Histological gonad analyses of late summer--early winter collections of bigeye tuna, Thunnus obesus, and yellowfin tuna, Thunnus albacares , from the northwest Atlantic and the Gulf of Mexico

JUNE 1981

HISTOLOGICAL GONAD ANALYSES OF LATE SUMMER-EARLY WINTER
COLLECTIONS OF BIGEYE TUNA, Thunnus obesus_ , AND
YELLOWFIN TUNA, Thunnus albacares , FROM THE NORTHWEST
ATLANTIC AND THE GULF OF MEXICO

Stephen R. Goldberg
and
Hillary Herring—Dyal

NOAA-TM-NMFS-SWFC- 14

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration
National Marine Fisheries Service

Southwest Fisheries Center

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0 0301 0063321 0

NOAA Technical Memorandum NMFS

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special purpose information, and have not received complete formal review, editorial control, or detailed editing

JUNE 1981

HISTOLOGICAL GONAD ANALYSES OF LATE SUMMER-EARLY WINTER
COLLECTIONS OF BIGEYE TUNA, Thunnus obesus , AND

YELLOWFIN TUNA, Thunnus albacares , FROM THE NORTHWEST
ATLANTIC AND THE GULF OF MEXICO

Stephen R. Goldberg
and
Hillary Herring—Dyal

National Marine Fisheries Service
Southwest Fisheries Center
La Jolla, California 92038

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Oceanic and Atmospheric Administration
Dr. John V. Byrne, Administrator

National Marine Fisheries Service
William H. Stevenson, Acting Assistant Administrator for Fisheries

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TABLE OF CONTENTS

Table 1 - Bigeye tuna: Histological Analysis Summary . .
Table 2 - Yellowfin tuna: Histological Analysis Summary
Faguresle =" Fishing? areas: <0 st sGg cenenis cuecie oe taes Jeena
Appendix A - Bigeye tuna: Statistical analysis summary...

Appendix B - Yellowfin tuna: Statistical analysis summary

$i 71

Histological gonad analyses of late summer-early winter collections
of bigeye tuna, Thunnus obesus, and yellowfin tuna Thunnus albacares,
from the Northwest Atlantic and the Gulf of Mexico

Stephen R. Goldberg
Department of Biology
Whittier College
Whittier, California 90608

Hillary Herring-Dyal
Southwest Fisheries Center
National Marine Fisheries Service, NOAA
La Jolla, California 92038

INTRODUCTION

There is little information available on the reproduction of yellowfin
and bigeye tuna in the Gulf of Mexico and adjacent Northwest Atlantic Ocean.
Previous reproductive studies on samples from the Pacific Ocean (Schaefer and
Orange, 1956; Orange, 1961; Yuen, 1955; Yuen and June 1957) provide some
information on the spawning of these species. In order to gain an
understanding of yellowfin and bigeye tunas' reproductive potential, a
histological gonadal analysis was conducted. This analysis will contribute to
a more complete picture of the reproductive biology of yellowfin and bigeye
tunas.

METHODS

Specimens were collected from the Gulf of Mexico and the Northwest
Atlantic Ocean (Figure 1). The collections were made from September 1978
through December 1979 by United States observers aboard Japanese longline
vessels fishing in the United States Fishery Conservation Zone.* Upon
capture, fishes were weighed to the nearest kilogram and measured ( fork-
length) to the nearest centimeter. A ventral longitudinal incision was made
on the absomen and the paired gonads were removed and placed in a 10% formalin
solution.“ Formalin-preserved gonads were weigheg to the nearest gram. A
sample from each gonad was embedded in Paraplast.~ Histological sections were

lcamples were obtained from the National Marine Fisheries Service,
Southeast Fisheries Center, Miami, Florida.

2 Fixation in large gonads was not uniform. This problem can be
avoided in future samples by making a slit along the length of the gonad
with a razor blade. This will facilitate penetration of the fixative.

3 The use of trade names does not imply endorsement by the
U.S. Government.

cut at 8 mm on a rotary microtome, mounted on slides and stained with iron
hematoxylin followed by an eosin counterstain. All gonads were histologically
classified according to their reproductive stages (Tables 1-2). Gonad
weights, gonosomatic indices and statistical analysis for bigeye and yellowfin
tuna specimens are given in Appendix A and B respectively.

RESULTS

Females -- Ovaries of all females (collected August through February)
were regressed (Tables 1-2) and consisted of primary oocytes arranged along
connective tissue septa. There was no vacuolization which typically occurs
prior to the beginning of yolk deposition for a new spawning cycle. Also, the
almost total absence of follicular atresia, a process in which follicles
undergo degeneration, suggests the ovaries had been reproductively inactive
for several months. Follicular atresia reaches its highest levels toward the
close of the reproductive season when follicles that initiated but did not
complete yolk disposition, degenerate. Follicles in various states of
atresia remain for some time after reproduction ceases.

Males -- Testes (collected August through February) were primarily
regressed (Tables 1-2) as one would expect during a time when no spawning was
occurring in the population. There were masses of residual sperm left in
several males as is typical in regressed testes. In other cases, limited
spermatogenesis was in progress. It is not unusual to find small quantities
of sperm formation in males when females are reproductively inactive.
However, the level of sperm formation was greatly reduced from what one would
have expected during peak spermatogenesis.

DISCUSSION

Although yellowfin tuna and bigeye tuna are reported to spawn most of the
year in tropical areas (Yuen, 1955; Yuen and June, 1957) we found no evidence
of spawning activity during August through February for yellowfin tuna and
September through February for bigeye tuna sampled from the populations under
study. Furthermore, the lack of follicular atresia in females during February
through September indicates they had not been spawning during the previous 6-8
weeks.

Prior to the onset of spawning, there is typically an increase in oocyte
size with a concomitant appearance of vacuoles. One would expect this
histological development to occur 8-10 weeks prior to spawning. This was not
observed in the samples of either species from December through February and
would push the postulated onset of spawning to April through May at the
earliest.

It thus appears based on these data, that the populations under study
undergo a brief spring spawning period similar to that of northern fishes
(Quasim, 1956) which have a restricted spawning season. In order to test our
hypothesis of a postulated spring spawning period, it will be necessary to
obtain additional female gonads from the period March through June. These
additional gonads would be of particular interest in obtaining other valuable
information regardless of the reproductive state. Assuming that spawning
females were obtained, we could then calculate fecundity estimates, minimum
size at sexual maturity, plot a seasonal gonosomatic index graph and obtain

information as to the kind of spawning cycle the populations undergo. For
example, are one or two modes of eggs spawned or is a mode of eggs matured and
gradually released? If the specimens were reproductively inactive this
information would be of interest and perhaps indicate the populations were
transitory and migrated into the area after spawning elsewhere. Other
factors, such as the possible existence of inadequate nutrition to permit
spawning in the study areas and the possibility of environmental pollution
inhibiting reproduction, need examining. These are questions that can only be
answered by additional collections, especially of spring specimens.

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LITERATURE CITED

Orange, C.J. 1961. Spawning of yellowfin tuna and skipjack in the
eastern tropical Pacific as inferred from studies of gonad develop-
ment. Inter-American Tropical Tuna Commission V(6):459-526.

Schaefer, M.B., and C.J. Orange. 1956. Studies of the sexual develop-
ment and spawning of yellowfin tuna (Neothunnus macropterus) and
skipjack (Katsuwonus pelamis) in three areas of the eastern Pacific
Ocean, by examination of gonads. Inter-American Tropical Tuna
Commission I(6):283-349.

Quasim, S.Z. 1956. Time and duration of the spawning season in some
marine teleosts in relation to their distribution. J. Cons. Int.
Explor. Mer. 21:144-154.

Yuen, H.S.H. 1955. Maturity and fecundity of bigeye tuna in the
Pacific. U.S. Dept. of Interior, Fish & Wildlife Service, Special
Scientific Report, Fisheries No. 150, 30 pp.

Yuen, H.S.H., and F.C. June. 1957. Yellowfin tuna spawning in the
central equatorial Pacific. U.S. Fish & Wildlife Service, Fish.
Bull. 57(112):251-264).

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APPENDIX A. Summary of yellowfin tuna gonad data collected 9/78 through
12/79 in the northwest Atlantic and Gulf of Mexico

—————

mean mean mean

fish gonad gonad No

Date Sex means SXx* et S¥* indice Sz* Samples

2 “Gy!

ee eee ee eee
9/78 3 24.0 1.00 67.50 6.8 28 04 2
9/78 g 25.0 2.00 43.00 8.9 a7) 02 2
10/78 3 - - - - - - 0
10/78 e 24.64 0.55 57.89 3.50 23 01 44
11/78 3 20.00 - 22.6 - 11 - 1
11/78 g 23.00 1.58 41.02 8.49 16 04 5
2/79 S - - - - - - 0
2/79 e 31.00 3.00 152.37 28.90 48 -05 3
8/79 é 19.00 - T5522 - 0.39 - 1
8/79 g 21.67 0.88 51.16 13.48 0.24 07 3
9/79 g 37.00 5.58 42.63 13.59 0.15 03 5
9/79 g 25.44 0.38 50.66 4.72 0.20 02 9
10/79 6 - ~ ~ - - - 0
10/79 e 29.88 2.42 73 .64 13.22 0.23 02 18
11/79 $ 23.75 1.65 11555 0.85 0.05 01 4
11/79 ? 26.14 135 61.29 9.49 0.23 03 7
12/79 $ - - - - - - 0
12/79 e 25.00 5.00 58.02 13.82 0.23 01 2
TOTAL 106

*S = variance

295

APPENDIX B. Summary of bigeye tuna gonad data collected 10/78 through
12/79 in the northwest Atlantic and Gulf of Mexico

mean mean mean
fish 3 gonad gonad No

Date Sex weight SX* weight s¥* indice SZ* Samples

X 7 z =(22)

a SN eee eee LS Se ee ee
10/78 3 - - - - - - 0
10/78 g 36.73 4.32 143.42 20.68 34 -03 22
11/78 3 58.44 Hail 37.14 She -07 -01 9
11/78 ° 54.16 2.64 215.64 18.88 41 -03 18
12/78 é 35.10 2.46 25.00 Seo7) -08 -01 25
12/78 g 33.19 1.85 119.43 10.87 .33 -02 57
9/79 3 - - - - - - 0
9/79 g 33.00 - 143.32 - -43 - 1
10/79 3 - - - - - - 0
10/79 g 25.60 0.87 74.62 11.59 .29 -03 5
11/79 6 20.75 4.03 16.46 2.05 .09 -02 4
11/79 g 30.88 3.36 120.78 20.37 36 -05 8
12/79 3 - - - - - - 0
12/79 g 26.11 225 72.61 10.25 -28 -02 9
TOTAL 158

*S = variance

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