coastal and marine natural values of the kimberley - wwf - Australia

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42. Coastal and Marine Natural Values of the Kimberley6.8.8 Habitat Separation in Three Oceanic SeabirdsAt smaller spatial scales, the patchiness of the ecosystem is further defined byseparation into distinct foraging areas for different species. Figure 16 shows thevariation in the distribution of three migrant (non-breeding) pelagic seabirds overa distance of about 120 nautical miles. Anecdotal evidence from several othertrips to the area support the view that this is a consistent pattern.Figure 16: Habitat separation amongst three migrant (non-breeding) pelagicseabirds off Scott Reef in 2004. This pattern of distribution has been observed in alleight expeditions to the area (Mike Carter, pers comm). Tahiti Petrels Pterodromarostrata are seen most often north of Scott Reef, whilst Matsudaira’s Storm PetrelOceanodroma matsudairae and Swinhoe’s Storm Petrel Oceanodroma monorhis(not pictured) are found to the south. Bulwer’s Petrel Bulweria bulwerii is morelikely in the steep shelf areas around the reef itself.These distributions are the result ofsubtle differences in the feeding ability ofdifferent species. Given the low overalllevel of productivity, if all species fed inthe same place, available resources woulddiminish. The area immediately northwestof Scott Reef appears depauperate withfew bird sightings. Nevertheless, Cuvier’sBeaked Whale and Rough-toothedDolphin have been observed here, thoughthey are likely to feed well below thesurface. Of the oceanic Procellariformeseabirds, Bulwer’s Petrel was relativelyabundant, which suggests this speciesis better adapted to feed in areas ofrelatively low surface productivity. Allthree species occurred at the shelf edge tothe south, where there were also severalsightings of Spinner Dolphins.6.8.9 Link between choice of nest siteand feeding areasA key factor that determines distributionin breeding seabirds is choice of nest site.Whilst on eggs or with chicks, birds arebound to return regularly to the centralcolony. If the availability of food withina certain distance drops below a criticalthreshold, breeding may not occur or islikely to be abandoned / unsuccessful.Red-footed Booby is of particular noteas it is confined to foraging within about100km of breeding sites (Jaquemet etal., 2005; Marchant & Higgins, 1990).Frigatebirds are similarly constrained.At Ashmore Reef, a high density of birdswere feeding in immediately adjacentshelf-edge waters (Figure 15) whereproductivity is notably higher thanthe surrounding ocean, particularly byOctober, when the Leeuwin current isexpected to have subsided (Dunlop etal., 1988). It is no coincidence that RedfootedBooby breeds at these locations.Boobies feed on relatively large prey and
Coastal and Marine Natural Values of the Kimberley43.in zones of higher productivity wherecompetition may be greater (Ballance etal., 1997). The species has a number ofstrategies to overcome this problem:1. It feeds on flying fish, which arepatchily distributed only in areas ofhighest zooplankton concentration(Piontkovski & Williams, 1995). Flyingfish are planktonivorous and a highenergy food source, found at a hightrophic level in the food chain; and2. It specialises by catching flying fish onthe wing (Marchant & Higgins, 1990),where they would be unavailable forother species.6.8.10 Turtle hatchling developmentin ocean currentsDuring juvenile stages, many marineanimals are at the mercy of currents anddo not actively swim or fly to where theyfeed. Despite their relatively large size,this is also true for turtle hatchlings. TheAustralian endemic Flatback Turtle isconfined to the shallow continental shelfand the Kimberley region is identified asone of six major aggregation areas (Limpuset al., 1983). The species lays fewer eggsthan other marine turtles but gives rise tomore fully developed young less prone topredation (Limpus et al., 1984). Like allmarine turtles young are swept in currentsto accumulate in productivity-rich areas(Luschi et al., 2003) but Flatback Turtlesdo not disperse into the ocean (Walker &Parmenter, 1990).It is therefore inherent in the evolution ofa stable breeding population that youngleaving Flatback Turtle nest sites endup in the right locations and do not getswept into currents offshore. As with allmarine species that exist at the populationthreshold of their environment, relativelyfew areas of high productivity are likelyto support a substantial proportion of theregional population.6.8.11 Benthic foraging in beaked whalesAt yet smaller spatial scales, theactivity of pelagic megafauna likeCuvier’s beaked whales is associatedwith undersea features such as deepcanyons (Waring et al., 2001), withrecent evidence that it feeds on benthos(Woodside et al., 2006). This means thatthere may be a direct link to the speciesfrom seabed communities, even in oceanover 1,000m deep (the species has beenrecorded foraging at almost 1,900mdepth) (Peter L. Tyack et al., 2006).Locations with top predators are generallyconsidered to be of high biodiversityvalue (Sergio et al., 2006) and thereis evidence that these habitats supportparticularly abundant aggregations offish (section 6.4)Figure 17 shows a sighting of Cuvier’sbeaked whale at Scott Reef (SimonMustoe, personal observations) situatedabove an area of complex bathymetry,including deep ocean canyons over adepth of about 850m.Figure 17: Location of Cuvier’s beaked whale Ziphius cavirostris sighting inOctober 2004. This species is a characteristic resident of deep ocean canyons.
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