125

Stock Assessment and ABC Calculation for Japanese Sardine(Sardinops Melanostictus) in the Northwestern Pacific underJapanese TAC System
Hiroshi NISHIDANational Research Institute of Fisheries Science, Fisheries Research Agency,
2-12-4 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japane-mail: hnishi@affrc.go.jp

Abstract
Stock assessment for the Japanese sardine (Sardinops melanostictus) in the northwestern Pacific
was conducted based on VPA and biomass surveys independent from commercial fisheries data. The
stock size of Japanese sardines in the northwestern Pacific has shown a continuous decrease from 1987
to 2003 and the stock biomass estimate for 2003 was about 130 thousand tons. The spawning success(RPS: number of recruits/spawning biomass) has been 22 inds./kg in average and fluctuated between 5
and 61 inds./kg during the recent 10 years. From the historical perspective, the current stock status
would not be in the conditions for rapid stock recovery. The acceptable biological catch (ABC), that isthe biological criterion for the TAC, is set to rebuild the spawning biomass at least to the level at which
the recruits are expected to be comparable with those of 1996 year-class. Some hypothetical scenarios
for controlling the catch amount were explored, in order to avoid intensive fishing efforts on dominant
recruits.
Key words: acceptable biological catch, Japanese sardine, Sardinops melanostictus, spawning success,virtual population analysis

1. Japanese TAC (Total Allowable Catch)
SystemAn entry limitation approach, which restricts fishing efforts, such as the number of vessels, period orarea of fishing operations, has been traditionally employed in the Japanese fisheries management system.
However, with the ratification of the United Nations
Convention on the Law of the Sea, an approach tocontrolling catch amounts directly by setting a total
allowable catch (TAC) was additionally introduced
into the Japanese fisheries management system in
1997.
TACs have been set for eight marine captured
species in the Japan EEZ: the Japanese sardine(Sardinops melanostictus), jack mackerel (Trachurusjaponicus), chub mackerel (Scomber japonicus), spotted mackerel (Scomber australasicus), Pacific saury(Cololabissaira),
walleyepollock(Theragra
chalcogrammus), Japanese common squid (Todarodes
pacificus), and snow crab (Chinoesetes opilio). TACs
for the chub mackerel and spotted mackerel are combined, with a common TAC set for both mackerels
Global Environmental Research9(2)/2005: 125-129
printed in Japanbecause they are not separated in catch statistics.
Under the TAC system, the National Fisheries
Research Institutes carry out stock assessments andABC calculations for 19 stocks of TAC species in theJapanese EEZ.

2. Distribution and Current Status of the
Pacific Stock of Japanese Sardine
The spawning area of the Pacific stock of the Japanese sardine covers a wide stretch of the waters off
Honshu and Shikoku islands. Juveniles are broadly
distributed in the Kuroshio/Oyashio Transition Zone
during spring as far east as 180° longitude (Fig.1,
Kinoshita, 1999). The distribution of adult sardines
extends to the central Pacific and the southern areas of
the Okhotsk Sea and Western Subarctic Gyre during
periods of high abundance.
The Japanese catch of the Japanese sardine peakedin 1936 and 1988 and fell to record lows in 1965 and2004. Watanabe et al. (1995) attributed the collapse of
the stock in the late 1980s to successive recruitment
failures caused by extremely low survival after the©2005 AIRIES126

H. NISHIDA

Fig. 1 Distribution and migration of the Pacific stock of the Japanesesardine. The spawning area of the Pacific stock of the Japanesesardine covers a wide stretch of the waters off Honshu and Shikoku
islands. Juveniles are broadly distributed in the Kuroshio/Oyashio
Transition Zone during spring. The distribution of adult sardines
extends to the central Pacific and to the southern areas of the Okhotsk
Sea and Western Subarctic Gyre.

feeding larval stage and before recruitment to fisheries
at age 1. Recruitment anomalies were negatively correlated with SSTs in the Kuroshio Extension (Noto &Yasuda, 2003). The SST anomalies were preceded byshifts in the mixed-layer depth in the Kuroshio Extension three years earlier (Yasuda et al., 2000). The winter mixed-layer depth in the Kuroshio Extension between 1967 and 1985 was deeper than the long-term
mean. Thus, sardine dynamics were associated withthe 1970 and 1988 shifts in the SST field (Yasunaka &Hanawa, 2002). In 1970 in particular, when SSTs in
the Kuroshio Extension were below average, zooplankton biomass in the Oyashio waters also increased(Odate, 1994), which may have been caused by intensified vertical mixing in winter associated with windstress (Sugimoto & Tadokoro, 1997). Increased production in the lower trophic levels in the Oyashio region may have enhanced the early growth and survivalof sardines.
Despite poor recruitment, fishing mortality of theJapanese sardine has increased drastically since theearly 1990s owing to strong incentives to continue
fishing for sardines (the landing price of sardines increased relative to the more abundant Japanese anchovy) (Yatsu et al., 2003).

3. Biomass Estimation for the Pacific Stockof the Japanese Sardine
Stockassessment

for

the

Japanesesardine(Sardinops melanostictus) was conducted based on a
virtual population anlysis (VPA). An age-structured
stock assessment was applied to fishery-dependentand fishery- independent biomass surveys to derive
estimates of population abundance and age-specificfishing mortality rates. Fishery-independent data include egg abundance as an index of spawners abundance, juvenile and 1-year-old fish biomass survey.
At first, for the estimate of age-specific selectivity,
VPA using Pope’s (1972) equation, was conductedbased only on catch at each age. Age was classified
ranging from 0 to 5 years and over. The monthly catchat each age was estimated from catch-by-size datausing the monthly age-length key. The yearly total
catch at each age from 1976 to 2003 was used for
VPA.
Na,y=Na+1,y+1 exp(M) + Ca,y exp(M/2)
Na,2003=Ca,2003 exp(M/2) / (1−exp(−Fa,2003))(the most recent year, in this case y =2003)
N4,y=(C4,yN5+,y+1 exp(M) / (C5+,y + C4,y)(4-year-old fish)+ C4,y exp(M/2)
N5+,y=(C5+,yN5+,y+1 exp(M) / (C5+,y + C4,y)(5-year-and-over fish)+ C5+,y exp(M/2)
Fa,y= −ln(1−(Ca,y exp(M/2) / Na,y))Stock Assessment and ABC Calculation for Japanese Sardine

Fa,2003=average value between Fa,1998 and Fa,2002(most recent year, in this case y=2003)

127

100,000

F5+,y=F4,y(4-year-old and 5-years-and-over fish)

Where N and C represent the number of fish andcatch, respectively, a their age, y the year, F the fishing mortality coefficient and M the natural mortality
coefficient (=0.4/year in the case of the Japanese sardine). F for the 5-years-and-over (5+) is assumed
equal to that of 4-year-old fish. The average value of F
between 1998 and 2002 was used for the 0-4 years-old
fish in the most recent year 2003. F for the age group
5+ in 2003 was set equal to F for the 4-year-old fish in
2003. Based on this calculation, age-specific selectivity was estimated and this result was used in the next
step as follows.
The next step, Fa,2003 was derived by minimizing∑i∑y{ln(Ii,y) – ln(qiPi,y) }2
The terms in the objective function (to be minimized) included the sum of squared differences in theobserved and predicted estimates from the catch ateach age and various sources of data used for tuning
VPA. I represents each fishery-independent abundanceindex and effective fisheries effort data, as follow; i,
the index number; y, the year q; the proportional coefficient; and P, the predicted estimates linked to eachindex.
Each index is as follows.
(1) Egg abundance during 1978 to 2003 were used todevelop the following time series of spawning
stock biomass. These data were derived fromNORPAC-net egg surveys performed by the Prefectural Fisheries Experimental Stations andNational Fisheries Research Institutes.
(2) Effective fisheries effort data (Japanese Fisheries
Information Center) during 1994 to 2003 wereused to develop the time series of F-bar (averaged F for all age group)(3) Fishery independent 1-year-old abundance survey data during 2002 and 2004 was used to develop a time series of the stock number of
1-year-old fish. These data were derived fromsurface-midwater trawl survey data in January(wintering area for young Japanese sardines).
Among these indices, (2) was double-weighted, theother two indices were single weighted. So, in thiscase, Fa,2003 was derived by minimizing,∑y{ln(I1,y) − ln(q1P1,y) }2 +2∑y{ln(I2,y) − ln(q2P2,y) }2+ ∑y{ln(I３,y) − ln(q３P３,y) }2Biomass (103 tons)

10,000

F5+,2003=average value between F4,1998 and F4,2002(5-years-and-over fish in 2003)

1,000
100
10
1975

1980

1985

1990

1995

2000

Fig. 2 Total stock biomass and spawning biomass
estimated for the Pacific stock of the Japanesesardine between 1976 and 2003. Blue line shows
the total biomass, and red line shows the spawning
biomass, respectively. The stock size of theJapanese sardine has shown a continuous decrease
from 1987 to 2003 and the stock biomass
estimated in 2003 was 130 thousand tons.

Total biomass and spawning stock biomass estimates based on the VPA are shown in Fig.2. The stock
size of the Japanese sardine in the northwestern
Pacific has shown a continuous decrease from 1988
(1,950 thousand tons) to 2003 with yearly fluctuations,and the stock biomass estimate in 2003 was about 130
thousand tons. Stock biomass estimates from the assessment conducted in 2004 have indicated the sardine
population has remained at a lower abundance level
than 1980s.
4. Spawning Success (Number of Recruits/
Spawning Biomass) of the Pacific Stock of The
Japanese Sardine
For calculating spawning biomass, maturation ofeach age group was estimated as follows. Maturation
rate of age-zero is 0; age-one, 0.1-0.2 during 1976 and1996 (Wada & Kinoshita, unpublished data), 0.5 during 1997 and 2003; and ages two to five and over, 1.
The relationship between recruits and the spawning
biomass is shown in Fig.3. It is clear that successive
poor spawning success between 1988 and 1991 caused
the collapse of this stock (Fig.3, left panel). In the past
10 years, the spawning success (RPS: number of
recruits/spawning biomass) has been 22 inds./kg inaverage and fluctuated between 5 and 61 inds./kg(Fig.3, right panel). This value is nearly as the samelevel as the early 1980s, so the current stock status isconsidered in the condition for stock rebuilding. In theearly 1970s, the RPS level was 100-1,000 inds./kg(Wada & Jacobson 1998), and it was an order ofmagnitude higher than in 1980s and 1990s. Considering the historical perspective, it would appear thatthe current stock status would not be in the condition
for rapid stock recovery.

350,000
300,000
250,000
200,000

1980

150,000
100,000
50,000
0

1988
0

5,000

10,00015,000Spawning biomass (103 tons)

Recruit No. (106 inds.)

H. NISHIDA

Recruit No. (106 inds.)

128

16,00014,00012,00010,000
8,000
6,000
4,000
2,000
0

1996

1993

2000

0

100

200

300

400

500

600

Spawning biomass (103 tons)

Fig. 3 The relationship beween spawning biomass and recruits. Successive poor spawning success between 1988 and1991 caused collapse of this stock (left panel). The spawning success (RPS: number of recruits/spawning
biomass) has been 22 inds./kg in average and fluctuated between 5 and 61 inds./kg during the recent ten years.

5. ABC Calculation for the Pacific Stock ofJapanese Sardine
To set the ABC for 2004 based on the stock status
in 2003, it is necessary to forecast stock estimates foreach year after 2004. Stock numbers for each age
group in 2004 were calculated as follows.
N0,y=SBy SSy
Na,y=Na-1,y-1 exp{− (Fa-1,y-1+M)}
Ca,y=Na,y{1− exp(−Fa,y)}exp(−M/2)
SBy=∑aMa,yNa,ySB and SS represent spawning biomass and spawning success (recruits per spawning biomass), respectively. M represents the maturation rate for each age.
Actually, for SS and F, the average value between
1999-2003 was adopted.
In the present TAC system, ABC levels are set tomaintain or rebuild stocks to a ‘suitable yield level
realized under the suitable control rule.’ Since 2000,
Japanese managers have set TAC levels in accordancewith harvest control rules based on precautionary andrisk averse approaches. The basis for this rule wasdiscussions in the United Nations, especially in FAO(Food and Agriculture Organization). Japanese scientists and managers examined case studies in the
United States and the North Atlantic areas, and since2000 have adopted the general harvest control rule
(ABC-rule) for each stock of TAC species. Figure 4
shows the ABC-rule which has been adopted since2004 (Fisheries Agency and Fisheries Research
Agency of Japan, 2004). In the ABC-rule, B limit, F
limit and B ban have to be set for each stock. When astock biomass is below the B limit, the stock is assumed to be overfished. In this case, F limit is thefishing mortality rate for stock rebuilding. When fishing mortality exceeds the F limit, overfishing is considered to be occurring and reduction of the current F

Fig. 4 General harvest control rule (ABC-rule) for eachstock of TAC species. In this rule, B limit, F limit
and B ban have to be set for each stock. When astock biomass is below the B limit, the stock is
assumed to be overfished. In this case, F limit is thefishing mortality rate for stock rebuilding. Whenfishing mortality exceeds the F limit, overfishing is
considered to be occurring and reduction of thecurrent F might be proposed. The B ban is theminimum biomass open to fishing and thresholdwhere fishery might be prohibited.

might be proposed. The B ban is the minimum biomass open to fishing and threshold where fishery
might be prohibited.
In the case of the Pacific stock of the Japanesesardine, the minimum spawning biomass for abundant
recruits was considered to be that of 1996 in a decadeof low stock abundance. The acceptable biologicalcatch (ABC), that is the biological criterion for the
TAC, is set to rebuild the spawning biomass at least to
the level at which the recruits are expected to be comparable with those of 1996 year-class. Based on such astock rebuilding idea, the ABC proposal for 2005 was
25,000 tons (‘ABC limit’ in Nishida et al., 2004) .
Here, B limit was set to the spawning biomass in 1996
and F limit was set to rebuild the stock to B limit levelin five years.

Stock Assessment and ABC Calculation for Japanese Sardine

Na,y=Ｎa-1,y-1 exp(− M) − CL a-1,y-1
N0,y=SBy SSy
CL represents catch amount limit adopted in the
hypothetical scenario. SB and SS represent spawning
biomass and spawning success (recruits per spawning
biomass), respectively. In this hypothetical scenario,
simulations were done with observed annual SS values (i.e., where the density-dependent effect was
ignored due to reduced abundance. See Fig.3) .
Figure 5 shows the observed trajectory of the stock
biomass and catch of the Pacific stock of the Japanesesardine under two types of the hypothetical control
scenario. The upper panel shows the case of the limitation of total catch set to 180 thousand tons since1997 for avoiding intensive fishing efforts on relatively dominant year-classes in 1996. 180 thousand
tons is the same as the landing in 1996. The lower
panel shows the case of the limitation of total catch set
to 200 thousand tons between 1999 and 2001, and set
to 100 thousand tons since 2002, as recruit abundance
had dropped to a level lower than before 2000.

800
1,000 tons

The major problem of the present TAC system isthat TACs are sometimes considerably larger thanABCs. Under the present legal system, marine capture
fisheries are managed by the minister and prefectural
governors. Therefore, many management bodies exist
for each stock, particularly if the stock is distributed
widely beyond prefectural jurisdictions. Each management body considers economic and social factors,
which tend to increase each individual TAC allocationand the sum of the TAC allocations for each stock.
A solution of this problem might be to modify thepresent management framework to include all stakeholders, administrators, and scientists who provide
scientific, economic and social information needed formaking decisions and executing management plans.
Under the present system, ABC calculation is conducted based on stock assessments mainly relying on
catch data from two years prior to the TAC year, andso uncertainty in the fluctuation of recruits in the twoyears (ABC calculation year and TAC year) is anothermajor problem in ABC calculation.
Here we propose catch amount control with the
purpose of avoiding intensive fishing efforts on dominant recruits. We examine the performance of two
types of hypothetical scenarios for catch amount control. In this case study, a retrospective analysis wasmade of stock numbers in the past years, so calculation of past stock numbers under catch amount control
was forecasted as follows.

1,000

600
400
200
0
1997

2000

2003

1997

2000

2003

1,000
800
1,000 tons

6. Examination of Some Catch Amount
Scenarios for the Purpose of Avoiding
Intensive Fishing Efforts on Dominant
Recruits

129

600
400
200
0

Fig. 5 The observed trajectory of the stock biomass and catchof the Pacific stock of the Japanese sardine. Red line:
stock biomass under the scenario, blue line: actual
stock biomass, orange line: catch under the scenario,
black line: actual catch.
The upper panel shows the case of the limitation of
total catch set to 180 thousand tons since 1997 foravoiding intensive fishing efforts on relatively dominant year-classes in 1996. 180 thousand tons is thesame as the landing in 1996. The lower panel shows thecase of the limitation of total catch set to 200 thousand
tons between 1999 and 2001, and set to 100 thousand
tons since 2002, as recruit abundance had dropped to alevel lower than before 2000.

In both cases, the speed of the stock biomass decrease was not as fast as the actual. The observed andpredicted trajectory of the stock biomass and catch of
the Pacific stock of the Japanese sardine are shown in
Fig.5. The simulated catch was less than the actual
catch in the first year of controls, however the total
catch after two or more years was higher than the actual catch. It appears that intensive fishing on theJapanese sardine probably prevented recovery of the
population, as was also the case with the Pacific stockof the chub mackerel Scomber japonicus (Kawai et al.,2002).

Acknowledgements
We thank members of the Prefectural Fisheries
Experimental Stations for providing us landing andbody size data for the Japanese sardine.

(Received 5 May 2005, Accepted 30 September 2005)