ICES Journal of Marine Science, 59: S122–S126. 2002
doi:10.1006/jmsc.2002.1210, available online at http://www.idealibrary.com on
Comparison of the development of coral and fish communities on
rock-aggregated artificial reefs in Eilat, Red Sea
Avigdor Abelson and Yehiam Shlesinger
Abelson, A., and Shlesinger, Y. 2002. Comparison of the development of coral and fish
communities on rock-aggregated artificial reefs in Eilat, Red Sea. – ICES Journal of
Marine Science, 59: S122–S126.
Despite potential advantages of artificial reefs in areas where natural coral reefs have
degraded, relatively little research has been undertaken in Eilat to improve our
understanding of the major factors governing the development of reef biota. We report
on the first study in a series aimed at increasing our knowledge of the effects of
morphology, substrate type, and location on the succession of reef organisms. The
development of stony corals and fish communities associated with two types of
constructions was examined, which were both made of aggregates of limestone rocks:
one randomly aggregated (RA) reef comprising relatively small rocks and an orderly
aggregated (OA) reef composed of relatively big rocks. Communities were censused
every 4–6 months for more than 4 years, with a final coral census being taken after 100
months. The OA reef attracted significantly more fish species and a higher number of
individuals than the RA reef, and reached its carrying capacity faster (30 months
versus 50 months). In contrast, number of reef-building corals on the RA reef was
significantly higher (in terms of both species and colonies) than on the OA reef, and the
plateau was not even reached after 100 months. We conclude that in the Gulf of
Aqaba, (1) for recreational purposes, small reefs (of a few cubic metres) may serve as
attractive sites because they support relatively rich fish communities, (2) larger rocks,
larger interstices and larger reef size induce higher species richness and greater
numbers of fish, and (3) structural complexity, as measured by fractal dimension, is an
important factor for the development of reef-building corals.
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
All rights reserved.
Keywords: artificial reef, complexity, coral reefs, fractal dimensions, recreational
diving, Red Sea, reef fish, stony corals.
Accepted 26 October 2001.
A. Abelson: Institute for Nature Conservation Research, Tel Aviv University, Ramat
Aviv 69978, Israel. Y. Shlesinger: Section of Environment, Municipality of Eilat, PO
Box 14, Eilat 88100, Israel. Correspondence to A. Abelson: e-mail:
[email protected]
Introduction
It is currently well recognized that coral reefs around
the world are experiencing massive deterioration
(Wilkinson, 1992; Richmond, 1993; Grigg, 1994;
Hughes, 1994; Hinrichsen, 1997). Wilkinson (1992),
based on assessments of global reef resources by UNEP/
IUCN and other published material, classified the coral
reefs of the world according to three categories of
disturbance: stable, threatened, or critical. The coral
reefs of the Gulf of Aqaba are considered to be in critical
condition, where ‘‘critical’’ is defined as being severely
damaged and in imminent danger of collapse or extermination (Wilkinson, 1992). The reefs of Eilat are
among the most deteriorated reefs in the Gulf, as
1054–3139/02/0S0122+05 $35.00/0
indicated by the reduction in live cover species diversity
and recruitment (Loya, 1975, 1986; Fishelson, 1973,
1995).
One way to counter the degradation of natural reefs is
to establish artificial reefs (AR) to provide alternative,
environment-friendly fishery sources and recreation sites
(Bohnsack and Sutherland 1985; Seaman and Sprague,
1991; Collins and Jensen, 1999). There are several
expected environmental benefits from AR. First, they
are expected to contribute to the conservation of natural
reefs by diverting human activities from them. Second,
they will offer refuges for rare and disappearing species
of invertebrates and fish. And third, they may provide
nursery grounds for young stages of reef species (Collins
and Jensen, 1999).
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
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Table 1. Summary of physical parameters of the orderly
aggregated (OA) and randomly aggregated (RA) reefs.
Parameter
Area covered (m2)
Bulk volume (m3)
Maximum hole diameter (m)
Dispersion*
Maximum height (m)
Rock size (diameter in cm)
Complexity–fractal dimension
OA
RA
12
15
0.6
0.083
2.5
49.512.3
1.230.05
4.9
3
0.05
0.047
1.5
18.95.6
1.500.1
*Dispersion=bulk volume/(area covereddepth).
100
Nature reserve
200
Fl.10s64m2IM
Hotel
Figure 1. Map of the northern part of the Gulf of Aqaba, Red
Sea, showing the artificial reef site (arrow) at the southern edge
of the natural reef of Eilat.
Despite the clear advantages of AR, in Eilat relatively
little has been done to improve our understanding of the
major factors governing the development of their biota. This is reflected in the lack of prior planning and
insufficient subsequent monitoring of the development
of the marine community, as well as the lack of
resources for optimum design and construction of reef
structures (but see Rinkevich, 1995). We hope to overcome the gap in our understanding of AR dynamics and
development by examining sound designs, incorporating
scientific knowledge of the important parameters that
determine the development of biota and the final
appearance of the reef.
This study is the first in a series aimed at increasing
our knowledge of the effects of reef structure, location,
morphology and substrate type on the succession of
organisms. Based on the comparison between two structures differing in their rock size and arrangement,
we examine the effectiveness of these designs in the
development of reef-building corals and fish communities and evaluate the relative importance of each feature
in supporting higher species richness and number of
individuals.
Materials and methods
The study site is situated in the southern part of the
Coral Reserve of Eilat off the natural fringing reef at a
depth of 15 m (Figure 1). The sea bottom at this site is
covered by small aggregations of corals. The two AR
constructions were designed as imitations of natural reef
knolls and deployed in December 1989 on a rubble-sand
bottom, 30 m apart and 100 m from the nearest natural
reef. The same substrate type present in local natural
reefs was applied (i.e. limestone rocks). One reef was
arranged as a randomly aggregated pile of relatively
small rocks (RA reef), while the other comprised an
orderly aggregated construction of relatively large
rocks with large interstitial spaces (OA reef). Physical
characteristics are given in Table 1.
Observations on the numbers of corals and fishes
associated with the two reefs were carried out by
SCUBA surveys every 4–6 months for over 4 years. An
additional coral survey was made after 100 months.
Each survey lasted for 7–10 d, during which three daily
dives were made: at dawn, midday, and at sunset. Visual
censuses of fish were performed along established lines
on both sides of each reef (Russell et al., 1978). Species
identification was based on Randall (1983) and Khalaf
and Disi (1997). Corals were censused visually by complete surveys of the rocks. Coral species identification
was based on Loya and Slobodkin (1971). Both reef
structures were divided into sections to enable reliable
censuses with no repeated counts of the same individuals. Results of fish counts are based on maximum
numbers of individuals during one dive, while number of
species was calculated for the entire period of each
observation. Comparison of fish and coral communities
at the OA reef (532 m high) and the nearest natural
reef was conducted in 1996, seven years after the deployment of the AR. Fish counts were performed using the
belt-transect technique (Russell et al., 1978). The size of
the belt was 53 m, which is the same order of magnitude as the OA. Data on number of fish and coral
species and number of individuals on the RA and OR
reefs were analysed using two-way ANCOVA and those
on the OA and natural reefs were using a t-test.
Assessment of the structural complexity of the reef
structures was carried out using the divider method to
calculate the fractal dimension index (D), a measure of
the decrease in apparent length of the transect as the
interval of measurement increases (Sugihara and May,
1990). Three 1-m-long transects were sampled randomly
S124
A. Abelson and Y. Shlesinger
70
30
(a)
60
(b)
# species
# species
50
20
10
40
30
20
OA
RA
10
0
0
400
800
300
200
100
0
(d)
# individuals
# individuals
(c)
10 20 30 40 50 60 70 80 90 100 110
Time (mo)
600
400
200
0
10
20
30
40
Time (mo)
50
60
Figure 2. Colonization of the randomly aggregated (RA) and orderly aggregated (OA) reefs based on time series of: (a) Number
of coral species (100 months); (b) number of fish species (56 months); (c) number of coral colonies (100 months); (d) number of
individual fish (56 months).
in each reef structure and the fractal dimension index
was calculated for each transect. The relationship
between apparent length and divider interval was estimated over the range of 4–20 cm with steps of 2 cm up
to intervals of 10 cm and 5 cm up to intervals of 20 cm.
The advantage of this method is that it includes an
assessment of structural complexity across a range of
intervals that may be relevant to diverse species.
Results
A maximum of 25 coral species and 60 fish species have
been observed at the OA reef, and 33 coral species and
34 fish species at the RA reef (Figure 2; species lists are
available upon request). These results indicate significant differences in the development in species richness of
corals (p<0.05) and fishes (p<0.01) between the two
reefs, with opposite signs. While more fish species were
seen at the OA reef, the RA reef favoured a larger
variety of coral species. The numbers of individuals
show similar trends to species richness (Figure 2). However, only in the case of fish was the difference significant
(p<0.001).
Comparing the patterns observed in coral and fish
community development, two major differences can be
pointed out. The initial phase of the development of the
fish community is characterized by a fast increase in the
number of both individuals and species, while the coral
communities are characterized by delayed development.
Likewise, during the final censuses the fish communities
appeared to have attained their climax, whereas the
coral communities still increased almost linearly up to
the last census carried out after 100 months.
A comparison of the communities present on the OA
reef after 7 years versus the neighbouring natural reef
showed opposite trends for fish and corals. The number
of fish species and individuals per transect (30 m3) on the
natural reef was significantly lower (p<0.01) than on the
OA reef (263 species and 15459 individuals versus
352 and 35779 individuals, respectively). For corals, both number of species and individuals per transect
were significantly higher (p<0.001) on the natural reef
than on the OA reef (184 species, 5814 individuals
versus 84, 178 individuals, respectively).
Discussion
The comparison between the RA and OA reefs indicates
opposite patterns of community development of corals
and fish. The higher species richness and larger population of fish on the OA reef versus higher species richness
and larger populations of corals on the RA reef may be
explained by the different morphological parameters.
Numerous studies have shown that reef volume
(amount of reef material) and the bottom area covered
are important design considerations. Ogawa et al. (1977)
Comparison of the development of coral and fish communities
noted that production increased proportionally with
reef size from 400 m3 to a maximum size of 4000 m3.
Rounsefell (1972) suggested that AR should be at least
5700 m3 to be able to maintain self-sustaining fish populations. Japanese researchers have suggested that the
minimum effective size for an artificial-reef set (i.e. the
aggregate of reef modules or blocks, clustered in groups
in a hierarchical arrangement) is 400 m3 (Ogawa et al.,
1977; Bohnsack and Sutherland, 1985). These studies
suggest that the reefs investigated here are much smaller
than the minimum size required to support selfsustaining fish communities. However, the comparison
with the neighbouring natural reef shows that even these
small constructions can improve the local situation in
terms of species richness and population sizes. Comparing the relative size of the two reefs studied, both in
terms of covered area and bulk volume, the OA reef was
much bigger than the RA reef (Table 1). This size
difference is believed to have played an important role in
the observed higher fish species richness as well as in the
higher number of individuals on the OA reef. In contrast, reef size seems unimportant for the development of
the coral community.
Conflicting reports exist on the effect of complexity on
the development of AR communities. Some studies
indicate that species diversity and biomass increase with
higher complexity and therefore conclude that complexity is an important design consideration (review by
Bohnsack and Sutherland, 1985). However, Crowder
and Cooper (1979) predicted that fish maximize their
feeding efficiency and growth at intermediate levels of
structural complexity. Generally, the term complexity
refers to an array of parameters, including spatial
arrangement, number of chambers and openings and the
amount of interstitial space (Bohnsack and Sutherland,
1985). However, these different aspects of complexity
should be considered separately, since they may have
different effects on reef inhabitants. Specifically, the size
and number of interstices have been found to affect
community structure and number of fish present
(Buckley, 1982), owing to different responses of different
species to their arrangement. The higher coral species
richness and development of numbers on the RA reef
cannot be explained by this type of interstitial complexity, but may be related to the higher fractal dimension (Table 1) as another measure of complexity, which
reflects the different rock size used and the random
arrangement. An important feature of a fractal curve (or
surface) is that its length (or area) becomes disproportionately large as the unit of measurement decreases.
This suggests that if a reef has a fractal structure, there
is more usable space for smaller animals living on the
substrate than for larger animals. This might explain
why the RA reef was favoured by settling corals, because
their larvae are extremely small. The relatively high
fractal values of the two artificial reefs compared with
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natural reefs (1.05–1.16 within the same measured scales;
Bradbury et al., 1984; Mark, 1984) may also explain
their higher fish species richness. The lower species
richness of corals of AR compared to natural reefs does
not contradict this explanation because their community
development is much slower (Figure 2): the community
has not reached a plateau after over 8 years). In contrast, the carrying capacity of the fish community was
reached after 30 months.
Another notable difference between fish and corals lies
in their initial development rates. Fish may be directly
attracted from natural reefs in the surrounding area,
whereas corals recruit solely by larvae. This explains the
longer initial delay in settlement. Similar differences
between fish and coral development on artificial structures have been seen in other coral reef areas (BaileyBrock, 1989; Brock and Kam, 1994).
Our results support the idea that complexity should be
evaluated and measured by more than one index to
describe structural and surface irregularities. At least
four indices have been proposed to examine the effects
of structural complexity on species within and among
habitats. These include ‘‘vector dispersion’’ (Carleton
and Sammarco, 1987), ‘‘chain and tape’’ (Connell and
Jones, 1991), ‘‘consecutive substratum height difference’’
(McCormick, 1994), and fractal dimension (Morse et al.,
1985; Sugihara and May, 1990). Beck (1998) compared
all four and found that fractal dimension was best
correlated with the density of gastropods, which were
the only animals larger than 5 mm present that were
directly associated with the habitat surface, and thus
best represented features of the habitat that affected
these animals. The higher coral recruitment to the RA
reef coinciding with higher fractal dimensions suggests
that this measure is also relevant for this group of
species, at least during the initial phase of development.
Finally, we offer some conclusions to serve as
practical guidelines for the future design of artificial
reefs, at least for this specific region. First, for
recreational purposes, small reefs (in the order of
magnitude of a few cubic metres) can serve as attractive
diving sites because they support relatively rich communities of fish and invertebrates. Therefore, a series of
small structures that can support greater numbers of
divers may be preferable above one larger reef. Second,
larger rocks, larger interstitial holes (presumably up to a
certain size). and larger reefs lead to higher species
richness and greater numbers of fish. This conclusion
corroborates previous studies that have shown the
effectiveness of interstices and reef size on community
structure. However, such reef configurations are not
required to produce high species diversity or numbers of
reef-building corals. Finally, to enhance recruitment of
reef-building corals fractal dimension of the surface
may be a good indicator of complexity when designing
surface structure.
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A. Abelson and Y. Shlesinger
Acknowledgements
We thank the director and staff of the Marine Biology
Laboratory at Eilat for their hospitality and the use of
laboratory facilities. We also thank Naomi Paz for her
editorial assistance.
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