Vertical Composition of Coral Reef Fish at Recruitment Using a New Light Trap Method

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1 Vertical Composition of Coral Reef Fish at Recruitment Using a New Light Trap Method Corey Phillis & Damon Wolf ABSTRACT: The vertical distribution of coral reef fish recruits entering Opunohu Bay, Moorea, French Polynesia was studied over the lunar period between November 14, 2002 and December 1 st Family and species composition differed between site, at depths within site, and over the lunar period that was sampled. Only the Bay Reef Site had significant results for both family and species composition (p= and ), however, all sites had depths within them with high predictive power. Bay Reef and Channel Sites were also predictably distinguishable, with the exception of the Channel Site on the species level (p= for family and p= for species). Family and species composition predictably differed between the crest and the inner crest patch reef, however neither the family or species' p-values were significant ( and ) because the crest site was only sampled three times due to strong currents and limited time. Number of species caught was greatest around the full moon, and some families were only found during this phase of the lunar cycle, while other species were present throughout the sampling period. Introduction Over the past two decades a considerable amount of attention and research has been paid to the spatial and temporal distribution of coral reef fishes prior to their settlement onto the benthic habitat. The majority of this research has been focused on the horizontal aspect of coral reef fishes distribution, with the intent of better understanding the level of coral reef fish population connectivity. However, very little information has been collected concerning the vertical distribution of pre-settlement fish, which may be equally important in determining community and population structure (Cowen, 2002). These studies have mostly focused on the pelagic larval stage and have produced results of coarse resolution of vertical stratification (surface/nonsurface) due to limited sampling capability and labor intensive sorting and identifying of specimens caught at different depths (Cowen, 2002). There is even less information on the vertical distribution of recruiting coral reef fish. In the case of pre-settlement stage distribution, a widely adopted method for collection is the semi-automated light trap (Leis and McCormick, 2002). This method, however, has only been used to determine the vertical distribution of pre-settlement coral reef fish in a single published study (Doherty and Carleton, 1997). However, light trap sampling is a passive method and it is unknown how the traps actually catch the larvae (Leis and McCormick, 2002), and to what extent (if any) they are biased toward catching certain taxa of fish and not others, such as Chaetodontidae versus the faster, highly mobile Acanthuridae. An approach that could possibly be more selective and complete for capturing pre- 1

2 settlement fish at night was developed inadvertently using dive lights and aquarium nets in waistdeep water with surprising success. This method was later optimized for use on SCUBA, in what we believe is the first application of a method of this kind. This new method, the Recruitment At Depth Census Using Light Traps on SCUBA (RADCULTS) allowed us to examine temporal, spatial, and vertical distribution patterns of recruitment on a coral reef in Opunohu Bay, Moorea, French Polynesia. The goal of our work was to determine if family and species composition of coral reef fish recruits differed at depths within site, between on-reef and off-reef sites, and over the lunar cycle. We also looked at family and species composition of recruits at the barrier reef crest compared to the composition after the recruits had been filtered (via predation or settlement) through the inner crest patch reef matrix. Methods Study sites: Our study consisted of four primary sampling sites within the lagoon of Moorea in the Society Islands of French Polynesia (Fig. 1). The Bay Reef Site (BRS, S 17 o 30' 345, W 149 o 51' 345) of Opunohu Bay was the first of the four locations to be sampled during each sampling night. BRS was sampled using SCUBA at depths of 50 and 20ft nine times between November 14 th and December 1 st. The 50ft site was performed on a steep slope of coral rubble adjacent to the fringing reef wall, the 20ft site was sampled directly above the reef of live/dead coral and volcanic rock. The Channel Site (CS, S 17 o 29' 557, W 149 o 52' 280) was located across the channel from the crest on the north end of the bay, at least 100 meters from any fringing reef. The CS was sampled 4 times, November 14 th, 16 th, 18 th, and 24 th, on the same night after the BRS was sampled. The CS terrain at 50ft had a slight slope and at thick layer of silt, while the 20ft site took place along the anchor line in the water column with no other substrates present. Data collected at the surface of both sites using light traps performed by A. Zacher and C. Engel provided 0ft depth samples (unpublished data, 2002). From the 26 th of November to the 1 st of December a second study was performed examining differences in species composition between the crest and the edge of the crest patch reef. These two sites were chosen within the West Opunohu crest; one very near the crest, Crest 0, and the other on the border of the inner-crest and the channel, Crest 15 (S 17 o 29' 310, W 149 o 52' 211). Crest 0 was located on the point of the crest nearest the Crest 15 site and was sampled while snorkeling in very shallow water just before the breaking waves of the crest. Crest 15 was sampled in less than 15ft of water on SCUBA at depth and snorkel at surface on a sandy terrain 2

3 within a patchy reef area. The study period spanned the waxing and waning phases around the full moon of the November lunar cycle, beginning eight days after the new moon and ending three days before the following new moon. BRS was sampled throughout this entire period, CS was sampled for the first half of this period (the final sample was performed four days after the full moon), and the Crest study occurred over the remaining days of the study period (beginning six days after the full moon). Sampling Method: Each dive site was sampled by a team of three divers using the RADCULTS method. Divers would place a light cannon in a stationary position at each sampling site and spend 15 minutes capturing as many recruits/larvae as possible with aquarium nets. Fish were transferred from the aquarium nets to Zip-Lock bags marked for the appropriate depth and placed in a goody bag. Each nights samples were separated into live and dead specimens; live fish were kept in aquariums labeled for depth and site and reared for more accurate classification. All fish were counted and classified to the best of our ability. In some cases fish were grouped by family when further classification was impossible by the naked eye. Certain Acanthuridae species (Acanthurus nigricauda, A. xanthopterus, A. triostegus, and A. olivaceus) were difficult to differentiate when they died before attaining characteristic markings or color and were lumped into the genus specific grouped category Acanthurus sp. A,B,C,D. Results A total of 1, 366 individual recruits, consisting of 63 species within 23 families were caught and identified over 8 sampling nights. The three most abundant families were Acanthuridae (513 individuals), Chaetodontidae (256 individuals), and Pomacentridae (178 individuals). The four most abundant species were Chaetodon trifasciatus (203), Chromis viridis (125), Acanthurus triostegus (93) and Abula vulpes (93). Discriminant function analyses were performed using the statistical program SYSTAT 9.0. Table 1. provides a summary of the 11 tests performed regarding family and species composition of coral reef fish at depth within sites, composition between sites, composition at depth between sites, and over the period of the lunar cycle that sampling took place. The results for all but the lunar cycle tests were also put into classification tables for easier interpretation. At Bay Reef Site, depths were highly descriptive of family (Table 2a; p=0.2990) and 3

4 species (Table 2b; p=0.0011) composition. For both family and species, the 50ft depth sample was most commonly confused with other depths, however, these sample were still quite descriptive (88 and 75 percent correct respectively). The channel site was less descriptive at most depths for both family and species (Table 3a,b; p= and respectively), but was 100% correct at the surface for family composition and 100% correct at 50ft. for the species composition. This is probably an artifact of the low sample size (only four dates), however the family classification was very nearly significant. Between sites, BRS was greatly classifiable from CS for both family and species, but CS was only moderately classifiable from BRS for family and barely classifiable for species (Table 4a,b; p= and respectively). For samples at different depths within and between sites, only BRS 20 and CS 50 were distinguishable from the other sites and depths for species (Table 5; p=0.0251). For the crest versus inner patch crest study, differences in family and species composition were 100% distinguishable in all cases but that of Crest 15 (inner patch crest), but was still classifiable 75% of the time (Table 6a,b; p= and respectively). However, this study suffered from a low sample size due to high currents that prevented us from snorkeling to the crest on one of our sampling nights. Finally, the number of species of recruits caught increased around the new moon (Fig. 4a). Figure 4b. describes the abundance of Factor 1 and Factor 2 families--statistcally generated groupings of families according to similar variables--over the lunar cycle sampled. Factor 1 represent families that appear to have episodic recruitment around the full moon and very little recruitment at other times during the lunar cycle. Factor 2 represents species that are common throughout the lunar cycle, but appear to be have less influence on the total family composition during the full moon. Discussion While methods were not designed to quantify abundance of recruits (due to nonstandardized methods), obtaining composition proved to be more than adequate. We found that we were able to accurately distinguish family and species composition between sites and at depths within sites. Not all tests were statistically significant due to our short sampling period, however predictive power was, in some cases, at, or near 100%. Obviously, a longer, and more frequent sampling period is needed to get higher resolution on the data to determine how predictive depth and site actually are for families and species. Although the RADCULTS method proved to have positive results, it should be modified further to allow for more efficient sampling time for divers as well as obtaining a greater collection of fish. A weighted, stationary underwater lantern apparatus would emit more light in a 360-4

5 degree direction, attracting greater numbers of fish than several flashlights; this will also free divers hands of flashlights, providing them with more dexterity. The aquarium nets should be modified to have a much longer net (several feet), which could be synched closed as needed, creating separate compartments of fish per site, in turn eliminating fish transfer and possible escape of fish. Due to similar color and pattern characterization of certain species of live recruits, and degradation of certain characteristics in dead recruits, identification at the species level can be very difficult. Rearing up unidentifiable species in an aquarium tank system would be the best technique to acquire accurate proper species recognition. Many new questions arose throughout the sampling and sorting processes. It is still unclear why such a high abundance and composition of species were found in the BRS site, and not within the CS. Sites chosen at different distances from reef sites in similar habitats would reinforce the comparison between the BRS and CS study. This may be the result of current patterns that are contrary to our initial assumption that currents drive fish recruitment to BRS from over the West Opunohu crest by way of CS, but rather comes westerly from across the bay. The surprisingly ease by which Acanthuridae were captured provided high enough abundance and composition to allow for further investigation on the importance of the recruitment patterns on the community and population structure. It may be interesting to compare temporal patterns and settlement of recruiting Acanthuridae to recruitment checks on otolith samples of adults of the same species to look at various population structure dynamics. Finally, we recommend the next course of action should be to test how well the RADCULTS method represents the actual family and species composition of recruitment events. We suggest using the RADCULTS method as well as different sampling methods (i.e. conventional light traps, crest nets, etc.) at different sites and habitats around the island to compare recruitment sampling methods, using resident fish composition surveys as a baseline for what is actually recruiting. Acknowledgements: We'd like to give our biggest thanks to Pete Raimondi and Giacomo Bernardi for their time, enthusiasm, helpfulness, patience and guidance throughout our project and for 5

6 providing the opportunity for such a great learning experience. Our next warmest thanks goes out to Pete Dal Ferro, for his time and excellent natural born RADCULTS skills, and to Alain Lo- Yat, whose knowledge and patience enlightened our fish identifying skills and saved us a world of time. Though Alain's help in larvae classification is more than appreciated, we took an even greater delight engaging in his wonderful cooking each and every night. Mark Readdie, Dawn Jech, and Jonna Engel also need a grope of hugs and kisses for their encouraging ideas and helping hands during the long hours on the night dives. And to every student in the class, your curiosity and best wishes were happily accepted. Bibliography Cowhen, R.K., (2002). Larval Dispersal and Retension and Consequences for Population Connectivity. In "Coral Reef Fishes" (P.F. Sale, ed) pp Academic Press, Elsevier Science (USA). Doherty, P.J., and Carelton, J.H., (1997). The Distribution and Abundance of Pelagic Juvenile Fish Near Grub Reef, Central Great Barrier Reef. Proc. 8 th Int. Coral Reef Symp., Vol. 2, pp Leis, J.M., and McCormick, M.I., (2002). The Biology, Behavior and Ecology of the Pelagic Larval Stage of Coral Reef Fishes. In "Coral Reef Fishes" (P.F. Sale, ed) pp Academic Press, Elsevier Science (USA). Lo-Yat, Alain, (2002). Variabite Temporelle de la Colonisation par les Larves de Poissons de L'atoll de Rangiroa (Tuamotu, Polynesie Francaise) et Utilisation de L'outil "Otolith" de ces Larves. (not yet published) 6

7 Table 1. Table Test Family/Species Source Pillai's Trace df p-value 2a. (Fig. 2) Bay Reef Site Family Depth , b. Bay Reef Site Species Depth , a. (Fig. 3) Channel Site Family Depth , b. Channel Site Species Depth , a. Bay vs. Channel Family Distance from Fringe Reef , b. Bay vs. Channel Species Distance from Fringe Reef , BRS 0,20,50 vs. CS 0,20,50 Species Depth x Distance , a. Crest 0 vs. Crest 15 Family Distance from Crest , b. Crest 0 vs. Crest 15 Species Distance from Crest , (Fig 4a). Bay Reef Site vs. Lunar Cycle Species Number of Species Caught (Fig 4b.) Bay Reef Site vs. Lunar Cycle Species Species Factors Table 2a. BRS: Family Depth %correct Total Table 2b. BRS: species Depth %correct Total Table 3a. Channel: Family Depth %correct Total

8 Table 3b. Channel: species Depth %correct Total Table 4a Bay vs. Channel: Family Depth Bay Channel %correct Bay Channel Total Table 4b. Bay vs Channel: species Depth Bay Channel %correct Bay Channel Total Table 5. BRS 0,20,50 vs. CS 0,20,50: Species Depth BRS 0 BRS 20 BRS 50 CS 0 CS 20 CS 50 %correct BRS BRS BRS CS CS CS Total

9 Table 6a. Crest Dive: Family Depth 0 15 %correct Total Table 6b. Crest Dive: species Depth 0 15 %correct Total

10 Figure 1. 10

11 Figure 2. Canonical Scores Plot By Family At Bay Reef Site ) 2 ( R O T C A F FACTOR(1) DEPTH

12 Figure 3. Canonical Scores Plot By Family For Channel Site ) 2 ( R O T C A F FACTOR(1) DEPTH

13 Figure 4. Number T Species Caught as a Function of Lunar Cycle H G U A C 30 S E I C E P S N LUNARPHASE 13

14 Figure 5. Lunar Phase Effects 1 and Factor 2 Families 5 4 e u l a V LUNARPHASE FACTOR1 FACTOR2 14

15 15