SCRS/2004/069
Col. Vol. Sci. Pap. ICCAT, 58(2): 470-483 (2005)
REPRODUCTIVE CHARACTERISTICS OF ATLANTIC BONITO (SARDA SARDA)
FROM THE SOUTH WESTERN SPANISH MEDITERRANEAN
D. Macías, M.J. Gómez-Vives, S. García & J.M. Ortiz de Urbina1
SUMMARY
The Atlantic bonito (Sarda sarda) is one of the most abundant small tuna species in the
Mediterranean Sea. This species is commercially exploited in the Spanish coast by traditional
fisheries. Nevertheless, the biological and reproductive information about this species is
currently scarce in the Western Mediterranean. In order to improve our knowledge about the
reproductive parameters (Sex-ratio, maturity, oocyte developmental features, etc) of this
population, gonads were collect along June (2003) from a Spanish Mediterranean trap (la
Azohia, Murcia). The fish collected ranging from 410 to 460 mm in fork length. The sex-ratio
was 1:1.2 F/M. All these fishes were mature and their reproductive characteristics indicate a
spawning area near to the trap location. These and other biological aspects (size distribution,
size/weight relations, etc) will be discussed in this paper.
RÉSUMÉ
La bonite à dos rayé (Sarda sarda) est l’une des plus abondantes espèces de petits thonidés en
Méditerranée. Cette espèce est commercialement exploitée sur la côte espagnole par les
pêcheries traditionnelles. On dispose néanmoins de peu d’informations sur la biologie et la
reproduction de cette espèce à l’ouest de la Méditerranée. Des gonades provenant d’une
madrague méditerranéenne espagnole (la Azohia, Murcie) ont été prélevées au mois de juin
(2003) afin d’améliorer nos connaissances sur les paramètres reproductifs (sex-ratio, maturité,
caractéristiques du développement des ovocytes, etc.) de cette population. Les poissons
recueillis mesuraient entre 410 et 460 mm de longueur à la fourche. Le sex-ratio était de 1 :1,2
F/M. Tous ces poissons avaient atteint le stade de maturité et leurs caractéristiques
reproductives indiquent une zone de frai proche de l’emplacement de la madrague. Ces aspects
biologiques et d’autres (distribution des tailles, rapports taille/poids, etc.) seront examinés
dans le présent document.
RESUMEN
El bonito (Sarda sarda) es una de las especies de pequeños túnidos más abundantes en el mar
Mediterráneo. Las pesquerías tradicionales explotan comercialmente esta especie en las costas
españolas. Sin embargo, actualmente existe poca información sobre la biología y reproducción
de esta especie en el Mediterráneo occidental. Se recogieron gónadas de una almadraba
española del Mediterráneo (La Azohía, Murcia, durante junio de 2003, con el fin de ampliar
conocimientos sobre los parámetros reproductivos (ratio de sexos, madurez, rasgos del
desarrollo de los oocitos, etc.) de esta población. La talla de los peces recogidos osciló entre
410 y 460 mm de longitud a horquilla. La ratio de sexos fue 1:1,2 H/M. Todos los peces habían
alcanzado la madurez y sus características reproductivas apuntaban hacia la existencia de una
zona reproductiva situada cerca de la almadraba. En este documento se discuten estas y otras
cuestiones biológicas (distribución por tallas, relaciones talla-peso, etc.).
KEYWORDS
Sarda sarda, Atlantic Bonito, spawning, sexual maturity, South Western Mediterranean.
1
Instituto Español de Oceanografía. C.O. de Málaga. Pto Pesquero s/n, 29640, Fuengirola, España.
470
1. Introduction
The Atlantic Bonito (Sarda sarda) inhabit temperate and tropical areas of both hemisphere of Atlantic Ocean,
Gulf of Mexico, the Black Sea and Mediterranean. It forms large mixed schools with other tuna species near the
surface (Collete and Nauen 1983; ICCAT 2003). The Atlantic bonito (Sarda sarda) is one of the most abundant
small tuna species in the Mediterranean Sea where has been commercially exploited since ancient times (Demir,
1963). In Spain, the species have been caught traditionally by seasonal coastal fisheries. Several fishing gears
have been used to catch it: Traps and other minor passive gears, purse seine and troll (Rey et al. 1984). In spite
of the importance to local economy of this species, little is currently known about the biology of this small tuna.
The first maturity size has been stated in 38 cm (FL) when the fish is one years old (Rey et al. 1984; ICCAT
2003). Several authors have observed that the species spawn in the Mediterranean, although it spawning areas
are better known in the eastern part of the Mediterranean and Black Sea (Mayorova and Tkacheva 1959; Demir
1963). Very little is known about reproductive areas of Atlantic bonito in the Western Mediterranean, although
have been noted a scarce number of larvae and eggs around the Balearic Islands and Algerian coast (Duclerc et
al., 1974; Piccinetti et al. 1996; Ehrenbaum 1924 and Piccinetti and Piccinetti Manfrin 1994). The spawning
period extends from May to July (Sanzo 1932; Rodriguez-Roda 1966). The species is a multiple spawner with
asynchronous oocyte development that carried out 3 or 4 spawning steps by reproductive (Majorova y Tkacheva
1959; Rey et al. 1984).
Information concerning to migration patterns is scarce, fragmented and in occasions contradictory (De Buen
1930; Rey and Cort 1981; Sabatés and Recasens 2001). The tagging-recoveries studies suggest a genetic
migration (from May to June) from the Atlantic Ocean to the spawning areas in the Mediterranean trough the
Gibraltar Strait. After, from June to September a post-reproductive migration takes place in a reverse direction
(Rey y Cort 1981). Nevertheless, other studies suggest that the Atlantic bonito is resident all over year in the
Mediterranean and that the mature specimens migrate from coastal areas to open sea to spawn (Sabatés and
Recasens 2001).
The aim of this paper is to describe and discuss the results obtained about the reproductive characteristics of the
Atlantic bonito in the study area.
2. Material and methods
2.1 Atlantic Bonito samples
2.1.1 Specimen collection
A total of 183 Atlantic Bonito were measured and weighed during a scientist survey developed during 2003 in
the “la Azohía” trap (Murcia) in the South Western Mediterranean. 40 specimens caught from June 2 to 10 were
diseccted to obtain gonad, liver and muscle tissue for reproductive studies and hard structures for age
determination like spines, vertebrae and otolithes. For each specimen fork length (FL) to the nearest millimetre,
weight and sex were determined. In addition the gonad weight to the nearest g was obtained with the purpose of
knowing the Gonad-somatic index (Kume and Joseph 1969) indicating the levels of sexual maturity of the
Bonito specimens. Gonads were collected from each fish.
Each couple of gonads (ovaries, testis) were removed from the gut cavity and identified as male or female, then a
section of 2 cm width from the central part of only one single gonad was removed and preserve in 10 % buffered
formalin and stored. A portion of each section preserved was washing in buffer solution, dehydrated in ethanol
and n-butanol series and embedded in paraffin. Then samples were sectioned at approximately 10 μm with a
microtome and then mounted on slides and stained with Mallory´s Trichrome Stain for a general assessment of
the histology of the gonad. Finally, the slides were examined and photographed in a Leyca photo-microscope.
2.1.2 Staging and measurement of oocytes
Five well-defined developmental stages of oocytes (perinucleolar, previtelogenic, partially yolked, totally yolked
and Hidrated oocytes) were defined using histology following several authors (Yamamoto 1956, Forberg 1982 &
Hunter et al. 1992). Developmental stage and measurement of whole oocytes were determined from 7 females in
different developmental stages. Oocyte sizes were obtained by measuring of two diameters of 100 oocytes of
each stage using an analysing system of only those oocytes that had been sectioned through the nucleus. Finally,
471
the histological analysis and oocyte diameter distribution in each female was used to assess the spawning
sequence (West 1990).
2.1.3 Histological classification
To estimate reproductive condition of Atlantic Bonito, two different histological classification systems were
used: one for estimating the sexual maturity and the other for assessing the activity stage of mature females.
Each ovary was histologically classified according to both systems (Hunter and Goldberg 1980; Hunter and
Macewicz 1980, 1985a, b; Hunter, Macevicz and Sibert 1986).
Sexual maturity
It is considered that a female is a mature one when has the capability to reproduce in a determinate spawning
season. Histological signs of maturity are the presence in the ovary of yolked oocytes, hydrated oocytes or
postovulatory follicles. The immature females have not reached the sexual maturity and are unable to reproduce
in a determinate season.
Sexual activity
Five different stages of activity have been taken into consideration in the Atlantic bonito female:
Inactive females: the histological analysis indicates that the ovary contains no yolked oocytes and no atresic
structures.
Active females: females were classified as active when the ovary contained yolked oocytes and there was no
atresia or only minor considered. Active females were further classified into other stages according to additional
criteria.
Ripening females (Maturng): Those females showing signs of sexual maturity (Yolked oocytes) but not signs of
imminent spawn or signs of past spawns steps.
Pre-spawning females (Ripe): Those females showing signs of an imminent spawning like hidrated or nuclear
migration phase oocytes but not postovulatory follicles or extended atresia. High density of oocytes in the ovary
can be seen.
Spawning females: spawning females were considered that whose ovaries present postovulatory follicles or
imminent spawning signs like hydrated or migratory-nucleus oocytes. The histological analysis shows signs of
past spawning (postovulatory follicles) and enough vitellogenic oocytes to complete more spawning.
Post-spawning females: Those females showing signs of past spawning (postovulatory follicles) but have not
enough vitellogenic oocytes to complete more spawning. Extended atresia in vitellogenic oocytes. Low oocytes
density in the ovary.
The gonadosomatic index was calculated according to Kume and Joseph (1969)
Atlantic bonito males:
Changes in the proportion of cysts containing spermatogonias, spermatocytes, spermatids and spermatozoa in the
Bonito testis characterises the maturity stage.
Males were classified as maturing when they showed a high incidence of cysts of spermatocytes and spermatids
in the tubule wall and a little amount of mature spermatozoa can be found in the lumen of their seminipherous
tubules.
The fluent males showed a high density of the sperm in the seminipherous tubules, lesser levels of meiosis and
low amount of cysts in seminipherous tubule walls.
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3. Results
3.1 Histological analysis
Ovary structure and Oocyte development
The ovary consists of a muscle wall and a lot of follicles in different stages of development inside connective
tissue. The follicles are made of an oocyte and a single layer of follicular cells. The type and number of follicles
vary with the maturity and activity stages.
The histologic analysis let us characterise five stages of oocyte development:
Perinucleolar stage (∅ 25-140 μm) (Figura 1 A). Oocytes displayed polygonal shape with numerous small
nucleoli near nuclear envelope. No alveoli or vesicles can be found in the ooplasm. The average diameter was 72
μm, the median 70 μm and the modal value was 68 μm.
Previtellogénic stage (Lipid cortical alveoli) (∅ 74-323 μm) (Figure 1 B). The previtellogenic oocytes exhibited
small lipid droplets and cortical alveoli in the peripheral ooplasm. No yolk granules can be found. The average
diameter of this stage was 190 μm, the median 192 μm and the modal value was 194 μm.
Partially yolked stage (∅ 160-427 μm) (Figure 1 C). These oocytes present small spherical yolk granules in the
peripheral ooplasm. The zona radiata increase it thickness and the follicular cell layer became cubic. The
ooplasm are not completely filled of yolk granules. The average value of the diameter was 296 μm, the median
294 μm and the modal value was 298 μm.
Totally yolked stage (∅ 318-690 μm) (Figure 1 D). Totally yolked oocytes display the ooplasm entire filled of
yolk granules. The zona radiata increase it thickness but the nucleus remains in the central location. The average
diameter was 485μm, the median 480μm and the modal value was 485μm.
Hidrated stage (∅ 450-871 μm) (Figure 1E, 1F). The final of the oocyte maturation is characterised by the
migration of the nucleus towards the animal pole, the coalescence of lipid and yolk globules and ooplasm
hydration. The average diameter of these oocytes was 732μm, the median 748 μm and the modal value was 749
μm.
Seven specimens displayed also post-ovulatory follicles (Figure 1 H). Atretic follicles can be found in a
different frequency in several ovaries. The atretic structures (Figure 1 G) had an average size of 242 μm and
their size vary from 85 μm to 655μm. These broad range indicate that atresia can be found in all stages of
oocytes development.
3.2 Maturity
Activity:
Ripening females. Three females (16.6%) were classified like maturating. These females displayed ovary profile
characterised by the presence of perinucleolar, previtellogenic and yolked oocytes in the ovary (Figure 2 A). No
atretic follicles and post-ovulatoy follicles can be found in the ovary. 82% of ovary follicles were not yolked
ones (Figure 2 B). No signs of imminent spawning activities can be found in the ovary (Figure 2 C).
Ripe females. Seven females (38.8%) were classified like totally mature. These females displayed ovary profile
characterised by the presence of perinucleolar, Previtellogenic, partially yolked and post-vitellogenic oocytes
(nuclear migration or hydrated oocytes) in the ovary (Figure 3 A). Minor atretic follicles can be found in the
ovary (0.9%) and no post-ovulatoy follicles was presented in the ovary. 70% of ovary follicles were not yolked
ones (Figure 3 B). No signs of imminent spawning activities can be found in the ovary (Figure 3 C).
Spawning females. Seven spawning females (38.8%) were caught in the sample. These females showed some
differential characteristics. All of these females showed perinucleolar, previtellogenic, yolked oocytes, atretic
structures and postovulatory follicles. But only some of these females presented hydrated oocytes. The location
of postovullatory follicles in all females indicate us recent spawning steps but not all of these females are in the
same stage of maturating for the next spawning step. The ovary profile is shown in Figure 4.
473
Post-spawning females. Only one female was post-spawning one (5.5%). This female showed perinuclear and
previtellogenic oocytes and large extended atresia of yolked and hydrated oocytes (Figure 5 A, 5 B & 5C).
Males. All males caught were maturing or mature. They showed a high incidence of cysts of spermatocytes and
spermatids in the tubula wall. Variable amount of mature spermatozoa can be found in the lumen of their
seminipherous tubules.
First maturity size
All female caught in this study were mature. All male present sperm in the testis so we can conclude that all of
them were also mature.
3.3 Sex ratio and Gonadosomatic index (GSI)
The fishes collected ranged from 41.5 cm to 48 cm in fork length (FL). Size distribution has been shown in
Figure 6 A. Detailed information on date, fork length (cm.), maturity, GSI and sex are given in Table 1. The
sex-ratio was 1:1.2 F/M. Males were more abundant than females in the studied sample. The proportion of
females increased with the size increment so the females are more abundant in the highest size classes (see
Figure 6B).
Mean IGS by size class are represented in the Figure 7 A. In female bonito the more size class grew up the more
GSI increased. Nevertheless, in the males no clear trends can be found in the GSI with the size increment.
Once females were classified using histology, mean IGS was calculated by activity stage. The results have been
summarised in the Figure 7 B. Mean IGS shows an increasing trend along maturating progress. The postspawning females, showed the minimum GSI values, followed by the maturating ones, and the totally mature
pre-spawning females reach the highest values.
3.4 Spawning pattern
The microscopic appearance of the oocytes and the oocyte size frequency distribution had been used to define
the spawning pattern shown by these species. The maturation of the Bonito follicles began in oocytes ranging
from 150 μm (Pre-vitellogenic oocytes) to 295 μm (early vitellogenesis). Vitellogenesis initiates at 160 μm but
the average size of partially yolked oocytes was 295 μm. In general fully yolked oocytes can be found at 318 μm
but the average size of this stage was 485 μm. The hydration started when the oocyte reach at least 450 μm in
diameter. Maximal size of hydrated oocyte in this study was 871 μm. The oocyte size frequency distributions
broadly overlapped each of the different stages of development. This characteristic indicated us that
perinucleolar stage oocytes were constantly recruited to the maturating stages. Postovulatory follicles appear
once the hydrated oocytes have been spawned. Atretic oocytes can be seen in all the developmental stages.
3.5 Length-weight relationships
Pairs of observations on length and weight (round - Figure 8A- and dressed - Figure 8 B) were used to fit the
following relationships:
Total weight = 4.6016E –5* FL 2.6772
Gutted weight = 0.00053525* FL 2.004417
Based on the determination coeffient, R2, model relating length and dressed weight seems to better fit the data,
which may be related to differential biological stage (and corresponding differences in liver and gonad weight)
between individuals.
4. Discussion
Oocyte and gonadal development
The Atlantic bonito ovary is considered asynchronous because oocytes at different levels of development were
present at the same time in the ovary. Developmental characteristics of oocyte maturation of Bonito are similar
to those described for other species with asynchronous development. Oogonia and perinuclear oocytes are found
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in all ovaries all year and are considered to be part of the general stock of the ovary. Later, when cortical lipid
alveoli develop pre-vitellogenic oocytes emerge. Yolk accumulation occurs in vitellogenic phase and, after
migration of the nucleus to the animal pole, hydration and ovulation occurs (Forberg 1982). In Bonito, hydrated
oocytes were used to define the mature pre-spawning stage according to Arocha´s (2002) criteria for swordfish.
After spawn step numerous post-ovulatory follicles are found in the ovary. Since post-ovulatory follicles become
soon indistinguishable from the connective tissue (Matsuyama et al. 1988), they have been commonly
interpreted as a sing of recent spawning. The mature spawning ovary never was empty of vitellogenic oocytes,
only in post-spawning (recovering ovaries) females we could observed empty ovaries.
Spawning pattern
The results of this study indicate that Bonito is a multiple spawner with asynchronous oocyte development.
These results agree with those of previous studies (Rey et al. 1984; Yoshida 1980). These studies suggest that
the Bonito could have a determinate fecundity and suffer only 3 or 4 spawn steps (Majorova y Tkacheva 1959).
However, our result indicates that there are no gaps between distributions of dominating stages of oocyte
development. We also found several modal values in yolked oocytes. These characteristics suggest that Atlantic
Bonito are a species with indeterminate fecundity. This fecundity pattern depends on estimates of batch
fecundity and spawning frequency to determinate potential annual fecundity (Hunter and Macewitz 1985; Hunter
et al. 1992).
First maturity size
Our results agree with those that indicated that Atlantic Bonito reached the first maturity size in the first year of
life (Rey et al. 1984; ICCAT 2003). All specimens caught in this study were mature.
Spawning season and areas
From the histology analysis of Atlantic Bonito ovaries we have deduced a reproductive activities pattern in the
studied area. During the first two weeks in June the females are spawning near the trap location. The spawning
season occurs mainly in later spring and early summer, with a peak in June. These results are consistent with
those reported previously in the Mediterranean (Demir 1963; Rodriguez Roda 1966; Rey et al. 1984). These
results suggest a short reproductive season in the Western Mediterranean and agree with the observations of
larvae of this species in the Catalonian coast along July (Sabatés and Recasens 2001).
The most important spawning areas in the Mediterranean had been located in the eastern Mediterranean
(Mayorova and Tkacheva 1959; Yoshida 1980). The catches of pre-spawning, spawning and recent postspawning mature female in the study area during June highly support the hypothesis of a reproductive area in the
South-western Spanish Mediterranean. Camiñas et al. (1986) reported the monthly landings of Sarda sarda
along a latitudinal gradient. According to this report, landings of Atlantic bonito decrease during the
reproductive season except in the area 37-39ºN where they could observe a high level of landing for this species
in May. These results agree with our observations of regular and important catches of this species by the “la
Azohía” trap during May and first week of June. These results could be interpreted as a migration to the
reproductive area of mature specimens during the reproductive season. We suggest that the Atlantic bonito
realised effective spawn in determined coastal areas. This hypothesis disagreed with the interpretation of Sabatés
and Recasens (2001) that suggested a migration to open sea areas to spawn. Finally our results agree with the
two most important hypothesis about reproductive migration of Atlantic bonito. First if according to Rey et al.
(1984) the Atlantic Bonito migrate from the Atlantic Ocean to the Mediterranean to spawn, the study area could
be a destination place to a part of these specimens. If, according to Sabatés and Recasens (2001) the Atlantic
bonito is a resident species in the Mediterranean then a genetic migration could concentrate the mature
specimens in certain coastal areas to spawn.
In conclusion our results unequivocally indicate that the Atlantic Bonito has a reproductive and spawning area in
coastal areas near to “la Azohía” in the Western Mediterranean.
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Table 1. Biological characteristics of Atlantic Bonito used in this study.
CODE
BON1
BON10
BON11
BON12
BON13
BON14
BON15
BON16
BON17
BON18
BON19
BON2
BON20
BON21
BON22
BON23
BON24
BON25
BON26
BON27
BON28
BON29
BON3
BON30
BON31
BON32
BON33
BON34
BON35
BON36
BON37
BON38
BON39
BON4
BON40
BON5
BON6
BON7
BON8
BON9
Length(cm) Weight (g) Dressed Weight
44,2
43,5
43,2
41,3
41,8
42,8
42,4
43,4
42
43,5
44,2
41,5
42,2
43
43
44
42,5
46
42,5
44
41,5
44
42,2
45
44,5
44
47
43,5
43
43
44,5
42,5
42,5
41,6
43,5
42,2
41
45
41,5
43
1154
1112
1082
970
1054
1182
1044
1146
1078
1006
1214
1008
1076
1156
1028
1102
1122
1280
1182
1186
1066
1164
1036
1146
1228
1102
1384
1160
1132
1096
1118
1050
1048
968
1098
1070
946
1240
1052
1110
GONAD W GSI
1060
1014
1008
896
976
1066
964
1052
998
934
1134
928
988
1028
940
1036
1024
1150
1118
1050
970
1058
962
1038
1078
1000
1264
1016
1044
1008
1050
960
974
882
976
972
872
1128
972
1018
478
SEXO Maturity
50,5 5,85 F
35,5 4,31 M
24 2,98 M
28 3,97 M
26 3,56 F
37,5 4,78 M
40 5,25 M
43 5,26 M
38 5,13 M
22,5 2,73 M
39,5 4,57 M
31 4,34 M
41 5,46 M
71 8,93 F
42 5,28 F
20,5 2,41 M
43,5 5,67 F
63 6,47 F
12 1,56 M
81 9,51 F
50,5 7,07 M
47 5,52 F
24 3,19 F
49 5,38 F
89 10,10 F
46 5,40 F
68,5 6,60 M
60 7,29 F
40 5,03 M
30,5 3,84 F
11 1,25 F
21,5 2,80 M
25 3,26 M
30 4,17 M
69 8,38 F
47 6,25 M
33 4,79 M
52 5,71 F
26 3,64 F
36,5 4,59 M
Ripening
mature
mature
mature
Spawning
mature
mature
mature
mature
mature
mature
mature
mature
Ripe
Ripe
mature
Spawning
Ripe
mature
Ripe
mature
Spawning
Spawning
Ripe
Ripe
Ripening
mature
Spawning
mature
Spawning
Post-spawning
mature
mature
mature
Ripe
mature
mature
Ripening
Spawning
mature
479
480
481
482
483