We thank more than two dozen fellow NOAA-certified divers for building the safety record of towed-diver surveys, the officers and crew of the NOAA ship Townsend Cromwell for logistic support and field assistance, and Greta Aeby for sharing belt-transect data. An integrated method for benthic habitat assessment is described, in which divers maneuver boards equipped with digital video, temperature, and depth recorders while being towed behind a small boat. The tow path is concurrently recorded by a GPS receiver, and a layback model is applied to more accurately map the data. Percent cover of salient benthic categories is quantified by whole-image analysis of still frames sampled at 30-s intervals. The results of 15 towed-diver surveys at Midway Atoll in theHawaiian Archipelago during a mass coral bleaching event are presented to exemplify the method and are compared to results derived from conventional methods. Towed-diver surveys bridge a gap between large-scale mapping efforts using satellite data and small-scale, roving diver assessments, providing a mesoscale spatial assessment of reef habitats. The spatial coverage of towed-diver surveys provides comprehensive data to managers concerning the extent, intensity, differential taxonomic response, and bathymetric correlates of bleaching.
Georeferenced towed-diver surveys covering more than 100,000 m 2 of benthic habitat and site-specific surveys at 30 sites during 2000-2002 determined distribution and abundance of scleractinian corals at French Frigate Shoals (FFS), Northwestern Hawaiian Islands. Percentage cover of corals was quantified by genus or species in forereef, backreef, and lagoon habitats and at La Perouse Pinnacles using three complementary methods: towed-diver surveys, video transects, and photoquadrats. Habitat-specific colony density and size-class distributions from measurements made within belt transects at fixed sites indicated that three coral genera, Porites, Pocillopora, and Acropora, accounted for more than 93% of total coral cover throughout the atoll, and their relative percentage cover, densities, and size distributions varied according to habitat and geographic location within the atoll. These descriptive data, which provide the most comprehensive overview yet of the scleractinian coral community at FFS, were used to assess the coral reefs' potential for resistance and resilience to selective stressors including bleaching, disease, and Acanthaster outbreaks. They also serve as a baseline for an ecosystem-based, long-term monitoring program with an objective of linking coral community change to other biological and physical factors.
Percent cover of shallow-water (< 20m) scleractinian corals at Maro Reef, an open atoll with no perimeter reef in the Northwestern Hawaiian Islands (NWHI), was quantified from analysis of imagery recorded along more than 81 km of benthic habitat by towed divers and at 18 sites surveyed with video transects and photoquadrats. Colony densities and size class distributions were determined from censuses within belt transects at the same sites. All three methods showed statistically significant differences in total coral cover and relative abundance of coral genera among four geographic sectors characterized by different exposure to prevailing wave regimes. Massive and encrusting growth forms of Porites dominated coral cover and density throughout all sectors, with Montipora the next most dominant genus. Patterns of highest coral cover differed from that of most other classic and open atolls in the NWHI. Populations of Pocillopora, though contributing little to percent cover, attained the largest colony sizes of any reef system in the NWHI surveyed with similar methods. Numerous indicators, including coral percent cover, recovery from bleaching, low coral disease prevalence, rarity of crown-of-thorns seastars, and high larval recruitment rates suggest healthy coral communities at Maro Reef, although the predicted accumulation of marine debris from outside sources remains problematic. These data enable the most comprehensive quantitative description of coral communities at Maro Reef produced to date and are a valuable baseline for assessing change.
The distribution and abundance of scleractinian corals at Pearl and Hermes Atoll (PHA), Northwestern Hawaiian Islands, were determined by georeferenced towed-diver surveys that covered more than 85,000 m of benthic habitat and site-specific surveys at 34 sites during 000 -00. Three complementary methods (towed-diver surveys, videotransects, and photoquadrats) were used to quantify percent cover of corals by genus or species in the fore reef, back reef, lagoon, and channel habitats. Three genera -Porites, Montipora, and Pocillopora-account for 97% of the coral cover throughout the atoll, though their relative abundances vary considerably according to habitat and geographic sector within habitats. Fore-reef communities are dominated by massive and encrusting Porites, while the back reef is dominated by Montipora and the lagoon by Porites compressa. All taxa show habitat-specific differences in colony density and size-class distributions as assessed through colony counts within belt transects at fixed sites. These demographic data, which provide the most thorough quantitative description of the coral communities at PHA to date, are used to focus a discussion on risks of reef degradation from salient contemporary hazards, including bleaching, disease, marine debris, and Acanthaster predation. Coral communities at PHA may be the most vulnerable in the Hawaiian Archipelago to bleaching and accumulation of marine debris, thus warranting special management attention. These data also provide a detailed baseline to which population parameters determined from long-term monitoring surveys can be compared to assess the direction, pace, and drivers of change.
Humpback whales (Megaptera novaeangliae) are well known for their complex song which is culturally transmitted and produced by males. However, the function of singing behavior remains poorly understood. Song was observed from 57 min of acoustic recording in the presence of feeding humpback whales aggregated in the near-shore waters on the west coast of South Africa. The structural organization of the song components, lack of overlap between song units, and consistency in relative received level suggest the song was produced by one “singer.” The unusual timing and location of song production adds further evidence of plasticity in song production.
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