Biologie et c o m p o r t e m e n t agrégatif cles p o i s s o n s
Current, catch and weight
composition of yellowfin tuna with FADs
off Okinawa Island, Japan
Shinichiro K a k u m a
Okinawa Fisheries Experimental Station, 1-3-1 Nishizaki, Itoman, Okinawa, Japan
[email protected]
Abstract
Yellowfin tuna (Thunnus albacares) is a main target for the fisheries at
Fish Aggregating Devices (FADs) off the south of Okinawa Island. Catch
and weight composition of the tuna were monitored at a fisheries cooperative market from 1989 to 1998. Some distinct w e i g h t groups (considered to represent cohorts) appeared in the catch and the weight of
these groups increased m o n t h l y presumably as individual tuna grew.
A l t h o u g h small, l i g h t weight fish were caught year-round, the 2 legweight g r o u p that was recruited in May grew to about 15 kg by May
of the next year. There was a significant seasonal cycle in the m o n t h l y
catch. The catch of heavier weight groups decreased in winter indicating
the tuna moved out of this FAD area. There was also a large fluctuation
in the annual catch. T h e n u m b e r of 15 k g - w e i g h t g r o u p in May and
the annual catch of that year were significantly correlated.
A t one of huge FADs (nirai), fifteen nautical miles southeast of Okinawa Island (depth about 1 300 m), a current meter (Aanderaa RCM-7)
recorded c u r r e n t and water t e m p e r a t u r e f r o m J u n e 1995 to March
1996. The current m e t e r was attached to the FAD at 4 m depth. W h e n
typhoons attacked Okinawa, the water temperature drastically dropped.
Typically, average current speed was 29 cm/s and eastward current was
most frequent. Being affected by tide, the current was averaged over
twenty-five hours. C u r r e n t speed was correlated w i t h the yellowfin
catch in 120 daily sets at the FAD; the weaker the current speed, the
greater the catch. A l t h o u g h not significant, the catch was greater with
northeastward current than w i t h south westward current.
Weight composition analysis
Yellowfin tuna (Thunnus albacares) is a main target for the FAD fisheries
in Okinawa. Flowever, very little is known about its m i g r a t i n g pattern
or a g g r e g a t i n g behaviour w i t h FADs. Particularly, those behaviours
may change w i t h size or age of the fish.
A l t h o u g h s p a w n i n g occurs year-round (Suzuki, 1991), yellowfin in
Okinawan waters have distinct size groups. T h e elata of weight of the
fish is far easier to obtain than the data of the length. So, we gathered
the weight elata to better understand the migration, the aggregation
behaviour with FADs and the g r o w t h of yellowfin.
492
Biology ancl behaviour of pelagic fish aggregations
Material and methods
At Itoman Fisheries Cooperative (FC), southern Okinawa Island, catch
records of FAD fisheries have been stored in a computer. Large yellowfin
are auctioned individually while smaller ones are sorted into similar size
groups, then auctioned. The weight of the groups ancl the number of the
fish in each group were recorded. The weight of the group was divided by
the number of the fish to obtain an average weight of the fish in the group.
The weight elata of 237,264 yellowfin caught from 1989 to 1998 was
used. The length from 30 cm to 150 cm was divided into 32 strata with
intervals of four centimetres. Then the weight was stratified so that
each boundary weight corresponds to the boundary length. The number
of the fish in every stratum was counted monthly to determine the
weight composition.
Figure 1
Weight composition
of yellowfin. One division of
Y axis indicates the number
of fish caught in that month
(600 max). Hatched modes
are G-May-2 kg and
G-May-15 kg. The modes in
May are hatched in black.
Results
In the weight composition from January 1990 to December 1998,
some weight groups (considered to represent cohorts) appeared in the
catch and the weight of these groups increased monthly presumably
as individual tuna grew (fig. 1).
; )-4-l_Lî ! ! ! ! !
' ' LW--J ! 'J' !
, :,, ;
: : ; : ; ; :W9?: : ;
; : : : ; ; : : : :
[J !::::::::::::::
,, ,
"
m .
I rjAJî : ' ' i i : : :
; ; ; ; i/X; ; ; ; ; ; ; ;
;: : :
Mj' 1 ' • ' :_L O k , :!;!LLLLL'
m
j : :: :; :; :::; ;:
:; ;: :; :: :;;; :;
::::::::::
: ;;;;;;;;;;;;;;
: : : : : : : : : u - f c j : j ; ; ; ; ; ;X9Q1 ; ; ;
::::!;::;:'(::;:
wmm
• • •; • • •
:;::;::::: i: : ;
: ; MSJJ
! 1 ! ! ' ! I ' j^v ! ! ::::::::::::::
! !!
;;;;;;;;;;;;;;
::::::!JJ
I!!!!!!
: : : 'jjl : : : : : ; : ; : ; : : ::
;i99o; ; :
!!
!
' ' Li ! ! ! ! ! 11 ! ! 1 ! ! i
; ; LUJ : i / { X : ; : : : ; ; ; : : ; ; : : ; i : ;
X / v i i I :: i : : : : : : : ;I i:
: :;: : : : : : : : : : : : : : : : : : : : :
!!!![/ A j j
! j f i ::::::;::
: : : i : : i : / ! :: : i : : : : : :
: :
iliimU-iiiJJJ^iiiliiiiliJ
:; ;; :: ;; ;; : : ;; ::
\J,\ : ; i : : i ; ; : : : : : : ; : ;
IIJUJÀL, : : ; v,
• 'Iru"
::::::::::::::::
: : j i i ! ; : 1 ! ! ! 1 1 ! ! ! 11 1 ! ! ! 1 ! ! ! ! !
i i m s
;
Dec.
Nov.
Oct.
Sep.
Aug.
Jul.
June
May
Apr.
Mar.
Feb.
Jan.
Dec.
Nov.
Oct.
Sep.
Aug.
Jul.
June
May
Apr.
Mar.
Feb.
Jan.
Dec.
Nov.
Oct.
Sep.
Aug.
Jul.
June
May
Apr.
Mar.
Fev.
Jan.
Weight (kg)
493
Biologie e t c o m p o r t e m e n t agrégatif cles poissons
Discussion
The modes of weight groups that were about 2 kg in May are clearly
identified in all years and they continued to be clear until October or
November (hereafter, these groups are called G-May-2kg). In winter,
the catch of the weight groups, especially heavier weight groups,
decreased. This indicates that those tuna moved out of this FAD area.
In May of some of the years, the modes of about 15 kg-weight groups
(hereafter G-May-15 kg) are clear. Presumed from growth rate, these
groups are the same cohorts of G-May-2 kg. Although light weight
groups are caught year-round, G-May-2 kg (=G-May-15 kg) are considered to be main cohorts in Okinawan waters.
Starting from about 2 kg in May, the clear modes of the weight groups
in every year demonstrate the growth pattern (fig. 2). From 1989 to 1995,
the growth patterns are similar, while in 1996 and 1997, the patterns
indicate slower growth rates.
Figure 2
Growth of yellowfin tuna
of G-May-2 kg
and G-May-15 kg.
Dark lines: from 1 9 8 9
to 1995.
Bright lines: 1 9 9 6 and 1997.
SP
20
Comparative growth curves are plotted in figure 3 from: (1) a study in
the Central and Western Equatorial Pacific ( C W E P ) (Lehodey & Leroy,
1999); (2) a study in the Philippines (Yamanaka, 1990); (3) this study
(an average from 1989 to 1995) to best fit the monthly weight from
2 kg to 5 kilogrammes. The growth rates among these places m i g h t
be different. However, if we assume the yellowfin around Okinawa
grow as fast as the other two growth curves, then the birth month of
G-May-2 kg is back calculated to be November of previous year for C W E P
growth curve, and August for the Philippines'growth curve.
494
Biology ancl behaviour of pelagic fish a g g r e g a t i o n s
Figure 3
Growth curves of yellowfin
tuna: the Central and
Western Equatorial Pacific
(CWEP, bright thin line);
the Philippines (bright thick
line); and Okinawa
(black line with dots).
^
Figure 4
Monthly catch of yellowfin
tuna at Itoman.
Hatched bars: monthly
catch. Curved line with dots:
annual average catch,
x x : very small catch.
x: small catch,
o : large catch,
o o : very large catch.
^
o
Because the prices of larger yellowfin are far higher than smaller ones,
the fishermen usually target larger yellowfin. The catch of yellowfin
tuna (> 10 kg) fluctuated seasonally and annually (fig. 4). Catches were
very small in 1990, small in 1991 and 1994, large in 1992, 1997 and
1998, very large in 1995. Figure 5 shows the yield (instead of the
n u m b e r of the fish) composition using the same weight strata.
40
O O
35
] Monthly catch
- Annual a v e r a g e
30
f
xx
251
% 20
ra
°
15 H
10
A
5
01
4
7
1989
10 1
HI
4
7
1990
10
1
4
7
1991
10
1
4
7 10 1
1992
4
7 10 I
1993
4
7
1994
10
1
4
7
10
1995
m
1
4
7
1995
10
rili
1
10
1997
1
4
7
10
1998
495
Biologie et c o m p o r t e m e n t agrégatif cles poissons
10 kg
10 kg
;2992 ; ; ;
; :
11 ! 1
11
10 kg
- • : o O .1995 i : i ; , ' i : : ; ; J_LLLJ : : : : :
^
x r n x f ; \ r :
: ; : : : : : / : Vj : :
1 ! ! • L ^ i
; ;
;: : :: :: :
;; ;: ;: ;; ; ; ;i;
11.
Aug.
; i
Jul.
June
May
>-H~U ! : L i i
:
^ ^ I T T Î T ;
v . '•
1
•:
; ;
:;:'
1
Apr.
Mar.
:
:;;;:;;:
;; : :; ;; ; ;;i; ; ;;;; ; ;
; ;
: :
; ;
.
; : ; : ; ; ;><19?4; ; ;
, , , , , , o 1997. ; ;
: ; ; ; ; ; :::::::: :
:;
; ; ;
: • : : j'-L' :
; : ; :,"
M
M
M
ijjj ; ; : ; ^
Sep.
f
:: ; :: i i J i L U j
Aug.
;i ;; ;;
; ; ; ;JJ/
Jul.
^
: ! : : : June
X
;;;;;;;;;;;;;;;
: ;
May
j^. ! ! ! ! 1 ! ! II ! ! ! 1
^ 11 ! ! 1 11 1 ! 1 ' 1 :I : ;
Apr.
;;;;;;;;;;;;;;;
; ;
Mar.
: : ; ; : : :j : ; ;
; ;; ;; ;: ; : : ; ;
; : : ; ; :
;;
iiiiilliLiiih
; : ; ; : *>;< i 9 9 q ; : ;
Feb.
; ; ! ; ; ; ; ; : ; ; ; ; i ;!; ; ; ; ; ; : : ; ; ; ; : ;; : ; ; : ; ; ; ; ; ; ; ; ;
;;;;;;;
. . . . . . . . >1993 • >
;;;;;;;;;;;;;
; ; I
;
Jan.
199e ;
Dec.
; ; ! I ; ; 11 ; ; : : : ; : U^ : ; ; : : Nov.
Oct.
LiLUiiJiiLLLL
;:;;;;:;_/
; : ; ; : : : i : : : : ; : i i ; ; ! ; ! ! ; ; ! ; i! !: ! ! ! ! ! ! ! ! ' , 1 ! ! !
Sep.
:: ;; :; ; : ;;: ; ;
! 11 !
; ; ; ;; ;;;
1
1
! 1,4.1 ! 1 1^ 1 1. 1 : : :
' : : !: i H ! ! ! ' ! ! '
:;
; ; ; ;; i^j ; ; ;
; il;;,' ; i ; UII; ;
1 11 11 1111 I ! IJ„.
: : >L: ; : : J i
;;;;;;;;;;;;;;;
Jul.
; • : June
:; ;: ;; ;;;:
May
Apr.
::::;:::;::::/:
Mar.
•••••••••niffL
Jan.
;;;;;;;;;;;;;;;
;;;;;;;;
Aug.
A . - :
;;;
;;;:;:
"77"
Dec.
Nov.
Oct.
; ! ; ; ; ; ; i ; ; ; 111
JlllLwilti,
:;
:
i : ; ; ! ; ; ! : 11 ; 11 !; 1 1 1J 11 ! ! 1 ! 1 II 1
1
; ; ;
;
Feb.
: : !: : : : : : ! : I : : : Jan.
! 1 \ \ \ \ 1 ! Ill ] ! 1
; ; ; ; : : :>i
! I I ! ill!
::::::::
Oct.
: : : : : Sep.
; i ; 11 ; ; ; ; ; ; ; 11 ;
' ' ' '
: ; ;
Nov.
;:;;;;:; ;/ri:jj : : : : :
: : :j j j
;: :; :
11 1 1
•
' 'X
: : : : : pi.
1998: : : Dec.
L U i ; ; ; ; ; ; ; i \ !_L-i_J 1 \
; i : ; ; ; ! i ; ;;; ; ;
: : ; ; ; : ; ; ; x.; i; i ; ; : :
Fev.
Weight (kg)
Figure 5
Weight composition
of yellowfin tuna expressed
by yield (kg) instead
of the number of the fish.
One division of Y axis
indicates the yield in
that month ( 6 0 0 0 kg max).
Hatched modes are weight
groups more than 10 kg.
The modes in May are
hatched in black.
In the good-catch year (1992, 1995, 1997 and 1998), the catches of
G-May-15 k g were large through fishing season (April to October). The
n u m b e r of G - M a y - 1 5 k g in May and the annual catch of that year are
significantly correlated (pcO.Ol). This correlation is significant not
only for the catch of I torn an FC but also the catch of whole southern
Okinawa's FCs (fig. 6). This suggests that we can predict the annual
catch if we obtain the w e i g h t composition at early stage of the fishing
season.
T h e n u m b e r of G - M a y - 1 5 k g in May and the n u m b e r of G - M a y - 2 kg
in May in previous year are not s i g n i f i c a n t l y correlated ( p > 0 . 0 5 ) .
A l t h o u g h they are p r e s u m e d the same cohorts, the m a g n i t u d e of
m i g r a t i n g schools to O k i n a w a is probably affected by environmental
factors (such as current and/or water temperature) rather than the stock
abundance of the cohorts.
496
Biology ancl behaviour of pelagic fish a g g r e g a t i o n s
Figure 6
400
-j
2 000
the number of G-May-15 kg
350
-
1 800
in May and the annual catch
of that year.
300
-
250
-
200
-
1 000
150
-
800
100
~
The relation b e t w e e n
1 600
1 400
Upper: yearly transition.
Lower: the relation.
1 200
The two factors
are significantly c o r r e l a t e d
o
(p<0.01).
s
C=
<
600
400
50
200
0
1
1
89
90
1
91
1
92
r
1
95
93
1
96
1
97
0
98
400
y = 0.1435x + 114.36
350
300
~r~= 0~86
-
250
200
150
100
50
0
T
0
500
T
1 000
\
1 500
2 000
Number of fish: May-15 kg
Current and Catch
According to m a n y FAD fishermen, the catch is strongly affected by
current or water temperature. However, there have been very few studies
on the relation between the catch of FAD fishing and environmental
factors.
W e have ten huge FADs (nirai) in 1999 that provide suitable platforms
for installing current meters. W e have measured the current ancl the
water t e m p e r a t u r e at nirai since 1995, while m o n i t o r i n g the catch
around the FADs.
Material and methods
From 15 J u n e to 23 O c t o b e r 1995 and f r o m 27 N o v e m b e r 1995 to
12 March 1996, at nirai 1, fifteen nautical miles south-east of Okinawa
Island ( d e p t h a b o u t 1 3 0 0 m), a c u r r e n t m e t e r (Aancleraa RCM-7)
recorded current ancl water temperature. T h e current meter was attached
to the FAD at 4 m d e p t h . Figure 7 shows the location of nirai 1.
T h e current speed ancl direction were compared with the daily yellowfin catch that was c a u g h t by fishermen using nirai 1 ancl FADs in the
vicinity.
(497
Biologie et c o m p o r t e m e n t agrégatif cles p o i s s o n s
Figure 7
The location and
the configuration of nirai 1.
The current m e t e r was
installed t o the nirai
with chains at 4 m depth.
Rpenife
^ b u u s
W h e n typhoons attack Okinawa, the surface water temperature drops
by the effect of surface water mixing. Figure S shows the water temperature at nirai 1. W h e n Typhoon 3 approached to Okinawa Island
on 22 July, the temperature drastically dropped about from 30 to
26degrees centigrade.
Figure 9 shows the current vectors, northward current composition
ancl eastward current composition. Typically, average current speed
was 29 cm/s ancl eastward current was most frequent.
Figure 10 shows the current from 15 June to 25 J u n e 1.995. The current
was affected by tide, and the current direction turned clockwise in
about one clay. So, the current was averaged over 25 h, then compared
with yellowfin daily catch.
498
Biology ancl behaviour of pelagic fish a g g r e g a t i o n s
typhoon 3
31 —i
- Temp.
g
29-
g
27-
«3
ai
CL
I
25
2321 •
1
„<o
1
h
I
A
I
I
A
I
>
I
«V
1
ft
/b
I
I
A
I
ft
I
ft
I
„ft
I
ft
I
/V
I
rfV
I
ft/
I
<V
I
I
r(V
I
I
nN
I
,£>
I
I
-S>
I I
(V
a>
A
°> 4
Figure 8 - The temperature at nirai 1 from 15 June 1 9 9 5 to 1 2 March 1 9 9 6 . The temperature drastically d r o p p e d when Typhoon
3 approached to Okinawa.
<oV <oV
v
°>N
^
Figure 9 - The current at nirai 1. Upper: expressed with stick diagram. The direction of the sticks indicates the current direction.
The length of the sticks indicates the current speed. Lower: northward (bright line) and eastward (dark line) current composition.
360
7
270
180
90
o
0
1
6/15
6/16
........—.
i
6/17
1
6/18
1
1
1
1
1
6/19
6/20
6/21
6/22
6/23
I
6/24
!
6/25
Northward composition
Eastward composition
Figure 10 - The current at nirai f r o m 15 June to 25 June 1 9 9 5 . Upper: current direction. The direction turned clockwise in about
a day. Lower: The northward (bright line) and eastward (dark line) current
composition.
(499
Biologie et c o m p o r t e m e n t agrégatif cles p o i s s o n s
Figure I I shows the relation between the current speed ancl the catches.
The catches are for two yellowfin tuna weight groups: (I) larger than
about 10 kg, ancl (2) smaller than about I O kilogrammes. The current
speed was stratified to five strata: 0-10; 10-20; 20-30; 30-40; ancl over
40 centimetres per second. T h e catches are average catches in the
corresponding current speed strata. The current speed was correlated with
the yellowfin catch in 120 daily sets, the weaker the current speed, the
greater the catch.
600
Figure 1 1
The relationships between
the current speed
500
and yellowfin tuna catch.
The dark line and dark dots
400
indicate yellowfin larger than
10 kg. The bright line and
bright dots indicate yellowfin
300
smaller than 10 kg.
Both of the groups show
negative correlation,
200
the weaker the current
speed, the greater the catch.
100
i
i
i
i
i
i
10
20
30
40
50
60
Current speed ( c m / s )
Figure 12 shows the relation between the current direction and the
average catches. The current direction was divided into eight groups:
N, NE, E, SE, S, SW, W, and NW. Although not significant, the catch was
greater with northeastward current than with southwestward current.
Figure 12
The relation between
the current direction
and yellowfin tuna catch.
The dark line: yellowfin
larger than 1 0 kg.
The bright line: yellowfin
smaller than 10 kg.
The farther the lines f r o m
the center, the greater
the catches.
500
Biology ancl behaviour of pelagic fish a g g r e g a t i o n s
Discussion
One of the reasons for the negative correlation between the current speed
ancl the catch seems that the yellowfin lost much energy to associate
with the FAD against the strong current, assuming without adequate food
sources.
Many FAD fishermen in Okinawa say that the catch is good when the
current direction is toward the islands. W e have not clearly confirmed
this yet. However, we have continued measuring the current at nirai 1 or
other nirai, ancl would add better understanding of the relation between
the current ancl the catch.
Bibliographic
References
Lehodey P., Leroy B., 1999. Age ancl g r o w t h of yellowfin tuna (Thunnus albacares) from the Western Central Pacific Ocean, as indicated
by daily g r o w t h increments ancl t a g g i n g data. SPC W o r k . Pap.,
SCTB12, YFT-2, 21 p.
Suzuki Z., 1991- A review of the biology ancl fisheries for yellowfin tuna
(Th/nrnns albacares) in the Western ancl Central Pacific Ocean. II. Interactions of Pacific tuna fisheries. FAO Fish. Tech. Pap., 336(2), 108137.
Yamanaka K.L., 1990. Age, g r o w t h ancl spawning of yellowfin tuna
in the southern Philippines. Indo-Pac. Tuna Program., IPTP Work.
Pap., 7, 6 6 p.
(501