Reef Fish Habitat Associations in the Gulf of California
Reef Fish Habitat Associations in the Gulf of California |
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Matt Levey
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Composition of reef fish assemblages is the result of several interacting processes that include biotic factors of recruitment, predation, competition, and abiotic factors such as habitat (Ebeling and Hixon 1991). How fishes associate with habitat has been the subject of much research, most focused on tropical coral reefs (Luckhurst and Luckhurst 1978, Chabanet and others 1997, Friedlander and Parrish 1998), though some studies have been conducted on deepwater temperate reefs (Stein and others 1994) and rocky bottom kelp forests (Ebeling and others 1980, Larson and DeMartini 1984). Diversity or abundance of reef fishes generally increased with habitat complexity or relief. There are differences among geographic regions and types of reefs so these results can only be applied to the region where the study was conducted. Because of this limitation, more research is needed directly comparing reef fish assemblages between temperate and tropical reefs, using the same methods (Ebeling and Hixon 1991, Ohman and others 1998), to determine to what extent correlations exist.
Comparing similar fish assemblages among geologically and structurally different habitat types allows the characteristics of habitat that influence reef fish assemblages to be isolated. Research of this type has been done in the Indian Ocean by Ohman (and others 1998) who reported differences in fish species diversity between sandstone and coral reefs. Off Sri Lanka, sandstone reefs were more structurally variable than nearby coral reefs. This was contrary to Ebeling and Hixon's (1991) characterization of temperate rocky reefs as having low spatial heterogeneity, lacking small-scale surface complexity, and having a low variety of microhabitats compared with coral reefs. This indicates that habitat configuration, not composition, was a more important factor influencing assemblages of reef fish.
My study will be conducted in the central and southern Gulf of California (Fig. 1). The Gulf contains some of the richest reef communities in the eastern Pacific, with most fish species being Panamic in origin (Walker 1960; Thomson and others 2000). Reef fish assemblages in the central and southern Gulf are similar, though different habitat exists in each region (Thomson and others 2000). Reef habitat of the central Gulf is comprised of rock outcrops, lava flows, boulders, walls, and pinnacles. Some hermatypic coral reefs grow in the southern Gulf.
Simplifying a complex system by grouping its smaller parts and studying the interactions among these groups can aid in understanding the whole (May 1984). For this study, the complex Gulf reef system will be partitioned into the smaller parts of individual fish species, fish trophic groups, fish assemblages, various types of rock habitat, and coral habitat. All sampling will occur at the 10-meter isobath to avoid the influence of depth, which can influence the structure of reef fish assemblages (Friedlander & Parrish 1998; Nuñez-Lara & Arias-González 1998). I will investigate reef fish - habitat associations at the scale of meters to tens of meters, as this is the scale most relevant to individual fish.
Research will be conducted for three field seasons during the months of September 2000-02. In the central Gulf, sampling will be conducted within the Parque Nacional Bahía de Loreto (PNBL). In the southern Gulf, sampling will be conducted within the Parque Marino Nacional de Cabo Pulmo (PMNCB).
PNBL, created in 1996, encompasses an area of 2,065 km2 and includes five islands of varying sizes. Surveys will be conducted off the three nearshore islands: Islas Coronados, Carmen, and Danzante. Macrohabitats (Greene and others 2000) at Isla Coronados and Isla Carmen consist of lava flows, vertical walls, ledges, caves, and crevices, microhabitats are cobble, boulder, sand, and mixed sediment. Isla Danzante has similar habitats, though is it also characterized by a steeply sloping bottom around most of the island (pers. obs.). The Loreto region experiences seasonal upwelling and wide fluctuations in sea surface temperatures (Maluf 1983).
Cabo Pulmo reef, the northernmost coral reef in the eastern Pacific, was officially protected by the creation of PMNCB in 1995. The Mexican government set aside 7111 ha for protection, of which 450 ha is coral reef (Reyes-Bonilla 1997). Cabo Pulmo reef is comprised of three sections, northern, central, and southern (Reyes-Bonilla & Calderon-Aguilera 1999). The northern section has sandy bottoms, low coral cover, and a seasonal influx of freshwater. The central section is comprised of granite, sandstone, and conglomerate that support a high coral cover. The southern section has calm waters because it is protected by the point of Los Frailes. Ten species of coral are found at Cabo Pulmo, with Pocillopora verrucosa and P. meandrina the most abundant.
Diversity, abundance, and distribution of fish species will be obtained during underwater stationary video surveys (Bohnsack & Bannerot 1986; Bortone and others 1991) using a Sony 3-CCD TRV900 miniDV digital video camera inside a Top Dawg underwater housing. Camera auto-focus will be disabled and an orange CY filter used to restore natural colors at depth. This method produces results with similar species diversity and greater abundance with non-cryptic Gulf reef fish than stationary visual censusing (Levey, unpublished manuscript).
Reliable bathymetry or habitat maps are non-existent for these areas, therefore sample sites will initially be chosen by reconnaissance snorkeling immediately before sampling. A habitat area will be 50 x 10 meters and contain at least an estimated 75% of one continuous habitat type. Five habitat types are to be sampled: lava flow, large boulders, small boulders, mixed (areas containing combinations of any of the above habitat types), and coral. Location coordinates will be recorded with a hand held GPS unit at the surface.
Three video surveys will be conducted per site; each centered 25 meters apart following the 10-m isobath. Using SCUBA, divers will descend to the bottom and swim toward the 10-m isobath. A diver will secure a 50 m transect tape to the bottom and extend the tape 5.6 m to demarcate the radius of the first survey circle (area = 100 m2). A three-minute waiting period will commence before recording to allow fish to acclimate to the presence of the divers. Recording will proceed counter-clockwise for five minutes, each quarter of the circle recorded for approximately 1 minute and 15 seconds. Using the same methods, the second survey will be conducted at 25 meters and the third survey at 50 meters.
Habitat variables will be measured immediately subsequent to recording and will be obtained using the following methods. Along the main 50-m transect, three 10-m increments will be chosen randomly (e.g., 0 m, 20 m, and 50 m). At these increments, a 10-m sub-transect will be placed across the 50-m tape, perpendicular to it, with five meters on either side. Habitat variables of rugosity, relief, boulder diversity (complexity), and percentage cover, height, and type of encrusting organisms will be recorded.
Rugosity will be determined using a modified chain method (Luckhurst & Luckhurst 1978). A third tape will follow the bottom contour, into holes and crevices and over rock and/or coral, along the 10 m sub-transect. The ratio of the straight-line distance to linear distance, measured to the nearest centimeter, will be used as an index of substrate rugosity. A crude measure of bottom relief will be determined by using a submersible depth gauge to measure bottom depth at both ends of the sub-transect (Núñez-Lara & Arias-González 1998). To obtain boulder diversity data, all boulders or coral heads directly below the sub-transect will be counted and categorized by size (Enric Sala, pers. comm. 2000). Percentage cover of encrusting organisms will be measured using a modified point-contact method (Schmitt & Holbrook 1986). The presence or absence of encrusting organisms will be recorded at every meter along the 10-m sub-transect. Height of the encrusting organisms will be estimated and recorded. Type of organism, either invertebrate or alga, also will be recorded. All data will be recorded on pre-printed Never-tear© sheets.
Video will be retained in digital form and reviewed on a high-resolution monitor to maintain picture clarity and colors. All non-cryptic fish within the 5.6 m radius circle (using the extended tape-measure as a guide) will be counted and identified to species. If one fish of a school enters the circle, all fish in that school will be counted (Brock 1954). The common damselfishes Stegastes rectifraenum and Stegastes flavilatus are difficult to differentiate, so these species will be recorded as Stegastes spp.
Each 50-m transect will be a replicate. Species and abundance data from the three surveys will be pooled to obtain cumulative species richness and mean abundance, as it is likely that highly mobile individuals would be counted more than once within the same transect. Habitat variable data for each transect will be pooled to obtain averages among the three sub-transects. Boulder diversity will be calculated using the Shannon - Wiener diversity index (Shannon & Weaver 1949; Ohman & Rajasuriya 1998). Assemblage and habitat data will be analyzed using canonical correspondence analysis (CCA), a method ideally suited for the analysis of habitat - assemblage relationships because it allows direct comparison of the two data matrices (Legendre & Legendre 1998). CCA was designed to identify synthetic gradients within ecological data. These gradients, linear combinations of environmental variables, allow the visualization of species - habitat associations using ordination diagrams. Statistical significance of identified associations can be determined by Monte Carlo permutation tests (Ter Braak 1995). CCA analysis will be performed with the computer program Canoco 4.0 (Ter Braak 1990) and ordination plots produced with CanoDraw (Smilauer 1992). In addition to species-habitat comparisons, all fish will be assigned trophic and mobility guilds (Friedlander & Parrish 1998) and these guilds analyzed using the above CCA methods. Guilds will be determined from reviewing the available literature on each species or its closest analog. Trophic guilds will consist of piscivores, planktivores, herbivores, corallivores, and invertebrate feeders. Mobility guilds will be based on residents, transients, and two groups of intermediate movement (10's of meters vs. 100's of meters).
This study will provide insight into the influence of habitat type and structure on fishes and fish assemblages in the Gulf of California. By directly comparing fishes between two distinctly different habitats, the characteristics of habitat that influence fish can be isolated. As more regions and types of reefs are studied, "universal" correlations may be found.
Knowledge of habitat use by fish species of interest is a priority for effective marine reserve management (Carr & Reed 1993; Rowley 1994; Murray et al. 1999). Fish species found within PNBL include the Gulf grouper (Mycteroperca jordani), and the leopard grouper, (Mycteroperca rosacea). These prized sportfishes are considered vulnerable because of fishing pressure, and have been placed on the World Conservation Union's (IUCN) Red List for marine fish species. Other popular sport and commercial fishes present within PNBL waters are the yellow snapper (Lutjanus argentiventris) and the Pacific dog snapper (Lutjanus novemfasciatus). All of the above fishes are part of the grouper-snapper complex, which is considered overfished in many areas (Coleman et al. 2000).
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