Characterization of mesophotic benthic habitats and associated reef communities at

Lang Bank, St. Croix

Jorge (Reni) García-Sais, Rene Esteves, Stacey M. Williams, Jorge Sabater-Clavell, Milton Carlo

149 th CFMC MeetingSt. Croix, USVI

April 2014

Study Objectives

• Provide a baseline quantitative and qualitative characterization of the benthic and fish communities associated with the principal mesophotic reef habitats within a depth range of 30 – 50 m at Lang Bank.

• Produce from direct diver observations a map of benthic habitats distributed within the 30 – 50 m depth range

• Analyze relationships between the benthic and fish community structure with physical factors, such as habitat type, depth, substrate rugosity and slope, and evaluate the function of mesophotic habitats in the life cycle of commercially important reef fish populations

• Provide a fisheries-independent survey of the composition and size frequency distributions of commercially important fish and shellfish (queen conch, spiny lobster) species associated with mesophotic (30 – 50 m) reef habitats at Lang Bank.

Sampling Design and Methodology

• Depth (30, 40, 50m) and physiographic zone stratified sampling approach based on preliminary analysis of multi-beam bathymetry blueprints of the study area

– Series of 10 roughly equidistant stations at the outer shelf-

edge with1-10 m linear photo-transect each at 30, 40 and 50 m – for sessile-benthic characterizations and 10 x 3m belt-transect for quantif. of small territorial fishes with extension to 20 x 3 m for large demersal comm. imp fishes and shellfishes (30 transects total).

– 10 transects at the inner wall (30, 40 m)– 43 transects at the deep basin (40, 50 m)– 2 transects at the spur and groove (SPAGG)

Sampling Design and Methodology (cont.)

30 – 100/200m long x 4 m wide belt transects for fishery independent surveys of comm. imp fishes and shellfishes

BCR – 9 (3,600 m2)

CPV – 14 (6,800 m2)

CRR – 3 (1,200 m2)

S & G – 4 (1,600 m2 )

Multivariate analyses of relative abundance data for sessile- benthic and fish populations

Benthic Habitats

• Bank Coral Reef• Colonized Pavement• Colonized Rhodolith Reef • Spur and Groove Reef• Sand

Bank Coral Reef

• Impressive and extensive bank reef formation of scleractinian corals distributed within the deep outer shelf basin of Lang Bank at depths of 35 – 50 m +

• Main structural build-up of Orbicella franksi growing as large table shaped colonies

• 28 scleractinian corals, 20 octocorals, 62 sponge, 67 fish species identified (preliminary count)

• Mean substrate cover by scl. corals: 29 %, O. franksi: 23.1 % (80% of total cover by corals)

• Within habitat similarity (highest measured at LB): 57.8%– O. franksi: 46.1%– P. astreoides: 4.0%

Live Coral Octocoral Sponge Benthic algae

Abiotic0

10

20

30

40

50

60

70

Benthic categories

Mea

n co

ver (

%)

Non-metric multidimensional scaling (nMDS) plot of Bray-Curtis similarities between benthic habitats based on the data of percent substrate cover by

sessile-benthic categories on photo-transects at Lang Bank, St. Croix, USVI. 2013

Transform: Square rootResemblance: S17 Bray Curtis similarity

HabitatCPVBCRPRFSAGCRRSND

2D Stress: 0.14

Colonized Pavement Habitat

• Low relief, hard bottom habitat distributed along at the edge of the deep shelf basin (30 – 33m) and at the insular shelf-edge and slope (33 – 50 m).

• 18 scleractinian corals, 18 octocorals, 52 sponges, 51 fish species identified

• Benthos mostly colonized by turf algae (72%), sand/rubble (14%), sponges (8%), octocorals, and scleractinian corals (3.6 %).

• Corals occurred as isolated coral heads not forming reefs, sometimes bioherms with sponges.

• Orbicella annularis, Porites astreoides, Siderastrea siderea main assemblage

• Within habitat similarity: 18.4%– O. franksi (5.8%)– P. astreoides (4.3 %)– S. siderea (2.9%)

Colonized Rhodolith Reef

• Mostly flat, homogeneous bottom of crustose algal nodules deposited over sand

• Distributed near the base of the inner walls – small mesophotic areal extension at Lang Bank

• Reef substrate cover dominated by benthic algae and abiotic categories

• 11 corals, 5 octocorals, 41 sponges, 32 spp fish identified– Bicolor Damselfish– Cherubfish– Creole Fish

• Important habitat for Queen Conch (Strombus gigas)Live Coral Octocoral Sponge Benthic

algaeAbiotic

010203040506070

Benthic categories

Mea

n co

ver (

%)

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

1

2

3

4

5

6Red Hind - Lang Bank

Den

sity

(ind

/1,0

00m

2)

10 15 20 25 30 35 400

0.3

0.6

0.9

1.2

1.5

1.8 Red Hind - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Total Length (cm)

Dens

ity (I

nd/1

,000

m²)

Survey Sites

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

0.5

1

1.5

2

2.5

3

3.5 Mutton Snapper - Lang BankDe

nstiy

(ind

/1,0

00m

2)

25 30 35 40 45 50 55 600

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5 Mutton Snapper - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Total Length (cm)

Den

sity

(Ind

/1,0

00 m

²)Survey Sites

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

1

2

3

4

5

6

7Queen Triggerfish - Lang Bank

Dens

ity (i

nd/1

,000

m2)

20 25 30 35 40 45 50 550

0.2

0.4

0.6

0.8

1

1.2

1.4Queen Trigger - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Total Length (cm)

Dens

ity (I

nd/1

,000

m²)

Survey Sites

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

1

2

3

4

5

6

7

8

9

10 Lionfish - Lang BankD

ensi

ty (i

nd/1

,000

m2)

5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

3

3.5 Lion Fish - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Total Length (cm)

Dens

ity (I

nd/1

,000

m²)

Survey Sites

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

10

20

30

40

50

60 Queen Conch - Lang BankDe

nsity

(ind

/1,0

00m

2)

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 280

0.2

0.4

0.6

0.8

1

1.2

1.4 Queen Conch - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Shell Length (cm)

Desn

sity

(Ind/

1,00

0 m

²)

Survey Sites

Bank Coral Reef Colonized Pavement

Colonized Rodolith Reef

Spur and Groove0

1

2

3

4

5

6Spiny Lobster - Lang Bank

Den

sity

(ind

/1,0

00m

2)

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 230

0.2

0.4

0.6

0.8

1

1.2 Spiny Lobster - All Sites

Abrir La Sierra

Bajo de Sico

Desecheo

Lang Bank

Shell Length (cm)

Den

sity

(Ind

/1,0

00 m

²)

Survey Sites

Preliminary Conclusions

• An impressive and extensive coral reef system is found within mesophotic (30-50m) depths at Lang Bank

• The predominant coral reef formation is a bank reef constructed by Orbicella franksi distributed throughout a deep outer shelf terrace before the shelf-edge

• The reef top at the shelf-edge is an important habitat for spiny lobster and red hind

• The colonized rhodolith reef habitat has limited extension, but where found it sustains dense populations of queen conch

• The shelf-edge and slope are mostly featureless and dominated by lightly colonized pavement

Questions?