ORIGINAL ARTICLE Biology

Age and maturation of jack mackerel Trachurus japonicusin the East China Sea

Mari Yoda • Tetsuro Shiraishi • Ryuji Yukami •

Seiji Ohshimo

Received: 7 June 2013 / Accepted: 28 November 2013

� The Japanese Society of Fisheries Science 2013

Abstract Growth and reproductive characteristics of

jack mackerel Trachurus japonicus collected in the East

China Sea were determined based on otolith readings and

gonad histology, respectively. Translucent and opaque

zones on sectioned otoliths were identified and opaque

rings counted. Von Bertalanffy growth parameters did not

significantly differ between males and females, and the

combined growth curve was: Lt = 401{1 - exp[-0.275

(t ? 1.149)]} (0.8 \ t \ 6.9), where Lt is fork length

(mm) at age t. The calculated lengths at age 1 in our

study were larger than those reported 50 years ago from

the East China Sea. The spawning period was evaluated

to be from December to June, but primarily from Feb-

ruary to May, based on the monthly changes in the go-

nadosomatic index and histological observations. The

minimum size and age at first maturity were smaller and

younger, respectively, than those reported in previous

studies.

Keywords Fecundity � Maturity age � Otolith � Ovary

histology � Spawning period

Introduction

Jack mackerel Trachurus japonicus is distributed on the

continental shelf waters in areas affected by the Tsushima

Warm Current and Kuroshio Current. In the East China Sea

(ECS), the species is distributed mainly on the continental

shelf, from the waters off Kyushu, Japan, to the northern

waters of Taiwan [1], and is one of the most important

target species in the ECS for Japanese purse seiners. The

total catch of jack mackerel caught by Japanese fisheries in

the ECS and southwestern Japan Sea reached 300–400

thousand metric tons in the early 1960s, but decreased to

170 thousand metric tons in the late 1960s [2]. The catch

was 140–190 thousand metric tons in the early 2000s. As a

means to manage the fisheries stock of this species, the

Japanese government introduced a total allowable catch

(TAC) system in 1997. However, jack mackerel is har-

vested by Japanese, Korean, and Chinese fishermen, and

there is to date no international fishery management system

in the ECS. Consequently, each country—independent of

the others—has implemented its own fisheries management

protocols.

Previous studies on jack mackerel in the ECS have

reported the age and growth based on whole otolith ana-

lysis [4] and estimated the spawning period from changes

in the weight of ovaries and in their morphology and color

[5]. However, the specimens on which these estimates are

based were caught during the period 1955–1969, which

includes the period of the highest stock level of jack

mackerel. The effects of population density and sea water

temperature on fish growth and maturity have been shown

for several fishes [6–10]. In addition, size-selective fishery

pressure has been shown to induce long-term shifts in the

life-history traits of fishes, particularly those of size and

age at maturity [11, 12]. Therefore, it is necessary to clarify

M. Yoda (&) � R. Yukami

Seikai National Fisheries Research Institute, Fishery Research

Agency, 1551-8 Taira, Nagasaki 851-2213, Japan

e-mail: yoda@fra.affrc.go.jp

T. Shiraishi

Okayama Fisheries Promotion Foundation, Urayasu-

Minamimachi, Okayama 702-8024, Japan

S. Ohshimo

National Research Institute of Far Seas Fisheries, Fishery

Research Agency, 5-7-1 Orido, Shimizu, Shizuoka 424-8633,

Japan

123

Fish Sci

DOI 10.1007/s12562-013-0687-5

the current biological characteristics of the jack mackerel

for future analyses and better management protocols. In

this study, we determined age using sectioned otoliths and

examined the reproductive cycle using histological

methods.

Materials and methods

Collection of samples

For the maturation analysis, 740 females were collected

between January 2001 and December 2004 by angling,

purse seine, and bottom trawl surveys conducted by the

Seikai National Fisheries Research Institute (Fig. 1). For

the otolith analysis, 636 specimens were collected between

February 2001 and December 2004 by purse seine, angling,

western pair trawler, set net and bottom trawl surveys

conducted by the Seikai National Fisheries Research

Institute (Tables 1, 2).

A decadal trend in jack mackerel catch has been

reported [3]. The stock of jack mackerel has gradually

recovered from 281 thousand metric tons in 2001 to 545

thousand metric tons in 2004 in the ECS and southwestern

Japan Sea. Therefore, our study period corresponds to a

period of increasing stock of jack mackerel. In a

preliminary study there was no appreciable evidence for

the effect of sampling location; thus, all data were

combined.

All specimens were measured to the nearest 1.0 mm in

fork length (L) and to the nearest 1.0 g of body weight

(BW). The ovary weight (OW) was measured to the nearest

0.1 g from females collected for maturation analysis, and

small pieces of the ovaries were fixed in Bouin’s solution.

The gonadosomatic index (GSI) was calculated as

GSI = (OW/BW) 9 100.

Otolith measurement

Saggital otoliths were removed, washed with tap water, and

then kept under dry conditions. For analysis, the otoliths

were subjected to the burn treatment at 150 �C for 15 min

125˚E 130˚E

25˚N

30˚N

35˚N

Southern ECS

Central ECS

Northern Kyushu

Fig. 1 Map showing the location of the study area in the East China

Sea (ECS)

Table 1 Number and fork length of jack mackerel (Trachurus

japonicus) collected for age determination, by area and month

Year Month Northern

Kyushu

Central ECS Southern ECS

n L range

(mm)

n L range

(mm)

n L range

(mm)

2001 February 38 169–342

March 16 230–262

April 22 276–339

May 18 321–392

August 16 260–334

October 11 205–220

November 16 247–292

2002 February 14 325–388

March 16 168–386

May 16 279–370

July 16 269–376

August 16 206–234

September 16 272–350

2003 January 16 270–373

February 9 196–214

March 99 144–272 29 194–270

April 18 152–190 26 256–294

May 10 184–203 15 237–278

July 38 248–320

August 10 274–294

September 14 201–224

October 7 268–303 10 238–270

December 9 232–260

2004 July 30 191–270

October 17 267–325

November 16 257–284

December 16 250–304 16 205-254

L Fork length, ECS East China Sea

Fish Sci

123

in a drying oven (DO-300A; AS ONE, Osaka, Japan),

embedded in epoxy resin (Epoxicure; Buehler, Lake Bluff,

IL) and sectioned transversely through the core with a

microcutter (MC-201; Maruto, Tokyo, Japan). The sec-

tioned otoliths were polished on the face of one side with a

waterproof sandpaper (#1200) and individually mounted on

a glass slide with epoxy resin. The other side was polished

with waterproof sandpaper (#400–#1200), lapping film

(9 lm), and alumina polishing powder (0.3 lm) until the

otolith rings were clearly visible (thickness 0.3 mm). Ot-

oliths were observed by transmission light microscopy.

Rings consisting of opaque and translucent zones were

observed on the surface of each of these sections. The

inside margin of the opaque zone was considered to be the

ring marks, and the number of rings (N) was counted.

Distances between the core and the n (= 1, 2, …, N)th ring

rn, and between the core and outer margin, R, were mea-

sured to the nearest 0.01 mm using an otolith measurement

system (ODRMS; RATOC, Tokyo, Japan) with transmitted

light.

To measure the precision of the ring count, the index of

average percent error (IAPE) [13] was calculated as

follows:

IAPE ¼ 100� 1=A

� �X1=T

� �XXij � Xj

�� ��=Xj

� �h i

where A is the number of fish counted, T is the number of

times each otolith was counted, Xij is the ring count from

the i-th reading of the j-th fish, and Xj is the average

number of rings of the j-th fish.

To define the period of ring mark formation, monthly

changes in the monthly increment (MI) rate was examined.

The MI rate was defined using R, rN, and rN-1,

MI ¼ R� rNð Þ = rN � rN�1ð Þ:

Fork length-at-age data were fitted to the von

Bertalanffy growth equation. The growth equation was

Lt ¼ L1 1� expf�K t � t0ð Þg½ �:

Where Lt is the fork length (mm) at age t, L? is the

asymptotic fork length, K is the growth coefficient, t is the

age (years), and t0 is the hypothetical age at which L = 0.

The von Bertalanffy growth parameters were estimated

using the least squares method [14].

Each otolith was examined three times, with a minimum

of 1 month between examinations. If two or more exam-

inations of otolith agreed in terms of the ring marks, this

number was recorded and used for the growth analysis.

Histological observations

The fixed ovaries were dehydrated and embedded in

Technovit resin (Kulzer, Wehrheim, Germany) for the

histological study. For light microscopy examination, 2- to

3-lm-thick sections were stained with 1 % toluidine blue

solution. The stained sections were observed under an

optical microscope. The ovaries were classified into six

maturity phases according to the histological characteris-

tics of the most advanced oocytes. The developmental

stages of the oocytes were categorized according to Yon-

eda et al. [15] as the following:

1. Immature phase (Im): previtellogenic oocytes in the

perinucleolus and yolk vesicle stage are present.

2. Developing phase (D): vitellogenic oocytes in the

primary to tertiary yolk stages are present.

Table 2 Number and fork length of female jack mackerel collected

for analysis of maturation, by area and month

Year Month Northern

Kyushu

Central ECS Southern

ECS

n L range

(mm)

n L range

(mm)

n L range

(mm)

2001 January 12 270–336

February 82 153–341

March 2 176–215

April 8 270–384

May 8 274–375 15 181–291

June 10 189–231

July 8 302–339

September 13 294–376

October 10 290–344

November 17 252–307

2002 February 40 234–379

March 14 168–358

April 43 222–342

May 6 279–350 4 238–265 2 242–265

June 10 191–210 2 189–250

July 34 276–331

August 31 172–234

September 42 214–338

October 34 213–390

2003 January 43 217–297

February 20 290–368 22 213–246 1 212

March 17 280–385 54 180–272

October 7 268–294

November 5 168–173 5 194–215

December 3 277–286

2004 February 10 148–204

April 29 167–258 26 180–295

May 4 198–204 5 169–178

June 5 188–197

July 7 226–264

November 7 257–282

December 19 253–298 4 226–255

Fish Sci

123

3. Mature phase (M): oocytes at the migratory nucleus or

mature stages are present.

4. Spawning phase (Spa): yolked oocytes and postovula-

tory follicles are present.

5. Spent phase (Spe): all yolked oocytes are in early

atretic stage.

6. Resting phase (R): late atretic stage oocytes and non-

yolked oocytes are present.

Sexual maturity

Size at maturity estimates were based on the histological

observations of a total of 412 females (L 148–385 mm)

collected from February to May. Sexually mature indi-

viduals were defined as females with ovaries in the

developing, mature or spawning phases. To estimate the L

at 50 % maturity (L50), a logistic function was fitted using

the generalized linear model (GLM) assuming binomial

error. Calculation of the GLM was carried out using GLM

procedure by statistical software of R (ver. 2.11.1; The R

Project for Statistical Computing, Vienna, Austria; avail-

able at: http://www.r-project.org/). The logistic equation

was

YL ¼1

1þ exp a� Lð Þ=bf g

where YL is the percentage mature at L, a is the L50, L is the

fork length (mm), and b is the slope.

Batch fecundity

Batch fecundity (BF) was estimated based on the number

of hydrated oocytes in the ovary according to the gravi-

metric method [16]. Females that had both new postovu-

latory follicles and hydrated oocytes at the same time were

not appropriate for batch fecundity estimation because such

females were considered to have released some part of a

batch of eggs before the time of capture. These specimens

were excluded from the batch fecundity estimation. In

total, 16 females collected from the northern Kyushu area

in February of 2001 and 2002 were available for the esti-

mation of the batch fecundity.

Results

Marginal increments

A total of 636 specimens were examined; in 41 (6.4 %)

specimens the rings could not be identified as rings or there

was discrepancy among the triplicate measurements. The

remaining 595 otoliths (93.6 %) showed distinct ring

marks (Fig. 2) and were used for the growth analysis. The

IAPE of 525 individuals (excluding for the L = 0 ring

otoliths) was 4.1 %. In specimens having only one ring

mark on the otolith, the MIs remained at a low level from

February to July (Fig. 3); in contrast, in specimens that had

otoliths with more than two ring marks, the MI decreased

rapidly from February to March. These results suggest that

ring marks form once per year, between February and

March. Ages were assigned to each fish according to the

number of ring marks (annuli) on the otolith, assuming a

hatch date of 1 March, which approximately corresponds to

the middle of the spawning season.

CC

Distal

Proximal

Ven

tral

Dor

sal

Fig. 2 Section of otolith of jack mackerel (Trachurus japonicus) with

four ring marks. Black arrows Ring marks, C core

0

0.1

0.2

0.3

0.4

0.5

MI

4 55 1810

16

1514

11

17(a)

0

0.2

0.4

0.6

0.8

1

1 2 3 4 5 6 7 8 9 10 11 12

MI

month

1639

47

38

(b)

44

6522

16

31 24 23

Fig. 3 Monthly changes in marginal increment (MI) of jack mackerel

otoliths by ring group. a one-ring group, b two- to six-ring group.

Vertical bars Standard error (SE). Number beside each data point

Number of fish examined each month

Fish Sci

123

Age and growth

To describe the growth, we fitted the von Bertalanffy growth

equation. As there were no significant differences in the

growth of males and females (F = 0.33, p [ 0.05), all the

individuals were pooled for the estimation of the growth

model. The estimated maximum age for males and females

was 6 years. The growth model determined from the best fit

of all observed L and the age (t) data (Fig. 4) were:

Lt ¼ 401 1� exp �0:275 t þ 1:149ð Þð Þf g 0:8\ t \6:9ð Þ:

Annual reproductive cycle

The mean GSI value in female jack mackerel remained at a

high level (GSI[2.0) from February to May. The maximum

mean value was 3.95 and was determined for females caught in

February and April. After May, the mean GSI value decreased

and remained low between June and October (Fig. 5).

Immature females were observed throughout the entire

year (Fig. 6), and females with developing ovaries occur-

red all year round except for August and September.

Females with ovaries in the mature phase were collected

during the period February–June, and those in the spawn-

ing phase were collected in the period November–May

except for January. Females with spent ovaries were col-

lected in all months with the exception of March, August,

September, and November, and those with resting ovaries

were found in all months except during the active spawning

season (February, March, May).

Size at female maturation

The minimum size at sexual maturity was 171 mm L. The

minimum size at specimens with mature stage oocytes was

180 mm L. The logistic equation was determined as follows:

YL ¼1

1þ exp 206:8� Lð Þ=18:4f g :

The L50 was found to be 206.8 mm, and all females with

L [300 mm were mature (Fig. 7).

Batch fecundity

Batch fecundity ranged from 5,738 to 102,469 eggs in fish

with an L of 180–379 mm. The equation of the relationship

between BF and BW is given by the equation: BF ¼7626:1� e0:0037BW 75\ BW \771ð Þ (Fig. 8).

Discussion

We have demonstrated that concentric annual rings are

formed in the sectioned otoliths of jack mackerel T.

Age0 1 2 3 4 5 6 7 8

For

k le

ngth

(m

m)

100

150

200

250

300

350

400

450

unidentified malefemale Growth Curve

Fig. 4 von Bertalanffy growth curves fitted to observed fork length

(L) and age (t) data for male (gray circles), female (inverted

triangles), and unidentified (black circles) jack mackerel specimens

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12

GS

I

month

55

175

87

106

44

2749

3155

51

34

26

Fig. 5 Monthly changes in the mean gonadosomatic index (GSI) for

female jack mackerel. Vertical bars SE. Number beside each data

point Number of fish examined each month

1

0.8

0.6en

cyR

Spe

0.4

Fre

qu Spa

M

0.2 D

01 2 3 4 5 6 7 8 9 10 11 12

Im

month

55 175 87 106 44 27 49 31 55 51 34 26

Fig. 6 Monthly changes in the frequency of occurrence of various

maturity stages of ovaries of jack mackerel. D Developing stage, Im

immature stage, M mature stage, R resting stage, Spa spawning stage,

Spe spent stage

Fish Sci

123

japonicus. The otoliths are characterized by thin opaque

zones and wider translucent zones, and the annuli of the

otolith sections are distinct and easy to interpret. This is the

first study to use sectioned otoliths from jack mackerel

specimens collected in the ECS and adjacent waters for age

determination. Several studies have pointed out that

transverse otolith sections are generally more accurate for

age estimates than those based on the whole otolith [17,

18]. Nishida and Hasegawa reported that it was difficult to

distinguish the annuli of the whole otolith in jack mackerel

aged [3 years [19]. In a study on the Japanese flounder

Paralichthys olivaceus, Masuda and Noro found that the

whole-otolith method was valid in male fish aged\4 years

and in female fish aged\5 years, but that it underestimated

age in older fish [18]. In our study, we distinguished six

annuli; thus, the sectioned otolith method would be a more

suitable method for all age ranges in jack mackerel. The

annulus formation season for jack mackerel in the waters of

the ECS was from February to March, which corresponds

to the spawning period of this species. We have demon-

strated there was no differences in growth between males

and females, which agrees with the results of a previous

study conducted in the central part of the Japan Sea [19].

The growth of jack mackerel at age 1 year estimated in

our study is similar to that reported in previous studies

conducted in Nagasaki and Wakayama prefectures [20, 21]

(Table 3). For jack mackerel aged[2 years, our estimate is

similar to that reported by Nakashima [4] for specimens

collected in the central ECS and northern Kyushu. In that

study, Nakashima [4] categorized the estimates of the

growth of jack mackerel in the ECS using whole otoliths

for age determination into three groups, namely, the

‘Central ECS’, ‘Northern Kyushu,’ and ‘Southern ECS’

groups, based on the appearance of the otoliths [4]. How-

ever, it is difficult to compare the results of this study with

those of our study as data on the locations where the

specimens were caught were not provided in the earlier

study. We have assumed that our sampling locations were

mainly the ‘Central ECS’ and ‘Northern Kyushu’ [22]. The

results from the earlier study suggest that the ‘Southern

ECS’ group was remarkably delayed in growth compared

with the other two groups (‘Central ECS’ and ‘Northern

Kyushu’) (Table 3); on the other hand, such differences are

commonly found when using the von Bertalanffy curve as

this curve is strongly determined by the values for L.inf and

t0, which are at the extreme points of the curve and usually

have the least data [23]. In our study, we used data aver-

aged across multiple locations to determine the growth of

jack mackerel. We strongly suggest that in future studies

on differences in the growth of fishes between different

areas sufficient data across an entire population should be

collected. On the other hand, the growth of marine fishes is

known to be variable and affected by density and/or

environmental factors, such as water temperature [24].

Growth-selective survival and hatch date in young jack

mackerel depend upon the temperature experienced during

150 200 250 300 350

0.0

0.2

0.4

0.6

0.8

1.0

Fork Length (mm)

Pro

port

ion

of m

atur

ity

Fig. 7 Frequency distribution of mature female jack mackerel

collected from February to May

Body Weight (g)0 100 200 300 400 500 600 700 800 900

Bat

ch F

ecun

dity

(x

103 )

0

20

40

60

80

100

120

Fig. 8 Relationship between batch fecundity and body weight of

female jack mackerel

Table 3 Estimates of fork length by age for jack mackerel in waters

around Japan

Sampling area Estimated L (mm) at age (years): References

1 2 3 4 5 6

East China Sea 179 233 272 303 326 345 Present

study

Northern

Kyushu

166 231 277 308 332 [4]

Central ECS 170 233 279 312 337 355

South ECS 154 193 223 246

Wakayama 187 252 296 325 344 356 [20]

Nagasaki 180 247 293 324 [21]

Niigata 148 206 249 282 [19]

Fish Sci

123

the growth season [25]. Additional studies are needed to

clarify the effects of density-dependent and environmental

factors on the growth of adults.

Our findings clearly show that jack mackerel in ECS

spawned mainly during the period February–May. Previous

studies conducted divided spawning jack mackerel into

four groups, namely, the ‘winter spawner,’ ‘spring spaw-

ner,’ ‘coldest season spawner,’ and ‘summer spawner’ [5],

with the peak of the spawning period being different for

each inhabited area. In our study, no mature specimens

were collected during the summer, and we therefore did not

detect the ‘summer spawner’ group, possibly due to the

restricted area of sampling and low number of specimens

collected. In contrast, as the catch level of jack mackerel in

the 1960s was much higher than that in the post-2000

years, the stock biomass can be assumed to have been

higher in the 1960s than in recent decades. Generally,

shrinkage of the distribution area and faster growth are

observed at lower levels of fisheries stock (e.g. [26]). The

extent of plasticity of life-history traits in jack mackerel

remains unclear, and the difference in biomass may be one

of the factors influencing the difference in the spawning

season.

Recent ichthyoplankton sampling surveys have shown

that one of the main spawning grounds of jack mackerel is

in the shelf-break region of the southern part of the ECS

south of 28�N during the period February–March, as the

just-hatched larvae were found in this region in abundance

[27, 28]. However, individuals with gonads in a sexually

mature phase were found during the period December–May

in the southern ECS (Yoda et al., unpublished data), sug-

gesting that the spawning period of this fish is not restricted

to a short period. Although additional research on the

estimate of total number of eggs produced and offspring

traits are needed to gain an understanding of the mecha-

nism of recruitment fluctuation in jack mackerel, the pro-

tracted spawning season observed here may be a robust

spawning strategy for environmental fluctuation.

The previously reported size at first maturity for female

jack mackerel in the ECS was 18.5–20.7 cm at age 2 years

[5]. Our data demonstrate that the minimum size at matu-

rity is 171 mm L and this was assumed for 1-year-old fish.

The size and age at first maturity was smaller and earlier,

respectively, than reported earlier for jack mackerel in the

ECS. In many temperate fish species, maturation normally

proceeds within a species-specific range of ambient tem-

perature [29, 30]. In captivity, jack mackerel did not

mature under 19 �C at age 1 year; on the other hand, at an

ambient temperature of [18 �C, fish aged 3–4 years had

atretic oocytes [31, 32]. A rise of sea surface temperature

(SST) in the East China Sea has been observed over a

100-year period, and the rates of SST increase in the seas

around Japan are higher than the global ocean average and

that for the western North Pacific (Japan Meteorological

Agency WEB: http://www.data.kishou.go.jp/). This

increased SST may be one explanation for our results of

earlier maturation and faster growth compared to the

1950s–1960s.

Our estimate of batch fecundity (5,738–102,469

oocytes) was smaller than that (184,292 oocytes) of a

previous study [33]; the size range of specimens in these

two studies also differed but to a lesser degree

(180–379 mm L vs. 220–400 mm L, respectively). Because

the difference of size between the two studies was not

great, we consider that the differences in batch fecundity

estimates is due to the difference in methodology. Chigi-

rinskiy [33] treated tertiary yolk stage oocytes as a first

batch. The jack mackerel has an ovary with asynchronous

oocyte maturation [34, 35]; thus if they spawned at one

time, tertiary yolk stage oocytes would still remain in the

ovary. We suspect that the batch fecundity of jack mack-

erel determined in past studies was an overestimation.

Although in indeterminate spawning species, annual

fecundity should be estimated from the number of oocytes

per spawn (batch fecundity), the percentage of females

spawning per day (spawning frequency), and the duration

of the spawning season, additional research on the estimate

of total number of eggs produced and offspring traits is

needed.

In conclusion, the results of our study show that jack

mackerel starts to spawn earlier (at age 1 year) and to grow

faster than previously reported for this species in the ECS.

Based on our findings, the spawning season in the ECS is

February–May, which corresponds with earlier reports. The

ECS is an internationally exploited common fishing

ground, and under these circumstances, monitoring bio-

logical characteristic is a simple and effective way for

detecting any potential collapse of stock (e. g. [26]). We

hope that our findings will provide a basis for future

research on T. japonicus as it is an internationally exploited

resource.

Acknowledgments We thank Dr. T. Sakai, Fisheries Research

Agency, for his valuable suggestions on the manuscript. This work

was supported by the Fisheries Agency, Government of Japan.

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