Stock Assessment of the Blue Swimming Crab (Portunus pelagicus) in East Java during 2016-2020

Bogor, 15 September 2020

Asosiasi Pengelolaan Rajungan Indonesia Indonesian Blue Swimming Crab Asosiation

Objective

This research aims to examine the population dynamics and stock status of the blue

swimming crabs (Portunus pelagicus) in East Java during 2016-2020 based on the

growth parameters, exploitation rates, and spawning potential ratio.

Outline

Sampling data

Carapace width composition

% Undersized and % Egg Berried Female

Growth parameters

Mortality and exploitation rates

Width at first maturity and width at first capture

Spawning Potential Ratio (SPR)

Table 1 Number of blue swimming crabs (Portunus pelagicus) samples in East Java

during 2016-2020

Sampling Data

No Site Period

2016 2017 2018 2019 2020

1 Pamekasan (4716)

Jul-Dec

(9262)

Jan-Jun

(7524)

Jan-Dec

(10875)

Jan-Dec

(9755)

Jan-Aug

2 Sumenep (4296)

Jul-Dec

(4591)

Jan-Apr

(4053)

Aug-Dec

(3877)

Jan-Jun

3 Gresik (5300)

Jan-Dec

(9342)

Jan-Dec

(4512)

Feb-Aug

Total 9012

(Jul-Dec)

13853 (Jan-Jun)

12824 (Jan-Dec)

24270 (Jan-Dec)

18144 (Jan-Aug)

Table 2 Width composition of male and female of blue swimming crabs in East Java

during 2016-2020

Carapace Width Composition

Year Site

Male (mm) Female (mm)

n min

size

max

size

mean±sd n min

size

max

size

mean±sd

2016 Pamekasan 2000 80 155 117.2 ± 11.7 2716 80 157 117.3 ± 10.5

Sumenep 2476 77 146 109.7 ± 10.4 1820 72 159 112.5 ± 10.8

2017 Pamekasan 4493 88 159 113.2 ± 9.9 4767 89 158 113.2 ± 9.6

Sumenep 2659 86 161 108.8 ± 11.3 1932 85 161 108.4 ± 12.0

2018 Pamekasan 3579 72.4 172 118.3 ± 12.2 3939 72 172.2 119.3 ± 11.6

Gresik 4243 86 148.1 107.7 ± 8.4 1057 88 148 110.0 ± 8.2

2019 Pamekasan 4305 75 172 118.3 ± 11.9 6563 72 170 119.9 ± 11.6

Sumenep 1145 74 169 121.2 ± 14.6 2906 72 164 123.9 ± 14.0

Gresik 7354 83 167.1 109.2 ± 11.0 1988 85.2 160.2 110.1 ± 11.1

2020 Pamekasan 3771 72 157 115.9 ± 12.6 5984 72 165 117.5 ± 11.7

Sumenep 1269 85 150 113.8 ± 10.8 2608 86 160 113.0 ± 10.8

Gresik 2126 85 162 117.7 ± 12.1 2386 81 163.2 116.0 ± 11.8

0

10

20

30

40

50

Pam

ekas

an

Sum

enep

Pam

ekas

an

Sum

enep

Pam

ekas

an

Gre

sik

Pam

ekas

an

Sum

enep

Gre

sik

Pam

ekas

an

Sum

enep

Gre

sik

% E

BF

The percentage of minimum legal size (MLS) and egg berried female

(EBF) of the blue swimming crabs in East Java during 2016-2020

% Undersized Crabs

(B)

2017 2018

2019 2020

0

10

20

30

40

Pam

ekas

an

Sum

enep

Pam

ekas

an

Sum

enep

Pam

ekas

an

Gre

sik

Pam

ekas

an

Sum

enep

Gre

sik

Pam

ekas

an

Sum

enep

Gre

sik

% M

LS

(A)

2017 2018 2019 2020

2016

% Egg Berried Female

7.0 8.1 6.7

9.4

5.8

2016 2017 2018 2019 2020

5.2 4.9 7.2

14.7 13.5

2016 2017 2018 2019 2020

2016

Growth Parameters

Growth of the blue swimming crabs by monthly carapace width distribution in

East Java during 2016-2020

Length infinity (L∞) = 179.55 mm

Growth rate per year (K) = 1.10 per year

Maximum age (tmax) = 2.639 year

Mortality and Exploitation Rates

Z = Total Mortality, M = Natural Mortality (Hoenig 1983), F = Fishing Mortality, E = Exploitation Rate

The E value >0.5 indicates the stock

status of BSC in East Java has

suffered over exploitation

The mean estimates of F/M from

East Java are all very high ≈ 4.

2017 2018 2019 2020

E=0.70 E=0.69 E=0.76 E=0.75

The estimated relative fishing mortality (F/M) from

the three sites in East Java during 2016-2020

2016

E=0.78

0

0.25

0.5

0.75

1

50 75 100 125 150 175

Pro

po

rtio

n

Mean Carapace Width (mm)

Width at First Maturity (Lm) and

Width at First Capture (Lc)

The maturity curve (orange) for BSC

fishery at East Java is positioned well to

the LEFT of the generic selectivity curve

(blue), indicating that the operation of the

fishery enables almost all BSC to mature

and spawn before entering fishery.

The mean carapace width (mm) at first capture (Lc) and mean carapace width at first maturity (Lm) of female BSC in East Java during 2016-2020

Lm

Lm 50 = 98.1 mm Lc 50 = 115.1 mm

Lc > Lm

Lc

Spawning Potential Ratio (SPR)

The SPR values of BSC in East Java are mostly

above the limit reference point of recruitment

level at SPR 0.2, which is required to stabilize

the population and the sustainability.

Gresik has the lowest estimates of SPR.

2017 2018 2019 2020

The estimated spawning potential ratio

(SPR) from the three sites in East Java

during 2016-2020

2016 2014/2015

24%

SPR 0.2

Catch Per Unit of Effort (CPUE)

0

2000

4000

6000

8000

10000

12000

75

81

87

93

99

105

111

117

123

129

135

141

147

153

159

165

171

Num

be

r o

f C

rab

s

Carapace Width (mm)

Bubu

Jaring

The size distribution of the BSC catch from the different fishing gears in East Java during 2016-2020

5.61

0

5

10

15

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 2016ave.

CP

UE

(kg/

trip

)

11.13

0

5

10

15

20

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 2017ave.

CP

UE

(kg/

trip

)

2.17

0

5

10

15

20

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 2018ave.

CP

UE

(kg/

trip

)

3.03

0

5

10

15

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 2019ave.

CP

UE

(kg/

trip

)

4.03

0

2

4

6

8

10

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec 2020ave.

CP

UE

(kg/

trip

)

Temporal abundance of blue swimming crab (Portunus

pelagicus) in East Java during 2016-2020

(n= 52172)

(n= 25881) Standard Effort=

Bubu (Trap)

∑ Catch (kg)

∑ E (trip)

CPUE (kg/trip)

FPI

Bubu 19339 5015 3.86 1.00

Jaring 3164 898 3.52 0.91

y = -0.0006x + 5.852 R² = 0.0678

0

4

8

12

0 1000 2000 3000 4000

CP

UE

(kg

/tri

p)

Effort (trip)

0

4000

8000

12000

16000

20000

2016 2017 2018 2019 2020

Cat

ch (

kg)

Year

Conclusion

1. The mean percentages of undersized blue swimming crabs (BSC) in East Java were all less than <10%, while the percentages of

the berried female were increasing over year.

2. The exploitation rates were higher than 0.5, implying the BSC stock had been over exploited.

3. The mean carapace width at first capture (Lc) was higher than the mean carapace width at first maturity (Lm), illustrating the

BSC was captured after mature and spawn.

4. The estimated spawning potential ratio (SPR) of BSC in East Java mostly occurred at greater than 0.2 (0.24 for Pamekasan, 0.22

for Sumenep, and 0.19 for Gresik). These values indicate that existing BSC fishery in Central Java was operating at above limit

reference point, however the appropriate measures are still needed to increase SPR at 0.3.

5. Bubu is the most effective fishing gears.

6. Fishing efforts continue to increase over year, however the productivity in terms of catch per unit efforts has been declined.

Kesimpulan

1. Rata-rata persentase jumlah rajungan berukuran kecil di Jawa Timur kurang dari 10%, sedangkan persentase rajungan

betina bertelur mengalami peningkatan dari tahun ke tahun.

2. Laju eksploitasi rajungan lebih dari 0.5 mengindikasikan bahwa sumber daya rajungan di Jawa Timur telah mengalami over

eksploitasi.

3. Lebar karapas rajungan pertama kali tertangkap (Lc) lebih tinggi dari lebar karapas pertama kali matang gonad (Lm)

menggambarkan kondisi rajungan yang tertangkap sudah dewasa dan memijah.

4. Hasil estimasi rasio potensial pemijahan (SPR) rajungan di Jawa Timur lebih dari 0.2 (0.24 untuk Pamekasan, 0.22 untuk

Sumenep, dan 0.19 untuk Gresik). Hal ini mengindikasikan perikanan rajungan di Jawa Timur telah memenuhi ambang

batas aman untuk rekruitmen, namun langkah-langkah strategis tetap dibutuhkan untuk meningkatkan SPR menjadi 0.3.

5. Alat tangkap Bubu lebih efektif dan selektif menangkap rajungan dibandingkan dengan alat tangkap lainnya

6. Jumlah usaha tangkapan meningkat namun produktivitas (catch per unit effort-CPUE) mengalami penurunan.

Terima Kasih