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

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 2002. 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=0.0542 and 0.0011), 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=0.1628 for family and p=0.0249 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 (0.3264 and 0.7892) 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 presettlement fish at night was developed inadvertently using dive lights and aquarium nets in waist-deep water with surprising success. This method was later optimized for use on SCUBA, 2 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 30' 345, W 149 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 and December 1. 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 29' 557, W 149 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, 16, 18, and 24, 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 of November to the 1 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 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 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 3 the Crest study occurred over the remaining days of the study period (beginning six days after the full moon).