Spatial and temporal variation in stock abundance of queen conch, Strombus

gigas, in the US Virgin Islands

Department of Planning and Natural Resources

Division of Fish and Wildlife

Research conducted by Gordon & Tobias

• About 3-3.5 years to reach sexual maturity • Reaches 12 inches in length• Mature conch has flared lip• Lifespan as long as 20 years (Berg, 1987)

Biology of Queen Conch

• Mate during the warm summer months

• Require at least 50 conch/hectare for reproduction (Stoner & Ray 1996)

• Produce about 400,000

eggs per spawn

• Spawn about 6 times

a summer

Conch Reproduction

Conch Reproduction

• Planktonic larvae stage (veligers) drift in water column for

2-3 weeks

• Settle to the bottom and change into regular conch form

Habitat & Feeding HabitsHabitat & Feeding HabitsCommonly found in seagrass and algae

habitat Can be found in all habitatsGraze on various species of algae and

seagrassReported to depths

of 400 feet Burying behavior

(juveniles)

Harvest size limits– -minimum length – 9 inches (23 cm) OR– -minimum lip thickness – 3/8 inch (9.5 mm)

Closed season extension* – June 1 – October 31 each year

*In 2008 VI government extended seasonal closure by 2 months for the territory because of overfishing concerns on STX

Territorial Regulations in the USVI

• Recreational quota

-6 conch per day per recreational fisher

-maximum of 24 per boat per day

• Commercial quota

-maximum of 200 conch per day per

registered commercial fishing vessel

• No sale of undersized conch shell permitted

Territorial Harvest Quota

• Landing quota developed in June 2008:

-50,000 lbs. annually per district

-Thereafter, the season will be closed until November 1 of that year.

All conch must be landed and reported in the district from which they were harvested.

Must be landed whole in the shell

Territorial Landing Restrictions

Total queen conch by fishing year. MSY is a range of 30,000 (CFMC) - 60,000 lbs (Olsen 2007). USVI MSY set at 50,000 lbs.

Reported queen conch commercial landings by fishing year, USVI (1995-2009)

0

50,000

100,000

150,000

200,000

250,000

1995

-96

1996

-97

1997

-98

1998

-99

1999

-00

2000

-01

2001

-02

2002

-03

2003

-04

2004

-05

2005

-06

2006

-07

2007

-08

2008

-09

Tot

al P

ound

s

STT/STJ STX

Esitimated range of MSY

Harvest size limits (9” length or 3/8” lip thickness) Recreational limit= 3 conch per person or 12 per boatCommercial limit= 150 conch per dayNo hookah Conch must be landed with meat and shell intactNo harvest in EEZ except Lang Bank, STX from

October 1 – June 30

Federal Regulations

USVI = total of 52 sites/114 transects (STT=12 sites/28T; STJ= 12/24T; STX=30 sites/62T)

Underwater Scooter Transects

Conch transect sites around St. Thomas/St. John, 2008-2009. Note: Two new sites were added to the original 22 based on fisher input.

Conch transect sites around St. Croix, 2009-2010. Note: Eight new sites were added to the original 22 based on fisher input and

areas of concern such as Lang Bank.

28 scooter transects around St. Thomas. *New sites based on commercial fisher input .

Density of adult and juvenile queen conch for 2008-09 observed on scooter transects around St. Thomas, USVI

0

1000

2000

3000

4000

Bolongo

Bay

*Brewer

s Bay

Fortuna

Bay

Inne

r Wate

r Is.

Lindq

vist

Bay

Mag

ens B

ay

Outer

Wate

r Is.

Perse

veran

ce B

ay

Pillsb

ury S

ound

*Saba

Secret

Har

bor

Wes

t End

Den

sity

(co

nch/

ha)

Adults Juveniles

24 scooter transects around St. John.

Density of adult and juvenile queen conch for 2008-09 observed on scooter transects around St. John, USVI

0

100

200

300

400

500

Den

sity

(con

ch/h

a)

Adults Juveniles

62 scooter transects around St. Croix. *New sites based on commercial fisher input and areas of concern (Lang Bank).

Density of adult and juvenile queen conch for 2009-10 observed on scooter transects around St. Croix, USVI.

0

200

400

600

800

1000

1200

14001 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

19A

19B

20A

20B 21 22

*CC

1*C

C2

*CC

3*C

C4

*LB

1*L

B2

*LB

3*L

B4

Den

sity

(co

nch

/ha)

Adults Juveniles

Is. Num. of Transects

Area (m2)

Num.

Conch

Avg. Adult

Density

Avg.

Juvenile

Density

Total

Density

STT 28 31,059 253 412.80 170.58 583.37

STJ 24 32,716 119 35.34 38.33 73.67

STX 62 101,789 641 28.88 34.09 62.97

Total 114 165,564 1013 28.21 33.00 61.19

Area surveyed, number and density of adult, juvenile, and total queen conch observed on scooter transects,

2008-2010.

Location Conch/ha ReferenceUS Virgin Islands

St. John

St. Thomas

St. Croix

73.67

583.37

62.97

Gordon and Tobias (2010)

Puerto Rico

Florida Keys

17.50

0.50

Jimenez (2007)

Berg et al. (1996)

Bahamas

Little Bahama Bank

Great Bahama Bank28.50

20.80

Smith and van Nierop (1984)

Conch densities throughout the Caribbean determined by extensive surveys

Queen conch density by habitat type in St. Thomas, USVI 2008-2010

0

100

200

300

400

500

600

Algae Coral Reef Pavement Rubble Sand Seagrass

conc

h/ha

Adults Juveniles

Note: Error bars are standard error

Queen conch density by habitat type in St. John, USVI 2008-2010

0

10

20

30

40

50

60

70

80

90

100

Algae Coral Reef Pavement Rubble Sand Seagrass

conc

h/ha

Adults Juveniles

Note: Error bars are standard error

Queen conch density by habitat type in St. Croix, USVI 2008-2010

0

20

40

60

80

100

120

140

Algae Coral Reef Pavement Rubble Sand Seagrass

conc

h/ha

Adults Juveniles

Note: Error bars are standard error

Adult and juvenile queen conch density in the USVI, 2008-2010

010

203040

506070

8090

0-6 7-12 13-18 19-24 25-30

Depth (m)

Den

sity

(con

ch/h

a)

Adults Juveniles Total

Overall queen conch density by island and depth in the USVI, 2008-2010

0

50

100

150

200

250

0-6 7-12 13-18 19-24 25-30

Depth (m)

Den

sity

(con

ch/h

a)

STT STJ STX

Shell length distribution for queen conch in the U.S. Virgin Islands, 2008-2010

0

10

20

30

40

50

60

70

80

90

100

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Shell Length (cm)

Num

ber

of I

ndiv

idua

ls

Juveniles

Adults

Shell length distribution of queen conch by island, 2008-2010

0

20

40

60

80

100

120

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Shell Length (cm)

Num

ber

of I

ndiv

idua

ls

STT STJ STX

Note: size at sexually maturity ~20cm (Stoner and Ray 1996)

Lip Thickness: Juvenile = no lip, New adult =1-7mm, Adult (sexually mature) = 8-15mm, Old = 16-33mm, Very Old = 34-59mm.

Maturity categories from Appeldoorn (1996) and Friedlander (1997).

Percent frequency of queen conch maturity categories from scooter transects, 2008 - 2010

0%

10%

20%

30%

40%

50%

60%

Juvenile New Adult Adult Old Very Old

% F

requ

ency

STT STJ STX

N=1013

Mean density of adult and juvenile queen conch observed at 10 common sites around St. Thomas, USVI (1990, 1996, 2001, 2008)

11.791.59 1.8832.19 24.16

120.90

31.46 36.040.00

50.00

100.00

150.00

200.00

250.00

1990 1996 2001 2008

Den

sity

(con

ch/h

a)

Adult Juvenile

Only 10 sites/transects common to all survey years were used. Error bars are standard errors.

Only 9 sites/transects common to all survey years were used. *Note: No juvenile data available for St. John in 1981 and 1985.

Mean density of adult and juvenile queen conch observed at 9 common sites around St. John, USVI (1981, 1985, 1990, 1996, 2001, 2008)

4.12 1.490.00 0.005.17 4.17

8.08

31.29

52.41

38.20

0.66

48.51

0

10

20

30

40

50

60

70

80

90

1981 1985 1990 1996 2001 2008

Den

sity

(con

ch/h

a)

Adults Juveniles

Notes: (1) only adults were sampled in 1981; (2) only 16 of the 22 original transects were surveyed in 2001; (3) all 22 original sites were surveyed again in 2009

Mean density of adult and juvenile conch at 22 original sites around St. Croix, USVI (1981, 2001, 2009)

7.60 27.35 46.0872.28 89.47

0.00

50.00

100.00

150.00

200.00

250.00

300.00

1981 2001 2009

Den

sity

(co

nch/

ha)

Adults Juveniles

USVI 2008-2010 overall conch densities are higher than previous years; however, adult densities <50 conch/ha.

Most juveniles were found in the 0-6m depth range, while most adults were found in the 25-30m depth range. Overall for USVI majority of the conch were in the 25-30m depth range.

Conch densities were greatest in seagrass habitat

Majority (>50%) of the conch found on scooter transects for USVI were sexually mature in terms of shell length (>20cm).

STX had the highest percent (%) frequency of sexually mature adults. STJ had the highest % frequency for old and very old conch, and STT and STX had the highest % frequency for juveniles.

SummarySummary

Questions???Questions???

St. Croix Conch Landings by Area

-

20,000

40,000

60,000

80,000

100,000

120,000

140,000

2000 2001 2002 2003 2004 2005 2006 2007 2008

Year

Po

un

ds

(lb

s)

Federal

Territorial

Unknown