Maizuru Fisheries Research StationFSERC

Kyoto University

Yoh Yamashita and Yutaka Kurita

Carrying capacity of nursery grounds for Japanese flounder in relation to stocking density

Stock enhancement program in Japan (since 1963)

＊Seventy governmental hatcheries for marine species

＊Fifteen billion juvenile stage animals of 80 marine species are released each year

Japanese flounder (Paralichthys olivaceus)Stocking started in 1977 and 25 million juveniles (4-10 cm TL) are annually released

Key criteria for successful stocking1．Seed quality

2．Genetic diversity

3．Release habitat

4．Release season

5．Fish size at release

6．Release method

7．Management of the released stock

8．Optimum stocking density(in relation to carrying capacity of the habitat)(surplus productivity of habitats available for stocked fish)

At the present state,

2. Stocking density is politically determined by local governments based on

(1) Government policy

(2) Production capacity of hatchery

(3) Request from local fisherman’s union

1. Because it is difficult to scientifically determine the optimum stocking density

* Without any consideration of the carrying capacity of the nursery grounds

Chiba Prefecture(1984 – 1995)

0

5

10

15

20

25

0 50 100 150

Number of Released Flounder (103)

Retu

rn R

ate

(%)

0

2

4

6

8

10

0 50 100 150

Number of Released Flounder (103)

Num

ber o

f Ret

urns

(103 )

*Data are shown as a local operation unit

Miyako Bay, Iwate Prefecture, (1987-2003)

0

5

10

15

20

25

0 50 100 150 200

Number of Released Flounder (103)

Retu

rn R

ate

(%)

0

5

10

15

20

25

0 50 100 150 200

Number of Released Flounder (103)

Num

ber o

f Ret

urns

(103 )

Fukushima Prefecture(1985-2002)

0

5

10

15

20

25

30

35

0 500 1000 1500

Number of Released Flounder (103)

Retu

rn R

ate

(%)

0

50

100

150

200

0 500 1000 1500

Number of Released Flounder (103)

Num

ber o

f Ret

urns

(103 )

2000-, 2001-year class

*Release operations and research on stocking effectiveness have been conducted on a prefecture level

2000-, 2001-year class

The relationship between stocking density and market return rate throughout Japan

0

5

10

15

20

25

30

35

0 500 1000 1500

Number of Released Flounder (103)

Retu

rn R

ate

(%)

0

50

100

150

200

0 500 1000 1500

Number of Released Flounder (103)

Num

ber o

f Ret

urns

(103 ) Fukushima Pref.

2000-, 2001-year class

*Data are from local governments throughout Japan

TaTaEffect

SalEffect

FishEnergy

Sal

AcxWfish

ArxWfish

DOeffect

MMSO

Wfish

Winberg

Mact

~DOobs

Mstd

GEFeed

MysidPopEner

q1 q2

Apc

FeedQual

Acmax

WastedEnergy

AwxWfish

MSgrowth

oxycal

Aa

MysidProd

As

Wwastes

NatMortality

AgScope

pHeffect

Eflux

AdxWfish

CompCons

sda

GEfish

pH

DOa

FeedRate

~pHobs

~Tobs

~Sobs

FishCons

SeedCons

MysidCalInd MysidDensArea

TemEffect

Comp0Energy

Comp0growth

Comp0Grate

Comp0N

Death0Comp1N

Death1

Comp0Ingestion

Comp1Energy

Comp1growth

Comp1Grate

death fraction

Comp1Ingestion

Mysid Mortality

FishNumber

deaths

anchovy

Ta 2TaEffect 2

SalEffect 2

SeedEnergy

Sal 2

AcxWseed

ArxWseed

DOeffect 2

MMSO 2

Wseed

Winberg 2

Mact 2

~DOobs 2

Mstd 2

GEFeed 2

q1 2 q2 2

Apc 2

Acmax 2

WastedEnergy 2

AwxWseed

MSgrowth 2

oxycal 2

Aa 2

As 2

Wwastes 2

AgScope 2

pHeffect 2

Eflux 2

AdxWfish 2

sda 2

GEseed

pH 2

DOa 2

FeedRate 2

~pHobs 2

~Tobs 2

~Sobs 2

Seed Number

TemEffect 2

SeedDeath

Predation

Predated N

Predated

~fWseed WtotalPredation

Wpredator

PredatorIngesRate

EncounterRate

~days14

mysids

Poss Consumption

Stand vs Active Metabolism

poss consumption2500

AcmaxXWseed

MysidDens in N

Total weight

Wild Flounder

Competitor

Predator

Diet

Released Flounder

Wild flounder Releasedflounder

CompetitorGrowth

Abiotic environment (temperature)

PreyMysids production + consumption

Small fishesconsumptionSurvival

Ecophysiological model to evaluate the process of natural biological production system (STELLA)

Application of the subpopulation growth model to Ohno Bay, northeastern Japan

Model Field• Ohno Bay (39 ºN, 142 ºE)• Area

500m x 500m (Depth: 0 – 10m)

• 76,000 hatchery-juveniles(mean TL; 8.9 cm)

• Sep. 5 – Nov. 9, 1989 (65 days)

• Temperature20.1 ºC → 14.8 ºC

• Salinity 31.5 – 32.8 ppt

• DO > 6 ppm

Comparison of growth between field data and output of the model (post –release sampling for 65 days)

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70Days aft er Release

Wei

ght

(g) Released

flounder

Wild flounder

Ohno Bay , 1989

Concept of Stock Enhancement→ Released fish use the surplusproductivity in the nursery habitats

Criterion of the optimum number of released flounder

→ Maximum number, which does not affect the growth of wild flounder

Estimate of the optimum stocking density using ecophysiological model

Relationship between number of flounder released and predicted growth of wild

flounder juveniles 65 days after release

0

10

20

30

40

50

0 20000 40000 60000 80000 100000Number of Flounder Released

Wei

ght

65 D

ays

afte

r R

elea

se (

g)

Density effect (5,000 fish)

Actual release 76,000 hatchery fish

No surplusproduction

Ohno Bay, 1989

Actual weight (wild flounder)

Over stocking!!

●

●

Regional difference in the carrying capacity for flounder juveniles

0

5

10

15

20

25

30

35

130 135 140

Longitude

Retu

rn R

ate

(%)

High return rates in northeastern

areas

●

●

●●●

●

●● Samplings of wild

flounder juveniles and mysids from 12 nurseries in

2000

●●

●●

In Northern Japan

1.Lower wild flounder juvenile density

2.Higher mysid abundance

3.Higher flounder feeding rate0.0 1.0 2.0 3.0 4.0 5.0

Nagasaki

Yamaguchi

Tottori

Fukui

Toyama

Nigata

Miyagi

Iwate

Aomori

Hokkaido

Juvenile density (ind/ min. tow)

0 100 200 300 400 500

Kagoshima

NagasakiYamaguchi

Tot tori

FukuiToyamaNiigata

MiyagiIwate

Aomori

Hokkaido

Mysid aboundance (mg/ m2)

1712.8

0 10 20 30 40 50 60

Nagasaki

Yamaguchi

Tot tori

Fukui

Toyama

Niigata

Miyagi

Iwate

Aomori

Hokkaido

Stomach Index (SI)

Wild flounder juvenile density

Mysid abundance

N

S

Feeding rate

Comparison of growth rates of wild Japanese flounder juveniles (3-6 cm TL) between northern and southern Japan

northernsouthern

DA

ILY

GR

OW

TH (m

m/d

ay) 2.0

1.5

1.0

0.5

0 14 16 18 20 22 24 26

WATER TEMPERATURE (℃ )

１．Higher growth rate in northern Japan (Tanaka et al., 1998)

2．No latitudinal difference in growth rate under the same rearing conditions

●

●

●●

●

●

●●

12

3 45

6

7

8Model prediction of the optimum stocking density for TL 8cm juveniles (thousands/25ha)

Nursery grounds in northern Japan have

higher surplus productivity for

stocking of flounder

Main factors0

20

40

60

80

1 2 3 4 5 6 7 8

Station

Opt

imum

Num

ber (

103 )

・ Lower density of wild flounder juveniles in northern Japan

・ Higher abundance and productivity of prey animals in northern Japan

1. Over-stocking of hatchery flounder has been commonly conducted at many stocking sites.

2. There are regional differences in the surplus productivity available for released flounder; Northern Japan has higher levels of surplus productivity in habitats than in southern Japan.

* However, stocking intensity is almost the same between these two areas.

Conclusions1. Each governmental institute which is

responsible for the stock enhancement program must conduct a follow-up study on the effectiveness of the program including effects of stocking density.

2. With the exception of the present study, the concept of the optimum stocking density in habitats has not been tested yet in the stock enhancement program in Japan. Appraisal of all ongoing stock enhancement programs from this point of view is needed.