department of planning and natural resources division of fish and wildlife
DESCRIPTION
Spatial and temporal variation in stock abundance of queen conch, Strombus gigas , in the US Virgin Islands. Research conducted by Gordon & Tobias. Department of Planning and Natural Resources Division of Fish and Wildlife. Biology of Queen Conch. About 3-3.5 years to reach sexual maturity - PowerPoint PPT PresentationTRANSCRIPT
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