Some coarse associations between the fish fauna and habitat type were detected, with a high diversity of families from temperate and tropical groups, but relatively low species diversity within families, and relatively low abundances (apart from sharks). These patterns were expected for the fish assemblages inhabiting a small island with a limited habitat complexity. The abundance of sightings of Galapagos whaler sharks was striking, greatly exceeding shark counts made with the same baited video technique in other marine parks (see Meekan and Cappo 2004).

With exception of the very high abundance of sharks, and the mix of tropical and temperate families, the functional structure of the assemblages identified on baited videos was broadly similar to the ray/wrasse/carangid assemblages known from algal reef communities on the exposed coasts of south-eastern and south-western Australia (Cappo et al. 2003). These fish assemblages typically prey on the small molluscs, crustaceans and worms that graze on algae, epiphytes or detritus. It is possible that the prevalence of sharks and the location of large predatory fishes in the darkness of the steep Lord Howe slopes are associated with the diurnal migrations of ommastrephid squid and slimy mackerel, but data on diets and prey distribution are lacking.

We also endeavoured to address the contention that these deeper waters may provide for replenishment of the shallow water reefal areas episodically degraded by bleaching, extreme weather events (e.g. Harriott and Smith 2002, Harriott and Banks 2002), temperature stress (e.g. Johannes et al. 1983) and crown-of-thorns starfish infestations (DeVantier and Andrews 1987, Harriott 1995). Lord Howe Island is surrounded by a drowned limestone ridge, interpreted as a fossil reef (Kennedy et al. 2002). It was evident that none of this substrate supported dominant hard coral communities but hard corals were conspicuous and quite prevalent in some areas. Close-up still images revealed small colonies which were invisible to the towed video camera and it is likely that hard corals are more prevalent in these deeper mid-shelf areas than indicated by the data. Deployment of the ROV would allow collection of better imagery to detect the finer scale community structure.

Over the summer months when algal growth is highest, corals might find it difficult to compete for space (Hatcher and Rimmer 1985). There was evidence that sea urchins and some fish may clear space for recruitment by grazing of the algae, and rough weather obviously has the same effect by tearing algae from their holdfasts. From the limited mid-shelf tows to the east of the island, where hard corals were relatively common, there appeared to be much greater topographic complexity and vertical relief than seen elsewhere. The available bathymetric data supports this notion (Dickson and Woodroffe 2002), and it may be that this area and the region south of Balls Pyramid has higher biological diversity associated with the wider range of habitats available in complex topographies.

The gorgonian-dominated benthic communities along the shelf edge were founded on protruding bedrock or stony and rocky seafloors. Filter feeding crinoids and ascidians were also recorded where this topography was interrupted by steeply graded silty sediments. These areas constituted a distinct habitat separated from the shallower reefal habitat by broad sandy regions with little or no benthic structure. This shelf-edge community type may be similar to other deeper water seamount communities where filter feeders predominate (Koslow et al. 1998).

Outcrops, walls and overhangs were observed to a depth of 200m. Of all the video tows, the surveys in this habitat, below the limits of sunlight penetration, produced the most sightings of fish – most notably large kingfish, Seriola lalandi, nannygai (redfish), Centroberyx sp., rosy jobfish, Pristipomoides multidens, and large unidentified groupers, Epinephelinae. These species are fished by islanders with droplines and jigs in this habitat.

Our deeper records of the Ballina angelfish to 200m have improved knowledge of the depth distribution of this species, which has uncertain conservation status. It was first recorded from a fish trap set off the Ballina bar in 1959 and has been recorded infrequently since then. Of particular relevance are the records from 25m depth at Balls Pyramid (Parker 1994) and 90m from the NORFANZ cruise in 2003 (http://www.oceans.gov.au/norfanz/).

In contrast, the towed and baited video surveys did not record large reservoirs of endemic fish species of importance to the local fishery, such as the double-header Coris bulbifrons, in the deeper water remote from the lagoon and fringing reef habitats. This implies that the management of species such as the double-header and bluefish, Girella cyanea, in commonly used fishing areas within the fringing reef and shallower habitats accessible from the shore, is a local priority. We found no evidence that they can be replenished by inshore movements from unfished aggregations in deeper shelf waters but the coverage achieved in this study was quite limited.