Mesophotic surveys of the flora and fauna at Johnston Atoll , Central Pacific Ocean

daniel wagner, randall k. kosaki, heather l. spalding, robert k. whitton, richard l. pyle, alison r. sherwood, roy t. tsuda and barbara calcinai NOAA, Papahānaumokuākea Marine National Monument, 1845 Wasp Boulevard, Building 176, Honolulu, HI 96818, USA, University of Hawai‘i at Mānoa, Department of Botany, 3190 Maile Way, Honolulu, HI 96822, USA, Bernice P. Bishop Museum, 1525 Bernice Street, Honolulu, HI 96817, USA, Universita Politecnica delle Marche, Dipartimento di Scienze della Vita e dell’Ambiente, Via Brecce Bianche, 60131, Ancona, Italy


I N T R O D U C T I O N
Located in the Central Pacific over 800 km from the Hawaiian Islands, 1,450 km from the Line Islands, and 2,500 km from the Marshall Islands, Johnston Atoll is the most remote atoll on Earth.Despite its remoteness, numerous expeditions have targeted the atoll in order to explore and characterize its marine biodiversity (reviewed by Amerson & Shelton, 1976;Lobel & Lobel, 2008).In 1880, a vessel of the North Pacific Guano Company collected 27 fish species from Johnston, including five undescribed species, which were subsequently described by scientists at the United States National Museum (Smith & Swain, 1882).In 1923, a partnership between the US Navy, the Bishop Museum and the Bureau of the Biological Survey (now US Fish and Wildlife Service) led to a series of expeditions to the north-western Hawaiian Islands, Wake Atoll and Johnston Atoll.As part of these expeditions, several scientists visited Johnston Atoll in 1923 aboard the US Navy minesweepers 'Tanager' and 'Whippoorwill' (Amerson & Shelton, 1976;Olson, 1996).These two minesweepers returned to the atoll in 1924 along with other US navy ships.Although no scientific personnel joined that expedition, biological specimens were collected for the Bishop Museum (Amerson & Shelton, 1976).Descriptions of the biota of Johnston Atoll, obtained as part of these efforts by the Bishop Museum to study the biodiversity throughout the Central Pacific, were later published in separate papers focusing on invertebrates (Edmondson et al., 1925;Clark, 1949), fish (Fowler & Ball, 1925), insects (Bryan, 1926;Chilson, 1953) and birds (Olson, 1996).
In the 1960s and 1970s, the Smithsonian Institution led the Pacific Ocean Biological Survey Program, a research programme that aimed to increase the knowledge about various islands throughout the Central Pacific.As part of the programme, several biologists visited Johnston Atoll between 1963 and 1973, and although the major focus was on seabirds, other biota were also studied and inventoried (Amerson & Shelton, 1976).Throughout the 1960s, the University of Hawai'i conducted a series of studies to measure the effects of dredging on the reef ecosystems of Johnston Atoll, and in particular on the prevalence of ciguatera around the atoll (Brock et al., 1965(Brock et al., , 1966;;Buggeln & Tsuda, 1969).
In 1983, the Hawai'i Undersea Research Laboratory brought its manned submersible 'Makali'i' to Johnston in order to study the deep waters surrounding the atoll.A total of 35 submersible dives were performed to maximum depths of 500 m, four of which were dedicated to geological studies (Keating, 1985), and the remainder centred on biological investigations (Randall & Ralston, 1984;Agegian & Abbott, 1985;Randall et al., 1985;Ralston et al., 1986;Chave & Mundy, 1994;Chave & Malahoff, 1998).In 2000, the Bishop Museum coordinated a major effort to inventory the shallowwater (,30 m) marine biota of Johnston as part of a survey of non-indigenous marine species of the atoll (Coles et al., 2001).Following these efforts, the US National Oceanic and Atmospheric Administration (NOAA) has regularly visited Johnston since 2004, in order to further characterize and monitor the shallow-water (,30 m) algae, fish, corals and other invertebrates of the atoll (Brainard et al., 2005;NOAA, 2006;Lobel & Lobel, 2008;Tsuda et al., 2010).
As a result of the multitude of scientific expeditions that have surveyed Johnston, there is ample information available on the marine biodiversity found in the shallow waters (,30 m) of the atoll (reviewed by Coles et al., 2001;Lobel & Lobel, 2008).Additionally, numerous records exist on the flora and fauna found in the deeper waters (100 -500 m) surrounding Johnston Atoll (Randall & Ralston, 1984;Agegian & Abbott, 1985;Randall et al., 1985;Ralston et al., 1986;Chave & Mundy, 1994;Chave & Malahoff, 1998).However, as in many regions around the world, little is known about the marine biodiversity found between these two depth ranges.This intermediate depth range hosts mesophotic coral ecosystems (MCEs), which are light-dependent coral reefs found below the depth limits of conventional SCUBA diving (.30 m) that extend to the deepest portion of the euphotic zone, which may be over 150 m in some oceanic regions with high water clarity like Johnston (Maragos & Jokiel, 1986;Kahng & Maragos, 2006;Kahng et al., 2010).The fixed upper depth limit of MCEs (30 m) corresponds to the depth limits of conventional SCUBA diving and does not represent a static ecological boundary (Kahng et al., 2014).As a result, shallow-water (,30 m) and mesophotic reefs (.30 m) can be quite similar in the distributions of their flora and fauna, particularly close to the depth boundary between these two ecosystems.
The only information on the mesophotic biodiversity of Johnston Atoll is derived from a limited number of surveys for reef fish at depths between 25 and 75 m (Kosaki, 1989;Kosaki et al., 1991).The purpose of this study was to survey this historically under-surveyed depth range at Johnston, in order to characterize and quantify the mesophotic flora and fauna of the atoll.Additionally, this study sought to compare the mesophotic biodiversity between Johnston Atoll and the Hawaiian Archipelago, because numerous previous studies have noted strong faunal and floral similarities between these two geographical areas (Gosline, 1955;Buggeln & Tsuda, 1969;Bailey-Brock, 1976;Grigg, 1981;Grigg et al., 1981;Randall et al., 1985;Maragos & Jokiel, 1986;Kosaki et al., 1991;Coles et al., 2001;Maragos et al., 2004;Tsuda et al., 2010).Due to these similarities, Johnston Atoll has been considered an important stepping stone for marine organisms reaching the Hawaiian Archipelago (Grigg, 1981;Grigg et al., 1981), an interpretation that is supported by both genetic (Rivera et al., 2004(Rivera et al., , 2011;;Timmers et al., 2011) and oceanographic studies (Kobayashi, 2006) that demonstrate strong connectivity between these two regions.

M A T E R I A L S A N D M E T H O D S
All dive surveys were performed using closed-circuit rebreathers on a research expedition to Johnston Atoll aboard the NOAA ship 'Hi'ialakai' in the summer of 2013 (HA-13-01).Dive sites were chosen using historical charts, as well as new multibeam data collected by the NOAA ship 'Hi'ialakai'.Chosen areas contained steep vertical drop-offs and hard substrate at depths between 32 and 78 m.A total of eleven dive sites were surveyed around Johnston Atoll (Figure 1).During each survey, one diver identified and counted all large, conspicuous, diurnally-active fish to the lowest possible taxonomic level along a 25 × 2 m belt transect Fig. 1.Map showing the location of the eleven sites that were surveyed at mesophotic depths (32-78 m) as part of surveys of the flora and fauna off Johnston Atoll.Areas in black show the four emergent land features of Johnston Atoll.(Kane et al., 2014).A second diver took photographs of the benthos at 25 randomly selected points along the transect using a 0.5 m 2 photoquadrat.Upon transect completion, both divers collected macroalgae, sponges and corals that could not be identified in situ as time permitted.No crustose coralline algae or turf algae were collected due to time constraints.Collected samples were photographed in situ, placed into separate bags and preserved for later identification by taxonomy experts.Additionally, both divers recorded the presence of fish and macrobenthic species that were not captured during the transect itself, and documented observations with video vouchers.Benthic cover was determined from photoquadrat images with the aid of Coral Point Count with Excel extensions software (CPCe) using 100 random points per image (Kohler & Gill, 2006).For this purpose, all macrobenthic organisms were identified to the lowest possible taxonomic level from photoquadrat images, using identifications of collected specimens by taxonomy experts where available.All species of the calcifying green alga Halimeda that were identified from collected specimens were grouped during the photoquadrat analysis, due to difficulties of differentiating individual species in photographs.Additionally, all identified organisms were classified post hoc based on their geographical distribution as being restricted to: (1) circumtropical or circumtemperate waters; (2) the Indo-Pacific; (3) the Indo-Pacific but not found in Hawai'i; or (4) Johnston Atoll and Hawai'i (Gosline, 1955;Randall et al., 1985;Kosaki et al., 1991).

Fish
A total of 99 fish species in 29 families were identified during mesophotic surveys, including 22 species which had not Fig. 2. Relative cover by macrobenthic group at the eleven sites that were surveyed at mesophotic depths (32 -78 m) off Johnston Atoll.
the mesophotic flora and fauna of johnston atoll Table 1.Species of macrobenthic organisms recorded during mesophotic surveys off Johnston Atoll.* , new record for Johnston Atoll; off transect, species recorded outside of transect and therefore benthic cover data were not collected; † , percentage cover at transect with the highest value; ‡ , mean percentage cover of all transects + standard deviation.previously been recorded from Johnston Atoll (Table 2).Additionally, we recorded Centropyge fisheri, a species with an uncertain previous sighting at Johnston Atoll (Randall et al., 1985).In terms of number of individuals, Chromis verater, Pseudanthias randalli, Acanthurus olivaceus, A. thompsoni, Centropyge nahackyi, Parupeneus multifasciatus, Pseudocheilinus evanidus and Sufflamen bursa were most common (Table 2).With the exception of A. thompsoni, which was only recorded at one site, this group of fish was also most widespread, being recorded from seven (Chromis verater) to three (Pseudanthias randalli) of the surveyed sites.Among species that were less abundant, Canthigaster jactator, Parapercis schauinslandii, Centropyge loricula, Bodianus albotaeniatus, Chaetodon tinkeri, Ctenochaetus hawaiiensis, Zanclus cornutus and Acanthurus dussumieri were widespread on the surveyed mesophotic reefs, being recorded at three or more of the surveyed sites.Of the 99 fish species that were identified, 74 (74.7%) are widely distributed throughout the Indo-Pacific including Hawai'i, 18 (18.2%)are restricted to Johnston Atoll and Hawai'i, six (6.1%) have widespread circumtropical distributions, and one (1.0%;Pseudanthias randalli) is found throughout the Pacific but not in Hawai'i (Table 2).

D I S C U S S I O N
In contrast to the shallow (,30 m) and deep-water (100-500 m) marine biodiversity of Johnston, which has been extensively surveyed (reviewed by Coles et al., 2001), only limited surveys have been performed at mesophotic depths surrounding the atoll (Kosaki, 1989;Kosaki et al., 1991).This study thus represents the first dedicated effort to inventory and quantify the mesophotic flora and fauna at Johnston Atoll.We recorded a total of 130 species from mesophotic depths, most of which (76.4%) have also been found during previous shallow-water (,30 m) surveys off Johnston Atoll (Tables 1 & 2).This indicates that the mesophotic flora and fauna of Johnston is not composed of a specialized group of species, but rather consists of a subset of species that is also found in shallow waters (,30 m).The only species which have not been found in previous surveys off Johnston Atoll include one black coral, one zoanthid, one squat lobster, two macroalgae, three sponges and 22 fish (Tables 1 & 2).With the exception of the three sponges, which have unknown geographical distributions (Table 1), all other species are commonly found in shallow-water reefs throughout the Indo-Pacific (Hoover, 2006;Huisman et al., 2007;Randall, 2007).Thus, it is likely that these species have been missed in previous shallow-water surveys at Johnston, and are not restricted to mesophotic depths at the atoll.This interpretation is consistent with most mesophotic surveys in the Hawaiian Islands, which indicate that the mesophotic flora and fauna are composed mainly of the same species, albeit in different proportions, that are also found in shallow waters (Brock & Chamberlain, 1968;Parrish & Boland, 2004;Rooney et al., 2010;Kane et al., 2014).However, in some Hawaiian locations, the mesophotic benthic flora and fauna appears distinct from shallow-water (,30 m) communities (Kahng & Kelley, 2007;Spalding, 2012).
Our surveys indicate that mesophotic reefs off Johnston are covered mostly by turf algae, crustose coralline algae, macroalgae and sand (Figure 2).These groups were also identified as the mesophotic flora and fauna of johnston atoll 7 being most abundant in previous surveys on many shallowwater (,18.3 m) reefs off Johnston Atoll (Brainard et al., 2005;NOAA, 2006).However, previous shallow-water surveys off Johnston have also noted high coral cover on several reefs of the atoll, particularly inside the lagoon, where coral cover can approach 100% (Maragos & Jokiel, 1986;Jokiel & Tyler, 1992;Brainard et al., 2005;NOAA, 2006).In contrast, coral cover at most of our mesophotic sites was generally low (0.56 -4.56%), with the exception of two sites which had 13.36% and 16.50% coral cover (Figure 1).The most commonly recorded corals during our mesophotic surveys were Acropora cytherea, Montipora capitata, Distichopora violacea and Millepora tenera (Table 1).
Despite its extreme geographical isolation, the very low level of endemism indicates that Johnston is relatively well (Tables 1 & 2).Additionally, this pattern was consistent amongst various taxonomic groups identified during our mesophotic surveys, as 100% of echinoderms, 99% of reef fish, 87% of corals and 78% of macroalgae are also known to occur in Hawai'i (Tables 1 & 2).In comparison, previous studies noted that 94% of reef fish species (Randall et al., 1985;Kosaki et al., 1991), 74% of coral species (Maragos et al., 2004) and 94% of algal species (excluding cyanobacteria) of Johnston Atoll are also found in Hawai'i (Roy Tsuda, unpublished data).It is currently unknown whether the three unidentified sponge species recorded during this study also occur in Hawai'i and elsewhere in the Indo-Pacific.Further investigations will be necessary to determine the taxonomic status and geographical affinities of the sponge species recorded during our surveys.
Coral reef ecosystems below the depth limits of conventional SCUBA diving remain scarcely surveyed worldwide, and particularly in remote locations like Johnston Atoll.This study represents the first dedicated effort to characterize the mesophotic flora and fauna at Johnston Atoll.Our results support the strong connectivity between Johnston Atoll and the Hawaiian Archipelago highlighted by previous studies.Finally, this study adds 30 new records to Johnston Atoll, thereby emphasizing the value of deep-diving technologies in surveying the largest portion of the depth range of coral reef ecosystems (30 -150 m), which remains largely unexplored.

Table 2 .
Fish species recorded during mesophotic surveys off Johnston Atoll.* , new species record for Johnston Atoll; off transect, species recorded outside of transect and therefore abundance data were not collected; † , abundance at transect with the highest value; ‡ , mean abundance of all transects + standard deviation; NR, depth not recorded.