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A video transect method for estimating reef fish abundance, composition, and habitat utilization at Gray's Reef National Marine Sanctuary, Georgia
Abstract
Reef fish communities at Gray's Reef National Marine
Sanctuary, Georgia, differed over five different habitat
types. Numbers of species and overall densities were highest
on ledge habitat, intermediate on live-bottom (three
categories of low relief less then15 cm rock outcroppings
covered by algae and macrofauna), and lowest over sand. On
average, abundance over ledges exceeded that over sand
bottoms by a factor of 50. Generally, community composition
at sites over ledges and dense live-bottom areas was similar
and distinct from sites found over sparse live-bottom and
sand. Many species were found in more than one habitat and
few individual species could be considered indicators of a
single habitat type. A nondestructive, repeatable procedure
of randomly dispersed video transects was devised for
assessing diurnally active fishes.
Richard 0. Parker Jr.
Alexander J. Chester
Russell S. Nelson
Beaufort Laboratory, Southeast Fisheries Science Center
National Marine Fisheries Service, NOM
Beaufort Laboratory, Beaufort, North Carolina 28516-9722
Manuscript accepted 22 February 1994.
Fishery bulletin 92:787-799.
Introduction
The geographic and depth distribution of fishes associated with reefs or hard bottom off the southeastern U.S. coast is generally known (Struhsaker, 1969; Huntsman and Manooch, 1978; Miller and Richards, 1980; Powles and Barans, 1980; Wenner, 1983; Chester et al., 1984; Sedberry and Van Dolah, 1984; Parker and Ross, 1986). Most of these studies have demonstrated changes in community structure associated with different depths and water temperatures. Although trawl collections over sand have been compared with collections over hard bottom (Wenner, 1983; Sedberry and Van Dolah, 1984), no quantitative in situ studies of the distribution of reef fishes by type of substrate have been published.
Gray's Reef National Marine Sanctuary (GRNMS), Georgia, one of 14 Marine Sanctuaries managed by the National Oceanic and Atmospheric Administration (NOAA), encompasses nearly 32 km2 at a depth of about 22 m. Compared with surrounding areas, Gray's Reef contains extensive, but patchy and discontinuous, hardbottom of moderate relief (up to 2 m). Rock outcrops or "ledges" have formed in a northwest to southwest direction. Ledges are often separated by wide expanses of sand and are subject to weathering, shifting sand, and slumping, which create a complex habitat with caves, burrows, troughs, and overhangs (Hunt, 1974). Sandy areas between the ledges consist of coarse shell with varying amounts of "rock-like" litter.
Reef fish assemblages are difficult to sample because of the diversity and mobility of the fauna and because of the variety of microhabitats within complex reef substrates (Russell et al., 1978). The applicability and limitations of various techniques for estimating reef fish abundance have been reviewed (Russell et al., 1978; Sale, 1980; Sale and Douglas, 1981; Brock, 1982; DeMartini and Roberts, 1982; Sale and Sharp, 1983; Kimmel, 1985; Sanderson and Solonsky, 1986; Bortone and Kimmel, 1991). Techniques include the use of traditional fisheries assessment gear (nets, traps, and hook-and-line), poisons, explosives, and visual observations. The need for repeatable surveys and the constraints of working in a national marine sanctuary necessitated the use of nondestructive survey techniques.
Diver observations are the most common method used in studies of reef fish assemblages (Brock, 1954; Bardach, 1959; Hobson, 1972; Chave and Eckert, 1974; Sale, 1975; Jones and Chase, 1975; Jones and Thompson, 1978; Anderson et al., 1981; Ogden and Ebersole, 1981; Sale and Douglas, 1981; Brock, 1982; Kimmel, 1985; Bohnsack and Bannerot, 1986; Parker, 1990). Although a variety of sampling techniques have been employed to make quantitative assessments of reef fish abundance, all rely on divers to identify and record fish species observed in a predefined area (transect and point counts) or over a period of time (rapid visual assessment techniques). Accuracy of the visual techniques is affected by light levels, water clarity, currents, fish species diversity and densities, substrate complexity, diver familiarity with the fishes, and number and size of the sampling units. Biases are induced by l) a tendency to undersample small, cryptic, and nocturnal species (Brock, 1982); 2) identification, counting, and recording errors (Brock, 1954; Russell et al., 1978); 3) attraction and aversion reactions of some species to the divers (Chapman et al., 1974); and 4) species differences in territory, home range, life history patterns, and behavior (Russell et al., 1978).
Remote observation techniques, using movie or closed-circuit television cameras deployed from vessels or carried by divers, have been used to estimate abundance of reef fish (Smith and Tyler, 1973; Alevizon and Brooks, 1975; Powles and Barans, 1980; Boland et al.2). A major advantage is that a permanent record of observed fishes is obtained without destroying the fauna. However, remote systems that are tethered to a surface vessel are severely limited by sea conditions. In addition, camera resolution, light levels, water clarity, depth, and lack of in situ guidance limit the effectiveness of remote observations, and biases are imposed by the attraction or avoidance of some species to the gear.
The objectives of this study were l) to develop a nondestructive, repeatable procedure for assessing diurnally active fishes inhabiting Gray's Reef National Marine Sanctuary, and 2) to describe and compare fish communities associated with ledge, live bottom, and sand habitats.
Discussion
Comparison of the fauna of GRNMS with that of other reefs off the southeastern U.S. coast suggests a high level of variability among reef communities. The fish species composition at GRNMS differs considerably from that of an intensely studied reef in 30 m of water, 44 km south of Beaufort Inlet, North Carolina (Parker, 1990). Of 113 species observed by divers at the two reefs, only 42 (37%) were common to both. Twenty-eight species were unique to GRNMS (Table l) and 43 species were seen only at the North Carolina site. Although more effort was expended at GRNMS (97 transects over 21 hours vs. 51 point counts over 17 hours), 15 more species were observed off North Carolina. The major difference appears to be that more temperate species usually associated with inshore environments (i.e. inshore lizardfish, toadfish, rock sea bass, pigfish, pearly razorfish, and Spanish mackerel) were present at GRNMS, whereas more tropical species (i.e. red grouper, harlequin bass, wrasse bass, white grunt, knobbed porgy, and queen angelfish) were seen at the North Carolina location. The warm waters of the Gulf Stream provide a mechanism for recruitment and survival of many tropical species (Briggs, 1974). GRNMS is 12 km closer to shore and 8 m shallower than the North Carolina site. More importantly, although the position of the Gulf Stream varies across the continental shelf, it generally follows the 200-m isobath which is much farther offshore from GRNMS (105 km) than from the North Carolina site (40 km).
The diversity of species collected is partly a reflection of the sampling method. Our observations on species abundance agree only partially with results obtained by trawling. The 10 most abundant and common species observed in this study included four (tomtate, black sea bass, cubbyu, and longspine porgy) of the most abundant species caught by trawling over hardbottom similar to GRNMS off the southeastern U.S. coast (Wenner, 1983; Sedberry and Van Dolah, 1984). Size, form, and behavior of three of the other six species may preclude their capture by trawls. Two of the three most abundant species' (round scad and slippery dick) are small and fusiform and can pass through the meshes of most trawls. Slippery dick and belted sandfish usually live close to the bottom where they are protected from trawls by the substrate. Round scad have been seen swimming freely in and out of the mouth of trawls towed up to 3.5 knots. A major source of unmeasured error in many visual assessments is observer error in sighting, identifying, counting, and recording. In a prior study of ledge fishes at GRNMS, 10 divers operating in pairs performed five counts of species and individuals. Although all divers were experienced in underwater surveys and were familiar with the fauna, the mean percent similarity for the five teams was only 57% and ranged from 47% to 64%. Videotaping reduces the variance in error among observers and allows virtually unlimited time for study of the images by many individuals (Ebeling et al., 1980).
In choosing between transect and point sampling, we
considered the particular conditions at GRNMS. When properly
applied, the precision of both procedures can be high (Keast
and Harker, 1976; Sale and Douglas, 1981; DeMartini and
Roberts, 1982; Bohnsack and Bannerot, 1986; Witzig, 1988).
Limited visibility at GRNMS was thought to bias point counts
for some species. Bohnsack and Bannerot ( 1986) found that
point samples with a radius of 2 m or less underestimated
abundances of 11 of 15 species observed. In contrast, Parker
(1990) found that during low visibility some species of reef
fish (e.g. gag, black sea bass, and white grunt) concentrate
under and around ledges. Extrapolating density of these fish
in a small visible area to the total population over an
entire reef that consists mostly of low profile, less than 1
meter rock outcroppings sparsely inhabited by fishes grossly
overestimates their abundance. Off bottom tidal currents,
frequently in excess of 20 cm/s at GRNMS, make it impossible
for the vessel to remain stationary for the 5 to 10 minutes
necessary to conduct enumerations. For these reasons we
developed a random transect technique that allowed us to
swim with the prevailing current, covering 86 to 544 m per
transect. Because visibilities at GRNMS can be consistently
less than 5 m, this technique allowed us to sample large
areas with minimum underwater time. The technique is a
consistent, repeatable procedure for assessing the
noncryptic, diurnally active fishes at GRNMS.
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