Clams (Quahogs) and Oysters: What drives the population?

Dr. John Kraeuter

Marine Sci. Center - Univerisity of New England and

Haskin Shellfish Research Lab. - Rutgers University

Hard clam (Northern Quahog)

(Mercenaria mercenaria) Infauna

Eastern oyster

(Crassostrea virginica)

Epifauna

Delaware Bay

Islip Sampling

• 6000 hectares divided into 400 grids

• About 350 grids sampled annually

• Duplicate 1.02 m2 clam shell bucket samples per

grid

• 6.4 mm mesh sieve, all live and dead enumerated

and all live and dead measured

• 25+ consecutive years of data

10ppt

28ppt

10ppt

28ppt

10ppt

28ppt

Delaware Bay Oyster Seed Beds

Delaware Bay Sampling

• Oyster dredge

• Random stratified design on each bed

• Composite bushel of 3 1 minute hauls

• Approximately 105 samples per year

• All live and dead oysters counted and measured

• Samples removed for disease and condition analysis

• 50 + consecutive years of data

0

100

200

300

400

500

600

700

800

900

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

2003

2005

2007

2009

Num

ber

m-2

Yearly and Average Oyster Abundance, Delaware Bay

Oysters/sq m

Spat/sq m

Mean oysters

Mean spat

0

100

200

300

400

500

600

700

800

900

Num

ber

m-2

Yearly and Average Oyster Abundance, Delaware Bay

Boxes/sq m

Oysters/sq m

Spat/sq m

Mean boxes

Mean oysters

Mean spat

0

50

100

150

200

250

300

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

2003

2005

2007

2009

Num

ber

m-2

Yearly and Average Oyster Abundance, Delaware Bay

Boxes/sq m

Oysters/sq m

Spat/sq m

Mean boxes

Mean oysters

Mean spat

11

90

68

Salinity

Disease

Predation

Spat Set

Growth

Survival

39.1

39.2

39.3

39.4

39.5

39.1

39.2

39.3

39.4

39.5

75.5 75.4 75.3 75.2 75.1

75.5 75.4 75.3 75.2 75.1

6.5 - 14.5 ppt

9.0 - 16.5 ppt

14.0 - 20.0 ppt

17.0 - 22.5

ppt

UPPER

UPPER CENTRAL

CENTRAL

LOWER

39.1

39.2

39.3

39.4

39.5

39.1

39.2

39.3

39.4

39.5

75.5 75.4 75.3 75.2 75.1

75.5 75.4 75.3 75.2 75.1

39.1

39.2

39.3

39.4

39.5

39.1

39.2

39.3

39.4

39.5

75.5 75.4 75.3 75.2 75.1

75.5 75.4 75.3 75.2 75.1

6.5 - 14.5 ppt

9.0 - 16.5 ppt

14.0 - 20.0 ppt

17.0 - 22.5

ppt

UPPER6.5 - 14.5 ppt

9.0 - 16.5 ppt

14.0 - 20.0 ppt

17.0 - 22.5

ppt

UPPER

UPPER CENTRAL

CENTRAL

LOWER

Dermo – Low

MSX – Very

Low

Dermo –

Moderate

MSX – Low

Dermo – High

MSX – Moderate

Dermo –

High

MSX –

High

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0

200

400

600

800

1000

1200

195319571961196519691973197719811985198919931997200120052009

Mort

alit

y

Num

ber

per

square

mete

r

Upper Central

Oyster Spat Mortality

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0

50

100

150

200

250

300

350

400

450

500

195319571961196519691973197719811985198919931997200120052009

Mort

alit

y

Num

ber

per

square

mete

r Central

Oysters Spat Mortality

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0

500

1000

1500

2000

2500

195319571961196519691973197719811985198919931997200120052009

Mort

alit

y

Num

ber

per

square

mete

r

Upper

Oyster Spat Mortaltiy

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0

50

100

150

200

250

300

195319571961196519691973197719811985198919931997200120052009

Mort

altiy

Num

ber

per

square

mete

r

Lower

Oysters Spat Mortality

0

100

200

300

400

500

600

700

800

900

1953 1957 1961 1965 1969 1973 1977 1981 1985 1989 1993 1997 2001 2005 2009

Nu

mb

er

pe

r s

qu

are

me

ter

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Mo

rta

lity

Oyster Spat Mortality

MSX

MSX

DERMO

0

1

2

3

4

5

6

7

8Spat/

Adult

Delaware Bay Seed Beds

Spat/adult

Mean spat/adult

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Num

ber

m-2

Islip Hard Clam Population

Harvest as a percentage of potential recruits

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Per

cen

t

1 Year Old

2 Year Old

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Num

ber

m-2

Islip Clam Population

Harvestable

Seed

Mean Harvestable

Mean Seed

0

0.05

0.1

0.15

0.2

0.25Age 2

Seed p

er

adult

Islip, New York

0

0.5

1

1.5

2

2.5

3

3.5

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

Num

ber

per

sam

ple

Islip New York

Live

Dead

Recruits

Mean Live

Mean Dead

Mean Recruit

.0.26

0.36

1.5

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0

0.5

1

1.5

2

2.5

3

3.5

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

% M

ort

alit

y

Num

ber

Islip Hard Clam

Harvestable

Recruit

% Mortality

Summary • Managing shellfish populations cannot be done without basic population data

collected on a regular schedule that is scaled to be in concert with the factors controlling the population.

• Standing stock is important but used alone it is a misleading indicator for managing the resource.

• All population measurements should include BOTH a recruitment and a mortality estimate.

• If the population is arrayed over a salinity gradient the processes controlling the population may vary along the gradient. Just because an area has high recruitment or good growth does not a priori mean that the spot is a good place to begin a restoration program.

• Sustainable fishing requires that the removal rate be based on the relationship between recruitment and mortality and NOT on standing stock.

• Too much effort is aimed at evaluating recruitment and not enough is directed to finding better ways of measuring mortality.