n t inclure la d i s p a r i t i o n d e s
m a c r o p h y t e s ) et une r é d u c t i o n d a n s la p r o p o r t i o n d e s a n g u i l l e s les plus g r a n d e s d e s d e u x
e s p è c e s a v e c en c o n s é q u e n c e u n e p r o l i f é r a t i o n d e la p e t i t e e s p è c e b e n t h i q u e
Gobiomorphus
cotidianus.
La s t r a t é g i e d e g e s t i o n s u g g é r é e est d e f a v o r i s e r a u m a x i m u m
le p r é l è v e m e n t d e m â l e s m i g r a t e u r s et d e réduire ainsi la c a p t u r e d e s f e m e l l e s qui s o n t
s u f f i s a m m e n t g r a n d e s pour être des prédateurs de
Gobiomorphus.
Mots-clés
: Anguilla
australis,
anguilles
à
courtes
nageoires,
longueur,
âge,
condition, migration.
INTRODUCTION
L a k e E l l e s m e r e ( F i g . 1) is a l a r g e (20 2 5 0 ha) c o a s t a l lake o n t h e e a s t c o a s t of t h e
S o u t h I s l a n d , N e w Z e a l a n d , w h i c h s u p p o r t s a n e x t e n s i v e c o m m e r c i a l e e l f i s h e r y for
s h o r t f i n n e d e e l s Anguilla australis.
Historically, t h e a n n u a l s e a w a r d m i g r a t i o n of m i g r a t o r y
e e l s f o r m e d a n i m p o r t a n t f i s h e r y to M a o r i ( G O O D A L L , 1 9 9 6 ) , a n d s i n c e t h e e a r l y 1 9 7 0 s
h a s b e e n a s i g n i f i c a n t c o m p o n e n t of t h e c o m m e r c i a l eel fishery, w i t h a n a v e r a g e of a b o u t
9 0 t of s h o r t f i n n e d e e l s t a k e n a n n u a l l y ( T O D D , 1 9 8 1 a ) . T h e lake is s e p a r a t e d f r o m t h e s e a
b y a g r a v e l b a r w h i c h is o p e n e d m e c h a n i c a l l y at p r e - d e t e r m i n e d levels ; a s t h e l a k e is
u s u a l l y c l o s e d d u r i n g m o s t of t h e m i g r a t i o n s e a s o n , f i s h e r s h a v e a c c e s s t o t h e m i g r a t i n g
e e l s t h a t c o n g r e g a t e at t h e s o u t h w e s t e r n c o r n e r of t h e lake, a d j a c e n t to t h e n a r r o w e s t p a r t
of t h e bar.
Figure 1
Map
of Lake Ellesmere.
Figure 1
C a r t e d u lac E l l e s m e r e .
Bull. Fr. Pêche
Piscic.
(1998)
— 143 —
349:141-152
A l t h o u g h t h e l a k e o n c e c o n t a i n e d l a r g e n u m b e r s of l o n g f i n n e d e e l s ,
Anguilla
dieffenbachii,
t h e s e h a v e l a r g e l y d i s a p p e a r e d , p o s s i b l y b e c a u s e t h e y a r e o l d e r at m i g r a t i o n
than shortfinned eels (JELLYMAN a n d T O D D , 1982) a n d are hence accessible to
c o m m e r c i a l f i s h e r s o v e r a l o n g e r p e r i o d ; t h e y a l s o a p p e a r m o r e v u l n e r a b l e t o f y k e netting
t h a n s h o r t f i n n e d e e l s ( N I W A , u n p u b l i s h e d d a t a ) . T h e e x t e n s i v e l o s s of m a c r o p h y t e b e d s
in t h e late 1 9 6 0 s h a s a l s o b e e n s u g g e s t e d a s a r e a s o n for t h e i r d e c l i n e ( J E L L Y M A N et al.,
1995). Shortfinned females were also formerly a b u n d a n t ( H O B B S , 1947), but their
n u m b e r s have also b e c o m e greatly diminished, and the migration fishery now
concentrates on shortfinned males which are caught from mid February through March.
M i g r a t o r y (silver) e e l s f o r m i m p o r t a n t f i s h e r i e s in a n u m b e r of E u r o p e a n c o u n t r i e s
( T E S C H , 1977). As such, they have attracted considerable research on factors associated
w i t h t h e t i m i n g of t h e i r d o w n s t r e a m m i g r a t i o n ( e . g . F R O S T , 1 9 5 0 ; L O W E , 1952 ;
V O L L E S T A D a n d J O N S S O N , 1 9 8 6 ; V O L L E S T A D et al., 1 9 9 4 ) - s i m i l a r s t u d i e s h a v e
b e e n c a r r i e d o u t in N e w Z e a l a n d by B U R N E T ( 1 9 6 9 ) a n d T O D D ( 1 9 8 1 a ) . S t u d i e s of
f a c t o r s a s s o c i a t e d w i t h t h e o n s e t of m i g r a t i o n in A. anguilla
(e.g. F R O S T , 1950 ;
V O L L E S T A D and J O N S S O N , 1986 ; V O L L E S T A D , 1992) have concluded that although
t h e r e is c o n s i d e r a b l e g e o g r a p h i c variability, m i g r a t i o n is m o r e likely t o b e a s s o c i a t e d w i t h
a c h i e v e m e n t of p a r t i c u l a r s i z e r a t h e r t h a n a p a r t i c u l a r a g e . T h e p r e s e n t p a p e r t e s t s t h e
h y p o t h e s i s t h a t t h e o n s e t of m i g r a t i o n of s h o r t f i n n e d m a l e e e l s in L a k e E l l e s m e r e i s m o r e
a s s o c i a t e d w i t h s i z e t h a n a g e , a n d e x p l o r e s r e a s o n s for t h e d e c l i n i n g s i z e of t h e s e e e l s
over time.
METHODS
S a m p l e s of m i g r a t i n g s h o r t f i n n e d m a l e e e l s w e r e c a u g h t in u n b a i t e d f y k e n e t s
( 1 2 m m m e s h ) , s e t o v e r n i g h t at T a u m u t u , L a k e E l l e s m e r e . D u r i n g t h e 1 9 7 0 ' s w h e n a n
extensive research p r o g r a m m e w a s underway, large s a m p l e s were m e a s u r e d a n d aged.
To c o m p l e m e n t t h e s e a n d o b s e r v e a n y c h a n g e s o v e r t i m e , f u r t h e r s a m p l e s w e r e c o l l e c t e d
d u r i n g 1 9 9 2 - 9 6 . F o r l e n g t h - f r e q u e n c y d a t a , e e l s w e r e a n a e s t h e t i s e d in t h e f i e l d , m e a s u r e d
(to 1 m m ) a n d r e l e a s e d . For s u b s a m p l e s r e t a i n e d for a g e i n g , e e l s w e r e t a k e n to t h e
l a b o r a t o r y w h e r e t h e y w e r e m e a s u r e d , w e i g h e d (to 0.1 g ) , a n d o t o l i t h s c o l l e c t e d . O t o l i t h s
w e r e p r e p a r e d b y c r a c k i n g a n d b u r n i n g after t h e m e t h o d of H U a n d T O D D ( 1 9 8 1 ) .
C o n d i t i o n w a s c a l c u l a t e d u s i n g k = w * 1 0 7 P w h e r e w = w e i g h t in g , a n d I = l e n g t h in m m .
M e a n g r o w t h r a t e s w e r e c a l c u l a t e d f r o m m e a n l e n g t h (or w e i g h t ) / m e a n a g e for e a c h year.
D a t a o n n o n - m i g r a t o r y e e l s c a m e f r o m e x t e n s i v e c o l l e c t i o n s of e e l s m a d e in t h e 1970's
(NIWA, unpublished data).
RESULTS
N u m b e r s of e e l s m e a s u r e d for t h e v a r i o u s y e a r s a r e g i v e n in T a b l e I. P r e v i o u s
analysis
(TODD,
1980)
showed
that
there
were
no consistent
differences
in
length
d i s t r i b u t i o n s of m a l e s c a p t u r e d at i n t e r v a l s t h r o u g h o u t a n y m i g r a t o r y s e a s o n . T h u s , in t h e
p r e s e n t p a p e r , all s a m p l e s t a k e n t h r o u g h o u t
a season were combined
into a
single
sample.
T h e m e a n l e n g t h h a s r e d u c e d f r o m 4 8 0 m m in 1 9 4 2 ( H O B B S , 1 9 4 7 ) t o 3 9 4 m m in
1 9 9 6 (Table I, F i g . 2 ) . Further, o v e r t i m e , t h e s h a p e of t h e l e n g t h f r e q u e n c y d i s t r i b u t i o n s
h a s r e m a i n e d n o r m a l but s h i f t e d in f a v o u r of s m a l l e r e e l s ( F i g . 3) ; s i m i l a r s h i f t s a r e
e v i d e n t f r o m w e i g h t d i s t r i b u t i o n s . T h e t r e n d for a c o n t i n u e d d e c l i n e in m e a n l e n g t h per
s e a s o n h a s b e e n p a r a l l e l e d by a s i m i l a r d e c l i n e in w e i g h t until t h e e a r l y 1 9 8 0 ' s , a f t e r w h i c h
w e i g h t h a s r e m a i n e d relatively c o n s t a n t . C o n d i t i o n w a s n o t c o r r e l a t e d w i t h l e n g t h ( p > 0 . 0 5 ) ,
Bull. Fr. Pêche
Piscic.
(1998)
—
349 :141-152
144
Table I
Length
distributions
(%/year)
of
migrating
male
shortfinned
eels
from
Lake
E l l e s m e r e , 1972-1996. < = < 0 . 1 .
Tableau I
R é p a r t i t i o n d e s l o n g u e u r s ( % / a n ) d e s a n g u i l l e s (Anguilla
du lac Ellesmere, 1972-1996. < = <0.1.
Size group
(mm)
330-339
340-349
350-359
360-369
370-379
380-389
390-399
400-409
410-419
420-429
430-439
440-449
450-459
460-469
470-479
480-489
490-499
500-509
510-519
520-529
N
Mean
SE
CV
1972
3.8
19
5 7
7.5
13.2
132
15.1
17.0
7.5
3 8
5 7
1.9
3 8
53
442.7
3.76
0.06
1974
1975
1976
1977
1.6
3.1
4 7
10.4
16.1
21.3
14.6
115
11.5
3 1
0 5
1 0
0.5
0 1
03
0.8
2 7
6 1
9 6
142
16 6
163
14.2
9.2
5.1
3.4
10
0 2
0.1
<
0 1
1 2
3 0
7 8
13.1
17 5
16.9
16.5
11.1
6.6
3.6
1 8
0 4
03
0.2
0 2
0.5
2 4
60
17 1
14 2
132
16.1
13.9
7.5
4 6
22
1.4
07
192
446.9
1.58
0.05
2524
439 4
0.47
0.05
2271
434.6
0.46
0.05
416
427.5
1.13
0.05
1978
0 1
0 1
0 7
2 3
5.9
12 4
17,9
18.4
16.4
12.7
6.6
3.3
1.6
0.8
0.5
0 1
02
2257
426.3
0.44
0.05
1979
Year
1980
<
<
<
04
37
85
14 7
18.7
19.2
14.1
8.6
5.5
3.6
1.5
07
0 1
0.3
0.2
0.3
0.8
2.6
6 2
13.3
188
19.6
15.3
11.7
63
26
09
0.8
04
0 2
0.1
0.2
2091
432.9
0.48
0 05
2229
424.9
0.45
0.05
australis)
1981
1982
1993
1994
0.2
13
2.6
5 8
11 9
17 5
19 1
16.0
11.8
78
2.8
1.9
0 6
0.5
0 1
0.2
02
1 7
4 3
9 4
17 3
17 1
173
126
6 6
60
4.3
1 5
04
0 4
0 6
0 2
1 3
5.1
8.9
14 5
17 0
174
102
9.8
9.4
4.7
17
0.6
13
8 1
21.3
18.1
20 0
188
6 3
25
1.3
0.6
857
416.0
0.75
0.05
469
412.1
1.18
0.06
235
392.9
1 49
0.06
mâles migrants
1995
0.7
03
3.7
7 5
13 6
23.5
21 8
16 0
7 5
4.1
1 0
0.3
1996
Total
<
2 8
5 6
11 1
17 9
17.1
17 1
7.9
7.1
16
0.4
0.8
02
06
13
29
57
105
14.6
165
15.9
13.1
85
4 9
2.8
1.6
0 5
0.2
02
0.1
294
389.7
1.05
005
225
394.0
1.26
0.05
14273
428.5
0.21
0.06
<
0 6
0.6
160
390.7
1.54
0.05
a n d t h e m e a n c o n d i t i o n of all y e a r l y s a m p l e s s h o w e d s i m i l a r t r e n d s to w e i g h t , e x c e p t t h a t
c o n d i t i o n i m p r o v e d m a r k e d l y o v e r r e c e n t y e a r s ( F i g . 2 ) . A g e d i s t r i b u t i o n s (Table II) s h o w a
r e a s o n a b l y c o n s t a n t m e a n a g e o v e r t h e 20 y e a r s for w h i c h d a t a a r e a v a i l a b l e , a l t h o u g h
d i f f e r e n c e s b e t w e e n y e a r s w e r e s i g n i f i c a n t ( A N O V A , F = 3 0 . 2 2 , p < 0 . 0 0 1 ) . M u c h of t h i s
d i f f e r e n c e will b e d u e t o v a r i a b l e a g e - c l a s s s t r e n g t h s - for e x a m p l e , a g e c l a s s 15 in 1 9 7 8
a l s o d o m i n a t e d t h e 1 9 7 9 s a m p l e ( a s age c l a s s 16) a n d p r o b a b l y a c c o u n t e d for t h e
g r e a t e s t a v e r a g e a g e , 1 5 . 6 , b e i n g r e c o r d e d in that y e a r ; r e m o v a l of t h e 1 9 7 9 d a t a f r o m
t h e a g e s a m p l e r e d u c e d t h e v a r i a b i l i t y in a g e b e t w e e n y e a r s , but d i f f e r e n c e s w e r e still
s i g n i f i c a n t (F = 1 1 . 5 6 , p < 0 . 0 0 1 ) . Of all a g e d m a l e s (n = 2 2 0 6 ) , 6 7 % w e r e 13 - 16 y e a r s o l d .
V a r i a b i l i t y in s i z e a n d a g e o v e r t h i s p e r i o d ( e x p r e s s e d a s c o e f f i c i e n t of v a r i a t i o n , CV,
T a b l e III) i n d i c a t e s t h a t w e i g h t is t h e m o s t v a r i a b l e p a r a m e t e r , f o l l o w e d by a g e a n d
c o n d i t i o n , w i t h l e n g t h b e i n g t h e l e a s t v a r i a b l e . T h e d i f f e r e n c e is e v e n m o r e m a r k e d w h e n
c o m p a r i s o n s a r e m a d e b e t w e e n i n d i v i d u a l y e a r s , a s v a r i a t i o n in l e n g t h r a n g e d f r o m 0.05 0 . 0 6 ( T a b l e I), w h e r e a s v a r i a t i o n in a g e r a n g e d f r o m 0 . 1 3 - 0 . 1 8 (Table II). To s e e w h e t h e r
migratory eels had a higher average condition than equivalent-sized non-migratory eels,
d a t a f r o m 1 9 7 8 w e r e u s e d ; in this year, a l a r g e s a m p l e of m i g r a t o r y m a l e s w a s m e a s u r e d
(n = 2 2 5 7 ) a n d a l a r g e s a m p l e of n o n - m i g r a t o r y e e l s of t h e s a m e l e n g t h r a n g e , 3 5 5 4 9 7 m m , w a s a v a i l a b l e (n = 4 3 6 0 ) . T h e resulting c o m p a r i s o n s h o w e d t h a t t h e c o n d i t i o n of
m i g r a t o r y e e l s w a s s i g n i f i c a n t l y g r e a t e r t h a n t h a t of n o n - m i g r a t o r y e e l s ( A N O V A ,
F = 295.95, p <0.001).
T h e r e w a s a s i g n i f i c a n t r e l a t i o n s h i p ( p < 0 . 0 0 1 ) b e t w e e n l e n g t h a n d a g e at m i g r a t i o n
for all d a t a (n = 2 2 0 6 ) , but it o n l y e x p l a i n e d 9 % of t h e variability. A s l e n g t h d i s t r i b u t i o n s
v a r i e d w i t h t i m e , t h e l a r g e s t s a m p l e a g e d , 1 9 7 9 (n = 5 1 4 ) w a s e x a m i n e d , w i t h s i m i l a r
Bull. Fr. Pêche
Piscic.
(1998)
3 4 9 : 1 4 1 -152
— 145 —
2
r e s u l t s i . e . R = 0 . 0 3 , p < 0 . 0 0 1 . In c o n t r a s t , t h e r e w a s a s t r o n g n e g a t i v e r e l a t i o n s h i p
b e t w e e n g r o w t h rate ( m m / y r ) a n d a g e at m a t u r i t y ( F i g . 4 ; n = 2 2 0 6 , R = 0 . 8 5 , p < 0 . 0 0 1 ) .
D i f f e r e n c e s in g r o w t h r a t e s in b o t h l e n g t h a n d w e i g h t w e r e s i g n i f i c a n t o v e r t h e 2 0 - y e a r
p e r i o d 1 9 7 5 - 9 5 ( l e n g t h g r o w t h rate : A N O V A , F = 2 4 . 2 6 , p < 0 . 0 0 1 ; w e i g h t g r o w t h rate :
A N O V A , F = 65.88, p<0.001) with both rates declining with time.
2
)
c
o
"•4—•
Condition ^
O
C
(0
<D
E
E,
2
sz
•*—•
CD
C
\
c
to
Weight
3
+-•
(1)
Length ^
C
(0
Year
Figure 2
M e a n a n n u a l l e n g t h , w e i g h t a n d c o n d i t i o n (± 2 S E ) o f m i g r a t i n g m a l e s h o r t f i n n e d
eels f r o m Lake Ellesmere for v a r i o u s y e a r s f r o m 1942-1996.
Figure 2
M o y e n n e a n n u e l l e d e s l o n g u e u r s , p o i d s e t c o n d i t i o n s (± 2 S E ) d e s m â l e s m i g r a n t s
d'Anguilla
australis
d u lac E l l e s m e r e p o u r d i f f é r e n t e s a n n é e s e n t r e 1942 et 1996.
Mean length = l o n g u e u r m o y e n n e ( m m ) ; Mean w e i g h t = p o i d s m o y e n (g) ; Mean
c o n d i t i o n = c o n d i t i o n m o y e n n e (k) ; Year = a n n é e .
Bull. Fr. Pêche
Piscic.
(1998)
25-
I
20-
— 146
349 :141 -152
—
25 ^
1975
n = 338
15
15
10
10
5
5
0300
0
450
500
550
1 975
n = 338
\
20
50
100
150
Ik*.
200
25
1993
250
300
1 993
n = 235
20
15
10
O
c
CD
CD
LL
550
50
300
Length (mm)
100
150
200
250
300
Weight (g)
Figure 3
L e n g t h a n d w e i g h t f r e q u e n c i e s of m i g r a t i n g male s h o r t f i n n e d eels f r o m Lake
E l l e s m e r e for 1975 a n d 1993-96. A r r o w s s h o w m e a n v a l u e s a n d t h e d o t t e d line
s h o w s the m i n i m u m c o m m e r c i a l weight for 1994-96.
Figure 3
F r é q u e n c e d e s l o n g u e u r s e t d e s p o i d s d e s m â l e s m i g r a n t s d'Anguilla
australis
d u lac E l l e s m e r e p o u r 1975 et 1993-96. L e s f l è c h e s i n d i q u e n t les v a l e u r s m o y e n n e s
e t la l i g n e p o i n t i l l é e d é s i g n e l e p o i d s c o m m e r c i a l m i n i m u m p o u r 1 9 9 4 - 9 6 .
F r e q u e n c y = f r é q u e n c e (%) ; L e n g t h = l o n g u e u r ( m m ) ; W e i g h t = p o i d s ( g ) .
Bull. Fr. Pêche
Piscic.
(1998)
349 :141 -152
— 147
—
T a b l e II
A g e d i s t r i b u t i o n (%/year) of m i g r a t i n g male s h o r t f i n n e d eels f r o m Lake Ellesmere,
1975-1995. < = <0.1.
T a b l e a u II
R é p a r t i t i o n d e s â g e s ( % / a n ) d e s a n g u i l l e s (Anguilla
lac Ellesmere, 1975-1995. < = <0.1.
Age
(years)
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
N
Mean
SE
CV
australis)
mâles migrants du
1.8
0.9
2.7
9.8
18.8
18.8
19.6
11.6
7.1
2.7
0.9
2.7
1.8
0.9
Total
(%)
<
0.3
1.3
3.9
8.6
15.3
20.1
18.0
14.1
9.7
4.9
2.3
1.0
0.2
<
112
14.6
0.24
0.18
2206
14.7
0.05
0.14
Year
1975
1978
1979
1980
1981
0.9
2.3
8.2
12.3
29.2
21.9
12.3
4.6
3.2
2.7
0.5
1.4
0.3
0.9
4.0
8.9
16.9
21.5
24.9
11.4
4.9
3.1
2.2
0.3
0.3
0.3
0.2
0.8
1.2
4.3
9.1
14.0
17.7
19.7
16.2
9.7
4.7
2.1
0.3
1.0
5.2
8.6
16.6
23.6
16.9
13.5
8.3
3.9
1.6
0.3
0.3
1.2
4.3
6.5
14.2
19.4
16.7
17.6
11.4
6.5
1.2
0.6
0.3
325
14.4
0.11
0.13
514
15.6
0.09
0.13
385
14.4
0.10
0.13
324
14.8
0.11
0.14
1982
0.4
0.7
3.6
7.3
18.1
23.6
19.9
10.5
6.9
6.9
1.1
0.7
0.4
1994
276
13.5
0.12
0.14
51
14.1
0.30
0.15
2.0
9.8
9.8
23.5
15.7
11.8
9.8
15.7
2.0
0.4
1995
0.5
219
14.6
0.13
0.14
T a b l e III
V a r i a t i o n in l e n g t h ( m m ) , w e i g h t (g), c o n d i t i o n , a g e a n d g r o w t h rate ( m m / y r , g/yr)
of m i g r a t i n g m a l e s h o r t f i n n e d e e l s f r o m L a k e E l l e s m e r e , 1 9 7 4 - 1 9 9 6 .
T a b l e a u III
V a r i a t i o n d e la l o n g u e u r ( m m ) , d u p o i d s ( g ) , d e la c o n d i t i o n , d e l ' â g e e t d u t a u x d e
c r o i s s a n c e ( m m / a n , g / a n ) d e s a n g u i l l e s (Anguilla
australis)
m â l e s m i g r a n t s d u lac
Ellesmere, 1974-1996.
N
Mean
Minimum
Maximum
SD
SE
CV
Length
14 273
428.5
330.0
529.0
24.8
0.21
0.06
Weight
3678
146.3
60.0
366.0
34.0
0.56
0.23
Condition
3678
1.99
1.00
4.05
0.24
0.01
0.12
Age
2206
14.65
8.0
28.0
2.11
0.05
0.14
Growth
rate
(length)
2206
29.30
14.46
47.44
4.11
0.09
0.14
Growth
rate
(weight)
2206
10.32
4.06
24.71
2.22
0.05
0.22
Bull. Fr. Pêche
Piscic.
(1998)
349 :141 -152
— 148
—
>.
E
E,
CD
•4-»
CO
O
O
Figure 4
R e l a t i o n s h i p b e t w e e n m e a n a n n u a l g r o w t h rate ( m m / y r ) a n d age of m i g r a t i n g male
s h o r t f i n n e d eels f r o m Lake Ellesmere.
Figure 4
R e l a t i o n e n t r e le t a u x d e c r o i s s a n c e a n n u e l m o y e n ( m m / a n n é e ) et l'âge d e s m â l e s
m i g r a n t s d'Anguilla
australis
d u lac Ellesmere.
G r o w t h rate (mm/yr) = taux de croissance (mm/année)
DISCUSSION
W h i c h factors are a s s o c i a t e d with t h e onset of m i g r a t i o n ?
O v e r t h e 2 3 y e a r s of r e c o r d , there h a s b e e n little c h a n g e in t h e m e a n a g e of
s h o r t f i n n e d m a l e s a l t h o u g h t h e r e is a range of 1 3 - 1 4 y e a r s in m o s t y e a r s (Table II). O v e r
t h e s a m e p e r i o d , a v e r a g e l e n g t h a n d weight h a v e b o t h d e c l i n e d s u b s t a n t i a l l y ( 1 2 a n d 3 9 % ,
r e s p e c t i v e l y ) a n d at first g l a n c e t h i s m i g h t b e i n t e r p r e t e d a s m e a n i n g t h a t a g e rather t h a n
s i z e d e t e r m i n e s t h e o n s e t of m i g r a t i o n . However, t h e c o e f f i c i e n t of v a r i a t i o n ( C V ) for l e n g t h
is still l e s s t h a n t h a t for a g e , i n d i c a t i n g t h a t l e n g t h r a t h e r t h a n a g e is a m o r e likely
d e t e r m i n a n t of m i g r a t i o n .
V a r i a t i o n in g r o w t h rate e x p l a i n e d 8 5 % of t h e v a r i a t i o n in l e n g t h at m i g r a t i o n , w i t h
s l o w g r o w t h a s s o c i a t e d w i t h g r e a t e r a g e but m o r e r a p i d g r o w t h a s s o c i a t e d w i t h l e s s e r a g e .
S i m i l a r r e l a t i o n s h i p s h a v e b e e n f o u n d for b o t h s e x e s of A. anguilla
(VOLLESTAD, 1992) ;
S V E D A N G et al. ( 1 9 9 6 ) i n t e r p r e t e d t h e s a m e t r e n d in f e m a l e A. anguilla
as indicating a
Bull. Fr. Pêche Piscic.
(1998)
349:141 -152
— 149
s t r a t e g y of m a t u r i n g at t h e e a r l i e s t p o s s i b l e o p p o r t u n i t y . T h i s is in a g r e e m e n t w i t h t h e o r y
t h a t m a l e s exhibit a t i m e - m i n i m i s i n g s t r a t e g y ( H E L F M A N et al., 1 9 8 7 ; L A R S S O N et al.,
1990 ; V O L L E S T A D , 1992), with e m p h a s i s on attaining maturity as quickly as possible.
U n l i k e f e m a l e s w h e r e g r e a t e r s i z e is a c c o m p a n i e d by i n c r e a s e d f e c u n d i t y (e.g. T O D D ,
1 9 8 1 b ) , t h e r e w o u l d s e e m t o b e little b e n e f i t for m a l e s t o g r o w l a r g e , a s o b s e r v a t i o n s of
artificial s p a w n i n g d o n o t i n d i c a t e m a t e s e l e c t i o n ( S O R E N S E N a n d W I N N , 1 9 8 4 ) w h e r e
l a r g e r size m i g h t o t h e r w i s e b e a d v a n t a g e o u s .
R e s e a r c h o n o t h e r s p e c i e s of Anguilla h a s s o m e t i m e s i n d i c a t e d t h a t m i g r a t i o n is
m o r e a s s o c i a t e d w i t h s i z e t h a n a g e (e.g. T E S C H , 1 9 7 7 ; V O L L E S T A D a n d J O N S S O N ,
1 9 8 6 ) . In c o n t r a s t , T O D D ( 1 9 8 0 ) s u s p e c t e d t h a t for m a l e A. australis,
migration might be
m o r e d e p e n d e n t o n a g e t h a n l e n g t h , a l t h o u g h t h e c o n s i d e r a b l e r a n g e in b o t h factors
m e a n t h e c o u l d n o t d r a w a n y f i r m c o n c l u s i o n s . F r o m t h e p r e s e n t study, it is c o n c l u d e d t h a t
l e n g t h is a b e t t e r p r e d i c t o r of t h e o n s e t of m a t u r i t y t h a n is a g e . A l t h o u g h t h e m i n i m u m
length recorded has continued to decline over time, m i n i m u m weight has b e e n reasonably
s t a b l e over t h e last 4 y e a r s of r e c o r d i n d i c a t i n g a p o s s i b l e m i n i m u m t h r e s h o l d size o f about
6 5 g , w h i c h e q u a t e s to 3 3 5 m m .
As migrating males from the present study had a significantly greater condition than
n o n - m i g r a t o r y e e l s of s i m i l a r s i z e , it is likely that a t t a i n m e n t of a t h r e s h o l d c o n d i t i o n is a
further prerequisite to maturation. T h u s , having attained an appropriate size, m a l e eels
a l s o n e e d t o b e of s u f f i c i e n t c o n d i t i o n ( t h r o u g h a c c u m u l a t i o n of s u f f i c i e n t lipid r e s e r v e s ) t o
u n d e r t a k e a m i g r a t i o n of a b o u t 5 0 0 0 k m ( J E L L Y M A N , 1 9 8 7 ) . In a s i m i l a r m a n n e r to t h e
w a y in w h i c h m e s e n t e r i c fat l e v e l s a r e i m p o r t a n t for r e g u l a t i n g m a t u r i t y in A t l a n t i c s a l m o n ,
Salmo salar ( R O W E elal., 1 9 9 1 ) , t h e fat c o n t e n t of m i g r a t o r y e e l s h a s b e e n s u g g e s t e d a s
i n f l u e n c i n g t h e o n s e t of m a t u r i t y in A. anguilla ( L A R S S O N et al., 1 9 9 0 ) .
Why
has the average size declined ?
T O D D ( 1 9 8 0 ) a s s u m e d t h a t t h e d e c l i n e in l e n g t h o b s e r v e d f r o m 1 9 4 2 to 1 9 8 0 w a s
d u e to c o m m e r c i a l f i s h i n g s e l e c t i v e l y r e m o v i n g t h e l a r g e r m a l e s . If t h i s w a s s o a n d t h e
larger males were caught w h e n they e x c e e d e d the current m i n i m u m c o m m e r c i a l size, then
it s h o u l d b e e v i d e n t f r o m a c h a n g i n g s k e w in s i z e f r e q u e n c y d i s t r i b u t i o n s , a n d t h e r e w o u l d
b e n o r e a s o n t o s u p p o s e a shift to s m a l l e r m i n i m u m s i z e s o v e r t i m e . A l t h o u g h t h e r e h a s
b e e n n o c h a n g e in t h e s h a p e of t h e l e n g t h - f r e q u e n c i e s o v e r t i m e , t h e m i n i m u m s i z e h a s
d e c l i n e d e.g. in 1 9 7 4 , t h e r e w e r e n o e e l s of 1 0 0 g o r l e s s , b u t by 1 9 9 6 t h i s s i z e group
c o m p r i s e d 3 2 % of s a m p l e s . F u r t h e r , t h e t i m e - f r a m e i n v o l v e d w o u l d s e e m t o m i t i g a t e
a g a i n s t c o m m e r c i a l f i s h i n g b e i n g t h e m e c h a n i s m i n v o l v e d i.e. i n t e n s i v e c o m m e r c i a l fishing
c o m m e n c e d in 1 9 7 1 ( 2 8 7 t ; J E L L Y M A N et al., 1 9 9 5 ) , but by t h e f o l l o w i n g y e a r ( t h e first
y e a r s i n c e 1 9 4 2 that l e n g t h d a t a w e r e r e c o r d e d ) m e a n l e n g t h s h a d d e c l i n e d by 3 3 m m
f r o m t h o s e r e c o r d e d by H O B B S ( 1 9 4 7 ) in 1 9 4 2 . It s e e m s u n l i k e l y t h a t c o m m e r c i a l fishing
c o u l d h a v e p r o d u c e d s u c h a r a p i d d e c l i n e w i t h i n o n e y e a r , a n d it is c o n c l u d e d t h a t t h e
d e c l i n e in a v e r a g e s i z e is m a i n l y d u e t o e e l s a c h i e v i n g m a t u r i t y at a s m a l l e r s i z e .
Why
has the g r o w t h rate declined ?
G r o w t h r a t e s of N e w Z e a l a n d e e l s s e e m l a r g e l y d e n s i t y d e p e n d e n t ( J E L L Y M A N ,
1997). As Lake Ellesmere eels w e r e heavily exploited during the mid 1970's (JELLYMAN
et al., 1 9 9 5 ) , it m i g h t h a v e b e e n e x p e c t e d t h a t g r o w t h r a t e s w o u l d h a v e i n c r e a s e d as a
result of c o m m e r c i a l f i s h i n g , in a s i m i l a r m a n n e r t o t h a t r e c o r d e d by C H I S N A L L a n d
H A Y E S ( 1 9 9 1 ) in L a k e W a a h i . In c o n t r a s t , g r o w t h r a t e s of m a l e m i g r a n t s (this s t u d y ) a n d
s m a l l n o n - m i g r a n t e e l s ( J E L L Y M A N et al., 1 9 9 5 ) h a v e b o t h d e c l i n e d . S u g g e s t e d
e x p l a n a t i o n s for r e d u c e d g r o w t h r a t e s a r e l o n g - t e r m e n v i r o n m e n t a l c h a n g e s w i t h i n t h e
lake, o r a n i n c r e a s e in t h e a b u n d a n c e of s m a l l e e l s .
Bull. Fr. Pêche
Piscic.
(1998)
349:141 -152
— 150
—
D u r i n g t h e p a s t 3 0 - 4 0 y e a r s , a n u m b e r of s i g n i f i c a n t e c o l o g i c a l c h a n g e s
occurred
within
the
lake,
with
the
most
significant
being
increased
have
eutrophication
( H U G H E S et al., 1 9 7 4 ; W A R D et al., 1 9 9 6 ) , c h a n g e s in salinity r e s u l t i n g f r o m c h a n g e d
o p e n i n g r e g i m e s in t h e 1 9 6 0 s ( G E R B E A U X , 1 9 9 3 ) , a n d t h e l o s s of m a c r o p h y t e s d u r i n g
the
1 9 6 0 ' s w i t h a s s o c i a t e d i n c r e a s e d turbidity. Possibly, s o m e of t h e s e c h a n g e s
have
b e e n d e t r i m e n t a l t o e e l s . For e x a m p l e , they c o u l d h a v e r e s u l t e d in c h a n g e s t o f o o d
composition,
s i m i l a r to c h a n g e s t h a t a c c o m p a n i e d
(HAYES and
RUTLEDGE,
i n c r e a s e d t u r b i d i t y in L a k e
1 9 9 1 ) . However, given t h e plasticity of e e l s diet
Waahi
(CAIRNS,
1 9 4 2 ; J E L L Y M A N , 1 9 8 9 ) , it s e e m s unlikely t h a t a n y c h a n g e s in f o o d c o m p o s i t i o n w o u l d
have disadvantaged eels.
I n c r e a s e d r e c r u i t m e n t s e e m s unlikely a s r e c r u i t m e n t is l a r g e l y d e t e r m i n e d by t h e
t i m i n g a n d d u r a t i o n of t h e artificial o p e n i n g of t h e lake, a n d this h a s not c h a n g e d
s i g n i f i c a n t l y o v e r t h e p a s t 2 0 y e a r s . However, a r e d u c t i o n in m o r t a l i t y of j u v e n i l e e e l s is a
d i s t i n c t p o s s i b i l i t y a s a r e s u l t of t h e selective r e m o v a l of l a r g e r e e l s of b o t h s p e c i e s w h i c h
a r e k n o w n p r e d a t o r s of j u v e n i l e e e l s ( B U R N E T , 1 9 5 2 ; J E L L Y M A N , 1 9 8 9 ) . T h e s c a l e of
t h e r e d u c e d a b u n d a n c e of l a r g e r e e l s is a p p a r e n t f r o m c o m p a r i n g t h e e s t i m a t e of H O B B S
( 1 9 4 7 ) of 5 0 0 t o n s (= 5 0 8 t) of s h o r t f i n n e d f e m a l e s m i g r a t i n g in 1 9 4 2 , w i t h a n e s t i m a t e of
o n l y 2 - 3 t d u r i n g 1 9 9 6 (T.J. G O U L D , pers. c o m m . ) . Similarly, the s e x p r o p o r t i o n s of
m i g r a t o r y e e l s h a v e c h a n g e d d r a m a t i c a l l y f r o m a d o m i n a n c e by f e m a l e s (4.6:1 ; c a l c u l a t e d
f r o m H O B B S , 1 9 4 7 ) , to a d o m i n a n c e by m a l e s ( 1 9 9 6 e s t i m a t e s = 2 3 5 : 1 ) .
T h e l a k e a l s o c o n t a i n s a d e n s e p o p u l a t i o n of c o m m o n bullies
(Gobiomorphus
cotidianus)
w h i c h e e l f i s h e r s c l a i m h a s i n c r e a s e d over r e c e n t y e a r s . T h i s r e p o r t e d i n c r e a s e
m a y a l s o b e a result of t h e r e d u c t i o n in the n u m b e r of l a r g e r e e l s that w o u l d f o r m e r l y h a v e
i m p o s e d s o m e p r e d a t o r y c o n s t r a i n t s o n bully n u m b e r s . A s b o t h bullies a n d j u v e n i l e e e l s
f r e q u e n t i n s h o r e a r e a s ( J E L L Y M A N et al, 1 9 9 5 ) a n d e a t s i m i l a r f o o d (e.g. c h i r o n o m i d
l a r v a e , m y s i d s , a n d s n a i l s ) , t h e y m i g h t d i r e c t l y c o m p e t e for f o o d . T h e net result of
i n c r e a s e d s u r v i v a l of j u v e n i l e e e l s a n d limited f o o d w o u l d b e a r e d u c t i o n in g r o w t h r a t e s of
juvenile eels.
Management implications
T h e L a k e E l l e s m e r e e e l f i s h e r y is m a n a g e d b y a Total A l l o w a b l e C o m m e r c i a l C a t c h
of 1 3 6 . 5 t, a n d a p r e s e n t m i n i m u m s i z e limit o f 1 6 0 g ( 1 9 9 5 / 9 6 s e a s o n ) w h i c h i n c r e a s e s at
a r a t e of 10 g / y e a r t o r e a c h 2 2 0 g in t h e 2 0 0 2 / 0 3 s e a s o n A s v i r t u a l l y all m i g r a t i n g m a l e
e e l s a r e l e s s t h a n t h i s s i z e ( 9 6 % in 1996), a l m o s t all e e l s h a r v e s t e d by t h e c o m m e r c i a l
f i s h e r y a r e , b y d e f i n i t i o n , f e m a l e s . If t h e t h e o r y is c o r r e c t t h a t t h e r e d u c t i o n in t h e n u m b e r
of l a r g e f e m a l e e e l s is at l e a s t p a r t l y r e s p o n s i b l e for t h e p r o l i f e r a t i o n of bullies ( w h i c h in
t u r n r e d u c e t h e f o o d a v a i l a b l e t o s m a l l eels), t h e n t h e f i s h e r y s h o u l d m a x i m i s e c a p t u r e of
m a l e e e l s a n d m i n i m i s e c a p t u r e of f e m a l e s . To e n a b l e t h i s , a n e x c l u s i o n z o n e w a s
g a z e t t e d in M a r c h 1 9 9 6 , for t h a t p a r t of t h e lake w h e r e m a l e e e l s c o n g r e g a t e in their
e n d e a v o u r s t o leave t h e l a k e . P r o v i d e d s u c h a m e a s u r e c a n b e i m p l e m e n t e d annually,
t h e r e s h o u l d b e a c o r r e s p o n d i n g i n c r e a s e in t h e a b u n d a n c e of f e m a l e e e l s , a n d h o p e f u l l y
a n i n c r e a s e in b o t h s i z e a n d g r o w t h r a t e s of m a l e s .
ACKNOWLEDGEMENTS
W e t h a n k f o r m e r M i n i s t r y of A g r i c u l t u r e a n d F i s h e r i e s staff, L U C H I H U a n d T I M
D O D G S H U N , for t h e i r p a i n s t a k i n g m e a s u r i n g a n d a g e i n g of t h e e x t e n s i v e s a m p l e s
c o l l e c t e d d u r i n g t h e 1 9 7 0 s a n d 8 0 s , a n d B E N C H I S N A L L , N I W A , H A M I L T O N , for a g e i n g
t h e 1 9 9 4 a n d 1 9 9 5 e e l s . W e a l s o w i s h to t h a n k M A R T Y B O N N E T T for h e l p f u l c o m m e n t s
o n the text.
Bull. Fr. Pêche Piscic.
(1998)
349 :141-152
— 151 —
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