OCEANOGRAPHIC PROCESSES OF CORAL REEFS: Physical and Biological Links in the Great Barrier Reef - Chapter 13 pptx

The Effects of Water
Flow around Coral Reefs
on the Distribution of
Pre-Settlement Fish (Great
Barrier Reef, Australia)
John H. Carleton, Richard Brinkman, and Peter J. Doherty
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
Helix Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Bowden Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Helix Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Bowden Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
Hydrodynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
Fish Distribution and Abundance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
Dispersion Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
INTRODUCTION
Coral reef fish, with very few exceptions, have planktonic egg, larvae, and pre-set-
tlement juvenile stages that vary in duration from weeks to months. Most reef fish
spawn buoyant eggs that have the potential to be transported many kilometers in
wind-driven surface currents before hatching into larvae capable of influencing their
dispersal. Others lay eggs in protected nests with the subsequent release to the water
column of actively swimming larvae or juveniles, thus minimizing the time their off-
spring are exposed to the vagaries of ocean or shelf currents and enhancing the
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chances of recruitment back to their natal reef (Jones et al., 1999). In either case, the
problem facing propagules expatriated from coral reefs is one of population
closure—of finding shallow coral reef habitat suitable for the juvenile/adult phases
in their life cycle.
We now know that the physical, chemical, and biological composition of the
water mass in the immediate vicinity of coral reefs is affected by fine-scale current
patterns generated through the interaction of reef topography with prevailing, far-
field currents (see Hamner & Wolanski, 1988 for review). Coral reefs, growing to
within a few meters of the sea surface, act as barriers to the flow of oceanic or shelf
currents. As currents approach they must diverge to flow around the reef edges, cre-
ating a zone of relatively stationary water immediately upstream which becomes
enriched with nutrients and plankton. At the reef face, topographical entrapment of
tidal currents by coral buttresses results in the advection of deep water up the reef
slope toward the crest. On the surface, wave turbulence mixes the chemical and par-
ticulate matter from deep layers with shallow wind-driven material just prior to push-
ing the mixture across the reef crest and onto the reef flat (Hamner et al., 1988). As
diverging currents accelerate around the reef, strong longshore currents are generated
close to and parallel with the reef sides. If longshore currents are strong enough, flow
separation occurs adjacent to sharp projections or indentations in the reef margin with
the resultant formation of particulate-rich eddies. In the lee of the reef, gyres and
eddies, depending on their size, location, strength, and duration, vary in their ability
to retain both neutrally buoyant material such as echinoderm larvae (Black, 1988) and
positively buoyant material such as coral eggs (Willis & Oliver, 1990). A number of
these reef-associated hydrodynamic processes must also affect the distribution and
abundance of pelagic, pre-settlement fish in the near-reef environment, and therefore
impact on the eventual success/failure of their recruitment back to suitable, coral reef
habitats.
To illustrate the role of flow dynamics on the retention of pre-settlement fish, we
present the findings from two independent studies at two physically distinct platform
reefs in the central section of the Great Barrier Reef (GBR). The first of these stud-

ies was of short duration and occurred at Helix Reef, a small, topographically simple,
oval-shaped reef; and the second, completed over a 3-month period, was at Bowden
Reef, a considerably larger, topographically more complex, elongated reef.
Synchronized light traps moored in close proximity to these reefs produced synoptic
views of fish distribution and abundance patterns at various times of the night and
states of the tide. By combining information on fish distribution patterns with physi-
cal oceanographic data, we gain an insight into which hydrodynamic processes con-
tribute most to the retention of juvenile fish near reefs.
MATERIALS AND METHODS
The light traps (Figure 1) were three-chambered devices similar in design to those
described by Doherty (1987). These traps have no moving parts and depend upon the
behaviour of photopositive organisms to effect their capture. Fish are attracted into
210 Oceanographic Processes of Coral Reefs
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the upper chamber through a number of tapered slits, then by a vertical array of oscil-
lating lights moved down through the middle chamber and into the lower chamber
where most of the fish remain alive until collected. Upon trap recovery, the catch is
washed from the lower chamber with filtered seawater, concentrated into a smaller
volume and fixed in 100% methylated ethanol. The lights were activated for three,
1-h periods each night (21:00 to 22:00, 24:00 to 01:00, and 03:00 to 04:00
GMT ϩ 10:00) around the new moon between November and January when seasonal
and lunar spawning patterns produce the largest catches (Doherty, 1991).
During the summer, the prevailing longshore currents in the central section of the
GBR are driven by the East Australia Current (EAC) and flow from the northwest to
the southeast, parallel to the major isobaths along the continental shelf (Andrews
& Furnas, 1986). Tidal currents which flow across the shelf isobaths, flooding to the
southwest and ebbing to the northeast (King & Wolanski, 1996), modulate the per-
sistent southward flow pushing the resultant current more to the east during falling
tide and more to the west during rising tide (Gay & Andrews, 1994). The interaction
of these far-field currents with the variety of reef shapes and sizes found in the cen-

tral GBR results in a complexity of flow pattern through the reef matrix.
HELIX REEF STUDY
Helix Reef (147° 18Ј E, 18° 38Ј S) is a small (Ͻ800 m diameter), relatively isolated
(Ͼ10 km to the nearest neighbouring reef), platform reef which rises to the surface
from a depth of 55 m (Figure 2). The surrounding seafloor is flat, composed of
mud/sand sediment and devoid of any outcrops. These topographical features result
in a relatively simple flow regime. The persistent, southerly set current splits around
the northern margin of Helix Reef, accelerates along the reef flanks, and sets up a
counterclockwise-rotating eddy in the lee (Figure 3, modified from Sammarco
& Andrews, 1988). Although tidal modulation of the shelf currents causes the lee
eddy to intensify or relax and to change its actual position, only during moderate to
strong southeasterly winds does the eddy degenerate (Sammarco & Andrews, 1989).
Pre-settlement fish were collected from the surface at 16 stations on the south-
ern, downstream side of Helix Reef. Stations A to C were located around the south-
ern reef margin within 50 m of the crest, while the remaining 13 traps were moored
in a regular grid pattern at a spacing of 350 m across a northwest to southeast axis
(see Figure 2). From the reef edge to the downstream side of the grid was 800 m and
from the northeast side to the southwest side was 1.4 km. Samples were collected
over three consecutive nights covering the new moon period in January 1992. On the
first night, an attempt was made to clear all traps after each sampling period. This
proved to be logistically very difficult and on the remaining two nights only the 11
traps closest to the reef (A to C, 1 to 8) were cleared after each period.
To discern pattern in fish associations, the log transformed abundance data (indi-
viduals h
Ϫ1
of trapping) from stations closest to the reef (A to C, 1 to 8) during the
three individual time periods of each night (21:00 to 22:00, 24:00 to 01:00, and
03:00 to 04:00) were subjected to agglomerative, hierarchical clustering techniques
(n ϭ 80 samples). Bray-Curtis dissimilarity coefficients (Bray & Curtis, 1957) were
The Effects of Water Flow around Coral Reefs 211

© 2001 by CRC Press LLC
calculated for every possible pair of samples, the resulting association matrix sub-
jected to the Ward’s incremental sum of squares fusion strategy (Belbin, 1987) and
the results summarized by a dendrogram. Diagnostic routines developed for use with
the Bray-Curtis metric (Cramer values) were applied to the results from the cluster
analysis to determine the level of fidelity of the various fish species to sample group-
ings (Abel et al., 1985). Catch rate, number of species, Shannon-Wiener diversity
index (HЈ), and Pielou’s evenness index (JЈ) (Pielou, 1969 and 1975) were deter-
mined for each of the sample groupings.
BOWDEN REEF STUDY
Bowden Reef (147° 56Ј E, 19° 02Ј S) is a much larger platform reef than Helix Reef
(6.0 km long ϫ 3.0 km wide), is crescent shaped with a continuous reef flat along the
northern, eastern, and southern sides, and has a semi-enclosed, shallow, sandy lagoon
(Figure 2). The seafloor surrounding Bowden Reef is flat, smooth, and has a depth of
40 to 50 m.
Light-traps were moored at the surface at 13 stations around the circumference
of Bowden Reef and at 4 stations within the lagoon (Figure 2). On the northern, east-
ern, and southern sides, two traps were anchored directly across from each other on
either side of the shallow reef flat with an additional two or three traps moored far-
ther out in deeper water, within 100 m of the reef crest. The outside near-crest traps
were placed just in front of the breaker zone and the inside traps located just behind
the reef flat. Traps were deployed for periods up to seven consecutive nights around
the new moon during the months of November to January 1992/1993. The traps were
activated each night for the same three 1-h periods as in the Helix Reef study, but
cleared only once per day and not after each sampling period.
Concurrently with trap sampling the strength and direction of far-field currents
were measured at half-hourly intervals by two Aanderaa RCM4-S current meters
moored at mid-depth to the west and south of Bowden Reef (Figure 2). Tidal height
data were collected by Aanderaa tide gauges placed on the seafloor at the base of each
current meter mooring, on the lagoon floor at Bowden Reef, and on two adjacent

reefs (Figure 2). In addition, data on the strength of the poleward flowing EAC were
obtained from a current meter moored at a depth of 35 m on the shelf slope seaward
of Myrmidon Reef, approximately 100 km to the northwest. Wind data were obtained
from a weather station at nearby Davies Reef.
The longest continuous sets of far-field current measurements were obtained in
November and January. These data, along with sea level and wind data, were used to
force a two-dimensional, depth-averaged, hydrodynamic numerical model (Wolanski
et al., 1989). This model was considered the most appropriate to simulate the flow
field at the time of biological sampling for a couple of reasons. First, the model has
successfully reproduced observed current fields at Bowden Reef in previous studies
(Wolanski et al., 1989, Wolanski & King, 1990); and second, this relatively simple,
two-dimensional model predicts very similar flow patterns at the sea surface, where
most nocturnal, pre-settlement reef fish occur (Doherty & Carleton, 1996), to com-
putationally more complex, three-dimensional models (Wolanski et al., 1997). The
212 Oceanographic Processes of Coral Reefs
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computational grid for the model was a square horizontal mesh of 386 m with the
X-axis aligned parallel to the margin of the continental shelf.
To provide an insight into how the reef-associated flow field may affect the dis-
tribution of pre-settlement fish around Bowden Reef, the computed current fields
from the two-dimensional hydrodynamic model were applied to a second-order
advection-diffusion model (Oliver et al., 1992). As in Wolanski et al. (1997), the
“simulated fish” were assumed competent and, in addition to passive advection by
local currents, were given a behavioral repertoire of swimming speeds typical of pre-
settlement coral reef fish (0.05, 0.1, and 0.2 ms
Ϫ1
: Leis & Carson-Ewart, 1997). Fish,
regardless of their size or swimming ability, within 3 km of the northern, eastern, and
southern sides swam directly toward the reef in response to low frequency sounds
generated by breaking waves (Leis et al., 1996) and only stopped swimming when

they were within 800 m of the crest. If fish were washed out of this 800-m-wide enve-
lope by local currents, they again swam directly toward the crest, stopping when they
reached the seaward margin of the envelope. Fish on the western, open-lagoon,
“quiet” side were not allowed a behavioral response to the reef. Pre-settlement fish
are known to appear in the vicinity of coral reefs immediately following sunset in
readiness for settlement (McIlwain, 1996). To incorporate this behavior into the
model, a plume of pre-settlement fish with a density of 100 fish per cell and extend-
ing across the entire width of the upstream model domain was released at 18:30 on
21 November and at 18:45 on 21 January. To calculate the time-integrated abundance
around the reef, propagules were counted every 15 minutes during simulation runs
along 13, evenly spaced, 1-km-long transects which projected seaward at right angles
from the reef margin. These counts were added to running totals for each transect.
Replicate abundance data were tested by three-way, fixed-factor analysis of vari-
ance for differences between November and January (those months in which flow
patterns were modeled), among stations around the reef circumference (stations 1 to
11, 16, and 17, Figure 2) and among three size classes (small Ͻ10 mm, medium Ն10
and Յ15 mm, large Ͼ15 mm). Prior to analysis, all data were log transformed to sta-
bilise variances (Sokal & Rohlf, 1981) and tested for heteroscedasticity by Cochran’s
procedures (Winer, 1971).
RESULTS
HELIX REEF STUDY
Over the three nights of sampling more than 15,000 individual fish were caught, rep-
resenting more than 160 species belonging to 31 families. However, a large number
of the species occurred only once or twice. The dataset was dominated by Clupeids
that accounted for 68% of all individuals, followed by Pomacentrids (10%), Nomeids
(7%), Apogonids (4%), Carangids (3%), and Gobies (2%).
Classification of all 80 samples identified four sample groupings highlighting
both spatial and temporal distribution patterns (Figure 4). The fish community struc-
ture at stations close to Helix Reef was consistent through time. All samples from sta-
tion A, regardless of time of night or night of sampling, clustered together in a distinct

The Effects of Water Flow around Coral Reefs 213
© 2001 by CRC Press LLC
group, while all the samples from station B and approximately 50% of those from sta-
tions C, 1, and 2 formed a second near-reef group. Fish community structure at sta-
tions farther from the reef (stations 3 to 8 and the remaining 50% of samples from
stations C, 1, and 2) varied with time of night. Approximately 70% of all late evening
samples (21:00 to 22:00) formed a cluster distinct from the remaining far-grid sam-
ples. Diagnostic routines indicated Spratelloides larvae, Apogonids, and Gobies con-
tributed most in the characterization of near-reef communities and that Psene
arafuensis was instrumental in distinguishing the late evening, far-grid community.
The relevance of these key taxa in defining fish associations is evident from the
composition of the four sample groupings (Table 1). Spratelloides larvae dominated
the two near-reef groups, Apogonids and Gobies occurred in relatively large numbers
only at station A, and P. arafuensis dominated the early evening, far-grid group. The
catch rate at station A was almost an order of magnitude higher than the other near-
reef group, which in turn was five to eight times higher than the far-grid groups.
Although species richness was highest at station A, the species diversity index was
214 Oceanographic Processes of Coral Reefs
TABLE 1
Composition of the Four Species Groupings from the Cluster Analysis
(H؅؍Shannon-Wiener diversity index and J؅؍Pielou’s evenness index)
Site A Abundance (%)
# h
Ϫ1
970.82 Spratelloides larvae 87
# Species 52 Apogonids 2.5
HЈ 0.7489 Gobies 2.5
JЈ 0.1895 Tripterygiids 2
Near Grid Group
# h

Ϫ1
175.54 Spratelloides larvae 83
# Species 24 Pomacentrus bankanensis 3.5
HЈ 0.8754 S. delicatulus 2.5
JЈ 0.2755 P. coelestis 2
Pomacentrids (unmetamorphosed) 1
Far Grid Group (22:00 to 23:00)
# h
Ϫ1
32.65 Psenes arafuensis 45
# Species 32 Pomcentrus bankanensis 9
HЈ 2.3036 Abudefduf vagiensis 8
JЈ 0.6647 Spratelloides delicatulus 6
Atherinids 3
Far Grid Group (01:00 to 05:00)
# h
Ϫ1
22.30 Pomacentrus bankanensis 28
# Species 26 Spratelloides larvae 22
HЈ 2.2518 S. delicatulus 10
JЈ 0.6911 S. gracilis 9.5
Pomacentrus unpigmented 7
P. coelestis 6
© 2001 by CRC Press LLC
the lowest (HЈ ϭ 0.75) due to the dominance of Spratelloides larvae (JЈ ϭ 0.19).
Species diversity indexes were highest at the far-grid groups (HЈ ϭ 2.30 and 2.25)
due to an even proportioning of abundance among species (JЈ ϭ 0.66 and 0.69).
Detailed scrutiny of catch rate data for each of the key taxa illustrates the con-
sistency of spatial and temporal patterns. The distribution of Spratelloides larvae in
the lee of Helix Reef becomes evident when the catch data from all three sampling

periods is integrated over the night of 5 to 6 January (Figure 5). In this figure the
columns and discs represent different information. The height and colour of the
columns are proportional to the number of individuals collected at each station at a
particular time (the numbers over each column are the actual catch rates). The diam-
eter of the discs represents the proportion of the total catch, from all stations over the
3 days of sampling, taken at each station, whereas the colour of the disc represents
the percentage of the station catch which was captured at that particular time. For
example, the relatively small disc at station 10 indicates that few Spratelloides larvae
were taken at this station—in fact, only one fish—and the bright red colour indicates
that the single individual captured on the night of 5 to 6 January represents 100%
of the station catch. A quick scan of disc diameters indicates that Spratelloides
larvae were most abundant at stations A, B, and 1. Although there was some tempo-
ral variation in catch rates, the spatial distribution pattern remained fairly con-
sistent (Animation 1). Apogonids and Gobies had similar distribution patterns to
Spratelloides larvae occurring primarily at stations A and B (Animations 2 and 3).
Nomeids and Pomacentrids were distributed quite differently. Psene arafuensis
avoided Helix Reef and was always most abundant at far-grid stations (Figure 6).
Although this spatial pattern was evident at all times, specific catch rates differed
consistently among sampling periods. The late evening samples (21:00 to 22:00)
always contained considerably more of this species (Animation 4). Pomacentrids
were distributed across the entire sampling grid but were most abundant at stations
A, B, and C. Catch rates were highest during the late evening and declined steadily
over the subsequent sampling periods (Animation 5).
BOWDEN REEF STUDY
Hydrodynamics
Tides during the sampling periods in November and January were similar in both
their amplitude (~2.5 m) and semi-diurnal nature. During November, winds were
from the north to northwest at 5.5 ms
Ϫ1
and a persistent, southeast flow at 0.47 to

0.59 ms
Ϫ1
was recorded on the shelf slope seaward of Myrmidon Reef. During
January, easterly winds prevailed (2.8 to 12.0 m
Ϫ1
) and the persistent, southeast cur-
rent was slightly weaker (0.26 to 0.49 ms
Ϫ1
).
For both November and January, computed current fields around Bowden Reef
were dominated by tidal forcing (Animation 6) and displayed similar characteristics
to previous observations and numerical studies (Wolanski et al., 1989). Current mag-
nitudes of 0.3 to 0.4 ms
Ϫ1
occurred in the far field during both maximum ebb and
flood tides producing zones of strong lateral velocity shear (Figure 7). During flood
The Effects of Water Flow around Coral Reefs 215
© 2001 by CRC Press LLC
tide, the net southerly current bifurcated at the northern end of the reef, accelerated
along the eastern and western flanks, and recombined to the south, resulting in
a relatively narrow region of reduced velocity immediately adjacent to the reef
(Figures 7a and d). During ebb tide, the net northeasterly current bifurcated to the
west of the reef, accelerated around the northern and southern ends, and recombined
some distance to the east, producing a wide region of still water along the reef face
(Figures 7b and e).
The size and strength of tidally generated hydrodynamic features were modu-
lated by prevailing winds and the low-frequency, southeasterly shelf flow, both of
which varied between November and January. The combination of northerly winds
and stronger southward shelf flow in November resulted in an enhanced net south-
ward flow which, during ebb tide, produced a number of re-circulation features

including the formation of a closed eddy to the southeast of the reef (Figure 7b).
Features of this strength were not evident in the computed circulation for January.
Removal of tidal effects by temporal averaging of the time-varying currents over a
tidal cycle revealed a stronger southward flow during November, with a more clearly
defined convergence zone and associated region of relatively reduced velocity to the
southeast of the reef (Figures 7c and f).
Fish Distribution and Abundance
Almost 50,000 individual fish, representing 45 families and over 300 species, were
captured at Bowden Reef during the new moon sampling periods in November and
January. As in the Helix Reef study, a large number of the species occurred only once
or twice. However, unlike the Helix Reef study, Pomacentrids were the most abun-
dant family comprising approximately 40% of the catch, followed by Clupeids (9%),
Apogonids (8%), Blennies (6%), and Gobies (3%).
Clupeids and Nomeids, families instrumental in defining near-reef and far-grid
communities at Helix Reef, were of less significance at Bowden Reef. Nomeids were
poorly represented, occurring only in extremely low numbers at the more exposed
stations, while Clupeids, comprised primarily of Spratelloides gracilis and S. deli-
catulus, were ubiquitous (Animations 7 and 8). As the effect of local currents on pre-
settlement fish was of primary interest, the Clupeidae were removed from all
subsequent analyses.
A shift in size frequency distribution toward larger fish in January (p Ͻ0.001,
␹
2
ϭ 1059; Figure 8) resulted in a highly significant interaction between months
and sizes (p Ͻ0.001, Table 2). Pre-settlement Pomacentrids and Blennies were
slightly larger in January but juveniles of the more open water families, Scombrids,
Carangids and Monocanthids, were substantially larger. The larger size in
Pomacentrids appears to be due primarily to a shift in species composition, from a
greater number of smaller species in November (e.g., Chrysiptera rollandi and
Dischistodus spp.) toward more medium-sized species in January (e.g., Pomacentrus

bankanensis), although some Pomacentrids, such as P. coelestis, were larger in
January. The other two highly significant interactions (months X stations, and sta-
tions X sizes, p Ͻ0.001, Table 2) reveal more as to the possible role of hydrodynamic
processes in the distribution of pre-settlement fish.
216 Oceanographic Processes of Coral Reefs
© 2001 by CRC Press LLC
The interaction between months and stations indicates a significantly different
distribution pattern around Bowden Reef between November and January. In
November, fish were more abundant at stations 1, 5, 6, 7, and 11 and less abundant at
station 3 (Figure 9). The November flow pattern averaged over a tidal cycle (Figure
7c) clearly shows an area of divergence and reduced flow near station 1; a large region
of reduced flow along the reef face surrounding stations 5, 6 and 7; and a narrow band
of still water immediately to the south near station 11. Station 3, to the north of the
reef, lies in an area of relatively strong currents. The tidally averaged flow regime in
The interaction between stations and sizes denotes a size-dependent distribution
pattern around the reef circumference. Small fish (Ͻ10 mm) had the greatest variabil-
ity in abundance among stations, large fish (Ͼ15 mm) had the most uniform distribu-
tion, and medium-sized fish (Ն10 to Յ15 mm) had a distribution similar in pattern to
small fish but less variable (Figure 10). Small fish were most abundant at stations 7 to
11 and at station 1. During flood tide, stations 8 to 11, which lie along the southern mar-
gin, are in a clearly defined convergence zone with an associated reduction in flow
velocity (Figures 7a and d). During ebb tide, these southern stations are in zone of flow
separation and subsequent eddy formation, and station 7, moored on the eastern reef
face, is surrounded by still water (Figures 7a and d). As noted previously, the tidally
averaged flow regime places station 1 in an area of divergence. The distribution of
medium-sized fish closely parallels that of small fish (Figure 10), but differences in
abundance among stations are much smaller. Large fish, with the exception of station
3, are uniformly distributed. Catch data for Chrysiptera rollandi and Pomacentrus
coelestis in November illustrate the distribution of a small/medium (mean ϭ 9.98 mm)
and large (mean ϭ 15.09 mm) fish (Animations 9 and 10). A plot of mean size at each

station for November and January data combined (Figure 11) shows that most fish were
captured at station 11 and that their mean size was significantly smaller than anywhere
else (p Ͻ0.05, Games and Howell multiple range procedure; Sokal & Rohlf, 1981).
The Effects of Water Flow around Coral Reefs 217
TABLE 2
Analysis of Variance Table Examining the Catch of Pre-Settlement Fish as a
Function of Month, Station, and Size
Source of Variation df MS F P
Month 1 0.071 0.52 ns
Station 12 0.905 6.66 ***
Size 2 20.591 151.60 ***
Month ϫ station 12 0.371 2.74 **
Month ϫ size 2 1.776 13.08 ***
Station ϫ size 24 0.364 2.68 ***
Month ϫ station ϫ size 24 0.159 1.169 ns
Note: ns ϭ not significant.
** ϭ p Ͻ0.01.
*** ϭ p Ͻ0.001.
© 2001 by CRC Press LLC
January, although similar in pattern, was substantially weaker (Figure 7f).
Dispersion Model
The theoretical distribution of pre-settlement fish around Bowden Reef, as predicted
by the second-order, advection-diffusion model, varied between November and
January (Figure 12). With the exception of passive, non-swimming particles in
November, fish were retained along the reef face from the northeast sector (location
0.3, Figure 12) around to the southern sector (location 0.8). However, the exact loca-
tion of maximum retention, the most abundant size class of fish, and the total num-
ber of propagules retained differed between months. In November, the primary peak
in retention for all swimming speeds occurred in the northeast sector; a secondary,
weaker peak was located on the eastern face near stations 5, 6, and 7; and abundance

declined steadily around the southeast reef margin toward the southern end (Figure
12). The difference in height between the primary and secondary peaks was most pro-
nounced for large, faster-swimming fish, indicating a fairly variable distribution, and
was least pronounced for small, slower fish, indicating a more even distribution. The
model also predicted a greater abundance of large rather than medium-sized or small
fish, and a total lack of retention in either the northwest or southwest sectors.
In January, a single peak was located to the southeast and a greater number of
medium-sized and small fish were retained (Figure 12). Within the sector of maxi-
mum retention, medium-sized fish (swimming speed of 0.1 ms
Ϫ1
) were most abun-
dant, while at other locations, such as in the northeast sector, predicted abundance
was proportional to swimming ability. Generally, the overall level of retention was
higher—the total number of fish was higher, retention occurred in the northwest and
southwest sectors, and a greater number of passive particles were trapped, especially
along the southern margin.
Detailed examination of the interaction between local currents and various
swimming abilities may help explain the resultant distribution pattern generated by
the diffusion model. If the “simulated fish” were treated as passive particles, they
were either swept past the reef or retained temporarily in regions of reduced velocity,
but without an increase in concentration (Animations 11a and 12a). If the fish were
allowed to swim to and remain near the reef, then “hot spots” of increased concen-
tration appeared at size-specific locations around the reef. Larger, strong-swimming
fish reached the reef first, quickly aggregating at a point on the upstream end in con-
centrations exceeding an order of magnitude above that of the incident plume.
Smaller, slow-swimming fish spent more time in the prevailing currents and were
advected farther downstream before making their first contact with the reef. These
aggregations developed more slowly, but were still at concentrations above that of the
incident plume.
Once near the reef, all fish, regardless of size, were subjected to the same suite

of tidally dominated local currents. Multiple, size-specific aggregations which had
formed as the incident plume was wrapped around the reef, were washed back and
forth along the reef face by the ebb and flood of the tide. Variability in current
strength pulled at and deformed these aggregations, but still they maintained their
cohesion. After a period of time, similar distribution patterns emerged for the “hot
spots” in each month, although concentrations within size-specific aggregations
218 Oceanographic Processes of Coral Reefs
© 2001 by CRC Press LLC
differed. Converging spatial patterns were most evident in January (Animation 12)
and, to a lesser extent, between medium-sized and large fish in November (Animation
11). Differences in abundance between size classes were due to a disproportionately
greater number of small- and medium-sized fish lost from the reef by fast currents
sweeping past the southern end (for example: 8:15 to 9:45 on 24/11/1992, Animation
11 and 10:00 to 12:00 on 24/1/1993, Animation 12). The zones of strong, lateral-
velocity shear were strongest around the ends of the reef in November, resulting in a
greater loss of smaller fish at that time. Although the location of fish aggregations was
generally restricted to still water along the reef face, smaller concentrations of large,
stronger-swimming fish were able to maintain their position on the more exposed
western side.
Inter-month hydrodynamic variability resulted in distinctive dispersal patterns in
each month. During the November simulations, passive particles were not retained in
the vicinity of the reef for more than 2 days, zones of aggregation remained on the
eastern face, and there was very little trapping on the western side or in the lagoon.
However, during January, passive particles were retained along the eastern perimeter
during the entire simulation, aggregations of fish were advected to the western side
by an anticyclonic re-circulation, and there was persistent trapping in the lagoon,
albeit at a low concentration.
From an Eularian, fixed-reference perspective, the most persistent zones of
retention were in the south and southeast sectors where aggregations of fish generally
remained at or above the concentration of the incident plume.

DISCUSSION
The findings of these two independent studies suggest that fine-scale hydrodynamic
features generated through the interaction of reef topography with prevailing, far-
field currents have a considerable impact on the distribution and abundance of pre-
settlement fish around coral reefs.
At Helix Reef, station A, located near the indentation in the southern margin,
consistently had the highest catch rates and greatest number of species. Overlaying
our station grid with the flow pattern generated by the time-averaged hydrodynamic
model of Sammarco and Andrews (1988) places station A at the centre of an eddy
(Figure 13). Although this flow regime was developed with the environmental condi-
tions that persisted during the mass coral spawning event in November 1983, the
combination of constant northerly winds, prevailing southerly flow, and falling tide
at the time of our sampling would most likely generate a very similar flow pattern.
Wind stress on the sea surface has the greatest influence on surface flow and the con-
stant northerly winds during this study (2.2 to 8.3 m s
Ϫ1
) would have accelerated the
flow around Helix Reef and enhanced the counterclockwise-rotating, lee eddy
(Sammarco & Andrews, 1988 and 1989). It is most likely, therefore, that the higher
catch rates and larger number of fish species at station A are due to the aggregating
effect of the eddy. This is not surprising as eddies are known to concentrate many
forms of meroplanktonic and holoplanktonic organisms behind reefs (Black, 1988;
The Effects of Water Flow around Coral Reefs 219
© 2001 by CRC Press LLC
Willis & Oliver, 1990), headlands (Alldredge & Hamner, 1980; Murdoch, 1989), and
islands (Hernandez-Leon, 1991). Also, Sammarco and Andrews (1988 and 1989)
found the highest concentration of coral spat (up to 90% of all settlement) on settle-
ment plates moored to the south of Helix Reef in areas of high water residence time,
again suggesting that lee eddies concentrated the planktonic coral planulae prior to
their settlement.

At Helix Reef we relied on the findings of earlier investigations to construct the
flow field around the reef at the time of fish trapping. At Bowden Reef, because phys-
ical oceanographic data were recorded simultaneously with biological sampling,
real-time flow patterns could be computed through the use of appropriate hydrody-
namic models. Comparison of computed flow regimes with concurrent fish distribu-
tion patterns highlighted which physical events had the greatest influence on the
retention of juvenile reef fish. Results from the Bowden Reef study suggest that the
development of site-specific fish concentrations was due to the interaction of hydro-
dynamic, physiological, and behavioral processes.
The particular numerical model we applied to the field data performed well. The
two-dimensional, depth-averaged model predicted the generation of realistic, local-
ized retention features at various temporal and spatial scales. Transient, tidally
induced features were similar between months but the persistent, low-frequency flow
in November was much stronger due to steady northerly winds and stronger southerly
flow of the EAC. The robustness of the model is evident from the concordance
between the predicted, counterclockwise circulation around the reef during January
and previous observations at Bowden Reef during times of diminished southward
flow (Wolanski et al., 1989).
The predicted distribution patterns from the second-order, advection-diffusion
model and the observed distribution patterns from the light trap catches varied in
agreement among the three size classes of fish. Predicted and observed distributions
of small/medium-sized fish were in broad, general agreement, whereas those for
large fish were not. The model predicted the greatest abundance of small/medium fish
along the reef face from the northeast sector around to the southern end; a more uni-
form distribution along the reef face in November; concentrated distribution along
the southeast sector in January; little retention on the exposed, western side in
November; and the advection of fish to the western side by an anticyclonic re-circu-
lation in January. The predicted peaks in both months occurred in sectors where there
were no light traps. Nonetheless, the light trap catches for smaller fish are in general
agreement with the predictions—high catch rates at station 7 on the reef face in

November (Figure 14); higher concentrations at southern stations in January; low
catch rates at western and more exposed northern stations in November; and elevated
catches at stations 16 and 17 on the western side in January. However, the model did
not predict high catch rates in both months at station 1 to the northeast of the reef, nor
the large catches at stations 4 and 11 in November. Currents at station 1 are very
strong during maximum ebb and flood tides and only the tidal-averaged flow pattern
indicates a zone of divergence at this location (Figure 7). Such zones are known to
accumulate plankton (Hamner & Hauri, 1981) and appear to do so for juvenile fish
as well. The stronger flow field in November would generate a more pronounced
220 Oceanographic Processes of Coral Reefs
© 2001 by CRC Press LLC
boundary layer of trapped water around the reef than the weaker currents in January
(Wolanski & Spagnol, 2000). This well-developed layer of still water would most
likely trap, retain, and concentrate pre-settlement juvenile fish, resulting in much
higher catches at those traps moored close to the reef crest.
Discrepancies between predicted and observed patterns of distribution for large
fish are a direct consequence of the model assumptions. All fish, regardless of size,
stopped swimming when they neared the reef and became passive. This behavior is
more realistic for smaller, weaker-swimming fish that are highly influenced by
hydrodynamic processes, but not for larger, more active individuals. In the period
prior to settlement, pelagic juveniles of reef fish can control their horizontal and ver-
tical positions around coral reefs and actively discriminate amongst habitats while
still in the plankton (Doherty & Carleton, 1996). Light trap catches for large fish,
with the exception of slightly elevated catch rates at station 3 (Figure 10), were
unvarying, suggesting a high degree of control over dispersion and an independence
from hydrodynamic forces.
The only skepticism we have with the observed distribution patterns arises from
a possible change in light-trap sampling efficiency when moored in areas with dif-
ferent current strength. Thorrold (1992) found that drifting traps consistently outper-
formed anchored traps and suggested that at higher current speeds, small fish have

difficulty swimming to and entering into moored traps. At Helix Reef, where fish
trapping covered a wide range of current regimes from sheltered sites behind the reef
to exposed sites in the open flow field, medium-sized, pre-settlement Pomacentrus
bankanensis were distributed across the entire sampling grid on all three nights
(Animation 13). If light-trap sampling efficiency varied among stations, we would
expect the mean body size of P. bankanensis to vary across the grid and to capture
only larger individuals at the more exposed, far-grid stations. This was not the case.
Mean body size did not differ significantly among stations (p Ͻ0.05, Games and
Howell multiple range procedure; Sokal & Rohlf, 1981), and both the largest and
smallest fish occurred at stations away from the shelter of the reef (Figure 15). Also,
if the patterns were generated solely by increased trap efficiency in still water and not
by true patterns in abundance, then catches of Psene arafuensis near Helix Reef
should have been highest in sheltered waters close to the reef. Instead this species
actively avoided the reef and was common throughout the far-grid stations and rare
at all times in the near-grid samples.
The advection-diffusion model did highlight the need for fish to actively partic-
ipate in the recruitment process. When incoming propagules were modeled as passive
particles, the incident plume was deformed as it moved past the reef, there was some
retention, especially in January, but concentrated “hot spots” did not form. For con-
centrations to develop it was necessary for fish to swim to and remain near the reef.
We now know, from a range of laboratory and field experiments, that such behavior
is not unreasonable. Laboratory studies have shown that pre-settlement reef fish
within the size range captured by light traps can sustain swimming speeds of 0.15 to
0.5 ms
Ϫ1
(Stobutzki & Bellwood, 1994) which, for the stronger swimmers, are well
above the maximum computed flood and ebb flows (0.3 to 0.4 ms
Ϫ1
). Pomacentrids,
the most abundant family taken around Bowden Reef, are some of the least resilient

The Effects of Water Flow around Coral Reefs 221
© 2001 by CRC Press LLC
swimmers but are capable, nonetheless, of covering distances greater than 20 km
(Stobutzki & Bellwood, 1997). Field studies have confirmed pelagic juvenile reef fish
are active swimmers during the day (Leis et al., 1996) and generally swim toward
coral reefs at night (Stobutzki & Bellwood, 1998).
Coral reef fisheries around the world are under increasing threat as traditional,
subsistent practices are abandoned for modern, high-yield methods (see Dalzell et al.,
1996 for review). In developing countries, population growth, the introduction of
more sophisticated technologies, and a shift toward cash economies have all con-
tributed to increased pressure on maximizing yields from local fisheries that have
existed for millennia. To protect against unsustainable development and the ultimate
failure of coral reef fisheries, managers have advocated the introduction of marine
protected areas or marine fisheries reserves as a means of maintaining suitable habi-
tats and critical numbers of large, healthy breeding stock (Russ & Alcala, 1996). In
order for these reserves to function correctly, they must be situated up-current from
suitable sink reefs—reefs that due to their location, size, shape, orientation, and local
flow fields, trap large numbers of incoming propagules. The findings of this study in
identifying which hydrodynamic features affect the distribution and abundance of
pre-settlement, juvenile fish around coral reefs are the first steps in the development
of robust, realistic models of dispersion and connectivity.
ACKNOWLEDGMENTS
We thank Jessie Brunskill, Michael Doherty, Bill Hamner, Ray McAllister, Natalie
Moltschaniwskyj, and the master and crew of RV Lady Basten for their assistance in
the field during the Helix Reef study, and David Booth, Gigi Beretta, and Christine
Schmidt for continued assistance during the Bowden Reef study. We thank John
Soles and Ray McAllister for technical assistance with the oceanographic aspect of
the Bowden Reef study, and Simon Spagnol, Felicity McAllister, and Patrick Collins
for their expertise with numerical modeling and visualization. We also thank Mary
Anne Temby for obtaining reference material. Bill Hamner is gratefully acknowl-

edged for his thorough and vigorous review of this manuscript.
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224 Oceanographic Processes of Coral Reefs
© 2001 by CRC Press LLC
The Effects of Water Flow around Coral Reefs 225
FIGURE 2 Map showing the location of Helix Reef
and Bowden Reef and the location of oceanographic
instruments (red diamonds are current meters, green
diamonds are tide gauges) within the central GBR.
Insets show three-dimensional detail of reef
topography and location of sampling stations.
FIGURE 3 Postulated, tidal-averaged flow pattern
around Helix Reef during sampling when the fall in
tidal height was approximately 150 cm, wind blew
from the north at 5 m s
Ϫ1
, and the persistent seasonal
flow was to the south at 40 cm s
Ϫ1
. Note accelerated
flow along the western flank, and eddy formation near
the southern indentation. (Adapted from Sammarco,

P.W. & Andrews, J.C. 1988 Science 239, 1422–1424.
With permission.)
FIGURE 4 Dendrogram summarizing the results from
the classification analysis of all 80 samples collected
during the study.
FIGURE 1 Diver with light trap.
© 2001 by CRC Press LLC
226 Oceanographic Processes of Coral Reefs
FIGURE 5 Typical distribution pattern for
Spratelloides larvae in lee of Helix Reef. See text for
detailed explanation of columns, discs, and colors.
FIGURE 6 Typical distribution pattern for Psene
arafuensis in lee of Helix Reef. Symbols as in Figure 5.
FIGURE 7 Synoptic plot of the current field around
Bowden Reef at times of (a) maximum flood,
(b) maximum ebb, and (c) time averaged over a tidal
cycle, during November, and at (d) maximum flood,
(e) maximum ebb, and (f) time averaged over a tidal
cycle, during January.
FIGURE 8 Size frequency distributions of fish
collected at Bowden Reef during November and
January.
FIGURE 9 Plots of fish abundance at stations around
the circumference of Bowden Reef in November and
January.
© 2001 by CRC Press LLC
The Effects of Water Flow around Coral Reefs 227
FIGURE 10 Plots of abundance for three size classes
of fish (small Ͻ10 mm, medium Ն10 and Յ15 mm,
large Ͼ15 mm) at each station around the

circumference of Bowden Reef.
FIGURE 11 Plots of mean size of fish at each station
around the circumference of Bowden Reef.
N ϭ sample size.
FIGURE 14 Size distribution of Pomacentrus
bankanensis across the sampling grid in the lee of
Helix Reef.
FIGURE 12 Predicted distributions of pre-settlement
fish around Bowden Reef for (a) November 1992, and
(b) January 1993. X-axis represents evenly spaced
distances around the reef circumference from the
northern extremity of the open, western lagoon
(x ϭ 0), around the eastern reef face, to the southern
extremity of the open lagoon (x ϭ 1). Predicted
relative abundance has been normalized against the
maximum abundance for both months of simulation.
FIGURE 13 Composite of postulated, tidal-averaged
flow pattern around Helix Reef superimposed over
sampling grid.
© 2001 by CRC Press LLC
228 Oceanographic Processes of Coral Reefs
ANIMATION 1 Distribution pattern for Spratelloides
larvae in lee of Helix Reef during each 1-h sampling
period (21:00 to 22:00, 24:00 to 01:00 and 03:00 to
04:00 GMT ϩ 10:00) over the three nights of
sampling in January 1992.
FIGURE 15 Plots of abundance for three size classes
of fish at each station around the circumference of
Bowden Reef in November and January.
ANIMATION 2 Distribution pattern for Apogonids in

lee of Helix Reef during the same time/date
combinations as in Animation 1.
ANIMATION 3 Distribution of Gobies in lee of
Helix Reef.
ANIMATION 4 Distribution of Psene arafuensis in
lee of Helix Reef.
© 2001 by CRC Press LLC
The Effects of Water Flow around Coral Reefs 229
ANIMATION 6 Simulated current fields around
Bowden Reef over a tidal cycle for November 1992.
ANIMATION 7 Distribution of Clupeids captured at
Bowden Reef in November 1992.
ANIMATION 8 Distribution of Clupeids captured at
Bowden Reef in January 1993.
ANIMATION 5 Distribution of Pomacentrids in lee
of Helix Reef.
© 2001 by CRC Press LLC
230 Oceanographic Processes of Coral Reefs
ANIMATION 13 Distribution pattern for
Pomacentrus bankanensis across the sampling grid in
lee of Helix Reef during the same time/date
ANIMATION 11 Predicted dispersal of a plume of
juvenile fish arriving at Bowden Reef during
November 1992. Simulations for four swimming
speeds of (a) 0.0 ms
Ϫ1
(passive particles),
(b) 0.05 ms
Ϫ1
, (c) 0.10 ms

Ϫ1
and (d) 0.20 ms
Ϫ1
.
ANIMATION 12 Predicted dispersal of a plume of
juvenile fish arriving at Bowden Reef during January
1993. Simulations for four swimming speeds of
(a) 0.0 ms
Ϫ1
(passive particles), (b) 0.05 ms
Ϫ1
,
(c) 0.10 ms
Ϫ1
and (d) 0.20 ms
Ϫ1
.
ANIMATION 10 Distribution of large pre-settlement
fish (mean ϭ 15.09 mm), represented by Pomacentrus
coelestis, at Bowden Reef in November 1992.
ANIMATION 9 Distribution of a small/medium-
sized pre-settlement fish (mean ϭ 9.98 mm),
represented by Chrysiptera rollandi, around Bowden
Reef in November 1992.
© 2001 by CRC Press LLC
combinations as in Animation 1.