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JUN 3 2 Rips

DLVISIGN OF FISHES
U. Se NATIONAL MUSEUM

Longline Fishing for Deep-Swimming
Tunas in the Marquesas Islands
and Adjacent Areas

By Howard O. Yoshida

——~

UNITED STATES DEPARTMENT OF THE INTERIOR ~~~

—__~

FISH AND WILDLIFE SERVICE -~~~

SO

AES Ge OE a ee a ee aa a a en et ee a ee Se ee eS on ne en ee eee ae ee ae a

UNITED STATES DEPARTMENT OF THE INTERIOR
Stewart L. Udall, Secretary
Charles F. Luce, Under Secretary
Stanley A. Cain, Assistant Secretary for Fish and Wildlife and Parks
FISH AND WILDLIFE SERVICE, Clarence F. Pautzke, Commissioner

BUREAU OF COMMERCIAL FISHERIES, Harold E. Crowther, Acting Director

Longline Fishing for Deep-Swimming
Tunas in the Marquesas Islands
and Adjacent Areas

By

HOWARD O. YOSHIDA

United States Fish and Wildlife Service

Special Scientific Report--Fisheries No. 546

Washington, D.C.
April 1967

CONTENTS

Page

Introduction il
Methods — 2
Distribution and abundance of tunas 3
Yellowfin tuna 3
Bigeye tuna 5)
Other tunas 5
Vertical distribution of tunas 6
Size of tunas 7
Summary 8
Literature cited vO

iii

Longline Fishing for Deep-Swimming
Tunas in the Marquesas Islands
and Adjacent Areas

By

HOWARD O. YOSHIDA, Fishery Biologist

Bureau of Commercial Fisheries Biological Laboratory
Honolulu, Hawaii 96812

ABSTRACT

Six hundred forty-two tuna, including 438 yellowfin tuna (Thunnus albacares),

102 bigeye tuna (T. obesus),

51 albacore (T. alalunga), and 51 skipjack tuna

(Katsuwonus pelamis), were caught by longline fishing on three cruises across
and around the Marquesas Islands (ca.

the Equator on long. 132° and 150° W.
long. 140° W.) between August 1956 and May 1958.

These cruises were part of

an investigation of fishery resources of the Marquesan area by the Bureau of
Commercial Fisheries Biological Laboratory at Honolulu,
The distribution and abundance of yellowfin tuna are considered in greatest

detail in this report, because this species dominated the catches.

Yellowfin tuna

were more abundant during the Southern Hemisphere summer than winter and on
long. 132° W. than on long. 150° W.; also, they were more numerous in the
“‘inshore,’’ <148 kilometers (80 nautical miles) from land, waters of the Mar-
quesas than in the adjacent ‘‘oceanic’’ (>148 kilometers from land) waters.
Their abundance differed seasonally in the insular waters of the Marquesas.

Although bigeye tuna were not as abundant nor as widely distributed, their
distribution was somewhat similar to that of yellowfin tuna.

No albacore were caught north of lat. 7° S. on long. 132° and 150° W. This
distribution appeared to be associated with a discontinuity of the oceanic struc-
ture extending east-west around lat. 10° S,

INTRODUCTION 1955; Shomura and Murphy, 1955).

Later in-

Investigations of the high-seas fishery re-
sources of the tropical and subtropical Pacific
Ocean by the Bureau of Commercial Fisheries
Biological Laboratory at Honolulul/ have in-
cluded sampling of deep-swimming tunas taken
by longline in equatorial waters. The early
investigations were designed to cover a vast
area of the equatorial Pacific to delimit the
general distribution and abundance of the deep-
Swimming tunas, This exploratory fishing
revealed a concentration of yellowfin tuna
(Thunnus albacares) along the Equator, with a
zone of high abundance between long. 140° and
160° W. (Murphy and Shomura, 1953a, 1953b,

1/

— Formerly known as the Pacific Oceanic Fishery
Investigations (POFI).

vestigations were centered on the distribution
and abundance of yellowfin tuna around the Line
Islands (Iversen and Yoshida, 1956, 1957), which
are located within the general zone of high
abundance.

After the completion of the investigations in
the central equatorial Pacific, exploratory fish-
ing was extended to the Marquesas Islands and
adjacent areas, The longline fishing in this
region, however, was ancillary to a more inten-
sive study of the surface tunas. In addition, a
small part of the fishing was expended towards
testing certain hypotheses that had developed
from results of previous work in the central
equatorial Pacific.

There were three cruises to the Marquesas
between August 1956 and May 1958 during which
at least part of the time was spent in longline

fishing (fig. 1). The seasons and areas of fish-
ing are given in table 1.

METHODS

Mann (1955) described in detail longline gear
Similar to that used for this study. The basic
unit, the basket, was composed of a 384-m.
mainline and eleven 5.5-m, hook droppers or
branch lines attached to the mainline at 27.4-m.
intervals. Forty-four to sixty of these baskets
were joined to make a set. The set was buoyed
by a series of floats attached to basket junc-
tures by 18.3-m. float lines. The bait was fro-
zen Pacific herring (Clupea harengus pallasi),
Murphy and Shomura (19538a) described in some
detail the manner in which longlines are set and
retrieved. As a rule, they are set at dawn and
retrieved at noon.

To estimate the depths fished by the longline,
five or six sounding tubes were used regularly
at each fishing station. They were attached to
the sixth dropper, the middle dropper of the
11-hook basket, and were distributed evenly,
throughout the set. For each longline station
the depths indicated by these sounding tubes
were averaged and the mean depths were used
to represent the mean maximum depth of the
longline. At many stations additional sounding
tubes were attached to the third, sixth, and ninth
droppers of one basket to get an estimate of the
configuration of the mainline.

The total lengths of all the tuna caught on the
cruises were obtained by the method described
by Marr and Schaefer (1949).

For some sections of this paper it was con-
venient to classify the fishing stations as
““oceanic’’ or ‘‘inshore.’’ The division between
oceanic and inshore fishing stations was made
at a distance of 148 kilometers (80 nautical

miles) from land (following Shomura and Murphy,
1955). By this criterion, all of the stations
fished on north-south transects on long. 132°
and 150° W. were oceanic. The inshore stations
were all near the Marquesas Islands.

Ltt LONGLINE STATIONS

eee:

>

/ ep 9p gh og

[Ze

Hareeetee .

140°

Figure 1.--Tracks of CHG (Charles H. Gilbert)
cruises 30 and 38, and JRM (John R. Manning)
cruise 34,

Table 1.--Summary of fishing by season and area, Charles H. Gilbert cruises 30 and 38 and
John R, Manning cruise 34

Cruise

Season

Area fished

Charles H. Gilbert
cruise 30

Winter (August-
September 1956)

Summer (January-
March 1957)

John R. Manning
cruise 34

Charles H. Gilbert Summer (February
cruise 38 1958)

ror Southern Hemisphere.

Long. 132° W. from lat., 1°32 N.. topl2eO7mesre
around Marquesas Islands and adjacent area.

Long. 132° Wo from lat. 4°29. Ne vto: el 2 O2RaiSme
around Marquesas Islands; long. 150° W. from
Lat. 16°344/Se to ySsig0N IN

Long. 150° W. from lat. 4°44' N. to 0°45' S.

In the analysis of the results of fishing, the
catch rates of yellowfin tuna and bigeye tuna
(I. obesus), by season and area, were subjected
to simple statistical tests. Catch rates from
two or more independent areas or seasons were
tested to see whether they could have been
drawn from the same population (Mann-Whitney
U test or the Kruskal-Wallis one-way analysis
of variance by ranks--Siegel, 1956). I concluded
that fish were relatively more abundant in one
area or season if differences in the catch rates
were Significant,

Unless otherwise stated, all the seasonal
designations in this paper refer to seasons in
the Southern Hemisphere.

DISTRIBUTION AND ABUNDANCE
OF TUNAS

The data on longline fishing from the three
cruises already have been published (Austin,
1957; Wilson and Rinkel, 1957; and Wilson,
Nakamura, and Yoshida, 1958). The tuna catch
of these cruises totaled 642 fish, comprising
438 yellowfin tuna, 102 bigeye tuna, 51 albacore
(IT. alalunga), and 51 skipjack tuna (Katsuwonus
pelamis).

The distribution and abundance of yellowfin
tuna are considered in greatest detail, since
this species dominated the catches.

Yellowfin Tuna

Previous investigations in the equatorial
Pacific Ocean disclosed the presence of con-
centrations of yellowfin tuna within a band of
latitude several degrees to the north and south
of the Equator, and also indicated north-south
and east-west shifts in these concentrations
(Murphy and Shomura, 1953a, 1953b, 1955;
Shomura and Murphy, 1955;Iversen and Yoshida,
1956).

In the winter of 1956, deep-swimming yellow-
fin tuna were caught from lat. 1°32’ N. to as far
south as lat. 13°26’ S., the southernmost fishing
station, on long. 132° W. (fig. 2). The catch
rates were highest at the northernmost fishing
station of the north-south transect and ranged
from 0 to less than 1 fish per 100 hooks for the
remainder of the transect. A rather discontin-
uous distribution is suggested.

The latitudinal range fished during the sum-
mer of 1957 on long. 132° W., was from lat.
4°29’ to 14°02’ S. Yellowfin tuna were found
throughout this range and were relatively more
abundant than in the previous winter (fig. 2 and

5

4 | OZ
WINTER 1956 |
3 -

0 Sisleose NRE ES
5 T 7

Ae 12a |
SUMMER 1957 |

é 2

ise 1 T

iy! auUo

eS 5

Boa 150° W.

I SUMMER 1957

& 3

oO

om ob A
5

4 150° W.
SUMMER 1958

15°S. 10° 5°
LATITUDE

Figure 2.--Oceanic catch rates (number of fish
per 100 hooks) for yellowfin tuna on long.
132° W., winter 1956 and summer 1957, and on
long. 150° W., summers 1957 and 1958.

‘table 2), The catch rate was highest at lat.

3°29’ S. In the same season, but farther to the
west, on long. 150° W., yellowfin tuna were less
abundant and were irregularly distributed be-
tween lat, 3°01’ N. and 16°34’ S, (fig. 2 and table
2). The highest catch rate, 1.6 fish per 100
hooks, was at lat. 0°06’ N. Another transect
was fished on this meridian the following sum-
mer. On this 1958 transect, fishing was con-
fined to a narrow band across the Equator, be-
tween lat. 4°44’ N. and 0°45’ S. Catches of yel-
lowfin tuna again were rather low and were
concentrated within a few degrees of latitude
north of the Equator.

In summary it is evident that yellowfin tuna
were relatively unavailable to longline fishing
on long. 150° W. At long. 132° W., where a
seasonal comparison can be made, yellowfin
tuna were relatively more abundant in the sum-
mer than in winter. They also were more
abundant to the east, at long. 132° W., than at
long. 150° W., at least during the summer of
1957. In addition to these seasonal and longi-
tudinal differences, relative abundance exhibited
latitudinal shifts.

Table 2.--Catch rates of yellowfin tuna by season and area

Area or season

Long. 132° W.

Between summer 1957 and
winter 1956

Between long. 132°
and 150° W.

Summer 1957

Between summer 1957
and winter 1956

Marquesas Islands

Between Marquesas Islands
and long. 132° W.

Winter 1956

Among Marquesas Islands
long. 132° and 150° W.

Summer 1957

CATCH PER 100 HOOKS

Figure 3.--Catch rates for tuna

(number of

yellowfin
fish per 100 hooks) at the in-
shore stations.

The daily catch rates (fish per 100 hooks) of
yellowfin tuna at inshore stations near the Mar-
quesas Islands are presented in figure 3. The
catch rate varied considerably from day to day
(0 to 8.4 fish per 100 hooks). Seasonally yel-
lowfin tuna were relatively abundant during the
summer (table 2). The catch rates during the
winter ranged from 0 to 2.2 fish per 100 hooks,
but in the summer the range was 0.9 to 8.4. The
Hawaiian longline fishery also shows a season-
ality in abundance; greatest numbers are caught
from May to September (Otsu, 1954). June
(1953) suggested that the longline fishery in
Hawaii is based on a spawning migration, for
the months which have the highest catch rates
for yellowfin tuna coincide with the period of
greatest ovarian maturity. No comparable data
On maturity are available from the Marquesas
Islands. The occurrence of fish larvae, how-
ever, has been taken to indicate spawning areas

Comparison of catch rates Test Result

Mann-Whitney U = 35.5, n, = ll, n

=" 135) py <0 002

1 2.
do UR= 7565 ny 13% ny = 15; p <0.002
do U = 4, n Seay ny = 9; p <0.002
do Lue sil n, = a5 ny = 25) 5D) 0.421
Reveral= X-.= 12.5078, d.f. = 2: pezonom

Wallis

and seasons by many workers, e.g., Matsumoto
(1958). The abundance of yellowfin tuna larvae
was observed during the periods covered by our
longline fishing. Strasburg (1960) gave data on
larvae captured in 1/2-hour surface plankton
tows in an area around the Marquesas Islands
bounded by lat. 7°32’ and 12°48’ S., long. 134°46’
and 143°55’ W. The larvae were captured at
the rate of 0.02 per 1,000 m.° of water strained
and 1.65 per 1,000 m.° of water
strained in summer. Furthermore, as will be
shown later, the higher catch rates on longlines
in summer may have resulted from an influx of
larger, and presumably more mature, yellowfin
tuna into the Marquesas area. They well may
converge on this area to spawn during summer.

Strasburg (1958: p. 348) noted several ways
in which the proximity of land can influence the
distribution of marine animals: ‘‘For certain
oceanic species the presence of land acts only
as a barrier to movements, whereas others
congregate about islands and other land masses
to feed or to reproduce. It occasionally happens
that oceanic fish abound in insular environ-
ments, either because of the islands’ intrinsic
nutritive richness or because they lie in the
path of rich oceanic currents.”’

My data, although rather meager, provide
information on the abundance of yellowfin tuna
close to the Marquesas Islands as contrasted
with the adjacent oceanic areas, For this com-
parison, the oceanic stations south of lat. 7° S.
were selected. I believe that the effect of the
equatorial enrichment system, in which yellow-
fin tuna are known to be relatively abundant, is
not discernible this far south. During summer,
the average catch rates were higher in the in-
shore waters of the Marquesas than in the ad-

in winter

jacent oceanic areas; catch rates differed little
during winter (fig. 4 and table 2). In the equa-
torial Pacific Ocean, near the Line Islands,
yellowfin tuna also tend to be more abundant
near land than in the open ocean (Shomura and
Murphy. 1955; Iversen and Yoshida, 1957).

WINTER

CATCH PER 100 HOOKS

Figure 4.--Catch rates (number of fish per
100 hooks) for yellowfin tuna in the Mar-
quesas Islands (inshore stations) and adja-
cent oceanic areas south of lat. 7° S.

Bigeye Tuna

The distribution of bigeye tuna differed from
that of yellowfin tuna on long. 132° W. in the
winter of 1956 (fig. 5), Their distribution was
relatively limited along this meridian (between
lat. 4° and 11° S.). whereas yellowfin tuna were
found from about lat. 2°N. to 13°S, In the
summer of 1957, however, bigeye tuna were
more abundant and more widely distributed (lat.
4°29’ N, to 14°02’ S.) than they were in the win-
ter of 1956 and their distribution closely paral-
leled that of yellowfin tuna (figs. 2, 5, and table
3).

Farther west, on long. 150° W., bigeye tuna
were not as abundant nor as widely distributed
as they were on long. 132° W. (fig. 5 and table
3). They were taken in small numbers between
lat. 5°51’ S. and 3°01’ N. on this transect but
were distributed discontinuously within these
latitudes. In contrast, yellowfin tuna were found
from lat, 16°34’ S, to 3°01’ N. on this meridian
during the same period. Along long. 150° W.

from lat, 4°44’ N. to 0°45’ S, the following sum -
mer, the distributions of bigeye and yellowfin
tunas were similar, except that the peak catch
of bigeye tuna was at lat. 0°53’ N. and the peak
catch of yellowfin tuna was at lat. 2°57’ N.

Only small numbers of bigeye tuna were
caught near the Marquesas Islands in both sum-
mer and winter. The catch rates varied from
0 to 0.8 fish per 100 hooks and, unlike the rates
for yellowfin tuna, displayed no marked season-
al difference,

Other Tunas

Small numbers of skipjack tuna and albacore
were caught along with yellowfin and bigeye
tunas. Deep-fishing longlines do not effectively
sample the skipjack tuna, which is a small,

3

° W. | |

WINTER 1956

tr

o

ida}

<4 1 7
S |
2 0 ZA wEOENEO D A
i]

ee cS}

a

a 150° W.
= 2 SUMMER 1957
&

S

3 |
150° W. |
4 SUMMER 1958
1
0 Was 0
15%S? 10° Se o° 5°N.

LATITUDE

Figure 5.--Oceanic catch rates for bigeye
tuna (number of fish per 100 hooks) on
long. 132° W., winter 1956 and summer 1957,
and on long. 150° W., summers 1957 and
1958.

Table 3.--Catch rates of bigeye tuna by season and area

Area or season Comparison of catch rates

Long. 132° W. Between summer 1957 and

winter 1956

Summer 1957 Between long. 132° and

150° W.

Test Result
Mann-Whitney U = 60, ny iis ny = 13; p <0.002
do U=" 43); n, = 135 ny = 15; p <0.002

surface-schooling species (Shomura and Murphy,
1955), Around the Marquesas Islands, where
they are abundant (Wilson, MS.2/), longlines
catch only small numbers. Catches of this fish
on long. 132° and 150° W. also were small and
sporadic,

Albacore first began to appear in the catches
at about lat. 7° S. on long. 150° W. and at about
lat. 11° S. on long. 132° W, The number caught
at any one station was small. This distribution
of albacore is interesting in light of the obser-
vations by Yamanaka (1956) in the western
South Pacific. Yamanaka noted a conspicuous
discontinuity, characterized by sharp changes
in water temperature, salinity, and sigma-t
values, in the oceanic structure running in an
east-west direction and centered on lat. 10°S,
He suggested that this discontinuity probably
limits distribution of albacore in the south-
west Pacific. Our observations on the oceanic
structure in this area indicate that this discon-
tinuity extends as far east as long. 132° W,

Only small numbers of albacore were taken
around the Marquesas Islands in both winter and
summer; the catch rates ranged from 0 to 2.2
albacore per 100 hooks,

VERTICAL DISTRIBUTION OF TUNAS

Iversen and Yoshida (1957) showed that the
longlines tend not to fish as deep near the Equa-
tor as they do several degrees to the north.
They suggested that this difference was caused
by the shear between the westerly South Equa-
torial Current at the surface and the easterly
Equatorial Undercurrent beneath. In general,
results were similar on the north-south fishing
transects across the Equator on long. 132° and
150° W. (fig. 6). The mean maximum depths
reached by the longlines within about 2° north
and south of the Equator were not as great as
those recorded farther north or south; this
trend was not strongly evident during Charles
H. Gilbert cruise 38.

The Pacific Equatorial Undercurrent first
was described by Cromwell, Montgomery, and
Stroup (1954). Knauss and King (1958), who
measured the velocities of the Undercurrent,
found that it was symmetrical about the Equator
and that the core of the current was flowing be-

eiisone Robert C. MS. The surface tuna re-
sources of the Marquesas Islands. Division of
Foreign Fisheries, Bureau of Commercial Fisher-
ies, Washington, D.C. 20240.

2
B
Zz
=
o
& |
5 |
= 200
eo al
= |
2 40 150° W.,
2 |
2 80}—
2 |JRM-34 :
2 120 \ : x =
g | ie oe’
\\
160 | “I
| CHG-38 |
|
2
200) 15° S: 10° 5¢ 0° 5°N.

LATITUDE

Figure 6.--Latitudinal variation of the mean
maximum depths of longline gear on long.
132° and 150° W. CHG is Charles H. Gilbert
and JRM is John R. Manning.

tween 2 and 3.5 knots at 0°, long. 140° W. They
also found that the Undercurrent was weaker on
either side of the Equator; the velocity was 0.6
knot at lat. 2° N. and 2° S, The depths reached
by our gear demonstrate this configuration, be-
cause the longlines usually were shallowest at
the Equator, where the velocity of the Under-
current is greatest, and became deeper on
either side, where the Undercurrent is weaker.

Longline catches have been used to evaluate
the vertical distribution of tunas (Murphy and
Shomura, 1953a, 1953b, 1955; Shomura and
Murphy, 1955). Chemical sounding tubes at-
tached to the gear have provided estimates of
the absolute depths of the longlines, and ac-
cordingly, the vertical distribution of tunas
(Shomura and Otsu, 1956; Iversen and Yoshida,
1957). Catches of tunas usually have been
greatest on the deeper hooks of the longline.
For example, Murphy and Shomura (1955: p. 27)
stated that ‘‘...yellowfin are usually but not
consistently more abundant at the deeper levels
in the equatorial Pacific, the best bigeye catches
are more regularly associated with the deeper
fishing levels, and albacore are clearly caught
in greatest abundance on the deepest fishing
hooks,’’

To determine the vertical distribution of
tunas, the catch rates for all stations were
combined and averaged by species according to

YELLOWFIN

BIGEYE ALBACORE

105-11I9WY

OCEANIC

DEPTH (M.)

WML

INSHORE

7 8

CATCH PER 100 HOOKS

Figure 7.--Vertical distribution of longline catches of

oceanic and inshore stations.
of stations used

the average maximum fishing depth of the long-
lines by 15-m. depth intervals. No zero catch
rates were used in calculating the averages.
The vertical distribution of tunas at oceanic
stations is shown in figure 7. Yellowfin tuna
were caught at all fishing depths; the highest
catch rate was at the 75- to 89-m. depth. Com-
plicating factors were latitudinal differences in
both the catch rates and the depths at which the
longlines fished. For example the depth at
which longlines fish is relatively shallow near
the Equator and here the catch rate of yellowfin
tuna was greatest. Therefore, the peak catch
rate at the 75- to 89-m. depth may be merely a
reflection of the greater abundance of yellowfin
tuna close to the Equator, or on the other hand,
it may indicate that the vertical distribution of
yellowfin tuna is real and that more are caught
here because the longlines fish the depths in
which tuna are most abundant,

Bigeye tuna, like yellowfin tuna, were caught
at all depths fished. The highest catch rates
were at 90 to 104m. Here again, it is difficult
fo assess the relative influence of the areal
distribution and the vertical distribution of the
ftunas.

As has been noted, no albacore were caught
north of lat. 7° S, It could be argued that the

tunas at
Figures in parentheses are number
in calculating the average catch rate (number
of fish per 100 hooks).

longlines did not fish deep enough north of lat.
7° S,to catch them. This argument is apparent-
ly not valid, however, for the longlines fished
as deep between lat. 2° and 5°N. and 4° and 7° S.,
as they did south of 7° S, It can be seen (fig. 7)
that albacore were distributed rather evenly
between 105 and 164 m.

The longlines fished relatively deep around
the Marquesas--112 to 166 m., average maxi-
mum depth. The vertical distribution of tunas
in this area is shown in figure 7. Although
yellowfin tuna were caught at all depths, greater
catch rates were within the two deepest strata.
Bigeye tuna were caught between105 and169 m.
with more fish taken in the 150- to 164-m.
range.

SIZE OF TUNAS

Longlines typically catch greater numbers
of the larger tuna than does pole and line or
troll fishing, either by inherent selectivity of
larger fish by the longlines or because the
deep-swimming populations are made up chiefly
of large fish (Murphy and Shomura, 19538a).
The length-frequency distributions of oceanic
and inshore catches of yellowfin tuna (fig. 8)
indicate essentially no difference. The length

0 aad Teal ele eee
LONGLINE
OCEANIC STATIONS
20 iy
Q=—s== | +
a 0 | Al |
a SO eae ] =]
oO
LONGLINE
INSHORE STATIONS | eral
20 | | N=228 | | cal |
| | | |
|
io} +— | a ifn
We We
Ifa fea eo |
| heel |
dk |
o 110 122 ig?
13.9 1259 185.9
i. lea Fe Pa Te] ey
POLE-AND-LINE | | | |
| MARQUESAS AREA | -- |aeleee|
BS 50 | ae eae | N#126 WY |
5S eee
5 1Z leer eal
|
Baro i
|
0 | zalenlera| leral eral
( L ; 98 no 122
53.9 5. 779 89.9 101.9 113.9 125.9

LENGTH (CM.)

Figure 8.--Length-frequency distributions of
yellowfin tuna caught by longlines at
oceanic stations, inshore stations, and on
pole and line in the Marquesas area (pole-
and-line data from Wilson and Rinkel, 1957,
and Wilson et al., 1958).

ranges were about equal and the dominant
sizes were similar. This situation is at vari-
ance with that around the LineIslands. Although
the catches there also are dominated by large
yellowfin tuna, more small fish (50-110 cm.)
are caught near the Line Islands than offshore
(Shomura and Murphy, 1955; Iversen and
Yoshida, 1957). No yellowfin tuna smaller than
114 cm. were caught by longlining around the
Marquesas, although their presence was shown
by catches made by pole-and-line fishing (fig.
8). Fish between 90 and 130 cm. were sparsely
represented in catches of both longline and pole
and line. This result may indicate that the
smaller yellowfin tuna in that size range were
not present in great numbers or that neither
fishing method adequately samples them.

The possibility has been mentioned that the
seasonally greater abundance of yellowfin tuna
around the Marquesas during the summer may
have been caused by an influx of large fish.

30

SUMMER

N=214
20

PERCENT
~)
6

0 110 122 146 158 170
113.9 1259 1499 1619 1139
LENGTH (CM)

Figure 9.--Length-frequency distribution of
yellowfin tuna in the Marquesas Islands,
arranged by season.

The length-frequency distribution of fish caught
in this area, arranged by season (fig. 9) shows
that no fish longer than 150 cm. were caught
during winter and that their modal size was be-
tween 130 and 138 cm. In the summer about
36 percent of the catch was composed of fish
longer than 150 cm., and the mode was at 146-
150 cm, These data should be viewed with
caution, however, for the winter sample con-
sisted of only 14 yellowfin tuna.

Length-frequency distributions of skipjack
tuna, albacore, and bigeye tuna are presented
in figure 10. These tunas were not caught in
sufficient numbers to warrant detailed com-
ments. The modes in the length-frequency dis-
tributions of bigeye tuna are indefinite; however,
85 percent of the bigeye tuna were between 126
and 178 cm.long. The size distribution of alba-
core was relatively compact: All of the speci-
mens measured between 90.6 and 111.6 cm., a
length range of only 21cm. Skipjack tuna ranged
from 46.6 to 83.1 cm., and had a mode at 74-
78 cm.

SUMMARY

1. This report is based on the tuna catches
made by longline fishing on three cruises
around the Marquesas Islands and across the
Equator on long. 132° and 150° W, Six hundred
forty-two tuna were caught, of which 438 were
yellowfin, 102 bigeye, 51 albacore, and 51
skipjack.

lalla nl ] Telhalhc)| lel Paral
1 i) | 1 Yi | | | |
35 KIPJACK Y - + eae loa foul ie ai
| N=48 g | |
30 leet Y) +t ical 1
| ZY | | |
25/4 Yy a ad = |
20} VW sop esi
Ly lial l |
15 i = i
| YY | | | Wall
10 i Y / t E
Af YZ
5 V _ a
o& AEA! | | je [ie]
5 35 qr TIP
i) |
© 30\—ALBACORE Lapse
a] N=46 [
imal lial
20} | q dette
Y) |
15} ++ ! | |
|
10;— |
T | | AV | |
| | | | ZA\ T
ot AAA_|_| L
BO SSP
] ]
io ___BIGEYE | 3
N:97 7A A
el jae VA g
o | T ~ Y ]
7 y
ou +! Z Z G as FA ler emai
50 62 74 86 98 110 122 134 146 158 170 192 194

539 659 +779 «2899 1019 1139
LENGTH (CM.)

Figure 10.--Length-frequency

distributions of

1259 1379 M499 1619 1739 1859 1979

skipjack tuna,

albacore, and bigeye tuna.

2. Yellowfin tuna were the most abundant and
widely distributed species of deep-swimming
tunas in the oceanic areas, Where areal and
seasonal comparisons could be made, they were
more abundant on long. 132° W,. than on 150° W.,
and during the Southern Hemisphere summer
than during winter. The concentrations also
showed apparent north-south shifts. Yellowfin
tuna also were more abundant in the inshore
waters of the Marquesas than in oceanic waters
south of lat. 7° S,

3. The abundance of yellowfin tuna around
the Marquesas varied seasonally. The presence
of greater numbers of larger, and presumably
more mature, fish and the occurrence of greater
numbers of the larvae during the summer, sug-
gest that they gather in this area to spawn.

4, Although bigeye tuna were not as abundant
or widely distributed, their distribution, in gen-
eral, was somewhat similar to that of yellowfin
tuna, The greatest relative abundance of bigeye
tuna was to the east (long. 132° W.) and during
the Southern Hemisphere summer; they also
showed north-south shifts in centers of concen-
tration. Only small numbers of this species
were taken near the Marquesas.

5. No albacore were caught north of lat.
7° S. on long. 132° and 150° W. There are indi-

cations that an east-west discontinuity of the
oceanic structure around lat. 10° S, somehow
may restrict the distribution, Like bigeye tuna,
albacore were not very abundant at the inshore
stations around the Marquesas.

6. Longlines did not fish as deep between
lat. 2°N, and 2°S, as they did farther to the
north and south. This fact was ascribed to
shearing between the westerly flowing South
Equatorial Current at the surface and the east-
erly flowing Equatorial Undercurrent beneath.

7. No conclusive statement could be made of
the depths at which oceanic yellowfin and bigeye
tunas were most abundant. Both species, how-
ever, were caught at all depths fished by the
longlines. Oceanic albacore south of lat. 7° S,
were evenly distributed between 105 and 164 m.
Near the Marquesas Islands catches of yellowfin
tuna were often greater when the longlines
fished deeper. Catches of bigeye tuna were
greatest between150 and 164 m. Albacore were
deeper than yellowfin or bigeye tunas.

8. The length-frequency distributions of
yellowfin tuna caught at oceanic stations and at
stations near the Marquesas Islands were sim-
ilar; yellowfin tuna from 90 to 130 cm. long
were sparsely represented in catches near the
Marquesas. Fish in this size range possibly

are not present in any great numbers in this
area.

(ERRE WAT WINE Glee)

AUSTIN, THOMAS S.

1957. Summary, oceanographic and fishery
data, Marquesas Islands area, August-
September, 1956 (EQUAPAC). U.S. Fish
Wildl. Serv., Spec. Sci. Rep. Fish. 217,
v + 186 p.

CROMWELL, TOWNSEND, R.B. MONTGOMERY,

and E. D. STROUP.

1954. Equatorial Undercurrent Pacific
Ocean revealed by new methods. Science
119(3097): 648-649.

IVERSEN, EDWIN S., and HOWARD O. YOSHIDA,

1956. Longline fishing for tuna in the central
equatorial Pacific, 1954.
Serv., Spec. Sci. Rep. Fish. 184, iv + 33 p.

1957. Longline and troll fishing for tuna in
the central equatorial Pacific, January
1955 to February 1956, U.S. Fish Wildl.
Serv., Spec. Sci. Rep. Fish. 203, v + 38 p.

JUNE, FRED C.

in

1953. Spawning of yellowfin tuna in Hawaiian
waters. Fish Wildl. Serv., Fish. Bull.
54: 47-64,

KNAUSS, JOHN A., and JOSEPH E. KING.

1958. Observations of the Pacific Equatorial
Undercurrent. Nature, London 182: 601-
602.

MANN, HERBERT J,

1955. Construction details of improved tuna
longline gear used by Pacific Oceanic
Fishery Investigations. Com. Fish. Rev.
WT (2) ee — 10.

MARR, JOHN C., and MILNER B. SCHAEFER.

1949. Definitions of body dimensions used in
describing tunas, Fish Wildl. Serv.. Fish.
Bull. 51: 241-244,

MATSUMOTO, WALTER M.

1958. Description and distribution of larvae
of four species of tuna in central Pacific
waters. U.S. Fish Wildl. Fish,
Bull. 58: 31-72.

MURPHY, GARTHI., andRICHARD S. SHOMURA,
1953a. Longline fishing for deep-swimming
tunas in the central Pacific, 1950-51.
U.S. Fish Wildl. Serv., Spec. Sci. Rep.

Fish. 98, iii + 47 p.
1953b. Longline fishing for deep-swimming

Serv.,

U.S, Fish Wildl.

10

tunas in the central Pacific, January-
June 1952. U.S. Fish Wildl. Serv., Spec.
Sci. Rep. Fish. 108, iii + 32 p.

1955. Longline fishing for deep-swimming
tunas in the central Pacific, August-
November 1952. U.S, Fish Wildl. Serv.,
Spec. Sci. Rep. Fish. 137, iv + 42 p.

OTSU, TAMIO.

1954, Analysis of the Hawaiian longline fish-
ery, 1948-52. Com. Fish. Rev. 16(9):
1-17.

SHOMURA, RICHARD S., andGARTHI, MURPHY.

1955. Longline fishing for deep-swimming
tunas in the central Pacific, 1953. U.S.
Fish Wildl. Serv., Spec. Sci. Rep. Fish.
157, v + 70 p.

SHOMURA, RICHARD §.,, and TAMIO OTSU.

1956, Central North Pacific albacore sur-
veys, January 1954-February 1955. U.S,
Fish Wildl. Serv., Spec. Sci. Rep. Fish.
173, v + 29 p.

SIEGEL, SIDNEY,

1956. Nonparametric statistics for the be-
havioral sciences. McGraw-Hill Book
Company, Inc., New York, Toronto,
London, 312 p.

STRASBURG, DONALD W.

1958. Distribution, abundance, and habits of
pelagic sharks in the central Pacific
Ocean. U.S, Fish Wildl. Serv., Fish.

Bull. 58: 335-361.

1960. Estimates of larval tuna abundance in
the central Pacific. U.S. Fish Wildl.
Serv., Fish. Bull, 60: 231-255.

WILSON, ROBERT C., and MURICE O, RINKEL.

1957. Marquesas area oceanographic and
fishery data, January-March 1957. U.S,
Fish Wildl. Serv., Spec. Sci.~Rep. Fish.
238, V+ 136 p.

WILSON, ROBERT C., EUGENE L, NAKAMURA,
and HOWARD O. YOSHIDA,

1958. Marquesas area fishery and environ-
mental data, October 1957-June 1958.
U.S. Fish Wildl. Serv., Spec. Sci. Rep.
Fish. 283, vi+105p.

YAMANAKA, HAJIME,

1956. Vertical structure of the ocean in rel-
evant to fishing conditions for albacore
adjacent to 10°S, in the western South
Pacific. Bull. Jap. Soc. Sci. Fish. 21(12):
1187-1193. /In Japanese with English
summary./

1576

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