Download Effects of Three Substrate Variables on Two

BULLETIN OF MARINE SCIENCE, 37(1): 129-142, 1985
EFFECTS OF THREE SUBSTRATE VARIABLES ON TWO
ARTIFICIAL REEF FISH COMMUNITIES
Charlie R. Chandler, Richard M. Sanders, Jr.
and Andre M. Landry, Jr.
ABSTRACT
Effects of substrate rugosity, percent substrate cover, and vertical rellief on resident and
semiresident ichthyofaunal communities associated with two sunken barges were investigated
off Panama City, Horida.
Measures of substrate rugosity and percentage substrate cover indicate Offshore Barge had
1.75 times the amount of surface area accessible to reef organisms for both sand and metal
substrates compared to that available at Inshore Barge. Offshore Barge also supported a
significantly greater abundance and species richness of resident fishes during summer (2: 21OC)
and winter (::s18OC)censuses. Both barges supported similar dominant species with no significant differences detected in the evenness of fish abundances. Midwater structures on
Inshore Barge supported significantly greater numbers of semiresident species during summer
censuses. However, greater abundance of semiresident species on Offshore Barge during winter
indicated that higher structural complexities take precedence over vertical relief in attracting
or concentrating these fishes when abundances (or temperatures) drop below a critical level.
Several researchers have attempted to explain relationships between various
reef substrate variables and the abundance and diversity of reef fish communities
(Hiatt and Strasburg, 1960; Talbot, 1965; Jones and Chase, 1975). Only recently,
however, have significant correlations been based on actual measurements of the
substrate variables (Risk, 1972; Luckhurst and Luckhurst, 1978) as the methods
used to collect these data are complex and require considerable diver-down time
to complete.
This study evaluates effects of substrate complexity on the structure ofichthyofaunal communities at two artificial reefs in the northeastern Gulf of Mexico. The
two reefs, known collectively as Twin Barges, were nearly identical in size and
shape and, because of their proximity to one another, were continuously exposed
to the same species pool and to similar environmental conditions. In addition,
the distance separating Twin Barges was sufficient to prevent migration of resident
individuals between reefs. Ichthyofaunal differences between Twin Barges were
therefore assumed to result from disparities in structural complexity of the two
reefs.
METHODS
Site Description. - U.S. Navy divers sank two nearly identical steel barges 3.3 km offshore Panama
City, Florida in 1964. The barges settled approximately 200 m apart in 22 m of water and were known
individually as Inshore (Latitude 30 03'53''N, Longitude 85°44'36"W) and Offshore Barges (Latitude
30"03'50"N, Longitude 85°44'36"W). Each barge covered 96.14 m2 of bottom surface area (Fig. I)
and was constructed of individual compartments held together by angle iron and steel I-beams.
The only major structural difference between the two reefs was the condition of their metal plating.
Deterioration of metal on Offshore Barge, especially the deck, resulted in every compartment being
exposed and accessible to fish and invertebrates (Fig. 2A). Inshore Barge, however, had few holes in
its metal plating which restricted access to regions within compartments (Fig. 2B). Both barges had
ample time (15 years) to establish climax biofouling communities and were assumed to be equal in
this respect.
0
Substrate Complexity. - Differences in substrate complexity between barges were categorized according
to vertical relief, percent substrate cover, and substrate rugosity.
129
130
BULLETIN OF MARINE SCIENCE, VOL. 37, NO. I, 1985
17.
x
x
Figure 1. Inshore Barge showing location of midwater structures (X) and transect ropes (-----).
Structural design of Twin Barges provided a uniform horizontal profile 1.25 m above the bottom
on each reef. Differences in vertical relief of the barges were created by anchoring 10 midwater structures
approximately 8 m in length to Inshore Barge on 28 August 1979. Each midwater structure consisted
of 10 automobile tires tethered together with galvanized steel cable and suspended to within 14 m of
the surface by an inflated inner tube in the uppermost tire. In order to minimize their effects on
resident ichthyofauna, eight midwater structures were initially positioned around the reef periphery
and two were anchored inside a central hole in the barge (Fig. I). Continual wear caused by corrosion,
currents, and strong winter wave surge made it difficult to maintain a full complement of 10 midwater
structures. However, at least one midwater structure was always suspended at each end of the barge
and the total number was never less than five. All vertical relief measurements were based on height
of a structure above the sand substrate.
Percent substrate cover is a measure of the relative proportion of bottom surface area occupied by
a substrate type. A rope marked off at 10-cm intervals and laid down the center of each census transect
(see Census Techniques) was used to obtain measurements of the total linear distance covered by each
substrate type on Twin Barges. Although not an actual measure of percent substrate cover, these linear
distances were a function of the total surface area covered by each substrate and were considered an
accurate representation of this variable.
The sum oflinear distances traveled over two substrate types, open sand and metal, represents the
total linear distance measured (TLD) on each reef. A third substrate type, protected sand, was defined
as that area of the bottom not seen from directly above because of overhanging metal plates. The
percent ofTLD occupied by protected sand was added to that occupied by open sand to estimate the
amount of sand substrate accessible to reef organisms.
Substrate rugosity is a measure of the total surface area utilized by reef organisms per square meter
of bottom area. Here again, a. measure which is a function of this variable was used because there is
no practical method of quantifying actual substrate rugosity. The technique used in this study was
similar to that described by Risk (1972) and Luckhurst and Luckhurst (1978). Divers used a fine link
brass chain to follow the contour of the substrate along a straight line to determine ratio of actual
surface distances relative to linear distances. The brass chain was wrapped with white cloth tape at
every fifth link and actual surface distances were recorded in number of chain links. These measures
were then converted to real distances by multiplying the number of chain links times average length
of each link (12.1 mm).
Census Techniques. - Ichthyofauna on Twin Barges was visually censused from 8 August 1979 through
I August 1980. Both reefs were always censused in succession on the same day and about I h apart
to insure that similar environmental conditions were affecting fish communities.
All censuses were conducted along three I-m-wide transects positioned along the sides and down
the Center of each barge (Fig. I) using a variation of the technique originally developed by Brock
(1954). Two divers first positioned themselves at diagonally opposite comers of the barge. Each diver
then swam the length of his transect recording identity and abundance of each species occurring
between the barge deck and I m above its surface. This portion of the census primarily included
larger, more mobile fishes (e.g., Carangidae). Divers next swam to the center transect and began moving
toward each other, counting all fish occurring on the barge deck or inside the barge. This portion of
the census was directed toward smaller, more sedentary fish species (e.g., Serranidae). Distance traveled
by each diver along the center transect of each barge varied between censuses and was never measured.
CHANDLER
ET AL.: REEF FISH COMMUNITY
STRUCTURE
131
A
B
Figure 2.
Decking on Offshore (A) and Inshore (B) Barges showing relative condition of metal plat'ing.
Therefore, counts from the two outside transects were averaged and combined with the sum of diver
counts from the center transect to produce a single census of all fish occurring in a volume I x 2 x
13.35 m on each reef during each census date.
A second census method was used tei count pelagic species occurring in midwaters above each reef.
After completing the transect census, both divers positioned themselves back-to-back over the center
of the barge and, upon ascent, censused all fish in their range of vision between a point I m above
the barge and the surface. Midwater counts by the two divers were summed to provide a census of
all fish occurring over the entire barge for each census date.
.
Bottom temperature was measured prior to ascent using a hand held field thermometer and recorded,
along with census counts, on white plastic slates. These data were later transferred to field notebooks.
132
BULLETIN OF MARINE SCIENCE. VOL. 37. NO. I. 1985
Table 1. Physical measurements
Measurements
of reef substrates
Inshore BaIge
Metal linear distance
Metal contour distance
Open sand distance
Protected sand distance
Total linear distance
Total contour distance
38.23
60.55
2.45
13.72
40.68
76.72
m
m
m
m
m
m
Offshore Barge
16.99
65.06
10.83
16.11
27.82
92.00
m
m
m
m
m
m
Ichthyofauna Classijication.-Determination
of how substrate complexity affected fish communities
required that each species be assigned to one of three classifications based on their abundances and
dependence on reef substrates. Resident fishes comprised the largest of the three classes and were
strongly dependent on reefs for basic niche requirements. This group primarily included demersal,
cryptic, and suprabenthic species which utilized the reef as a spawning site, source of food, or for
protective cover (e.g., Apogonidae, Blenniidae). Their abundances were fairly constant and easily
counted by divers, varying only slightly between each census date. The semiresident classification
included those species which were less dependent on reefs for food or shelter and did not appear to
maintain permanent residency on either barge. Instead, these fishes used the reefs as a visual reference
point or as a visual distractant against predators. This group was represented primarily by highly
abundant schooling species which were pelagic (e.g., Carangidae) or suprabenthic in nature (e.g.,
Rhomboplites aurorubens). Transient species, which constitute the third class, occurred on Twin Barges
so infrequently that their dependence on reefs could not be determined. Though the effect which these
species had on other reef fish populations was generally negligible, these fish were excluded from all
analyses to avoid any bias which they might generate in the results.
AnalYSis of Data. - Four parameters of community structure were used to compare ichthyofaunas at
the Twin Barges: density, species richness, evenness, and similarity. Density (N) is the total number
of individuals and species richness (S) is the total number of species counted during each census.
Evenness (E) was measured using an index developed by Heip (1974):
eH' - 1
E=-S - 1
where e is the base of natural logarithms and H" is species diversity as calculated using the ShannonWeiner information function (Pielou, 1966). Similarity (C) of ichthyofauna occurring on Twin Barges
for each census date was determined using the Morisita index (Wolda, 1981):
Only those classes which were directly affected by the three substrate variables were used to calculate
parameters of community structure. Two variables, substrate rugosity and percent substrate cover,
are largely responsible for determining availability of space resources on each barge. Therefore, counts
of resident fishes were used to calculate N, S, E, and C from transect census data because space
resources are used primarily by these taxa. Disproportionally high abundances of a select few species
as well as reduced species richness negated the analysis of S, E, and C for semiresident ichthyofauna.
However, N for each transect and midwater census was used to show relative distribution of semiresident populations in these two zones. All comparisons using densities of semiresident species were
based on natural logarithm transformations of N.
RESULTS
Substrate Cornplexity.-Deck
surface area was the single most important factor
distinguishing the Twin Barges. Physical measurements (Table 1) of reef substrates
indicate that 94% of the total linear distance (TLD) on Inshore Barge was metal
substrate, as most of its decking was still intact. An average of 1.49 contour meters
of barge metal was measured for each linear meter of this reef. By comparison,
large sections of decking had eroded away on Offshore Barge and the metal sub·
CHANDLER
ET AL.: REEF FISH COMMUNITY
133
STRUCTURE
24
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10
20
M
5
c:
75
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1· .~,
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t:: ~
t "'.
50
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l
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,.......... "'~---"---"" •.........•....•..-
- 16
'
o
UG
SEP
OCT
NOV
DEC
.IAN
FEB
- 15
MAR
APR
MAY
Jim
JUL
MI)NTH
Figure 3. Bottom water temperatures (dotted line) and total abundance (N) of resident fishes observed
during transect censuses on Inshore (dashed line) and Offshore (solid line) Barges between 8 August
1979 and I August 1980.
strate comprised only 61.1 % of that reefs TLD. However, access to more protected metal and sand substrates inside the compartments was greatly enhanced
and, as a result, Offshore Barge had an average of 2.34 m of metal contour per
linear meter of reef.
Sand substrates represented 39.7% ofInshore Barge TLD. Open sand on this
reef accounted for only 6% of its TLD and the remaining 33.7% was protected
sand generally accessible only through small holes in the metal plating ofInshore
Barge. The internal area of Offshore Barge was completely exposed with open
(38.8%) and protected sand (58%) substrates together comprising 96.8% ofTLD.
Ratio of contour to linear distances for all reef substrates was 1.89 and 3.31 on
Inshore and Offshore Barges, respectively. These estimates of substrate rugosity
indicate that Offshore Barge had 1.75 times as much accessible substrate area as
Inshore Barge.
Hydrographic Data.-Effects
of certain environmental variables (e.g., temperature, turbidity, current) on the two reef fish communities have been discussed
(Sanders, 1983) and are not considered in this paper. However, the considerable
differences between summer and winter fish populations made it necessary to
separate censuses into "summer" (~21°C) and "winter" (::5; 18°C) collections. Periods between seasonal data sets were marked by rapid changes in water temperatures. Bottom water temperature dropped 6°C in 6 wk reaching its lowest point
(15°C) 10 January 1980. A gradual warming trend followed with water temperature
rising to 17.5°C on 11 May 1980. Bottom water temperatures rose 6.5°C over the
next 3 wk and never fell below 21°C for the remainder of the study. Water
temperatures of 19°C were recorded during censuses of 12 and 19 December 1979
and these data were not included in analysis of seasonal data sets. Censuses taken
between 27 December 1979 and 11 May 1980 were considered as winter data;
all other censuses are considered as summer data.
Community Structure of Resident Ichthyofauna. - Total abundance of resident
ichthyofauna was greater at the Offshore Barge for all but one transect census date
134
BULLETIN OF MARINE SCIENCE, VOL. 37, NO.1.
1985
Table 2. Frequency of occurrence and average abundance of fishes seen on Inshore and Offshore
Barges during summer (~21°C) transect censuses
Inshore Barge (21 censuses)
Taxon
Dasyatis Spp.
Family Congridae
Sardinella aurita
Opsanus pardus
Family Ogcocephalidae
Centropristis ocyurus
C. striata
Diplectrum formosum
Epinephelus morio
Mycteroperca microlepis
M. phenax
Serranus sub/igarius
Rypticus maculatus
Apogon pseudomaculatus
Caranx bartholomaei
c. crysos
C. ruber
Decapterus punctatus
Seriola dumerili
Lutjanus campechanus
L. griseus
Rhombop/ites aurorubens
Family Haemulidae
Haemulon auro/ineatum
H. plumieri
Orthopristis chrysoptera
Archosargus probatocephalus
Calamus leucosteus
Diplodus holbrooki
Lagodon rhomboides
Equetus acuminatus
Chaetodipterus faber
Chaetodon ocellatus
C. sedentarius
Holacanthus spp.
Chromis enchrysurus
C. scotti
Pomacentrus variabilis
Ha/ichoeres caudalis
Family Scaridae
Sphyraena barracuda
Family Blenniidae*
Coryphopterus
punctipectophorus
Scorpaena plumieri
Para/ichthys albigutta
Balistes capriscus
Lactophrys quadricornis
Chi/omycterus schoepfi
Unknown fry
All residents
All semiresidents
All transients
Class·
T
R
SR
cR
T
cR
R
T
R
cR
R
cR
cR
cR
SR
SR
SR
SR
SR
cR
cR
SR
T
SR
cR
SR
cR
T
cR
R
cR
SR
R
R
cR
cR
cR
cR
cR
R
SR
cR
cR
R
cR
cR
R
R
SR
R&cR
SR
T
Fre-
Percent
fre-
quency
Quency
0
0
0
3
2
10
2
5
2
10
0
I
18
14
3
3
2
I
6
12
14
3
10
0
21
17
13
4
4
0
2
6
12
0
0
14
2
0
II
21
2
I
16
5
0
5
19
I
I
I
a
0
14
10
48
10
24
10
48
5
86
67
14
14
10
5
29
57
67
14
48
0
100
81
62
19
19
0
10
29
57
0
0
67
10
0
52
100
10
5
76
24
0
24
90
5
5
5
Average
abundance
0.0
0.0
0.0
0.14
0.14
2.24
0.1
0.36
0.1
3.62
0.05
3.67
1.6
0.29
0.24
0.55
0.1
1,210.4
2.83
2.0
0.14
1,044.07t
0.0
1,950.93
1.55
7.9
0.19
0.19
0.0
0.05
0.38
2.74
0.0
0.0
0.95
0.19
0.0
1.33
21.45
0.14
0.02
4.38t
0.67
0.0
0.29
2.98
0.02
0.02
71.36
48.5
4,291.14
0.69
Offshore Barge (20 censuses)
Fre-
Percent
fre-
Average
quency
quency
abundance
1
I
2
17
a
8
0
0
1
14
2
20
14
15
0
4
1
5
12
14
11
12
1
20
19
0
11
4
5
0
16
13
2
2
20
4
6
20
20
0
0
20
14
I
6
19
0
I
4
5
5
10
85
0
40
0
0
5
70
10
100
70
75
a
20
5
25
60
70
55
60
5
100
95
0
55
20
25
0
80
65
10
10
100
20
30
100
100
0
0
100
70
5
30
95
0
5
20
0.05
0.05
274.95
1.85
0.0
1.8
0.0
0.0
0.05
3.9
0.05
10.45
2.73t
3.05t
0.0
0.65
0.03
3,049.97t
3.55
1.0
1.25
485.9
0.03
2,671.17
2.35
0.0
1.08
0.35
0.15
0.0
1.0
4.25
0.5
0.5
3.55
0.35
0.6
3.7
27.43
0.0
0.0
8.95
13.9
0.05
0.7
4.53
0.0
0.03
616.4
95.55
7,106.87
0.43
• R = resident; cR = common resident; SR = semiresident; T = transient.
t Includes one extreme census count (> mean ± two standard deviations). Adjusted average abundances are: R. maculatus = 2.21; A.
pseudomaculatus = 1.84; D. puncta/us ~ 315.77; R. aurorubens = 583.79; Family Blenniidae
Represents combined. counts of Parablennius marmoreus and Hyp/eurochilus geminatus.
*
= 2.78.
CHANDLER lOTAL.: REEF FISH COMMUNITY
135
STRUCTURE
Table 3. Frequency of occurrence and average abundance of fishes seen on Inshore and Offshore
Barges during winter (~ 18OC)transect censuses
Inshore Barge (II censuses)
FreClass·
Taxon
Family Congridae
Opsanus pardus
Centropristis ocyurus
Mycteroperca microlepis
M. phenax
Serranus sub/igarius
Rypticus maculatus
Family Echeneidae
Decapterus punclalus
Seriola dumerili
Lutjanus campechanus
L. griseus
Rhombop/ites aurorubens
Haemulon auro/ineatum
H. plumieri
Orthopristis chrysoptera
Archosargus probalocephalus
Calamus leucosteus
Equetus acuminatus
Chaetodipterus faber
Chaetodon ocellatus
Holacanthus spp.
Pomacenlrus variabilis
Ha/ichoeres cauda/is
Family Blenniidaet
Coryphopterus
punctipectophorus
Paralichthys albigutla
Syacium papillosum
Ba/istes capriscus
Chilornycterus schoepfi
All residents
All semi residents
All transients
• R = resident; cR = common
resident;
quency
4
4
4
11
1
5
0
0
0
0
7
0
cR
cR
cR
cR
R
cR
cR
T
SR
SR
cR
cR
SR
SR
cR
SR
cR
T
cR
SR
R
cR
cR
cR
cR
I
4
9
5
2
4
0
0
0
10
0
3
2
cR
R
R
cR
R
R&cR
SR
T
SR
= semiresident;
0
2
1
II
I
T
Percent
freQuency
Average
abundance
Offshore Barge (II censuses)
Frequency
Percent
fre-
Average
quency
abundance
36
36
36
100
9
45
0
0
0
0
64
0
9
36
82
45
18
36
0
0
0
91
0
27
18
0.45
0.55
0.36
4.82
0.09
1.09
0.0
0.0
0.0
0.0
1.64
0.0
4.55
39.77
4.05
13.05
0.14
0.27
0.0
0.0
0.0
0.91
0.0
0.55
0.36
0
10
1
9
1
10
3
1
2
3
3
3
7
9
9
I
5
0
6
6
1
11
5
8
7
0
91
9
82
9
91
27
9
18
27
27
27
64
82
82
9
45
0
55
55
9
100
45
73
64
0.0
2.45
0.09
3.36
0.27
5.32
0.91
0.09
2.45
0.32
0.45
0.5
78.18
289.95
1.64
0.09
0.5
0.0
1.14
5.45
0.14
2.32
0.45
2.0
1.64
0
18
9
100
9
0.0
0.18
0.09
4.41
0.05
19.74
57.37
0.27
10
1
0
10
1
91
9
0
91
9
8.64
0.09
0.0
6.32
0.14
38.37
376.44
0.09
= transient.
t Represents combined counts of Parablennius marmoreus and l/ypleurochilus geminatus.
(Fig. 3). Seasonal mean abundances at Offshore Barge (summer-95.6;
winter38.4) were twice as large as those at Inshore Barge (summer-48.5;
winter-19.7).
Of the 32 resident taxa seen on the Twin Barges during summer censuses, 21
occurred with a frequency ~20% on at least one barge and were therefore considered to be common (Table 2). Nineteen common residents occurred on Inshore
Barge but only two had average abundances higher than those of Offshore Barge.
All 21 common residents were present on Offshore Barge. Similar trends were
seen during winter when 22 resident taxa were censused on the barges (Table 3).
Sixteen of the 17 taxa considered common residents were observed on Offshore
Barge, 12 of which occurred in higher densities compared to those at Inshore
Barge. Only 12 taxa were common on Inshore Barge and five of these had higher
abundances. Details concerning distribution of individual taxa were discussed by
Chandler (1983) and are not considered in this paper.
Similarity of species between the two barges for each census date was initially
136
BULLETIN OF MARINE SCIENCE. VOL. 37, NO. I. 1985
17.5
15.0
'"
'"..,
'"
12.5
~
u
;;;
10.0
..,~
'"
u
..,
7.5
..'"
5.0
2.5
AUG
SEP
OCT
NOV
OEC
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
MONTH
Figure 4. Species richness (S) of resident fishes observed during transect censuses on Inshore (dashed
line) and Offshore (solid line) Barges between 8 August 1979 and I August 1980.
high during August and September (0.7-1.0) but underwent a gradual decline until
early May 1980 when it began to exhibit extreme fluctuations.
Offshore Barge maintained a greater species richness of resident ichthyofauna
for all but one transect census date (Fig. 4). Averages of 14.7 and 9.2 resident
species were seen per summer census on Offshore and Inshore Barges, respectively.
These averages were reduced to 10.5 and 6.5, respectively, for winter censuses.
Evenness was the only parameter not to exhibit well defined differences between
the two barges (Fig. 5). Ninety-five percent Bonferroni confidence intervals (Timm,
1975) calculated for both winter and summer censuses indicate no significant
differences existed between reefs for this parameter.
Community Structure of Semiresident Ichthyofauna.-Although
resident fishes
were by far the most stable and diverse portion of the ichthyofaunal community,
they represented only a small fraction of the fishes censused. Excluding transient
taxa, semiresident fishes accounted for more than 98% of all fish observed during
summer transect censuses on either reef (Table 2). Despite reductions in population densities during colder months, these fish maintained their dominance by
representing 90.8 and 74.4% of winter transect abundances on Offshore and Inshore Barges, respectively (Table 3). Dominance by semiresident species was even
more pronounced in midwater censuses which had infrequent occurrences of
resident and transient species.
Inshore Barge supported nearly 75% of the ichthyofauna seen on both barges
during summer midwater censuses (Table 4). The same reef, however, held less
than 1% of the semiresidents seen in winter midwater censuses, while Offshore
Barge continued to support moderate populations of semiresident species (Table
5). Offshore Barge supported 53.5% of the semiresident ichthyofauna seen on both
reefs during summer transect censuses (Table 2). Offshore Barge was again the
most productive reef during colder months supporting an average of73.3% of the
semiresident fish seen in each winter transect census (Table 3).
Paired t-tests were performed for each season and census type to determine if
the two barges supported equal proportions (arcsin transformed) of semiresident
137
CHANDLER ET AL.: REEF FISH COMMUNITY STRUCTURE
.9
.8
1-
t.
•7
<oJ
'"'"
<oJ
.6
'"'"
<oJ
>
.
'"
•5
.4
.
.•...... ...
--
•3
AUG
~~p
OCT
NOV
D
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
Figure 5. Evenness (E) of resident fishes observed during transect censuses on Inshore (dashed line)
and Offshore (solid line) Barges between 8 August 1979 and I August 1980.
fishes seen during each census date (Table 6). The results indicate semiresident
fishes occurred in significantly greater abundances on Inshore Barge during summer midwater censuses but showed no distinct preference for either reef in summer transect censuses. Losses due to emigration and predation, however, were
more severe on Inshore Barge during colder months. As a result, significantly
greater abundances of semiresident fishes were observed on Offshore Barge for
both mid water and transect winter censuses.
DISCUSSION
The structural complexity of Twin Barges was comparable to that of Caribbean
coral reefs. Luckhurst and Luckhurst (1978) used techniques similar to those
employed in this study to obtain multiple substrate rugosity measurements (SR)
from 16 quadrats (each 9 m2) on coral reefs at Cura9ao and Bonaire, Netherlands
Antilles. These workers reported a maximum mean SR value (3.62) which was
slightly greater than that found on Offshore Barge (3.31) but reported individual
measurements as high as 4.6 for the most complex quadrates. The SR of Inshore
Barge (1.89) was considerably less than that of Offshore Barge but was similar to
the mean SR (1.93) for all of Luckhurst and Luckhurst's 16 quadrats.
Our results indicate that the greater availability of space resources allowed
Offshore Barge to consistently support a larger, more diverse resident ichthyofaunal community than Inshore Barge. Other environmental variables were similar on both reefs for each census date and should not have contributed to differences in the ichthyofaunal communities. Availability of certain food resources
(i.e., infaunal invertebrates and biofouling communities) was undoubtedly higher
on Offshore Barge but this too was dependent on the amount of sand and metal
substrates occurring on each reef.
It is possible that presence of midwater structures on Inshore Barge in some
way affected the recruitment and distribution of resident individuals on that reef.
However, preliminary observations and initial diver censuses indicate that dif-
138
BULLETIN OF MARINE SCIENCE, VOL. 37, NO. I, 1985
Table 4. Frequency of occurrence and abundance offishes seen on Inshore and Offshore Barges during
summer (~21°C) midwater censuses
Inshore Barge (17 censuses)
Offshore Barge (17 censuses)
Percent
FreTaxon
Sardinella aurita
Family Engraulidae
Mycteroperca microlepis
Rachycentron canadum
Caranx bartholomaei
c. crysos
Decapterus punctatus
Seriola dumerili
Lutjanus campechanus
Rhomboptites aurorubens
Haemulon aurolineatum
Chaetodipterus faber
Euthynnus alletteratus
Balistes capriscus
• R = resident; SR = semiresident; T
quency
quency
SR
3
2
18
12
6
0
12
29
53
71
6
47
88
24
6
41
T
R
I
T
0
2
5
9
12
SR
SR
SR
SR
R
SR
SR
SR
=
I
8
15
4
T
I
R
6
Percent
fre-
Class·
Average
abundance
70,646.82
6,470.47
0,06
0.0
0.65
22.91
134,822.45
62.26
0.47
3,064.35
16,885.59
16.56
0.21
0.65
FreQuency
3
0
2
I
0
5
6
15
I
5
II
9
2
8
frequency
18
0
12
6
0
29
35
88
6
29
65
53
12
47
Average
abundance
18,823,24
0.0
0.12
0,03
0.0
21.53
12,052,59
111.41
0.29
764.5
3,918.65
98.27
0.24
2.09
transient.
ferences already existed in resident populations prior to deployment of midwater
structures.
Distribution of some fishes on Twin Barges may be associated with availability
of specific space resources. Luckhurst and Luckhurst (1978) investigated the relationship between three gobiid species and percentage substrate cover on a reef
at Curac;ao, Netherlands Antilles. Two of the species, Coryphopterus glaucofraenum and Gnatholepis thompsoni, were always associated with bottom substrates and showed significant correlations between their abundances and area of
sand available. A similar relationship was observed for C. punctipectophorus on
Twin Barges. This species was found exclusively on sand substrates inside barge
compartments and exhibited a cryptic coloration similar to its surroundings which
made it very difficult to see from above. Ninety-seven percent of total linear
distance (TLD) measured (i.e., bottom surface area) on Offshore Barge was sand
substrate so it is not surprising that C. punctipectophorus was seasonally either
Table 5. Frequency of occurrence and abundance of fishes seen on Inshore and Offshore Barges during
winter (:::;18°C) mid water censuses
Inshore Barge (11 censuses)
Species
Mycteroperca microlepis
Rachycentron canadum
Decapterus punctatus
Seriola dumerili
Lutjanus campechanus
Rhomboptites aurorubens
Haemulon aurotineatum
Chaetodipterus faber
Batistes capriscus
• R
=.
Class'
R
Frequency
2
Average
Fre-
Percent
fTe-
quency
abundance
quency
quency
18
T
a
SR
SR
R
SR
SR
SR
R
0
a
a
a
a
3
27
I
9
0
18
resident; SR = semiresident; T = transient.
a
a
2
Offshore barge (II censuses)
Percent
fTe-
a
0.55
0.0
0.0
0.0
2.64
0.0
90.82
0.0
1.45
0
I
2
2
2
3
3
7
2
a
9
18
18
18
27
27
64
18
Average
abundance
0.0
0.09
999.82
13.55
7.36
2,772.45
968.1
207.45
0.45
CHANDLER
ET AL.: REEF FISH COMMUNITY
139
STRUCTURE
Table 6. Paired i-tests on semiresident fish abundances censused at Inshore and Offshore Barges.
Listed are the total number of censuses a species was observed on at least one reef (N), i-test value
(1), probability of getting a greater absolute value of T (PR), and mean of the differences
Census type
N
T
PR
Mean
Summer midwater
Winter midwater
Summer transect
Winter transect
17
9
20
11
3.64
-38.16
-0.58
-2.39
0.0022
0.0001
0.566
0.0408
0.581
-1.531
-0.09
-0.67
the first or second most abundant fish on this reef. In comparison, less than 40%
of the TLD on Inshore Barge was sand substrate and only small populations of
this goby were found on the reef. Methods used to measure substrate variables
did not allow divers to determine relative amounts of horizontal and vertical
metal plating on each barge. However, the decking on Inshore Barge was almost
completely intact and provided about twice as much external horizontal area
compared to Offshore Barge. It is likely that particular niche requirements of some
resident fish included a need for this type of habitat and may account for the
distribution of those few species (e.g., Centropristis ocyurus) having greatest abundances on Inshore Barge.
Our conclusions regarding resident ichthyofaunas of Twin Barges are based on
the assumption that any differences which occurred in fish community structure
resulted from inequalities in vertical relief or available space resources. All other
environmental variables (e.g., temperature, turbidity, depth, etc.) were nearly
identical at both reefs for each census date and did not appear to differentially
impact abundance or distribution of individual species. However, validity of this
assumption is somewhat dependent on the extent to which random processes
affected their populations.
Sale and his coworkers (Sale and Dybadahl, 1975; 1978; Sale, 1977; 1978; 1980;
Talbot et al., 1978) have shown that when space resources on a reef are limited,
composition ofichthyofaunal communities occupying a given area is determined
by chance arrival of larval recruits capable of filling spaces vacated through predation. These researchers believe greater interspecific competition for space increases the influence of stochastic processes in determining structure of coral reef
fish communities, and that such reefs may be less stable and predictable than has
been traditionally thought. However, the reef recruitment studies listed above
were concerned primarily with larval colonization of small unoccupied reefs by
tropical species and did not consider effects which long term biological interactions
might have on stability or composition of fish communities (Helfman, 1978).
Inshore and Offshore Barges, on the other hand, were moderate in size and supported well established fish communities. It is likely that resident individuals had
a significant impact on the structure of their fish communities through selective
predation and social facilitation of incoming recruits. Furthermore, species richness on Twin Barges was less than that in tropical areas and interspecific competition for space resources should be considerably reduced.
Seasonal changes in evenness and similarity indicate structure of fish communities on Twin Barges was affected primarily by recruitment of new individuals
during summer months, and by predation or mortality rates in winter. The greater
influx of new recruits for some resident fishes (e.g., Ha/ichores caudalis) during
summer produced communities dominated by a relatively few number of species.
Furthermore, similarity offish communities tended to be higher during this period
140
BULLETIN OF MARINE SCIENCE, VOL. 37, NO. I, 1985
because both reefs recruited from the same species pool. With declining water
temperatures, losses due to predation and mortality began to exceed population
gains due to recruitment at each barge. These losses were apparently greatest for
more dominant species as evenness increased in winter months. Lack of recruits
in this season also caused reductions in similarity of resident ichthyofaunas as
the structure of each reef fish community became more dependent on the type
and availability of habitats offered by each reef.
Extremely low similarities «0.1) encountered in early June 1980 resulted from
a sudden influx of new recruits to each reef. Availability of space resources was
highest during this period and the structure of resident communities was totally
dependent on random recruitment of larval fishes. As available space resources
became filled, resident communities were structured by more deterministic processes such as predation and competitive interaction which favor more aggressive
speCIes.
Distribution of semiresident fish was such that they formed an "envelope" over
each barge, the thickness of which varied with abundances observed during each
census. Greatest densities always occurred immediately above each barge and
thinned rapidly as distance above the deck increased. Summer censuses indicate
that Inshore Barge supported more semiresident fish by allowing the envelope to
extend higher on mid water structures. Midwater censuses produced densities of
these fish which were slightly greater on Inshore Barge for most summer census
dates. Transect censuses, however, indicate no difference in fish densities between
the two barges for the region immediately above the decking.
Semiresident fishes began to emigrate from the study area as water temperatures
dropped and the height of envelopes above both barges decreased. Semiresident
fish were virtually absent from midwater and transect censuses over Inshore Barge
during winter, but were nearly always present above Offshore Barge.
Semiresident fish appeared to prefer bottom reef habitat over that provided by
midwater structures. As long as semiresident fish density exceeded the abundances
which could be supported by metal substrates, excess individuals remained on
Inshore Barge and utilized suspended tire structures as points of reference or for
predator avoidance. Offshore Barge began to support greater densities of semiresident fish when abundances fell to a level less than or equal to that which could
be supported by metal substrates. Predator-prey interactions observed on each
reef indicate that the broken pattern of the Offshore Barge decking was a better
visual distractant to predators and provided a greater number of escape routes
for potential prey than did that on Inshore Barge.
Our results indicate each reef may have been too small to attract or support
schooling pelagic predators. Abundances of semiresident fishes were generally
large compared to resident populations, but frequency of occurrence for pelagic
predators on Twin Barges was relatively low compared to results of other studies
conducted in the area. Although the uppermost tires of midwater structures were
usually visible to divers floating at the surface directly above the reef, the tops of
these structures were still 14 m below the surface. This, along with size of the
barge, made it necessary for pelagic fish schools to come relatively close before
locating and orienting to Inshore Barge. Once present, many of the high energy
pelagic predators may have maintained only short residency times requiring a
larger reef area for orientation. In effect, midwater structures may not have been
high enough nor the barges large enough to attract large numbers of pelagic
predators.
The lack of large pelagic predators may also be related to the specific depth
preferences of these fish and to annual variations in their abundances. Klima and
CHANDLER IT AL.: REEF FISH COMMUNITY
STRUCTURE
141
Wickham (1971) examined attraction of pelagic fish to small artificial structures
off Panama City, Florida and found that small baitfish were attracted more to
structures suspended in midwaters rather than at the surface, while larger predators
tended to prefer surface structures. Wickham and Russell (1974) examined attraction of pelagic fishes to suspended artificial structures in an area not far from
Twin Barges, but only one of the three dominant predators observed in this study
was the same as those observed by Klima and Wickham 2 years earlier. Hastings
et al. (1976) also noted annual differences in the occurrence of pelagic baitfish
attracted to two structures resembling oil platforms located offshore Panama City.
Very little has been written concerning effects of substrate complexity on pelagic
schooling species, and importance of this relationship may have been underestimated. Availability of funds has generally limited organizations to the most
inexpensive materials for construction of artificial reefs. Although cost requirements for aquisition and placement of complex structures may be high, long term
cost effectiveness using these substrates to attract demersal as well as pelagic fishes
may outweigh losses in volume of material deposited. Midwater structures certainly help to attract greater numbers of pelagic species to existing reefs but, once
present, these fish (especially small bait fish) may associate more with bottom
than suspended structures. In such instances, deployment of midwater structures
around the periphery of existing reefs may be most effective.
ACKNOWLEDGMENTS
We thank T. Traviesa, D. Grizzard, and the staff of the Panama City Marine Institute for financial
and logistical support of this study. Special thanks also go to E. Nakamura, and the National Marine
Fisheries Service at Panama City, Florida and Galveston, Texas for furnishing boat time and photographic equipment. We are also indebted to D. Pitts, G. Guillen, and C. K. Chandler for their help
in coUecting data. M. Muehsam served as statistical consultant and assisted in the analysis of data.
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DATEACCEPTED: November
16, 1984.
ADDRESSES:(C.R.C.) LGL Ecological Research Associates. 1410 Cavitt. Bryan. Texas 77801; (R.M.S.)
Kansas Fish and Game Commission, 512 E. 9th, Lawrence, Kansas 66044; (A.M.L.) Department of
Marine Biology, Texas A&M University at Galveston, Galveston. Texas 77550.