Growth and feeding ecology of groupers on artificial and natural reefs in the northern
Gulf of Mexico
Charlotte H. Rivas, Lynne Wetmore, Jay Rooker
(Undergraduate) Department of Marine Biology, Texas A&M University at Galveston.
♦ Abstract
The main objective of this study was to understand the habitat value of three different reef types (coral reefs, clay banks, oil platforms) for demersal fishes in the Gulf of Mexico. Two ecological components of habitat quality, growth and diet, were assessed for
two species of grouper: graysby (Cephalopholis cruentata) and rock hind (Epinephelus adscensionis). Otolith microstructure analysis was used to determine age, and age-length relationships were developed to estimate growth rates. Diet was determined by
conventional gut content analysis. Growth rates obtained for graysby were similar between clay banks and oil platforms, but significantly lower for individuals from coral reefs. Growth rates for rock hind were equivalent between clay banks and oil platforms;
however, this species was rarely observed at the coral reef sites; only three specimens were collected. Percent gut fullness was lowest for graysby and rock hind collected from coral reefs. Diets of graysby and rock hind from oil platforms and clay banks
contained mainly small fishes, while prey species of grouper on coral reefs were primarily invertebrates. Results from this study indicate that the diverse reef types may serve different functional roles for demersal fishes in the northern Gulf of Mexico.
♦ Introduction
Hard bottom reef habitats play a significant role in the preservation of demersal fish
populations, but they constitute less than 3.0% of the total area in the northwestern Gulf of
Mexico (GOM) (Parker et al. 1983); hence, understanding the ecological value of these
habitats is important to maintain integrity of fish communities. Diet and growth are two
ecological components that are often used as proxies to evaluate habitat quality for marine
fishes. Thus, the main objective of this study was to characterize the relative habitat value of
two natural reef types (coral reefs, clay banks) and one artificial reef type (oil platforms) in the
Gulf of Mexico by comparing the growth and dietary composition of two small grouper
species: graysby (Cephalopholis cruentata) and rock hind (Epinephelus adscensionisa).
Results were also compared to previously published growth models from other geographic
locations in order to assess regional variability in growth for these two grouper species.
♦ Gut Content Analysis
♦ Materials & Methods
♦ Discussion & Conclusions
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♦ Results
Field collections: In 2009-2010, graysby (n = 65) and rock hind (n = 142)
were collected from:
• 2 coral reefs (East & West Flower Garden Banks)
• 2 clay banks (Sonnier Bank, Stetson Bank)
• 4 oil platforms (HI-389, VR-315A, VR-331A, VR-340A)
Fig 2. Age-length relationship for
graysby.
Growth rates were similar between clay
banks and oil platforms, but significantly
slower for graysby at coral reefs
(ANCOVA, P < 0.05). Thus, fishes could
not be pooled across habitats in growth
models.
The von Bertalanffy model was used to
create an age-at length curve for graysby
at coral reefs, and growth at this reef type
in the GOM was substantially slower than
predicted by the two previously published
models for graysby in other regions.
Because of limited samples, separate
growth curves were not created for clay
banks and oil platforms. However, in the
size range available, growth of graysby at
these two reef types in the GOM
appeared similar to graysby in other
regions.
Results indicate that growth of graysby
may differ significantly among habitats in
the GOM, and fishes at coral reefs may
grow substantially slower than graysby in
other regions.
Results from this study indicate that different reef types may fill different ecological roles for groupers in the GOM, possibly as a function of food availability. Growth rates for graysby were similar at clay banks and oil
platforms but significantly slower at coral reefs, and it appears that this observed variability in growth may be driven by dietary differences among habitats. Gut fullness was lowest at coral reefs, potentially indicating lower
foraging efficiency, and while the diets of graysby at both clay banks and oil platforms were composed entirely of fish, the primary prey item at coral reefs was invertebrates, which typically have a lower nutritional value.
Like graysby, growth rates of rock hind were similar between clay banks and oil platforms; however, this species was extremely rare at coral reefs, where only three specimens were collected, and all three of these fish had
empty stomachs. Although much of the conservation and management efforts for demersal fishes in the GOM have focused on coral reefs, the increased growth rates and dietary differences observed at clay banks and oil
platforms emphasize the potential importance of these two reef types as essential habitat for groupers in the region, particularly for rock hind, which was only found almost exclusively at these two habitats.
The marked differences in growth for both graysby and rock hind in the northern GOM compared to previously published models from other geographical locations (i.e. southeastern USA, Curacao) indicate that regional
variability in growth for these two grouper species may be substantial. If this is the case, effective management of these species will require region-specific life history information in order to develop accurate fishery models,
and the growth models presented here may represent valuable baseline data for graysby and rock hind in the northern GOM.
Graysby
Rock
Hind
Fig 3. Age- length relationship for rock
hind.
No significant differences in growth were
detected among habitats (ANCOVA, P >
0.05), and fishes from all three reef types
were pooled to create a single age-at-
length curve using the von Bertalanffy
growth model.
Growth of rock hind at all three reef types
in the GOM appeared to be substantially
slower than predicted by the only
previously published growth model
Results indicate that growth of rock hinds,
although similar among habitats, may vary
substantially from region to region.
L
t
= 248(1 -
e
-0.17(t + 1.00)
)
Southeastern USA
(Potts & Manooch 1999)
Curacao
(Nagelkerken 1979)
Northern Gulf of Mexico
(GOM)
(Coral reef only)
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14
Total length (mm)
Age (years)
Graysby
Clay Bank
Coral Reef
Platform
L
t
= 212(1 - e
-0.46(t + 0.66)
)
Southeastern USA
(Potts & Manooch 1995)
Northern Gulf of Mexico
(GOM)
(All habitats pooled)
0
50
100
150
200
250
300
350
400
450
0 1 2 3 4 5 6 7 8
Total length (mm)
Age (years)
Rock Hind
Clay Bank
Coral Reef
Platform
Coral zone
Coral
zone
Coral
Zone
Clay/Siltstone
Coral Reef Clay Bank
Oil Platform
Reef habitats available to demersal fishes in the GOM
• Large reef crest area
• Uniform, high (50-60%)
coral cover
• Located shelf-edge
• Smaller reef crest
• Limited coral /sponge cover
(mainly clay flats)
• Located mid-shelf
• Small area of coral/ sponge
cover on crossbeams
• Located throughout
continental shelf
• A single otolith was chosen randomly from each fish,
embedded in epoxy resin and sectioned using a
Beuhler Isomet saw
• Otolith sections were mounted on glass slides, and
annual growth increments (annuli) were enumerated
under a compound light microscope using the software
Image Pro Plus
Otolith microstructure analysis:
Gut content analysis:
• Stomachs were dissected under a microscope, and gut
fullness was recorded for each fish: Empty (0 prey
items) or Full (≥ 1 prey item)
• All prey items were identified to the lowest possible
taxon, and each fish was assigned to a dietary
category: “Fish”, “Fish/Invert”, or “Invert,” based on
prey item composition
Fig 5. Dietary analysis
for rock hind.
The percentage of fish
with full guts (Figure A)
was 0% at coral reefs, but
substantially higher (≈
42%) at both clay banks
and oil platforms.
Prey item composition
was similar between clay
banks and oil platforms.
The majority (55-56%) of
rock hinds collected at
both of these reef types
contained only fish in their
stomachs, while a minority
contained invertebrates
(21-22%) or fish and
invertebrates (22-24%).
Fig 4. Dietary analysis
for graysby.
The percentage of fish
with full guts (≥1 prey item
in stomach) (Fig 4A) was
lowest at coral reefs
(25%) but higher at clay
banks (41%) and oil
platforms (67%).
There were also marked
differences in prey item
composition among
habitats (Fig 4B). Diet
was similar between clay
banks and oil platforms,
with 100% of graysby at
these reef types
containing only fish in
their stomachs. However,
at coral reefs, the majority
of graysby (60%)
contained only
invertebrates in their
stomachs, while a
minority contained only
fish (20%) or a
combination of fish and
invertebrates (20%).
SUMMARY
•Graysby: growth at coral reefs was
significantly slower than at other reef types
in the GOM, and also slower than graysby
in other regions
•Rock hind: growth at all three reef types
in the GOM was substantially slower than
in other regions
SUMMARY
•Percent gut fullness was
lowest at coral reefs
•The primary prey item at
coral reefs was inverts, but
the primary prey at clay
banks and oil platforms was
fish
Figure 1. Whole otolith with
yellow lines showing sectional
cuts (A) and transverse otolith
section (B) and detail (C) from a
240 mm graysby (age =11 years)
collected from East Flower
Gardens Bank. Arrows denote
annual growth increments
(annuli).
B
C
Age 1
Age 2
Age 3
Age 11
Age 4
A
