Chromis

Abstract: Three underwater stereo-video techniques were used to sample the relative densities and species richness of temperate reef fish assemblages at three reef locations and two habitats (high- and low-relief reef) within Hamelin Bay, south-western Australia. The three techniques compared were diver-operated stereo-video strip transects, baited remote stereo-video and unbaited remote stereo-video. While unbaited remote stereo-video and diver-operated stereo-video transects recorded greater species richness at high compared to low-relief reefs, baited remote stereo-video recorded similar species richness at the two habitat types. The diver-operated stereo-video system was manoeuvred through caves and under overhangs recording small, cryptic, cave-dwelling species that were not recorded by either remote video techniques (Trachinops noarlungae, Trachinops brauni, Chromis klunzingeri, Trachichthys australis). Both remote video techniques recorded greater species richness and relative density of the most common species of Labridae, Ophthalmolepsis lineolatus. Baited remote video recorded the rarer, large predatory fish species (e.g. Seriola hippos, Glaucosoma hebraicum, Heterodontus portusjacksoni). None of the techniques sampled small cryptic fish families such as Gobiidae or Blenniidae. A combination of survey techniques is recommended for comprehensive fishery-independent studies that aim to sample broad components of fish assemblages.

Abstract: Fish assemblages associated with nearshore Posidonia oceanica seagrass beds, rocky–algal reefs and unvegetated sandy substrates were studied at two sampling localities, Otranto (Apulian coast) and S. Domino (Tremiti Islands), located in the Southern and Central Adriatic Seas (Mediterranean Sea), respectively. Data were collected in situ by using non-destructive diver visual census methodology. A higher species richness and fish density were observed over thePosidonia seagrass and, in turn, the rocky–algal reef and unvegetated sand habitats. Planktivorous fish species (Spicara maena, Spicara smaris and Chromis chromis) showing patchy distributions were dominant in terms of abundance at both localities. The two former species did not show any particular habitat preference, while C. chromis was common over both the Posidonia and rocky–algal habitats and was only occasionally recorded over sand.Symphodus ocellatus , Diplodus annularis and Spondyliosoma cantharus were the most common species in theP. oceanica seagrass beds. Symphodus roissali, Symphodus tinca and Diplodus sargus were mainly associated with the rocky–algal reef habitats, while other species, such as Gymnammodytes cicerellus,Lithognathus mormyrus , Gobius geniporus, Mullus barbatus and Uranoscopus scaber, were censused over the unvegetated sand habitats. A cluster analysis performed on the entire fish density data set showed distinct groupings for seagrass, rocky–algal and bare sand fish assemblages, regardless of season and sampling site. Fish assemblages in the more structured (seagrass and rocky–algal reef) habitats were relatively similar to each other but quite different from those of the unstructured bare sands. With regard to small-sized specimens (juvenile stages including recruits), those of S. ocellatus, Symphodus mediterraneus, Serranus cabrilla, D. annularis, S. cantharus andSarpa salpa were mainly censused over P. oceanica beds, while juveniles of C. chromis inhabited predominantly rocky–algal bottoms rich in crevices. Small individuals of Coris julis were censused over both P. oceanica and rocky–algal habitats. No juveniles of any species were observed over bare sand. These results suggest that differences in fish species richness and abundance are primarily related to habitat structure. Different habitat preferences were evidenced for the juveniles and adults of several fish species. The ecological importance and need for protection of such shallow inshore habitats are discussed in relation to their crucial role as nurseries for many fish species, and their overall importance in maintaining littoral fish populations and species diversity is emphasized.

Abstract: Little is known of the sources of mortality that affect local population dynam~cs of coralreef fishes. To examine the role of predat~on, resident p~scivorous fishes [moray eels (Muraenidae), large squirrelfishes (Holocentridae), groupers (Serranidae), and snappers (Lutjanidae)] were removed from 3 of 6 isolated patch reefs of living coral near Lee Stocking Island, Bahamas, in 1992. All 6 reefs were then seeded with natural densities of newly settled recruits of 3 species: Chromis cyanea (blue chromis, Pomacentridae), Halichoerespictus (rainbow wrasse, Labridae), and Thalassoma bifasciatum (bluehead wrasse, Labridae). Controls showed that any secondary effects of transplanting new recruits were negligible. Over the next month, survivorship of C. cyanea (mean 41.3%) and H. pictus (80.8%) on the predator-absent reefs was significantly greater than on the predator-present (control) reefs (9.4% for C. cyanea and 43.2% for H. pictus). No statistical difference was evident for T bitasciatum (48.5 vs 37.8% survival), perhaps because juveniles of this species are cleaner fish and/or because they were less conspicuous to predators than the other species. Although the size distributions of the wrasses did not differ between treatments, the size distribution of C. cyanea shifted significantly. At the end of the experiment, surviv.ing C. cyanea were slightly larger on the predator-present reefs (mode = 4.0 cm total length, TL) than on the predator-absent reefs (mode = 3.5 cm TL), despite no significant difference at the start of the experiment (mode for both treatments = 3.0 cm TL). We interpret this size shift as predators differentially consuming smaller recruits and/or surviving recruits growing faster after densities were reduced by predators. Preliminary remote video monitoring of the activity of transient p~scivores [mostly jacks (Carangidae)] over the experimental reefs suggested that such predators may regularly visit isolated reefs in search of prey. If so, transient predators may have accounted for the surprisingly low first-month survivorship (about 40 to 80%) of new recruits on reefs where resident predators were removed. In any case, we conclude that resident predators can substantially alter the local density and size structure of reef fishes shortly after they settle from the plankton. Because piscivores differentially affected the survivorship of different species, predation may also influence the structure of reef-fish communities by altering the relative abundances of prey species established at the time of settlement. K E Y WORDS: Coral-reef fishes . Predat~on . Remuitment . Size distribution . Survivorship