Download Enhancement of Recruitment to Coral Reefs Using Light

BULLETIN OF MARINE SCIENCE, 63(3): 581–588, 1998
CORAL REEF PAPER
ENHANCEMENT OF RECRUITMENT TO CORAL REEFS USING
LIGHT-ATTRACTORS
Philip L. Munday, Geoffrey P. Jones, Marcus C. Öhman
and Ursula L. Kaly
ABSTRACT
Methods that enhance larval settlement are required to examine the importance of
recruitment in the dynamics of coral reef fish populations. Although it is known that
larval reef fishes are attracted to light, here we show for the first time that a light-attraction device positioned above patch reefs at Lizard Island (Great Barrier Reef) significantly increased the number of fish settling on the reefs below. The device was a modified light trap with a tube allowing the vertical movement of larvae from the trap to the
reef. The number of species of settling fishes, and the abundance and diversity of immigrant fishes were also greater on the light-enhanced reefs. By comparison, the alternative
technique of enhancing recruitment using surface buoys moored to reefs was unsuccessful. Further studies are now required to determine whether enhanced recruitment using
light-attractors leads to a longer-term increase in population size, as opposed to temporarily concentrating juveniles on the reef.
The theory that open marine populations are “recruitment-limited” was first postulated
for coral reef fishes (Doherty, 1981; Victor, 1983), but is now accepted for many marine
organisms with a dispersive larval stage (Gaines and Roughgarden, 1985; Underwood
and Fairweather, 1989; Booth and Brosnan, 1995). This theory predicts that population
dynamics will primarily be driven by the magnitude and variation in supply of larvae to
the population, rather than processes acting after settlement. Theoretical models support
the notion that recruitment will usually influence, if not “limit” per se, the dynamics of
open populations (Warner and Hughes, 1988; Caley et al., 1996). Also, there is an increasing amount of empirical evidence that the dynamics of many populations of coralreef fishes are strongly influenced by the availability of recruits (Doherty and Williams,
1988; Doherty and Fowler, 1994). However, despite agreement that recruitment influences the dynamics of open populations, there is still little agreement about the relative
importance of recruitment versus post-settlement processes in determining the size and
variability of local populations of coral-reef fishes (Jones, 1991; Caley et al., 1996). Experiments are required to test the importance of recruitment relative to post-settlement
processes such as predation, competition and disturbance, on the population dynamics of
coral-reef fishes (Caley et al., 1996). However, in order to implement such experiments,
techniques for enhancing recruitment in a realistic way must be developed (Caley et al.,
1996). Here we examine techniques for artificially enhancing recruitment.
Late-stage fish larvae and pelagic juveniles from a wide range of taxa are attracted to
bright lights at night (Doherty, 1987; Victor, 1991; Thorrold, 1992). This behavior has
been exploited to sample assemblages of fish larvae by using light traps that attract and
catch large numbers of larval fish (Doherty, 1987; Choat et al., 1993). Given the potential
of light traps to attract large numbers of settlement stage fish larvae over a considerable
distance, we hypothesized that they could be a useful tool for enhancing recruitment to
areas of coral reef. In this study we tested this idea by using a basic light trap design
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Figure 1. Illustration of light-attractor with tube attached to permit movement of larval fish from
the attractor to reefs.
(similar to Doherty, 1987), but modified it so that it acted as an attractor rather than a trap,
with a delivery tube to focus juveniles onto the reef below (Fig. 1).
Fish larvae are also known to congregate around floating or partially submerged objects (Kingsford and Choat, 1985; Leis, 1991) and surface and mid-water fish aggregation devices have been shown to significantly enhance recruitment to artificial reefs immediately below (Beets, 1989; Brock and Kam, 1994). We compared the effectiveness of
unlit surface floats to the use of light attractors to enhance recruitment of coral-reef fishes
to patch reefs.
MATERIALS AND METHODS
To determine whether the number of fish settling on coral reefs can be artificially enhanced, we
deployed floats and modified light traps over replicate patch reefs at Lizard Island on the Great
Barrier Reef (14°40'S,145°28'E ) during January and February 1996. Three different treatments
were established, (i) a light-attractor reef, (ii) a float reef, and (iii) a control reef. These treatments
were replicated in time, by rotating the treatments among the three different reefs. The light-attractor
treatment consisted of a transparent perspex light trap (Fig. 1) suspended from a large surface float
with a plastic mesh tube extending from the collecting box to approximately 0.15 m above the patch
reef. This tube allowed larval fish to leave the light-trap in close proximity to the reef. The modified
light trap contained a 12 v fluorescent light tube which was turned on at dusk and off at dawn
throughout the study period. The unlit float reef treatment consisted of a taught 8 mm nylon rope
MUNDAY ET AL.: ENHANCED RECRUITMENT USING LIGHT-ATTRACTORS
583
Table 1. Probability values for main effects using split-plot ANOVA design. Apogonids were
excluded from the analysis of numbers of settlers to remove the effect of one exceptional
settlement event. Note the interaction effect is not tested in a two factor split-plot design.
Main effects
Time
Treatment
Number of settlers Species abundance
(settlers)
NS
NS
P = 0.036
P = 0.007
Number of
non-settlers
P = 0.002
P = 0.025
Species abundance
(non-settlers)
P = 0.005
P = 0.043
suspended from a large surface float (40 cm diam) above the reef, with 10 cm diameter floats tied at
0.5 m intervals along the rope. Floats and light-attractors were moored over each patch reefs by a
rope harness attached to four sets of concrete building blocks (two blocks per set). The control reef
treatment consisted of a patch reef with the concrete mooring blocks but no float or light trap. To
assess the composition and density of the larval fish assemblage present over the study period, an
unmodified light trap was also positioned near the study site. This light trap was suspended 1.5 m
below a large surface float in approximately three to six metres of water.
To provide uniform habitats on which newly settled fish could be easily censused, patch reefs
were constructed on sandy substratum in 4 m of water (high tide), near the entrance to the lagoon on
the western side of Lizard Island. Each reef was approximately 1m in diameter and 0.4 m height in
the middle and was built from dead Pocillopora coral. Patch reefs were approximately 50 m apart
and were 50 m from larger areas of reef that could have attracted larval fishes.
The number of fish present on each reef was estimated by visual census each morning during six
replicate, three-day time periods. At the end of each 3-d period, treatments were rotated among
reefs to exclude any bias associated with any particular reef. Separate recordings were made of
newly settled fish, and juveniles or adults that had immigrated from surrounding areas. Time periods were consecutive except when repairs to light-attractors or traps were required at the end of a 3d period. All fish were collected from each reef on the third day of each time period by surrounding
the reef in a fine net and anaesthetising the fish with quinaldine. To ensure that all fish had been
collected each reef was pulled apart and then reassembled.
Data were analysed by ANOVA using a split-plot design. To determine the effect of time and
treatment on the number of settlers and non-settlers on reefs, the numbers of fish collected on the
final day of each three-day period were analysed. To determine the effect of time and treatment on
species abundance, the maximum number of species observed on each treatment reef during each
three-day period was used.
RESULTS
A greater number of larval fish settled on reefs under the light-attractor, compared with
the float reefs and control reefs throughout the study period, irrespective of which individual reef had the light attractor (Table 1, P = 0.036). Light-attractor reefs consistently
had a greater abundance of settlers (Fig. 2), with an average of 3 times higher settlement
than float reefs and control reefs. There was no appreciable difference between the abundance of settlers on the unlit float and control reefs throughout the study (Fig. 2). In
addition, there was a significant effect of light enhancement on the total number of species of settlers observed on reefs during each time period (Table 1, P = 0.007). Three
times as many species (mean = 7.1 ) settled on light-attractor reefs than on either float
(mean = 2.1) or control reefs (mean = 2.5).
A similar total number of larvae were caught in the light trap and on the light-attractor
treatment over the study (380 and 370, respectively), but the species composition of the
fish caught in the light trap differed considerably from the three treatment reefs. Ser-
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Figure 2. Cumulative totals of newly settled fishes on treatment reefs (apogonids not included).
Arrows indicate when all fish were cleared from reefs and treatments were re-randomised among
reefs at the start of a 3-d time series.
ranids (Epinephelinae) were the major component of the light trap catch (52%), followed
by chaetodontids (17%) and pomacentrids (10%). In comparison, apogonids were the
major component on all treatment reefs (65%), followed by serranids (10%) and
tetraodontids (7%). This indicates that recruitment of the different taxa was not enhanced
in proportion to the numbers actually attracted to the light on the surface.
A considerable number of juvenile and adult fish (immigrant, non-settlers) were also
observed on the patch reefs. The abundance of non-settlers changed significantly through
time (Table 1, P = 0.002) and differed among treatments (Table 1, P = 0.025). More nonsettlers were observed on the light-attractor treatment than other treatments and, apart
from the first time series, there was a general increase in the number of non-settlers
observed on each reef type in each successive time series (Fig. 3). The number of nonsettler species present also differed through time and among treatments (Table 1, P =
0.005 and P = 0.043). More non-settler species were observed on the light-attractor treatment (mean = 4.7) than on the float reef (mean = 3.2) or control reef (mean = 2.5). A high
proportion of the non-settlers that moved to the patch reefs were potential predators of
larval fishes, such as labrids, lutjanids, nempiterids and pinguipedids.
DISCUSSION
It is well known that larval reef fishes are attracted to light during the later stages of
development (Doherty, 1987; Choat et al., 1993). However, this phenomenon has so far
been employed only as a method of collecting larvae and examining spatial and temporal
patterns in their distribution and abundance (e.g., Milicich et al., 1992; Doherty et al.,
1994). Our study is the first to demonstrate that the abundance and diversity of juveniles
settling on the reef itself can be enhanced by placing a light attractor above a reef over
successive nights. The critical test of whether the local dynamics of open marine popula-
MUNDAY ET AL.: ENHANCED RECRUITMENT USING LIGHT-ATTRACTORS
585
Figure 3. Cumulative totals of adult and juvenile fish (immigrant, non-settlers) on treatment reefs.
Arrows indicate when all fish were cleared from reefs and treatments were re-randomised among
reefs at the start of a three day time series.
tions are determined by recruitment relies on some means of experimentally enhancing
recruitment (Caley et al., 1996). The use of light attractors provides a methodology to
carry out such tests. By enhancing recruitment in a realistic manner, while holding other
factors constant, it will be possible to determine whether additional recruitment results in
a proportional increase in adult numbers or whether additional recruitment is offset, wholly
or in part, by post-settlement processes. Importantly, additional recruitment achieved in
this manner would be the result of natural settlement, thereby strengthening the relevance
of results to real populations.
The effect of increased settlement on post-settlement rates of mortality and movement
were not examined in this study. Several studies have indicated that mortality may be high
in the first few days after settlement (Sale and Ferrell, 1988; Hixon, 1991; Carr and Hixon,
1995) and this could greatly reduce the effect that enhanced settlement has on subsequent
population sizes. Where post-settlement mortality is constant or density-independent,
enhanced settlement may be an effective method of increasing population size. If, however, an increase in the density of settlers increases post-settlement mortality rates, then
the effectiveness of recruitment enhancement programmes will be reduced. Further studies are required to determine if enhanced recruitment can lead to sustained increases in
population sizes.
The greater abundance and diversity of immigrant non-settlers found on the lightattractor reef, many of which were piscivorous species, suggests that enhanced recruitment may not lead to greater population densities. The increase in piscivore numbers on
patch reefs was probably a short term re-distribution of fish from nearby reefs. Relationships between the abundance of prey and the abundance of piscivores have been detected
on other experimental reefs (Hixon and Beets, 1993). If an increased supply of recruits
was maintained in this study then the local abundances of predators may also increase,
leading to an increase in mortality rates.
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Settlement may be enhanced on individual patch reefs using modified light-traps (this
study) or large fish attraction devices (FADs) (Beets, 1989; Brock and Kam, 1994). The
small floats used in this study were not successful, perhaps because they lacked sufficient
physical structure to attract larvae. The different scales over which these two methods
operate are as yet unknown. Light-traps attract larvae from within the range of horizontal
underwater visibility and may not attract additional settlement stage larvae at the scale of
whole reefs, unless large numbers of light attractors are deployed. Therefore, there may
be a redistribution of settlement patterns within reefs rather than a net increase in settlement if light attractors were used to enhance settlement at this spatial scale. The same
conclusion may apply to the use of FADs.
Many species of fishes are known to make precise habitat choices during settlement
(Sale et al., 1984, Ohman et al., 1998) and this may explain the larger proportion of
chaetodontids and serranids in the light trap catch than on the patch reefs. The substratum
of dead Pocillopora used to construct the patch reef may have been unsuitable for settlement of these species. Even if these taxa were attracted to the illuminated site, they may
have returned to the plankton after leaving the light-attractor tube rather than settling to a
reef composed of dead coral. Corallivorous chaetodontids in particular might be expected
to exhibit a preference for settlement on reefs where live corals are present.
The large proportion of apogonids that settled on all patch reefs in comparison to the
proportion collected in the light trap suggests that apogonids were attracted to the structure of the reefs rather than the light source per se. The greater physical structure provided
by the light-attractor apparatus, compared to the control reef, could potentially have increased settlement to the light trap reef rather than the presence of a light source. Constraints on the number of light-traps available precluded the use of a light-trap control
reef in the experimental design. However, similarity in the number of settlers on the float
reef and control reef suggest that increased physical structure alone did not significantly
increase settlement to the reef. Also, the moored light trap consistently had greater numbers of larvae at settlement stage than the float and control reefs, indicating that the light
source was an important factor attracting fish to the light-trap reef.
This study has demonstrated that settlement of coral-reef fishes may be artificially
enhanced with lights. This method may be suitable for testing the demographic consequences of enhancing recruitment and could potentially be a useful technique for assisting the recovery of locally depleted populations of coral reef fishes. However, the effect
of increased settlement on subsequent population sizes has not been sufficiently tested to
determine the relative effectiveness of the technique. The success of recruit enhancement
is likely to be scale-dependent and although recruitment enhancement may be possible at
the scale of patch reefs, the potential for light-traps to enhance recruitment at larger spatial scales is unknown.
ACKNOWLEDGMENTS
J. Leis and I. Stobutzki helped with the identification of larval fishes and, in conjunction with B.
Edwards, provided invaluable assistance with the maintenance of light-traps. E. Broadhurst, S.
Slade, C. Hutchings and D. Wilson assisted with field work. Figure 1 was drawn by B. Mayrhofer.
This research was supported by an Australian Research Council grant.
MUNDAY ET AL.: ENHANCED RECRUITMENT USING LIGHT-ATTRACTORS
587
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DATE SUBMITTED: January 2, 1997.
DATE ACCEPTED: November 3, 1997.
ADDRESSES: (P.L.M., G.P.J., U.L.K.) Department of Marine Biology, James Cook University of North
Queensland, Townsville, Queensland, Australia 4812. (M.C.Ö.) Department of Zoology, Stockholm
University, 106 91 Stockholm, Sweden. CORRESPONDING AUTHOR: (P.L.M.) Tel. +61-77-815341, Fax:
+61-77-251570, E-mail: [email protected].